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Physical cosmology
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Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the universe and allows study of fundamental questions about its origin, structure, evolution, and ultimate fate.[1] Cosmology as a science originated with the Copernican principle, which implies that celestial bodies obey identical physical laws to those on Earth, and Newtonian mechanics, which first allowed those physical laws to be understood.
Physical cosmology, as it is now understood, began in 1915 with the development of Albert Einstein's general theory of relativity, followed by major observational discoveries in the 1920s: first, Edwin Hubble discovered that the universe contains a huge number of external galaxies beyond the Milky Way; then, work by Vesto Slipher and others showed that the universe is expanding. These advances made it possible to speculate about the origin of the universe, and allowed the establishment of the Big Bang theory, by Georges Lemaître, as the leading cosmological model. A few researchers still advocate a handful of alternative cosmologies;[2] however, most cosmologists agree that the Big Bang theory best explains the observations.[3]
Dramatic advances in observational cosmology since the 1990s, including the cosmic microwave background, distant supernovae and galaxy redshift surveys, have led to the development of a standard model of cosmology. This model requires the universe to contain large amounts of dark matter and dark energy whose nature is currently not well understood, but the model gives detailed predictions that are in excellent agreement with many diverse observations.[3]
Cosmology draws heavily on the work of many disparate areas of research in theoretical and applied physics. Areas relevant to cosmology include particle physics experiments and theory, theoretical and observational astrophysics, general relativity, quantum mechanics, and plasma physics.
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Modern cosmology developed along tandem tracks of theory and observation. In 1916, Albert Einstein published his theory of general relativity, which provided a unified description of gravity as a geometric property of space and time.[4] At the time, Einstein believed in a static universe, but found that his original formulation of the theory did not permit it.[5] This is because masses distributed throughout the universe gravitationally attract, and move toward each other over time.[6] However, he realized that his equations permitted the introduction of a constant term which could counteract the attractive force of gravity on the cosmic scale. Einstein published his first paper on relativistic cosmology in 1917, in which he added this cosmological constant to his field equations in order to force them to model a static universe.[7] The Einstein model describes a static universe; space is finite and unbounded (analogous to the surface of a sphere, which has a finite area but no edges). However, this so-called Einstein model is unstable to small perturbations—it will eventually start to expand or contract.[5] It was later realized that Einstein's model was just one of a larger set of possibilities, all of which were consistent with general relativity and the cosmological principle. The cosmological solutions of general relativity were found by Alexander Friedmann in the early 1920s.[8] His equations describe the Friedmann–Lemaître–Robertson–Walker universe, which may expand or contract, and whose geometry may be open, flat, or closed.
In the 1910s, Vesto Slipher (and later Carl Wilhelm Wirtz) interpreted the red shift of spiral nebulae as a Doppler shift that indicated they were receding from Earth.[12][13] However, it is difficult to determine the distance to astronomical objects. One way is to compare the physical size of an object to its angular size, but a physical size must be assumed in order to do this. Another method is to measure the brightness of an object and assume an intrinsic luminosity, from which the distance may be determined using the inverse-square law. Due to the difficulty of using these methods, they did not realize that the nebulae were actually galaxies outside our own Milky Way, nor did they speculate about the cosmological implications. In 1927, the Belgian Roman Catholic priest Georges Lemaître independently derived the Friedmann–Lemaître–Robertson–Walker equations and proposed, on the basis of the recession of spiral nebulae, that the universe began with the "explosion" of a "primeval atom"[14]—which was later called the Big Bang. In 1929, Edwin Hubble provided an observational basis for Lemaître's theory. Hubble showed that the spiral nebulae were galaxies by determining their distances using measurements of the brightness of Cepheid variable stars. He discovered a relationship between the redshift of a galaxy and its distance. He interpreted this as evidence that the galaxies are receding from Earth in every direction at speeds proportional to their distance from Earth.[15] This fact is now known as Hubble's law, though the numerical factor Hubble found relating recessional velocity and distance was off by a factor of ten, due to not knowing about the types of Cepheid variables.
Given the cosmological principle, Hubble's law suggested that the universe was expanding. Two primary explanations were proposed for the expansion. One was Lemaître's Big Bang theory, advocated and developed by George Gamow. The other explanation was Fred Hoyle's steady state model in which new matter is created as the galaxies move away from each other. In this model, the universe is roughly the same at any point in time.[16][17]
For a number of years, support for these theories was evenly divided. However, the observational evidence began to support the idea that the universe evolved from a hot dense state. The discovery of the cosmic microwave background in 1965 lent strong support to the Big Bang model,[17] and since the precise measurements of the cosmic microwave background by the Cosmic Background Explorer in the early 1990s, few cosmologists have seriously proposed other theories of the origin and evolution of the cosmos.
Energy of the cosmos
[edit]The lightest chemical elements, primarily hydrogen and helium, were created during the Big Bang through the process of nucleosynthesis.[18] In a sequence of stellar nucleosynthesis reactions, smaller atomic nuclei are then combined into larger atomic nuclei, ultimately forming stable iron group elements such as iron and nickel, which have the highest nuclear binding energies.[19] The net process results in a later energy release, meaning subsequent to the Big Bang.[20] Such reactions of nuclear particles can lead to sudden energy releases from cataclysmic variable stars such as novae. Gravitational collapse of matter into black holes also powers the most energetic processes, generally seen in the nuclear regions of galaxies, forming quasars and active galaxies.
Cosmologists cannot explain all cosmic phenomena exactly, such as those related to the accelerating expansion of the universe, using conventional forms of energy. Instead, cosmologists propose a new form of energy called dark energy that permeates all space.[21] One hypothesis is that dark energy is just the vacuum energy, a component of empty space that is associated with the virtual particles that exist due to the uncertainty principle.[22]
There is no clear way to define the total energy in the universe using the most widely accepted theory of gravity, general relativity. Therefore, it remains controversial whether the total energy is conserved in an expanding universe. For instance, each photon that travels through intergalactic space loses energy due to the redshift effect. This energy is not transferred to any other system, so seems to be permanently lost. On the other hand, some cosmologists insist that energy is conserved in some sense; this follows the law of conservation of energy.[23]
Different forms of energy may dominate the cosmos—relativistic particles which are referred to as radiation, or non-relativistic particles referred to as matter. Relativistic particles are particles whose rest mass is zero or negligible compared to their kinetic energy, and so move at the speed of light or very close to it; non-relativistic particles have much higher rest mass than their energy and so move much slower than the speed of light.
As the universe expands, both matter and radiation become diluted. However, the energy densities of radiation and matter dilute at different rates. As a particular volume expands, mass-energy density is changed only by the increase in volume, but the energy density of radiation is changed both by the increase in volume and by the increase in the wavelength of the photons that make it up. Thus the energy of radiation becomes a smaller part of the universe's total energy than that of matter as it expands. The very early universe is said to have been 'radiation dominated' and radiation controlled the deceleration of expansion. Later, as the average energy per photon becomes roughly 10 eV and lower, matter dictates the rate of deceleration and the universe is said to be 'matter dominated'. The intermediate case is not treated well analytically. As the expansion of the universe continues, matter dilutes even further and the cosmological constant becomes dominant, leading to an acceleration in the universe's expansion.
History of the universe
[edit]The history of the universe is a central issue in cosmology. The history of the universe is divided into different periods called epochs, according to the dominant forces and processes in each period. The standard cosmological model is known as the Lambda-CDM model.
Equations of motion
[edit]Within the standard cosmological model, the equations of motion governing the universe as a whole are derived from general relativity with a small, positive cosmological constant.[24] The solution is an expanding universe; due to this expansion, the radiation and matter in the universe cool and become diluted. At first, the expansion is slowed down by gravitation attracting the radiation and matter in the universe. However, as these become diluted, the cosmological constant becomes more dominant and the expansion of the universe starts to accelerate rather than decelerate. In our universe this happened billions of years ago.[25]
Particle physics in cosmology
[edit]During the earliest moments of the universe, the average energy density was very high, making knowledge of particle physics critical to understanding this environment. Hence, scattering processes and decay of unstable elementary particles are important for cosmological models of this period.
As a rule of thumb, a scattering or a decay process is cosmologically important in a certain epoch if the time scale describing that process is smaller than, or comparable to, the time scale of the expansion of the universe.[clarification needed] The time scale that describes the expansion of the universe is with being the Hubble parameter, which varies with time. The expansion timescale is roughly equal to the age of the universe at each point in time.
Timeline of the Big Bang
[edit]Observations suggest that the universe began around 13.8 billion years ago.[26] Since then, the evolution of the universe has passed through three phases. The very early universe, which is still poorly understood, was the split second in which the universe was so hot that particles had energies higher than those currently accessible in particle accelerators on Earth. Therefore, while the basic features of this epoch have been worked out in the Big Bang theory, the details are largely based on educated guesses.
Following this, in the early universe, the evolution of the universe proceeded according to known high energy physics. This is when the first protons, electrons and neutrons formed, then nuclei and finally atoms. With the formation of neutral hydrogen, the cosmic microwave background was emitted. Finally, the epoch of structure formation began, when matter started to aggregate into the first stars and quasars, and ultimately galaxies, clusters of galaxies and superclusters formed. The future of the universe is not yet firmly known, but according to the ΛCDM model it will continue expanding forever.
Areas of study
[edit]Below, some of the most active areas of inquiry in cosmology are described, in roughly chronological order. This does not include all of the Big Bang cosmology, which is presented in Timeline of the Big Bang.
