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Cosmic noise
Cosmic noise
Cosmic noise
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Cosmic noise, also known as galactic radio noise, is radio-frequency electromagnetic radiation from sources outside of the Earth's atmosphere. Its characteristics are comparable to those of thermal noise. Cosmic noise occurs at frequencies above about 15 MHz when highly directional antennas are pointed toward the Sun or other regions of the sky, such as the center of the Milky Way Galaxy. Celestial objects like quasars, which are super dense objects far from Earth, emit electromagnetic waves in their full spectrum, including radio waves. The fall of a meteorite can also be heard through a radio receiver; the falling object burns from friction with the Earth's atmosphere, ionizing surrounding gases and producing radio waves. Cosmic microwave background radiation (CMBR) from outer space is also a form of cosmic noise. CMBR is thought to be a relic of the Big Bang, and pervades the space almost homogeneously over the entire celestial sphere. The bandwidth of the CMBR is wide, though the peak is in the microwave range.

History

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Karl Jansky, an American physicist and radio engineer, first discovered radio waves from the Milky Way in August, 1931. At Bell Telephone Laboratories in 1932, Jansky built an antenna designed to receive radio waves at a frequency of 20.5 MHz, which is a wavelength of approximately 14.6 meters.

After recording signals with this antenna for several months, Jansky categorized them into three types: nearby thunderstorms, distant thunderstorms, and a faint steady hiss of an unknown origin. He discovered the location of maximum intensity rose and fell once a day, which led him to believe he was detecting radiation from the Sun.

A few months went by following this signal thought to be from the Sun, and Jansky found that the brightest point moved away from the Sun and concluded the cycle repeated every 23 hours and 56 minutes. After this discovery, Jansky concluded the radiation was coming from the Milky Way and was strongest in the direction of the center of the galaxy.

Jansky's work helped to distinguish between the radio sky and the optical sky. The optical sky is what is seen by the human eye, whereas the radio sky consists of daytime meteors, solar bursts, quasars, and gravitational waves.

Later in 1963, American physicist and radio astronomer Arno Allan Penzias (born April 26, 1933) discovered cosmic microwave background radiation. Penzias's discovery of cosmic microwave background radiation helped establish the Big Bang theory of cosmology. Penzias and his partner, Robert Woodrow Wilson worked together on ultra-sensitive cryogenic microwave receivers, originally intended for radio astronomy observations. In 1964, upon creating their most sensitive antenna/receiver system, the Holmdel Horn Antenna, the two discovered a radio noise they could not explain. After further investigation, Penzias contacted Robert Dicke, who suggested it could be the background radiation predicted by cosmological theories, a radio remnant of the Big Bang. Penzias and Wilson won the Nobel Prize in Physics in 1978.

NASA's work

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The Absolute Radiometer for Cosmology, Astrophysics, and Diffuse Emission (ARCADE) is a device designed to observe the transition out of the "cosmic dark ages" as the first stars ignite in nuclear fusion and the universe begins to resemble its current form.[1]

ARCADE consists of 7 precision radiometers carried to an altitude of over 35 km (21 miles) by a scientific research balloon. The device measures the tiny heating of the early universe by the first generation of stars and galaxies to form after the Big Bang.

Sources of cosmic noise

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Cosmic noise refers to the background radio frequency radiation from galactic sources, which have constant intensity during geomagnetically quiet periods.[2]

Sun flares

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Cosmic noise can be traced from solar flares, which are sudden explosive releases of stored magnetic energy in the atmosphere of the Sun, causing sudden brightening of the photosphere. Solar flares can last from a few minutes to several hours.

During solar flare events, particles and electromagnetic emissions can affect Earth's atmosphere by fluctuating the level of ionization in the Earth's ionosphere. Increased ionization results in absorption of the cosmic radio noise as it passes through the ionosphere.