Very early universe
[edit]
The early, hot universe appears to be well explained by the Big Bang from roughly 10−33 seconds onwards, but there are several problems. One is that there is no compelling reason, using current particle physics, for the universe to be flat, homogeneous, and isotropic (see the cosmological principle). Moreover, grand unified theories of particle physics suggest that there should be magnetic monopoles in the universe, which have not been found. These problems are resolved by a brief period of cosmic inflation, which drives the universe to flatness, smooths out anisotropies and inhomogeneities to the observed level, and exponentially dilutes the monopoles.[27] The physical model behind cosmic inflation is extremely simple, but it has not yet been confirmed by particle physics, and there are difficult problems reconciling inflation and quantum field theory.[vague] Some cosmologists think that string theory and brane cosmology will provide an alternative to inflation.[28]
Another major problem in cosmology is what caused the universe to contain far more matter than antimatter. Cosmologists can observationally deduce that the universe is not split into regions of matter and antimatter. If it were, there would be X-rays and gamma rays produced as a result of annihilation, but this is not observed. Therefore, some process in the early universe must have created a small excess of matter over antimatter, and this (currently not understood) process is called baryogenesis. Three required conditions for baryogenesis were derived by Andrei Sakharov in 1967, and requires a violation of the particle physics symmetry, called CP-symmetry, between matter and antimatter.[29] However, particle accelerators measure too small a violation of CP-symmetry to account for the baryon asymmetry. Cosmologists and particle physicists look for additional violations of the CP-symmetry in the early universe that might account for the baryon asymmetry.[30]
Both the problems of baryogenesis and cosmic inflation are very closely related to particle physics, and their resolution might come from high energy theory and experiment, rather than through observations of the universe.[speculation?]
Big Bang Theory
[edit]Big Bang nucleosynthesis is the theory of the formation of the elements in the early universe. It finished when the universe was about three minutes old and its temperature dropped below that at which nuclear fusion could occur. Big Bang nucleosynthesis had a brief period during which it could operate, so only the very lightest elements were produced. Starting from hydrogen ions (protons), it principally produced deuterium, helium-4, and lithium. Other elements were produced in only trace abundances. The basic theory of nucleosynthesis was developed in 1948 by George Gamow, Ralph Asher Alpher, and Robert Herman.[31] It was used for many years as a probe of physics at the time of the Big Bang, as the theory of Big Bang nucleosynthesis connects the abundances of primordial light elements with the features of the early universe.[18] Specifically, it can be used to test the equivalence principle,[32] to probe dark matter, and test neutrino physics.[33] Some cosmologists have proposed that Big Bang nucleosynthesis suggests there is a fourth "sterile" species of neutrino.[34]
Standard model of Big Bang cosmology
[edit]The ΛCDM (Lambda cold dark matter) or Lambda-CDM model is a parametrization of the Big Bang cosmological model in which the universe contains a cosmological constant, denoted by Lambda (Greek Λ), associated with dark energy, and cold dark matter (abbreviated CDM). It is frequently referred to as the standard model of Big Bang cosmology.[35][36]
Cosmic microwave background
[edit]The cosmic microwave background is radiation left over from decoupling after the epoch of recombination when neutral atoms first formed. At this point, radiation produced in the Big Bang stopped Thomson scattering from charged ions. The radiation, first observed in 1965 by Arno Penzias and Robert Woodrow Wilson, has a perfect thermal black-body spectrum. It has a temperature of 2.7 kelvins today and is isotropic to one part in 105. Cosmological perturbation theory, which describes the evolution of slight inhomogeneities in the early universe, has allowed cosmologists to precisely calculate the angular power spectrum of the radiation, and it has been measured by the recent satellite experiments (COBE and WMAP)[37] and many ground and balloon-based experiments (such as Degree Angular Scale Interferometer, Cosmic Background Imager, and Boomerang).[38] One of the goals of these efforts is to measure the basic parameters of the Lambda-CDM model with increasing accuracy, as well as to test the predictions of the Big Bang model and look for new physics. The results of measurements made by WMAP, for example, have placed limits on the neutrino masses.[39]
Newer experiments, such as QUIET and the Atacama Cosmology Telescope, are trying to measure the polarization of the cosmic microwave background.[40] These measurements are expected to provide further confirmation of the theory as well as information about cosmic inflation, and the so-called secondary anisotropies,[41] such as the Sunyaev-Zel'dovich effect and Sachs-Wolfe effect, which are caused by interaction between galaxies and clusters with the cosmic microwave background.[42][43]
On 17 March 2014, astronomers of the BICEP2 Collaboration announced the apparent detection of B-mode polarization of the CMB, considered to be evidence of primordial gravitational waves that are predicted by the theory of inflation to occur during the earliest phase of the Big Bang.[9][10][11][44] However, later that year the Planck collaboration provided a more accurate measurement of cosmic dust, concluding that the B-mode signal from dust is the same strength as that reported from BICEP2.[45][46] On 30 January 2015, a joint analysis of BICEP2 and Planck data was published and the European Space Agency announced that the signal can be entirely attributed to interstellar dust in the Milky Way.[47]
Formation and evolution of large-scale structure
[edit]Understanding the formation and evolution of the largest and earliest structures (i.e., quasars, galaxies, clusters and superclusters) is one of the largest efforts in cosmology. Cosmologists study a model of hierarchical structure formation in which structures form from the bottom up, with smaller objects forming first, while the largest objects, such as superclusters, are still assembling.[48] One way to study structure in the universe is to survey the visible galaxies, in order to construct a three-dimensional picture of the galaxies in the universe and measure the matter power spectrum. This is the approach of the Sloan Digital Sky Survey and the 2dF Galaxy Redshift Survey.[49][50]
Another tool for understanding structure formation is simulations, which cosmologists use to study the gravitational aggregation of matter in the universe, as it clusters into filaments, superclusters and voids. Most simulations contain only non-baryonic cold dark matter, which should suffice to understand the universe on the largest scales, as there is much more dark matter in the universe than visible, baryonic matter. More advanced simulations are starting to include baryons and study the formation of individual galaxies. Cosmologists study these simulations to see if they agree with the galaxy surveys, and to understand any discrepancy.[51]
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Other, complementary observations to measure the distribution of matter in the distant universe and to probe reionization include:
- The Lyman-alpha forest, which allows cosmologists to measure the distribution of neutral atomic hydrogen gas in the early universe, by measuring the absorption of light from distant quasars by the gas.[52]
- The 21-centimeter absorption line of neutral atomic hydrogen also provides a sensitive test of cosmology.[53]
- Weak lensing, the distortion of a distant image by gravitational lensing due to dark matter.[54]
These will help cosmologists settle the question of when and how structure formed in the universe.
Dark matter
[edit]Evidence from Big Bang nucleosynthesis, the cosmic microwave background, structure formation, and galaxy rotation curves suggests that about 23% of the mass of the universe consists of non-baryonic dark matter, whereas only 4% consists of visible, baryonic matter. The gravitational effects of dark matter are well understood, as it behaves like a cold, non-radiative fluid that forms haloes around galaxies. Dark matter has never been detected in the laboratory, and the particle physics nature of dark matter remains completely unknown. Without observational constraints, there are a number of candidates, such as a stable supersymmetric particle, a weakly interacting massive particle, a gravitationally-interacting massive particle, an axion, and a massive compact halo object. Alternatives to the dark matter hypothesis include a modification of gravity at small accelerations (MOND) or an effect from brane cosmology. TeVeS is a version of MOND that can explain gravitational lensing.[55]
Dark energy
[edit]If the universe is flat, there must be an additional component making up 73% (in addition to the 23% dark matter and 4% baryons) of the energy density of the universe. This is called dark energy. In order not to interfere with Big Bang nucleosynthesis and the cosmic microwave background, it must not cluster in haloes like baryons and dark matter. There is strong observational evidence for dark energy, as the total energy density of the universe is known through constraints on the flatness of the universe, but the amount of clustering matter is tightly measured, and is much less than this. The case for dark energy was strengthened in 1999, when measurements demonstrated that the expansion of the universe has begun to gradually accelerate.[56]
Apart from its density and its clustering properties, nothing is known about dark energy. Quantum field theory predicts a cosmological constant (CC) much like dark energy, but 120 orders of magnitude larger than that observed.[57] Steven Weinberg and a number of string theorists (see string landscape) have invoked the 'weak anthropic principle': i.e. the reason that physicists observe a universe with such a small cosmological constant is that no physicists (or any life) could exist in a universe with a larger cosmological constant. Many cosmologists find this an unsatisfying explanation: perhaps because while the weak anthropic principle is self-evident (given that living observers exist, there must be at least one universe with a cosmological constant (CC) which allows for life to exist) it does not attempt to explain the context of that universe.[58] For example, the weak anthropic principle alone does not distinguish between:
- Only one universe will ever exist and there is some underlying principle that constrains the CC to the value we observe.
- Only one universe will ever exist and although there is no underlying principle fixing the CC, we got lucky.
- Lots of universes exist (simultaneously or serially) with a range of CC values, and of course ours is one of the life-supporting ones.
Other possible explanations for dark energy include quintessence[59] or a modification of gravity on the largest scales.[60] The effect on cosmology of the dark energy that these models describe is given by the dark energy's equation of state, which varies depending upon the theory. The nature of dark energy is one of the most challenging problems in cosmology.
A better understanding of dark energy is likely to solve the problem of the ultimate fate of the universe. In the current cosmological epoch, the accelerated expansion due to dark energy is preventing structures larger than superclusters from forming. It is not known whether the acceleration will continue indefinitely, perhaps even increasing until a Big Rip, or whether it will eventually reverse, lead to a Big Freeze, or follow some other scenario.[61]
Gravitational waves
[edit]Gravitational waves are ripples in the curvature of spacetime that propagate as waves at the speed of light, generated in certain gravitational interactions that propagate outward from their source. Gravitational-wave astronomy is an emerging branch of observational astronomy which aims to use gravitational waves to collect observational data about sources of detectable gravitational waves such as binary star systems composed of white dwarfs, neutron stars, and black holes; and events such as supernovae, and the formation of the early universe shortly after the Big Bang.[62]
In 2016, the LIGO Scientific Collaboration and Virgo Collaboration teams announced that they had made the first observation of gravitational waves, originating from a pair of merging black holes using the Advanced LIGO detectors.[63][64][65] On 15 June 2016, a second detection of gravitational waves from coalescing black holes was announced.[66] Besides LIGO, many other gravitational-wave observatories (detectors) are under construction.[67]
Other areas of inquiry
[edit]Cosmologists also study:
- Whether primordial black holes were formed in our universe, and what happened to them.[68]
- Detection of cosmic rays with energies above the GZK cutoff,[69] and whether it signals a failure of special relativity at high energies.
- The equivalence principle,[32] whether or not Einstein's general theory of relativity is the correct theory of gravitation,[70] and if the fundamental laws of physics are the same everywhere in the universe.[71]
- Biophysical cosmology: a type of physical cosmology that studies and understands life as part or an inherent part of physical cosmology. It stresses that life is inherent to the universe and therefore frequent.[72]
See also
[edit]References
[edit]- ^ For an overview, see George FR Ellis (2006). "Issues in the Philosophy of Cosmology". In Jeremy Butterfield & John Earman (ed.). Philosophy of Physics (Handbook of the Philosophy of Science) 3 volume set. North Holland. arXiv:astro-ph/0602280. Bibcode:2006astro.ph..2280E. ISBN 978-0-444-51560-5.
- ^ "An Open Letter to the Scientific Community as published in New Scientist, May 22, 2004". cosmologystatement.org. 1 April 2014. Archived from the original on 1 April 2014. Retrieved 27 September 2017.
- ^ a b Navas, S.; Amsler, C.; Gutsche, T.; Hanhart, C.; Hernández-Rey, J. J.; Lourenço, C.; Masoni, A.; Mikhasenko, M.; Mitchell, R. E.; Patrignani, C.; Schwanda, C.; Spanier, S.; Venanzoni, G.; Yuan, C. Z.; Agashe, K. (1 August 2024). "Review of Particle Physics: Big-Bang Cosmology". Physical Review D. 110 (3): 444. doi:10.1103/PhysRevD.110.030001. hdl:11384/149923. ISSN 2470-0010.
- ^ "Albert Einstein – Biographical". Nobel Prize. Archived from the original on 22 January 2025. Retrieved 23 January 2025.
- ^ a b Liddle, A. (2003). An Introduction to Modern Cosmology. Wiley. p. 51. ISBN 978-0-470-84835-7.
- ^ Vilenkin, Alex (2007). Many worlds in one: the search for other universes. New York: Hill and Wang, A division of Farrar, Straus and Giroux. p. 19. ISBN 978-0-8090-6722-0.
- ^ Jones, Mark; Lambourne, Robert (2004). An introduction to galaxies and cosmology. Milton Keynes Cambridge, UK; New York: Open University Cambridge University Press. p. 228. ISBN 978-0-521-54623-2.
- ^ Jones, Mark; Lambourne, Robert (2004). An introduction to galaxies and cosmology. Milton Keynes Cambridge, UK; New York: Open University Cambridge University Press. p. 232. ISBN 978-0-521-54623-2.
- ^ a b "BICEP2 2014 Results Release". The BICEP / Keck CMB Experiments. 17 March 2014. Retrieved 18 March 2014.
- ^ a b Clavin, Whitney (17 March 2014). "NASA Technology Views Birth of the Universe". NASA. Retrieved 17 March 2014.
- ^ a b Overbye, Dennis (17 March 2014). "Detection of Waves in Space Buttresses Landmark Theory of Big Bang". The New York Times. Archived from the original on 1 January 2022. Retrieved 17 March 2014.
- ^ Slipher, V. M. (1922). "Further Notes on Spectrographic Observations of Nebulae and Clusters". Publications of the American Astronomical Society. 4: 284–286. Bibcode:1922PAAS....4..284S.
- ^ Seitter, Waltraut C.; Duerbeck, Hilmar W. (1999). Egret, Daniel; Heck, Andre (eds.). "Carl Wilhelm Wirtz – Pioneer in Cosmic Dimensions". Harmonizing Cosmic Distance Scales in a Post-Hipparcos Era. ASP Conference Series. 167: 237–242. Bibcode:1999ASPC..167..237S. ISBN 978-1-886733-88-6.
- ^ Lemaître, Georges (1927). "Un Univers homogène de masse constante et de rayon croissant rendant compte de la vitesse radiale des nébuleuses extra-galactiques". Annales de la Société Scientifique de Bruxelles (in French). A47: 49–59. Bibcode:1927ASSB...47...49L.
- ^ Hubble, Edwin (March 1929). "A Relation between Distance and Radial Velocity among Extra-Galactic Nebulae". Proceedings of the National Academy of Sciences of the United States of America. 15 (3): 168–173. Bibcode:1929PNAS...15..168H. doi:10.1073/pnas.15.3.168. PMC 522427. PMID 16577160.
- ^ Hoyle, Fred (1948). "A New Model for the Expanding Universe". Monthly Notices of the Royal Astronomical Society. 108 (5): 372–382. Bibcode:1948MNRAS.108..372H. doi:10.1093/mnras/108.5.372.
- ^ a b "Big Bang or Steady State?". Ideas of Cosmology. American Institute of Physics. Archived from the original on 12 June 2015. Retrieved 29 July 2015.
- ^ a b Burles, Scott; Nollett, Kenneth M.; Turner, Michael S. (May 2001). "Big Bang Nucleosynthesis Predictions for Precision Cosmology". The Astrophysical Journal. 552 (1): L1 – L5. arXiv:astro-ph/0010171. Bibcode:2001ApJ...552L...1B. doi:10.1086/320251. S2CID 118904816.
- ^ Burbidge, E. M.; Burbidge, G. R.; Fowler, W. A.; Hoyle, F. (1957). "Synthesis of the Elements in Stars". Reviews of Modern Physics. 29 (4): 547–650. Bibcode:1957RvMP...29..547B. doi:10.1103/RevModPhys.29.547.
- ^ Frautschi, S. (13 August 1982). "Entropy in an expanding universe". Science. 217 (4560): 593–599. Bibcode:1982Sci...217..593F. doi:10.1126/science.217.4560.593. PMID 17817517. S2CID 27717447.
- ^ Kirshner, R. P. (2003). "Throwing Light on Dark Energy". Science. 300 (5627): 1914–1918. Bibcode:2003Sci...300.1914K. doi:10.1126/science.1086879. PMID 12817141. S2CID 43859435.
- ^ Frieman, Joshua A.; Turner, Michael S.; Huterer, Dragan (2008). "Dark Energy and the Accelerating Universe". Annual Review of Astronomy & Astrophysics. 46 (1): 385–432. arXiv:0803.0982. Bibcode:2008ARA&A..46..385F. doi:10.1146/annurev.astro.46.060407.145243. S2CID 15117520.
- ^ e.g. Liddle, A. (2003). An Introduction to Modern Cosmology. Wiley. ISBN 978-0-470-84835-7. This argues cogently "Energy is always, always, always conserved."
- ^ Ojeda, P.; Rosu, H. (June 2006). "Supersymmetry of FRW barotropic cosmologies". Int. J. Theor. Phys. 45 (6): 1191–1196. arXiv:gr-qc/0510004. Bibcode:2006IJTP...45.1152R. doi:10.1007/s10773-006-9123-2. S2CID 119496918.
- ^ Springel, Volker; Frenk, Carlos S.; White, Simon D.M. (2006). "The large-scale structure of the Universe". Nature. 440 (7088): 1137–1144. arXiv:astro-ph/0604561. Bibcode:2006Natur.440.1137S. CiteSeerX 10.1.1.255.8877. doi:10.1038/nature04805. PMID 16641985. S2CID 8900982.
- ^ "Cosmic Detectives". The European Space Agency (ESA). 2 April 2013. Retrieved 25 April 2013.
- ^ Guth, Alan H. (15 January 1981). "Inflationary universe: A possible solution to the horizon and flatness problems". Physical Review D. 23 (2): 347–356. Bibcode:1981PhRvD..23..347G. doi:10.1103/PhysRevD.23.347.
- ^ Pogosian, Levon; Tye, S.-H. Henry; Wasserman, Ira; Wyman, Mark (2003). "Observational constraints on cosmic string production during brane inflation". Physical Review D. 68 (2) 023506. arXiv:hep-th/0304188. Bibcode:2003PhRvD..68b3506P. doi:10.1103/PhysRevD.68.023506.
- ^ Canetti, Laurent; et al. (September 2012). "Matter and antimatter in the universe". New Journal of Physics. 14 (9) 095012. arXiv:1204.4186. Bibcode:2012NJPh...14i5012C. doi:10.1088/1367-2630/14/9/095012. S2CID 119233888.
- ^ Pandolfi, Stefania (30 January 2017). "New source of asymmetry between matter and antimatter". CERN. Retrieved 9 April 2018.