Solar wind

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Solar wind is a flux of particles, protons and electrons together with nuclei of heavier elements in smaller numbers, that are accelerated by the high temperatures of the solar corona to velocities large enough to allow them to escape from the Sun's gravitational field.[3]

Solar wind causes sudden bursts of cosmic noise absorption in the Earth's ionosphere. These bursts can only be detected only if the magnitude of the geomagnetic field perturbation caused by the solar wind shock is large enough.[4]

See also

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  • Intergalactic space – Void between celestial bodies
  • Interplanetary space – Void between celestial bodies
  • Interstellar medium – Matter and radiation in the space between the star systems in a galaxy
  • Radio astronomy – Subfield of astronomy that studies celestial objects at radio frequencies

References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Cosmic noise

View on Grokipedia
from Grokipedia
Cosmic noise, also known as galactic radio noise, refers to the random radio-frequency originating from extraterrestrial sources beyond Earth's atmosphere, encompassing a broad spectrum of natural emissions that form the background in observations. First detected serendipitously in 1932 by Karl Jansky at Bell Telephone Laboratories while investigating shortwave interference, it manifested as a steady "hiss" peaking toward the at frequencies around 20 MHz, initially attributed to thermal emission but later recognized as nonthermal in nature. This discovery laid the foundation for , revealing the radio-emitting universe and enabling studies of celestial phenomena invisible at optical wavelengths. The primary sources of cosmic noise include produced by relativistic electrons spiraling in interstellar magnetic fields, particularly within the ; free-free emission () from hot ionized hydrogen regions (H II regions) around young stars; and the (CMB), a uniform 2.73 K filling the as a relic of the , peaking near 220 GHz. Additional contributions arise from discrete sources such as the Sun—especially during flares, which can produce intense bursts at frequencies above 15 MHz when antennas are directed toward it—and extragalactic objects like radio galaxies and quasars, though these are often superimposed on the diffuse galactic background. Cosmic noise exhibits characteristics similar to thermal noise but with a power-law , decreasing with frequency, and it dominates observations in the radio band from approximately 10 MHz to 1 THz, limited at lower frequencies by ionospheric absorption and at higher by atmospheric opacity. Historically, Jansky's 1931–1933 measurements at 20.5 MHz marked the inadvertent birth of the field, followed by Grote Reber's 1937 construction of a 9.5-meter parabolic dish in , which produced the first radio maps of the sky at 160 MHz, confirming the galactic origin and nonthermal spectrum of the noise. World War II radar developments spurred postwar expansion, with key groups forming in 1945: Stanley Hey's team in detecting solar noise; and advancing interferometer techniques and meteor studies at and , respectively; and Pawsey's Radiophysics Laboratory in mapping southern skies. These efforts culminated in major discoveries, including the 21 cm hydrogen line predicted by Hendrik van de Hulst in 1944 and observed in 1951, transforming cosmic noise from mere interference into a vital probe of galactic structure, cosmology, and the early universe.

Introduction and Fundamentals

Definition and Characteristics

Cosmic noise refers to radio-frequency originating from extraterrestrial sources beyond Earth's atmosphere, encompassing emissions from the and other cosmic structures detectable primarily at frequencies above approximately 10 MHz, where the becomes transparent to such signals. This noise manifests as a steady background in radio observations, distinct from discrete sources, and was first noted as a persistent "hiss" in early detections. Key characteristics of cosmic noise include its frequency dependence, with intensity increasing toward lower frequencies below 100 MHz due to the power-law spectrum of dominant emission processes, typically following a α ≈ -0.7 to -0.8 where flux density S ∝ ν^α. The angular distribution is anisotropic, peaking along the and toward the , reflecting the structured distribution of emitting regions within the . In terms of , it corresponds to an equivalent blackbody temperature of roughly 2 × 10^5 at frequencies around 100 MHz, decreasing at higher frequencies as the emission becomes less dominant. Additionally, cosmic noise exhibits , particularly from nonthermal components, with the polarization direction aligned perpendicular to local magnetic fields in the emitting regions. The primary physical mechanisms generating cosmic noise are nonthermal and free-free emission. arises from relativistic electrons spiraling in interstellar with strengths on the order of B ≈ 10^{-6} Gauss (10 μG), producing the bulk of the continuum emission at frequencies below about 30 GHz; these electrons, part of the cosmic-ray population, follow a power-law energy distribution n(E) ∝ E^{-δ} with δ ≈ 2.5, leading to the observed spectral steepness. Free-free emission, in contrast, is from electrons scattering off ions in hot ionized gas, such as in H II regions at temperatures around 10^4 K, contributing a flatter spectrum (α ≈ -0.1) that becomes optically thick at lower frequencies. Together, these processes account for over 90% of the Galactic radio continuum at gigahertz frequencies, with dominating at lower frequencies.