- ^ Peebles, Phillip James Edwin (April 2014). "Discovery of the hot Big Bang: What happened in 1948". The European Physical Journal H. 39 (2): 205–223. arXiv:1310.2146. Bibcode:2014EPJH...39..205P. doi:10.1140/epjh/e2014-50002-y. S2CID 118539956.
- ^ a b Boucher, V.; Gérard, J.-M.; Vandergheynst, P.; Wiaux, Y. (November 2004). "Cosmic microwave background constraints on the strong equivalence principle". Physical Review D. 70 (10) 103528. arXiv:astro-ph/0407208. Bibcode:2004PhRvD..70j3528B. doi:10.1103/PhysRevD.70.103528. S2CID 1197376.
- ^ Cyburt, Richard H.; Fields, Brian D.; Olive, Keith A.; Yeh, Tsung-Han (January 2016). "Big bang nucleosynthesis: Present status". Reviews of Modern Physics. 88 (1) 015004. arXiv:1505.01076. Bibcode:2016RvMP...88a5004C. doi:10.1103/RevModPhys.88.015004. S2CID 118409603.
- ^ Lucente, Michele; Abada, Asmaa; Arcadi, Giorgio; Domcke, Valerie (March 2018). "Leptogenesis, dark matter and neutrino masses". arXiv:1803.10826 [hep-ph].
- ^ Collaboration, Planck; Ade, P. A. R.; Aghanim, N.; Arnaud, M.; Ashdown, M.; Aumont, J.; Baccigalupi, C.; Banday, A. J.; Barreiro, R. B.; Bartlett, J. G.; Bartolo, N.; Battaner, E.; Battye, R.; Benabed, K.; Benoit, A.; Benoit-Levy, A.; Bernard, J. -P.; Bersanelli, M.; Bielewicz, P.; Bonaldi, A.; Bonavera, L.; Bond, J. R.; Borrill, J.; Bouchet, F. R.; Boulanger, F.; Bucher, M.; Burigana, C.; Butler, R. C.; Calabrese, E.; et al. (2016). "Planck 2015 Results. XIII. Cosmological Parameters". Astronomy & Astrophysics. 594 (13): A13. arXiv:1502.01589. Bibcode:2016A&A...594A..13P. doi:10.1051/0004-6361/201525830. S2CID 119262962.
- ^ Carlisle, Camille M. (10 February 2015). "Planck Upholds Standard Cosmology". Sky and Telescope. 130 (1). Sky & Telescope Media: 28. Bibcode:2015S&T...130a..28C. Retrieved 9 April 2018.
- ^ Lamarre, Jean-Michel (2010). "The Cosmic Microwave Background". In Huber, M. C. E.; Pauluhn, A.; Culhane, J. L.; Timothy, J. G.; Wilhelm, K.; Zehnder, A. (eds.). Observing Photons in Space. ISSI Scientific Reports Series. Vol. 9. pp. 149–162. Bibcode:2010ISSIR...9..149L.
- ^ Sievers, J. L.; et al. (2003). "Cosmological Parameters from Cosmic Background Imager Observations and Comparisons with BOOMERANG, DASI, and MAXIMA". The Astrophysical Journal. 591 (2): 599–622. arXiv:astro-ph/0205387. Bibcode:2003ApJ...591..599S. doi:10.1086/375510. S2CID 14939106.
- ^ Hinshaw, G.; et al. (October 2013). "Nine-year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Parameter Results". The Astrophysical Journal Supplement. 208 (2): 19. arXiv:1212.5226. Bibcode:2013ApJS..208...19H. doi:10.1088/0067-0049/208/2/19. S2CID 37132863.
- ^ Naess, Sigurd; Hasselfield, Matthew; McMahon, Jeff; Niemack, Michael D.; et al. (October 2014). "The Atacama Cosmology Telescope: CMB polarization at 200 < l < 9000". Journal of Cosmology and Astroparticle Physics. 2014 (10): 007. arXiv:1405.5524. Bibcode:2014JCAP...10..007N. doi:10.1088/1475-7516/2014/10/007. S2CID 118593572.
- ^ Baumann, Daniel; et al. (2009). "Probing Inflation with CMB Polarization". CMB Polarization Workshop: Theory and Foregrounds: CMBPol Mission Concept Study. Vol. 1141. pp. 10–120. arXiv:0811.3919. Bibcode:2009AIPC.1141...10B. doi:10.1063/1.3160885.
- ^ Scranton, R.; Connolly, A. J.; Nichol, R. C.; Stebbins, A.; Szapudi, I.; Eisenstein, D. J.; et al. (July 2003). "Physical Evidence for Dark Energy". arXiv:astro-ph/0307335.
- ^ Refregier, A. (1999). "Overview of Secondary Anisotropies of the CMB". In de Oliveira-Costa, A.; Tegmark, M. (eds.). Microwave Foregrounds. ASP Conference Series. Vol. 181. p. 219. arXiv:astro-ph/9904235. Bibcode:1999ASPC..181..219R. ISBN 978-1-58381-006-4.
- ^ Overbye, Dennis (25 March 2014). "Ripples From the Big Bang". The New York Times. Archived from the original on 1 January 2022. Retrieved 24 March 2014.
- ^ Planck Collaboration (2016). "Planck intermediate results. XXX. The angular power spectrum of polarized dust emission at intermediate and high Galactic latitudes". Astronomy & Astrophysics. 586 (133): A133. arXiv:1409.5738. Bibcode:2016A&A...586A.133P. doi:10.1051/0004-6361/201425034. S2CID 9857299.
- ^ Overbye, Dennis (22 September 2014). "Study Confirms Criticism of Big Bang Finding". The New York Times. Archived from the original on 1 January 2022. Retrieved 22 September 2014.
- ^ Cowen, Ron (30 January 2015). "Gravitational waves discovery now officially dead". nature. doi:10.1038/nature.2015.16830.
- ^ Heß, Steffen; Kitaura, Francisco-Shu; Gottlöber, Stefan (November 2013). "Simulating structure formation of the Local Universe". Monthly Notices of the Royal Astronomical Society. 435 (3): 2065–2076. arXiv:1304.6565. Bibcode:2013MNRAS.435.2065H. doi:10.1093/mnras/stt1428. S2CID 119198359.
- ^ Cole, Shaun; Percival, Will J.; Peacock, John A.; Norberg, Peder; Baugh, Carlton M.; Frenk, Carlos S.; et al. (2005). "The 2dF Galaxy Redshift Survey: power-spectrum analysis of the final data set and cosmological implications". Monthly Notices of the Royal Astronomical Society. 362 (2): 505–534. arXiv:astro-ph/0501174. Bibcode:2005MNRAS.362..505C. doi:10.1111/j.1365-2966.2005.09318.x. S2CID 6906627.
- ^ Percival, Will J.; et al. (2007). "The Shape of the Sloan Digital Sky Survey Data Release 5 Galaxy Power Spectrum". The Astrophysical Journal. 657 (2): 645–663. arXiv:astro-ph/0608636. Bibcode:2007ApJ...657..645P. doi:10.1086/510615. S2CID 53141475.
- ^ Kuhlen, Michael; Vogelsberger, Mark; Angulo, Raul (November 2012). "Numerical simulations of the dark universe: State of the art and the next decade". Physics of the Dark Universe. 1 (1–2): 50–93. arXiv:1209.5745. Bibcode:2012PDU.....1...50K. doi:10.1016/j.dark.2012.10.002. S2CID 119232040.
- ^ Weinberg, David H.; Davé, Romeel; Katz, Neal; Kollmeier, Juna A. (May 2003). "The Lyman-α Forest as a Cosmological Tool". In Holt, S.H.; Reynolds, C. S. (eds.). AIP Conference Proceedings: The Emergence of Cosmic Structure. AIP Conference Series. Vol. 666. pp. 157–169. arXiv:astro-ph/0301186. Bibcode:2003AIPC..666..157W. CiteSeerX 10.1.1.256.1928. doi:10.1063/1.1581786. S2CID 118868536.
- ^ Furlanetto, Steven R.; Oh, S. Peng; Briggs, Frank H. (October 2006). "Cosmology at low frequencies: The 21 cm transition and the high-redshift Universe". Physics Reports. 433 (4–6): 181–301. arXiv:astro-ph/0608032. Bibcode:2006PhR...433..181F. CiteSeerX 10.1.1.256.8319. doi:10.1016/j.physrep.2006.08.002. S2CID 118985424.
- ^ Munshi, Dipak; Valageas, Patrick; van Waerbeke, Ludovic; Heavens, Alan (2008). "Cosmology with weak lensing surveys". Physics Reports. 462 (3): 67–121. arXiv:astro-ph/0612667. Bibcode:2008PhR...462...67M. CiteSeerX 10.1.1.337.3760. doi:10.1016/j.physrep.2008.02.003. PMID 16286284. S2CID 9279637.
- ^ Klasen, M.; Pohl, M.; Sigl, G. (November 2015). "Indirect and direct search for dark matter". Progress in Particle and Nuclear Physics. 85: 1–32. arXiv:1507.03800. Bibcode:2015PrPNP..85....1K. doi:10.1016/j.ppnp.2015.07.001. S2CID 118359390.
- ^ Perlmutter, Saul; Turner, Michael S.; White, Martin (1999). "Constraining Dark Energy with Type Ia Supernovae and Large-Scale Structure". Physical Review Letters. 83 (4): 670–673. arXiv:astro-ph/9901052. Bibcode:1999PhRvL..83..670P. doi:10.1103/PhysRevLett.83.670. S2CID 119427069.
- ^ Adler, Ronald J.; Casey, Brendan; Jacob, Ovid C. (July 1995). "Vacuum catastrophe: An elementary exposition of the cosmological constant problem". American Journal of Physics. 63 (7): 620–626. Bibcode:1995AmJPh..63..620A. doi:10.1119/1.17850.
- ^ Siegfried, Tom (11 August 2006). "A 'Landscape' Too Far?". Science. 313 (5788): 750–753. doi:10.1126/science.313.5788.750. PMID 16902104. S2CID 118891996.
- ^ Sahni, Varun (2002). "The cosmological constant problem and quintessence". Classical and Quantum Gravity. 19 (13) 304: 3435–3448. arXiv:astro-ph/0202076. Bibcode:2002CQGra..19.3435S. doi:10.1088/0264-9381/19/13/304. S2CID 13532332.
- ^ Nojiri, S.; Odintsov, S. D. (2006). "Introduction to Modified Gravity and Gravitational Alternative for Dark Energy". International Journal of Geometric Methods in Modern Physics. 04 (1): 115–146. arXiv:hep-th/0601213. Bibcode:2007IJGMM..04..115N. doi:10.1142/S0219887807001928. S2CID 119458605.
- ^ Fernández-Jambrina, L. (September 2014). "Grand rip and grand bang/crunch cosmological singularities". Physical Review D. 90 (6) 064014. arXiv:1408.6997. Bibcode:2014PhRvD..90f4014F. doi:10.1103/PhysRevD.90.064014. S2CID 118328824.
- ^ Colpi, Monica; Sesana, Alberto (2017). "Gravitational Wave Sources in the Era of Multi-Band Gravitational Wave Astronomy". In Gerard, Augar; Eric, Plagnol (eds.). An Overview of Gravitational Waves: Theory, Sources and Detection. pp. 43–140. arXiv:1610.05309. Bibcode:2017ogw..book...43C. doi:10.1142/9789813141766_0002. ISBN 978-981-314-176-6. S2CID 119292265.
- ^ Castelvecchi, Davide; Witze, Witze (11 February 2016). "Einstein's gravitational waves found at last". Nature News. doi:10.1038/nature.2016.19361. S2CID 182916902. Retrieved 11 February 2016.
- ^ Abbott, B. P.; et al. (LIGO Scientific Collaboration and Virgo Collaboration) (2016). "Observation of Gravitational Waves from a Binary Black Hole Merger". Physical Review Letters. 116 (6) 061102. arXiv:1602.03837. Bibcode:2016PhRvL.116f1102A. doi:10.1103/PhysRevLett.116.061102. PMID 26918975. S2CID 124959784.
- ^ "Gravitational waves detected 100 years after Einstein's prediction". www.nsf.gov. National Science Foundation. Retrieved 11 February 2016.
- ^ Overbye, Dennis (15 June 2016). "Scientists Hear a Second Chirp From Colliding Black Holes". The New York Times. Archived from the original on 1 January 2022. Retrieved 15 June 2016.
- ^ "The Newest Search for Gravitational Waves has Begun". LIGO Caltech. LIGO. 18 September 2015. Retrieved 29 November 2015.
- ^ Kovetz, Ely D. (2017). "Probing Primordial Black Hole Dark Matter with Gravitational Waves". Physical Review Letters. 119 (13) 131301. arXiv:1705.09182. Bibcode:2017PhRvL.119m1301K. doi:10.1103/PhysRevLett.119.131301. PMID 29341709. S2CID 37823911.
- ^ Takeda, M.; et al. (10 August 1998). "Extension of the Cosmic-Ray Energy Spectrum beyond the Predicted Greisen-Zatsepin-Kuz'min Cutoff". Physical Review Letters. 81 (6): 1163–1166. arXiv:astro-ph/9807193. Bibcode:1998PhRvL..81.1163T. doi:10.1103/PhysRevLett.81.1163. S2CID 14864921.
- ^ Turyshev, Slava G. (2008). "Experimental Tests of General Relativity". Annual Review of Nuclear and Particle Science. 58 (1): 207–248. arXiv:0806.1731. Bibcode:2008ARNPS..58..207T. doi:10.1146/annurev.nucl.58.020807.111839. S2CID 119199160.
- ^ Uzan, Jean-Philippe (March 2011). "Varying Constants, Gravitation and Cosmology". Living Reviews in Relativity. 14 (1) 2. arXiv:1009.5514. Bibcode:2011LRR....14....2U. doi:10.12942/lrr-2011-2. PMC 5256069. PMID 28179829.
- ^ Dick, Steven J. (2020). "The Biophysical Cosmology: The Place of Bioastronomy in the History of Science". Space, Time, and Aliens. Cham: Springer International Publishing. pp. 53–58. doi:10.1007/978-3-030-41614-0_4. ISBN 978-3-030-41613-3.
Further reading
[edit]Popular
[edit]- Greene, Brian (2005). The Fabric of the Cosmos. Penguin Books Ltd. ISBN 978-0-14-101111-0.
- Guth, Alan (1997). The Inflationary Universe: The Quest for a New Theory of Cosmic Origins. Random House. ISBN 978-0-224-04448-6.
- Hawking, Stephen W. (1988). A Brief History of Time: From the Big Bang to Black Holes. Bantam Books, Inc. ISBN 978-0-553-38016-3.
- Hawking, Stephen W. (2001). The Universe in a Nutshell. Bantam Books, Inc. ISBN 978-0-553-80202-3.
- Ostriker, Jeremiah P.; Mitton, Simon (2013). Heart of Darkness: Unraveling the mysteries of the invisible Universe. Princeton, NJ: Princeton University Press. ISBN 978-0-691-13430-7.
- Singh, Simon (2005). Big Bang: The Origin of the Universe. Fourth Estate. Bibcode:2004biba.book.....S. ISBN 978-0-00-716221-5.
- Weinberg, Steven (1993) [1978]. The First Three Minutes. Basic Books. ISBN 978-0-465-02437-7.
Textbooks
[edit]- Cheng, Ta-Pei (2005). Relativity, Gravitation and Cosmology: a Basic Introduction. Oxford and New York: Oxford University Press. ISBN 978-0-19-852957-6. Introductory cosmology and general relativity without the full tensor apparatus, deferred until the last part of the book.
- Baumann, Daniel (2022). Cosmology. Cambridge: Cambridge University Press. ISBN 978-0-19-852957-6. Modern introduction to cosmology covering the homogeneous and inhomogeneous universe as well as inflation and the CMB.
- Dodelson, Scott (2003). Modern Cosmology. Academic Press. ISBN 978-0-12-219141-1. An introductory text, released slightly before the WMAP results.
- Gal-Or, Benjamin (1987) [1981]. Cosmology, Physics and Philosophy. Springer Verlag. ISBN 0-387-90581-2.
- Grøn, Øyvind; Hervik, Sigbjørn (2007). Einstein's General Theory of Relativity with Modern Applications in Cosmology. New York: Springer. ISBN 978-0-387-69199-2.
- Harrison, Edward (2000). Cosmology: the science of the universe. Cambridge University Press. ISBN 978-0-521-66148-5. For undergraduates; mathematically gentle with a strong historical focus.
- Kutner, Marc (2003). Astronomy: A Physical Perspective. Cambridge University Press. ISBN 978-0-521-52927-3. An introductory astronomy text.
- Kolb, Edward; Michael Turner (1988). The Early Universe. Addison-Wesley. ISBN 978-0-201-11604-5. The classic reference for researchers.
- Liddle, Andrew (2003). An Introduction to Modern Cosmology. John Wiley. ISBN 978-0-470-84835-7. Cosmology without general relativity.
- Liddle, Andrew; David Lyth (2000). Cosmological Inflation and Large-Scale Structure. Cambridge. ISBN 978-0-521-57598-0. An introduction to cosmology with a thorough discussion of inflation.
- Mukhanov, Viatcheslav (2005). Physical Foundations of Cosmology. Cambridge University Press. ISBN 978-0-521-56398-7.
- Padmanabhan, T. (1993). Structure formation in the universe. Cambridge University Press. ISBN 978-0-521-42486-8. Discusses the formation of large-scale structures in detail.
- Peacock, John (1998). Cosmological Physics. Cambridge University Press. ISBN 978-0-521-42270-3. An introduction including more on general relativity and quantum field theory than most.
- Peebles, P. J. E. (1993). Principles of Physical Cosmology. Princeton University Press. ISBN 978-0-691-01933-8. Strong historical focus.
- Peebles, P. J. E. (1980). The Large-Scale Structure of the Universe. Princeton University Press. ISBN 978-0-691-08240-0. The classic work on large-scale structure and correlation functions.
- Rees, Martin (2002). New Perspectives in Astrophysical Cosmology. Cambridge University Press. ISBN 978-0-521-64544-7.
- Weinberg, Steven (1971). Gravitation and Cosmology. John Wiley. ISBN 978-0-471-92567-5. A standard reference for the mathematical formalism.
- Weinberg, Steven (2008). Cosmology. Oxford University Press. ISBN 978-0-19-852682-7.
External links
[edit]
From groups
[edit]- Cambridge Cosmology – from Cambridge University (public home page)
- Cosmology 101 – from the NASA WMAP group
- Center for Cosmological Physics. University of Chicago, Chicago, Illinois
- Origins, Nova Online – Provided by PBS
From individuals
[edit]- Gale, George, "Cosmology: Methodological Debates in the 1930s and 1940s", The Stanford Encyclopedia of Philosophy, Edward N. Zalta (ed.)
- Madore, Barry F., "Level 5 : A Knowledgebase for Extragalactic Astronomy and Cosmology". Caltech and Carnegie. Pasadena, California.