Distinction from Other Noise Types

Cosmic noise is fundamentally distinguished from by its extraterrestrial origin, whereas arises from natural processes within Earth's environment, primarily discharges in thunderstorms that produce sporadic, radio interference. This is confined to the planet's atmosphere and , exhibiting peak intensities in the (VLF) to (HF) bands, typically below 30 MHz, where it can overwhelm signals by 15 dB or more compared to cosmic contributions during active periods. In contrast, cosmic noise persists uniformly from celestial sources and becomes the primary sky noise component above approximately 15 MHz in directional observations, unaffected by local weather or terrestrial events. Man-made noise, another terrestrial interferent, stems from human activities such as electrical appliances, automotive ignition systems, vehicles, and communication transmissions, generating intentional or unintentional emissions that are highly variable in intensity and . Unlike the isotropic and steady of cosmic noise across the , man-made noise is localized to populated or industrialized areas, often dominating at lower frequencies (below 100 MHz) in urban settings where it can exceed natural noise floors by orders of magnitude, necessitating remote observatory sites for . Its impulsive and characteristics further differentiate it from the more continuous cosmic emissions, which maintain relevance even in protected bands above 100 MHz. Receiver noise, in opposition to the external cosmic input, originates internally within the detection and is dominated by following the Nyquist relation Pn=[k](/page/K)TBP_n = [k](/page/K) T B, where [k](/page/K)[k](/page/K) is Boltzmann's constant, TT is the physical temperature, and BB is the bandwidth, alongside limits at higher frequencies. This intrinsic lacks any extraterrestrial signature and is independent of sky conditions, contributing to the overall system TsysT_{sys} as Tsys=TA+TrxT_{sys} = T_A + T_{rx}, where TAT_A includes the cosmic TcT_c from the antenna. For example, in quiet rural skies, cosmic can surpass residual atmospheric contributions above 100 MHz, elevating TAT_A to levels where it significantly influences TsysT_{sys} in sensitive observations, unlike the fixed, equipment-limited receiver .

Historical Development

Discovery and Early Observations

In the 1920s, amateur radio operators in the United States and Britain occasionally reported episodes of unexplained "hissing" static interference on , particularly around 28 MHz, which some attributed to natural atmospheric phenomena but which hinted at extraterrestrial sources without clear identification. These anecdotal observations gained scientific traction through the work of Karl at Bell Telephone Laboratories in Holmdel, , where he was tasked with investigating sources of shortwave interference that could disrupt transatlantic communications. In 1931, using a large rotatable array operating at approximately 20 MHz, Jansky systematically recorded and analyzed static disturbances, distinguishing three main types: nearby thunderstorms, distant thunderstorms, and a faint, steady "hiss" that peaked every 23 hours and 56 minutes, aligning with the Earth's sidereal rotation rather than its solar day. By directing the antenna toward the constellation Sagittarius in the direction of the 's center, Jansky pinpointed the source as extraterrestrial radio emission from our galaxy, a finding he attributed to thermal processes, such as the agitation of charged particles in hot stars within the Milky Way. Jansky's discovery was formally announced in his seminal 1933 paper, marking the birth of radio astronomy as a discipline, though it initially received limited attention from the astronomical community focused on optical observations. Seeking to confirm and extend these results, amateur astronomer and engineer constructed the world's first purpose-built parabolic in his backyard in , in 1937—a 9.5-meter dish made from galvanized supported by wooden scaffolding. Operating initially at higher frequencies without success, Reber achieved detections at 160 MHz in 1939, mapping the intensity of galactic across the sky and producing the first radio survey of the heavens, which revealed strong emissions concentrated along the Milky Way's plane. Reber's 1940 publication of these contour maps solidified cosmic noise as a verifiable astronomical , paving the way for further exploration.