- Tyler, Pat, and Newman, Phil, "Beyond Einstein". Laboratory for High Energy Astrophysics (LHEA) NASA Goddard Space Flight Center.
- Wright, Ned. "Cosmology tutorial and FAQ". Division of Astronomy & Astrophysics, UCLA.
- Musser, George (February 2004). "Four Keys to Cosmology". Scientific American. Retrieved 22 March 2015.
- Burgess, Cliff; Quevedo, Fernando (November 2007). "The Great Cosmic Roller-Coaster Ride". Scientific American (print). pp. 52–59.
(subtitle) Could cosmic inflation be a sign that our universe is embedded in a far vaster realm?
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Physical cosmology
View on GrokipediaHistorical Development
Ancient and Medieval Concepts
Early conceptions of the cosmos in ancient Greece emphasized a geocentric model, with Earth at the center of a finite, spherical universe composed of nested celestial spheres carrying the stars, planets, Moon, and Sun in uniform circular motion. Aristotle (384–322 BCE) developed this framework in his treatise On the Heavens, positing an eternal universe without beginning or end, where the sublunary realm of Earth was composed of the four elements (earth, water, air, fire) subject to change and decay, while the superlunary realm consisted of incorruptible aether in perfect, eternal rotation. Greeks such as Pythagoras and Plato had earlier argued for Earth's sphericity based on observations like the curved shadow during lunar eclipses and the changing positions of stars with latitude, a view solidified by Aristotle's arguments that the sphere represented the most perfect geometric form.[5][6][7] In the 3rd century BCE, Aristarchus of Samos proposed an alternative heliocentric model, suggesting that Earth and the planets orbited the stationary Sun, with Earth rotating daily on its axis to explain the apparent motion of the stars; however, this idea was largely rejected in favor of the geocentric view due to inconsistencies with observed planetary retrogrades and the lack of stellar parallax. By the 2nd century CE, Claudius Ptolemy refined the Aristotelian geocentric system in his Almagest, incorporating epicycles—smaller circular orbits whose centers moved along larger deferents centered near Earth—to account for irregular planetary motions, achieving predictive accuracy for astronomical tables that dominated for over a millennium. The physical interpretation in the Aristotelian-Ptolemaic tradition envisioned up to 55 nested spheres for the seven classical planets, Sun, Moon, and fixed stars, all driven by a divine "prime mover."[8][9][10] During the medieval period, Islamic scholars preserved and advanced Greek cosmology through translations and innovations, often integrating it with theological perspectives on a created yet ordered universe. In the 11th century, Al-Biruni (973–1048 CE) accurately measured Earth's radius at approximately 6,339 km using trigonometric methods from mountain elevations and horizon dip angles, a value remarkably close to modern estimates of 6,371 km and demonstrating empirical precision beyond Ptolemy's approximations. Islamic astronomers compiled extensive zij (astronomical handbooks) with tables for planetary positions, eclipses, and timekeeping, such as Al-Battani's Zij (9th century), which corrected Ptolemaic parameters and influenced later European works; these tables supported practical needs like determining prayer times (qibla directions) and calendars, blending observation with religious requirements.[11][12][13] In medieval Europe, Ptolemaic cosmology was adopted via Arabic translations, harmonized with Christian theology by figures like Thomas Aquinas (1225–1274), who in Summa Theologica reconciled Aristotle's eternal spheres with biblical creation by positing God as the ultimate cause of motion in a finite, hierarchical universe centered on Earth as humanity's divinely appointed domain. This synthesis reinforced geocentric orthodoxy, viewing celestial perfection as reflective of divine order, though tensions arose from scriptural interpretations favoring a created cosmos over Aristotle's eternity. By the Renaissance, accumulating observational anomalies—such as the precession of equinoxes and inconsistencies in planetary predictions—fostered doubts about the Ptolemaic system, culminating in Nicolaus Copernicus's 1543 publication of De revolutionibus orbium coelestium, which revived heliocentrism by proposing circular orbits around the Sun to simplify celestial mechanics, though still retaining some epicycles.[14][15][16]Modern Foundations and Key Discoveries
The foundations of modern physical cosmology were laid in the early 20th century with Albert Einstein's development of general relativity, published in 1915, which provided a new framework for understanding gravity as the curvature of spacetime. In 1917, Einstein applied this theory to the universe as a whole in his paper "Cosmological Considerations in the General Theory of Relativity," proposing a static, finite, and unbounded model to maintain a stable cosmos, as an expanding or contracting universe seemed incompatible with the prevailing astronomical observations at the time. To achieve this static solution, Einstein introduced the cosmological constant term (Λ) into his field equations, acting as a repulsive force to counterbalance gravitational attraction, though he later reportedly called it his "greatest blunder" after evidence of expansion emerged. This static model was challenged by Alexander Friedmann in 1922, who derived solutions to Einstein's equations showing that the universe could expand or contract dynamically, depending on the initial conditions and matter density, thus introducing the concept of an evolving cosmos without the need for a cosmological constant. Building on Friedmann's work, Georges Lemaître proposed in 1927 the "primeval atom" hypothesis, envisioning the universe as originating from a single, hot, dense state that expanded over time, akin to the modern Big Bang theory, and he connected this to observations of nebular redshifts suggesting recession. Empirical confirmation came in 1929 with Edwin Hubble's discovery of the velocity-distance relation, v = H₀ d, where galaxies recede at speeds proportional to their distance d, with H₀ as the Hubble constant, establishing the expanding universe observationally. A pivotal prediction of the Big Bang model was the existence of a cosmic microwave background (CMB) radiation, first theoretically forecasted by George Gamow, Ralph Alpher, and Robert Herman in 1948 as the remnant glow from the early hot universe, cooled to a few kelvins by expansion. This was serendipitously discovered in 1965 by Arno Penzias and Robert Wilson, who detected a uniform 2.7 K microwave signal across the sky while investigating radio noise, later interpreted as the CMB confirming the hot Big Bang origin. In 1963, the identification of quasars—highly luminous, distant objects signaling active galactic nuclei powered by supermassive black holes—further expanded the observable universe's scale and highlighted energetic processes in the early cosmos.[17] Additionally, the 1948 Alpher-Bethe-Gamow paper predicted Big Bang nucleosynthesis, successfully accounting for the observed abundances of light elements like helium. The cosmological constant, initially discarded, saw revival in the late 20th century as a possible explanation for dark energy driving accelerated expansion.Fundamental Components and Energy Budget
Observable Universe and Scale
The observable universe refers to the spherical region centered on Earth from which electromagnetic radiation has had sufficient time to reach us since the Big Bang, approximately 13.8 billion years ago. Due to the expansion of space, the comoving radius of this region is estimated at about 46.5 billion light-years, representing the distance light from the cosmic microwave background (CMB) has traveled to us today. This limit defines the particle horizon, beyond which no information can have reached observers on Earth, as calculated using cosmological parameters from CMB observations.[18] On large scales, the observable universe exhibits a hierarchical structure shaped by gravitational instability from primordial density fluctuations. Galaxies, numbering around 2 trillion in total, serve as the basic building blocks, aggregating into clusters of hundreds to thousands of members, which in turn assemble into superclusters spanning tens of megaparsecs. These superclusters are linked by elongated filaments of galaxies and gas, forming the cosmic web, while expansive voids—regions of low density—occupy much of the volume, creating a filamentary network that permeates the observable universe. Notable examples include the Sloan Great Wall, a vast filamentary structure extending over 1.4 billion light-years and containing multiple superclusters.[19][20] Mapping this structure relies on redshift surveys that measure galaxy velocities via Doppler shifts, enabling distance estimates through the Hubble relation for nearby objects and more sophisticated integrals for distant ones. Surveys like the Sloan Digital Sky Survey (SDSS) have cataloged millions of galaxies, providing three-dimensional maps that reveal clustering patterns. Complementary methods include angular diameter distance, which relates an object's physical size to its observed angular extent, and luminosity distance, derived from flux and intrinsic brightness comparisons, both essential for calibrating cosmic scales across redshifts.[21] A key challenge in understanding the observable universe's uniformity is the horizon problem: distant regions, separated by angles exceeding the causal horizon at recombination (when the universe was about 380,000 years old), exhibit nearly identical temperatures in the CMB, implying correlations without prior causal interaction under standard Big Bang expansion. This uniformity underscores the need for mechanisms to explain large-scale isotropy within the observable volume.[22]Baryonic Matter and Radiation
Baryonic matter, the ordinary matter composed of protons and neutrons, constitutes approximately 5% of the universe's total energy density. This fraction is determined from measurements of the cosmic microwave background (CMB) anisotropies, yielding a present-day baryon density parameter .[23] The primordial composition of baryonic matter, established during Big Bang nucleosynthesis (BBN), is dominated by hydrogen and helium, with mass fractions of about 75% for hydrogen-1 and 25% for helium-4, alongside trace amounts of deuterium, helium-3, and lithium-7. Heavier elements, or "metals," make up less than 2% by mass and are produced later through stellar nucleosynthesis. Within the baryonic component, the distribution spans various forms, with only a small portion locked in luminous structures. Stars account for roughly 0.5% of the total critical density, representing about 10% of all baryonic matter, primarily in galaxies. The majority resides in diffuse states: intergalactic and intracluster hot gas contributes around 50-60%, cold neutral and molecular gas in galaxies and the intergalactic medium adds another 20-30%, while dust and stellar remnants like white dwarfs and neutron stars form minor fractions. Supermassive and stellar-mass black holes, formed from collapsed stars and mergers, harbor an estimated 1-2% of baryonic matter, though their exact contribution remains uncertain due to incomplete censuses. Relic radiation, including photons and neutrinos from the early universe, comprises a negligible but precisely measured portion of the energy budget, about 0.01% today. The CMB photons dominate the radiation density, with neutrinos contributing significantly due to their relativistic nature. The total radiation density parameter is given by , where from CMB data, consistent with three neutrino species.[23] This radiation imprints key signatures in large-scale structure, such as baryon acoustic oscillations (BAO), which are frozen density waves from the early plasma era manifesting as a characteristic scale of ~150 Mpc in galaxy clustering. BAO serve as standard rulers for distance measurements, confirming the baryon density independently.Dark Matter
Dark matter constitutes approximately 27% of the universe's total energy budget, comprising the majority of the non-baryonic matter that influences gravitational dynamics without interacting electromagnetically.[23] This invisible component is inferred to cluster on small scales, providing the gravitational scaffolding for the formation of galaxies and large-scale structures.[23] The primary evidence for dark matter arises from discrepancies in galactic dynamics, where observed rotation curves of spiral galaxies remain flat at large radii, indicating the presence of unseen mass far beyond the visible stellar distribution. Pioneering spectroscopic observations in the 1970s by Vera Rubin and her collaborators demonstrated that stars in the outer regions of galaxies like Andromeda orbit at velocities inconsistent with the luminous matter alone, requiring an extended dark matter halo to account for the gravitational pull.[24] Further confirmation comes from gravitational lensing in colliding galaxy clusters, such as the Bullet Cluster (1E0657-558), where weak lensing maps reveal mass concentrations offset from the hot intracluster gas, directly separating the gravitational effects of dark matter from baryonic components during the merger.[25] Additionally, the power spectrum of cosmic microwave background (CMB) anisotropies shows distinct peaks that align with a universe dominated by cold dark matter, as the third acoustic peak's amplitude and position constrain the matter density to support structure growth.[23] Dark matter is characterized by its density parameter , derived from CMB temperature and polarization data, which dictates its contribution to the universe's expansion history.[23] In the standard paradigm, it is predominantly "cold," consisting of non-relativistic particles with low velocity dispersion that allow efficient clustering into dense halos on sub-galactic scales, facilitating the hierarchical formation of cosmic structures.[26] Variants include "warm" dark matter, with mildly relativistic particles that suppress small-scale structure formation due to free-streaming lengths on the order of dwarf galaxy scales, and "hot" dark matter, highly relativistic species that would smooth out fluctuations too effectively to match observations.[26] Leading candidates for dark matter particles include weakly interacting massive particles (WIMPs), hypothetical fermions with masses around 10-1000 GeV that could have frozen out in the early universe via weak-scale interactions. Axions, ultralight pseudoscalar bosons proposed to solve the strong CP problem in quantum chromodynamics, offer another possibility with masses near eV and coherent field oscillations. Sterile neutrinos, right-handed counterparts to active neutrinos with keV-scale masses, represent a warm dark matter option that could explain X-ray emission lines from galaxy clusters. Detection efforts span collider searches at the Large Hadron Collider (LHC) for WIMP production in high-energy proton collisions, which have set exclusion limits on supersymmetric models without direct discovery,[27] and direct underground experiments like XENONnT and LUX-ZEPLIN (LZ), which use multi-tonne liquid xenon detectors to probe WIMP-nucleus scattering and have established limits below cm² for 30 GeV/c² masses as of 2025, with no detection reported.[28] As an alternative, primordial black holes formed in the early universe's high-density fluctuations have been proposed to account for all dark matter if their mass spectrum peaks around asteroid sizes, though microlensing constraints limit their abundance.[29]Dark Energy
Dark energy is a hypothetical form of energy permeating all of space that exerts a negative pressure, driving the accelerated expansion of the universe on cosmological scales. It is inferred to dominate the current energy budget of the universe, comprising approximately 68% of the total energy density, with the remaining contributions from dark matter (about 27%) and ordinary matter (about 5%). This component contrasts with the attractive gravitational effects of matter, instead producing a repulsive force that counteracts the universe's tendency to decelerate due to gravity. The Friedmann equations incorporate dark energy through an additional term, often denoted as the cosmological constant Λ, which modifies the dynamics of cosmic expansion. Recent observations from the Dark Energy Spectroscopic Instrument (DESI) as of 2025 show a 3.1-sigma preference for evolving dark energy over a constant Λ, though the ΛCDM model remains the prevailing framework.[30] The existence of dark energy was first evidenced in 1998 through observations of type Ia supernovae, which serve as standard candles for measuring cosmic distances. Two independent teams, led by Adam Riess and Saul Perlmutter, analyzed high-redshift supernovae and found that these distant explosions appeared fainter than expected in a decelerating universe, indicating an accelerating expansion. Their results constrained the cosmological constant density parameter to Ω_Λ ≈ 0.7 at high confidence, implying a positive value for this repulsive component rather than a matter-dominated slowdown. Subsequent measurements, including those from the cosmic microwave background, have refined this to Ω_Λ ≈ 0.68 ± 0.01. Dark energy is characterized by its equation of state parameter w = p/ρ, where p is pressure and ρ is energy density; for the simplest model, a cosmological constant, w = -1 exactly, meaning the energy density remains constant as the universe expands. This arises naturally from the vacuum energy predicted by quantum field theory, where virtual particle fluctuations contribute a pervasive energy density. However, the theoretical vacuum energy density exceeds the observed value by up to 120 orders of magnitude, posing the infamous cosmological constant problem that remains unresolved. Alternative models, such as quintessence, propose a dynamic scalar field with time-varying w close to -1 but potentially evolving, allowing the energy density to decrease slowly over cosmic time. Other theoretical interpretations include modified gravity theories, such as f(R) gravity, where the acceleration emerges from alterations to general relativity's action rather than an additional energy component, effectively mimicking dark energy effects on large scales. In extreme cases, phantom dark energy models with w < -1 predict an escalating repulsion that could culminate in a "Big Rip" singularity, where the universe's expansion tears apart galaxies, stars, and eventually atoms in finite time. Observational probes of dark energy include the integrated Sachs-Wolfe (ISW) effect, where the decay of gravitational potentials in an accelerating universe imprints temperature anisotropies on the cosmic microwave background through photon interactions with evolving large-scale structures.Cosmological Models and Equations
Friedmann Equations and Geometry
Physical cosmology relies on the Friedmann equations to describe the dynamics of the universe's expansion within the framework of general relativity. These equations emerge from applying Einstein's field equations to a homogeneous and isotropic universe, modeled by the Friedmann-Lemaître-Robertson-Walker (FLRW) metric. The FLRW metric assumes spatial uniformity and isotropy, capturing the large-scale structure of the cosmos through a time-dependent scale factor that governs the relative separation of comoving coordinates.[31] The FLRW metric takes the form where is the speed of light, is cosmic time, are comoving spatial coordinates, and is the curvature parameter determining the geometry: for flat (Euclidean) space, for closed (spherical) space, and for open (hyperbolic) space. This metric was independently developed by Friedmann, Lemaître, Robertson, and Walker in the 1920s and 1930s as a solution to Einstein's equations under the cosmological principle.[31][32] To derive the Friedmann equations, substitute the FLRW metric into Einstein's field equations, where is the Ricci tensor, is the Ricci scalar, is the cosmological constant, is the gravitational constant, and is the stress-energy tensor for a perfect fluid with energy density and pressure . Assuming a diagonal metric and perfect fluid form (with four-velocity ), the non-zero components yield the dynamical equations for . The first Friedmann equation relates the Hubble parameter to the contents of the universe: The second Friedmann equation describes the acceleration: These equations were first derived by Alexander Friedmann in 1922 using an early form of the metric without the cosmological constant, later generalized by others.[32][33] The curvature term encodes the spatial geometry: positive implies a finite, closed universe; zero a flat, infinite one; negative an open, infinite one. The total density parameter , where , determines flatness: for , for closed, and for open geometries. The critical density defines the threshold for flatness in a matter-dominated universe without or curvature, such that yields . Density parameters like for matter and for the cosmological constant thus parameterize deviations from flatness and drive the expansion dynamics.[32][34]Lambda-CDM Model
The Lambda-CDM model serves as the standard concordance framework in physical cosmology, integrating the Friedmann equations with observational constraints to describe the universe's composition and evolution, though recent observations hint at possible refinements. It posits a flat universe dominated by a cosmological constant representing dark energy, cold dark matter (CDM), baryonic matter, and relativistic radiation as the key energy components, with dark energy comprising approximately 68% of the current energy budget, dark matter about 27%, baryons 5%, and radiation a negligible fraction today. This model assumes the cosmological principle, positing that the universe is homogeneous and isotropic on large scales greater than about 100 Mpc. It further incorporates Gaussian initial conditions for primordial density perturbations, originating from quantum fluctuations amplified during cosmic inflation. Recent data from the Dark Energy Spectroscopic Instrument (DESI), as of March 2025, provide evidence at approximately 4.2σ significance that dark energy may evolve over time rather than remaining constant, potentially indicating deviations from the standard ΛCDM paradigm.[35] The Lambda-CDM framework is characterized by six primary parameters that encapsulate its free variables: the present-day Hubble constant , the physical baryon density , the physical cold dark matter density , the scaled angular size of the sound horizon at recombination , the Thomson optical depth to reionization , and the scalar spectral index of the primordial power spectrum. These parameters are constrained through Bayesian inference applied to datasets such as the cosmic microwave background (CMB), baryon acoustic oscillations, and supernova distances, with recent integrations including DESI 2025 results refining estimates while highlighting tensions. A key prediction is a nearly scale-invariant power-law initial spectrum of curvature perturbations, with , which seeds density fluctuations that grow under gravity. In the Lambda-CDM paradigm, cold dark matter enables hierarchical structure formation, wherein small-scale density perturbations collapse first into dwarf galaxies and dark matter halos, which subsequently merge to build larger structures like galaxy clusters over cosmic time. The model also forecasts distinct acoustic peaks in the CMB temperature and polarization power spectra, arising from baryon-photon oscillations in the early universe's plasma, with the first peak corresponding to the sound horizon scale at recombination. These predictions align well with observations, as evidenced by the Planck 2018 analysis combined with later datasets, which yield km/s/Mpc (from CMB), for the root-mean-square matter fluctuation amplitude on 8 Mpc scales as of 2025, providing a high-confidence fit to CMB anisotropies and large-scale structure data.[23] Despite its successes, the Lambda-CDM model faces tensions in parameter estimates, notably the discrepancy, where CMB-derived values exceed those from weak lensing and galaxy clustering surveys by about 2-3, suggesting potential refinements in small-scale physics or systematics, alongside emerging evidence for dynamic dark energy.Horizons and Expansion Metrics
In physical cosmology, horizons derived from the expansion of the universe delineate the boundaries of causality and observability. The particle horizon marks the proper distance to the farthest point from which light could have reached an observer since the Big Bang, limiting the causal past of any event. It is mathematically expressed aswhere is the scale factor normalized to 1 at the present time , and is the speed of light. In the CDM model, this integral accounts for the evolving expansion history dominated sequentially by radiation, matter, and dark energy, yielding a current particle horizon of approximately 46 billion light-years.[36] This boundary defines the theoretical extent of the observable universe and underscores the horizon problem—regions of the cosmic microwave background that appear causally connected despite exceeding the particle horizon—which cosmic inflation addresses by enabling superluminal expansion in the early universe.[36] For universes undergoing accelerated expansion due to dark energy, an event horizon emerges as the maximum proper distance from which light emitted at the present time can ever reach the observer in the infinite future. It is given by
In the CDM framework, the dominance of the cosmological constant ensures this integral converges, resulting in a current event horizon of about 16 billion light-years, beyond which events are forever unobservable.[36] This horizon implies that acceleration isolates future cosmic evolution from our causal influence, a feature absent in decelerating models.[37] The Hubble horizon, also known as the Hubble sphere, provides a snapshot of the expansion at any epoch, defined as the proper distance , where is the Hubble parameter. Regions beyond this distance recede faster than light relative to the observer, though this does not violate relativity due to the uniformity of expansion. Currently, in CDM, the Hubble horizon measures roughly 14 billion light-years, shrinking during matter domination and approaching the event horizon asymptotically as dark energy prevails.[36] The age of the universe, , quantifies the duration of cosmic expansion from the Big Bang to the present and is computed via
with incorporating contributions from radiation, matter, and . Planck Collaboration measurements yield billion years, reflecting the integrated effects of these eras: brief radiation domination (), extended matter domination (), and recent -driven acceleration.[23] Conformal time simplifies the treatment of light propagation in expanding spacetimes by transforming the metric into a conformally flat form, where null geodesics follow straight lines. It is defined as
directly relating to the particle horizon via . In CDM, the current conformal time is approximately 47 billion years (in units where ), saturating toward a finite maximum of about 64 billion years due to acceleration, which aids in analyzing photon paths and cosmological perturbations.[36]
Early Universe Dynamics
Inflationary Epoch
The inflationary epoch represents a brief period of exponential expansion in the early universe, posited to resolve key inconsistencies in the standard Big Bang model. Proposed by Alan Guth in 1981, this phase involves a scalar field, known as the inflaton φ, whose potential energy V(φ) drives the universe's scale factor to grow by a factor of e^{60} to e^{70} over approximately 10^{-36} to 10^{-32} seconds after the Big Bang.[22][38] This rapid growth dilutes any pre-existing inhomogeneities and relics, setting the stage for the subsequent hot Big Bang phase. The mechanism relies on the inflaton field slowly rolling down its potential under gravity-dominated conditions, characterized by the slow-roll approximation. In this regime, the field's kinetic energy is negligible compared to its potential, leading to quasi-de Sitter expansion where the Hubble parameter H remains nearly constant. The slow-roll parameters quantify deviations from exact exponential growth: where primes denote derivatives with respect to φ (in reduced Planck units, M_{Pl} = 1), and inflation occurs when both ε ≪ 1 and |η| ≪ 1. This dynamics, refined by Andrei Linde and others in 1982, ensures prolonged expansion without fine-tuning. Inflation addresses the flatness problem by dynamically attracting the universe's density parameter Ω to unity, the horizon problem by allowing causally disconnected regions to achieve thermal equilibrium through prior sub-horizon coherence, and the monopole problem by expanding GUT-scale relics beyond observable scales.[22] Quantum fluctuations in the inflaton field during this epoch seed density perturbations, yielding a nearly scale-invariant power spectrum P(k) ∝ k^{n_s - 1} with scalar spectral index n_s ≈ 0.965, consistent with cosmic microwave background observations.[39] Specific models illustrate the versatility of inflation. Chaotic inflation, introduced by Linde in 1983, assumes arbitrary initial field values, making inflation a generic outcome of quantum fluctuations in a quadratic or higher-order potential. Hybrid inflation, proposed by Linde in 1994, incorporates two fields where a "waterfall" transition ends inflation, facilitating integration with particle physics symmetries. Eternal inflation variants, also from Linde in 1986, suggest perpetual expansion in some regions due to stochastic quantum effects, leading to a multiverse of bubble universes. The epoch concludes via reheating, where the oscillating inflaton decays into relativistic particles, transitioning to radiation domination.[22]Big Bang Nucleosynthesis
Big Bang nucleosynthesis (BBN) refers to the production of the lightest elements in the universe during the first few minutes after the Big Bang, when the temperature and density allowed nuclear reactions to occur in thermal equilibrium. This process took place between temperatures of approximately 1 MeV (corresponding to about 1 second after the Big Bang) and 0.1 MeV (around 3 minutes), as the universe expanded and cooled rapidly.[40] The key input parameter is the baryon-to-photon ratio, η ≈ 6 × 10^{-10}, which determines the overall abundance of baryonic matter relative to radiation and influences the efficiency of nuclear binding.[40] BBN provides a stringent test of the standard cosmological model, as its predictions depend critically on the weak interaction rates and the expansion rate H(T) ∝ √g_* T^2 / M_Pl, where g_* is the effective number of relativistic degrees of freedom and M_Pl is the Planck mass.[40] The sequence begins with the freeze-out of the neutron-proton ratio, which occurs when the weak interaction rate for interconversion (n ↔ p + e^- + ν_e) falls below the expansion rate at T ≈ 1 MeV. At equilibrium, the initial n/p ratio is exp(-Δm / T) ≈ 1/6, where Δm ≈ 1.293 MeV is the neutron-proton mass difference; free neutron decay then reduces it to about 1/7 by the start of nucleosynthesis.[40] This sets the seed for helium production, as nearly all available neutrons are eventually captured. The onset of heavier element formation is delayed by the deuterium bottleneck: although the reaction p + n ↔ D + γ has a low binding energy threshold of 2.224 MeV, the high photon-to-baryon ratio (η^{-1} ≈ 10^{10}) maintains a tail of high-energy photons that photodissociate deuterium until T ≈ 0.08 MeV, when the reaction rate exceeds the destruction rate.[40] Once sufficient deuterium accumulates, it acts as a catalyst for subsequent reactions, including D + p → ^3He + γ and D + D → ^3He + n or ^3H + p, rapidly building up to ^4He via processes like ^3He + n → ^4He + γ and ^3H + p → ^4He + γ.[40] The primary output is ^4He, with nearly all neutrons incorporated into it due to its high binding energy of 28.3 MeV, yielding a primordial mass fraction Y_p ≈ 0.247. Trace amounts of other light elements are also produced: deuterium survives at D/H ≈ 2.45 × 10^{-5}, helium-3 at ^3He/H ≈ 1.04 × 10^{-5}, and lithium-7 at ^7Li/H ≈ 4.82 × 10^{-10}, with smaller contributions from ^7Be (which decays to ^7Li post-BBN).[40] These predictions, first outlined in foundational work by Wagoner, Fowler, and Hoyle, have been refined through detailed numerical simulations incorporating precise nuclear cross-sections and neutron lifetime measurements (τ_n = 879.4 ± 0.6 s). Observations of primordial abundances in low-metallicity regions, such as D/H ≈ (2.547 ± 0.033) × 10^{-5} from quasar absorption lines and Y_p ≈ 0.2449 ± 0.0040 from extragalactic H II regions, show excellent agreement for deuterium and helium-4, supporting the standard model with three neutrino species.[40] However, the "lithium problem" persists, as the observed ^7Li/H ≈ (1.6 ± 0.3) × 10^{-10} in halo stars is a factor of 3–4 lower than predicted, potentially indicating gaps in stellar depletion models or nuclear rates, though BBN consistency for other elements remains robust.[40]| Element | Predicted Abundance | Observed Primordial Abundance | Agreement |
|---|---|---|---|
| ^4He (Y_p) | 0.247 | 0.2449 ± 0.0040 | Excellent |
| D/H | 2.45 × 10^{-5} | (2.547 ± 0.033) × 10^{-5} | Excellent |
| ^7Li/H | 4.82 × 10^{-10} | (1.6 ± 0.3) × 10^{-10} | Discrepancy (factor ~3) |