Key Contributions and Milestones

Following Karl Jansky's foundational detection of cosmic radio emission in the 1930s, advancements in technology profoundly influenced the study of cosmic noise by repurposing surplus military equipment for astronomical observations. In 1942, British physicist James Stanley Hey and his team at the Army Operational Research Group identified intense radio bursts from the Sun using anti-aircraft systems operating at wavelengths of 4-8 meters, marking the first deliberate detection of solar radio noise and demonstrating its interference with signals. Independently, in 1945, Australian physicist Edward George "Taffy" Bowen and colleagues at the Council for Scientific and Industrial Research Radiophysics Laboratory utilized modified receivers to confirm solar radio emissions at 200 MHz, observing correlations between these signals and optical activity that established as primary radio sources. These wartime efforts, leveraging declassified innovations, transitioned cosmic noise studies from incidental discoveries to systematic solar radio astronomy. A major theoretical and observational milestone came with the prediction of the 21 cm hyperfine transition line of neutral hydrogen by Dutch astronomer Hendrik van de Hulst in 1944. This line was first detected in 1951 by Edward Purcell and Harold Ewen at using a , and independently by a Dutch team led by Christiaan Muller. The 21 cm line allowed astronomers to map the distribution of atomic hydrogen in the and beyond, significantly advancing the use of cosmic noise for studying and galactic dynamics. The 1950s saw international expansion in radio astronomy infrastructure and precise measurements of discrete cosmic noise sources, solidifying the field. The establishment of major observatories, such as the in the , culminated in the completion of its 76-meter in 1957, which enabled high-resolution observations of galactic and extragalactic emissions and supported early space tracking efforts. Concurrently, teams led by at Cambridge University conducted interferometric measurements of strong discrete sources, including Cygnus A, revealing its compact, non-thermal nature and angular size of about 1 arcminute at 81.5 MHz by 1952, which advanced understanding of extragalactic radio structures. These developments fostered global collaboration, with observatories in , the , and the contributing to surveys that mapped the distribution of cosmic noise across the sky. NASA's entry in the introduced space-based platforms to overcome terrestrial limitations like ionospheric absorption. The Explorer-1 (RAE-1) , launched in July 1968, provided the first low-frequency (0.2-20 MHz) observations from above the , measuring galactic and solar cosmic noise with unprecedented sensitivity and detecting broadband continuum emission from the that extended below ground-based cutoffs. This mission yielded data on over 10^10 bits annually, enabling studies of Jupiter's decametric emission and solar type III bursts free from atmospheric interference. Theoretical progress in the mid-20th century elucidated the mechanisms behind cosmic noise. In 1953, Soviet astrophysicist Iosif Shklovskii proposed that non-thermal radio emission from sources like the arises from produced by relativistic electrons spiraling in , a model that explained the power-law spectra observed in galactic cosmic noise and was soon extended to extragalactic sources. By the 1970s, polarization observations of synchrotron emission, pioneered by surveys using telescopes like the Effelsberg 100-meter dish, allowed mapping of the Milky Way's large-scale structure, revealing ordered fields of approximately 1-10 microgauss aligned with spiral arms through Faraday rotation measures and polarization angles. These milestones transformed cosmic noise from a radar nuisance into a probe of interstellar physics.

Sources of Cosmic Noise

Galactic and Extragalactic Emissions

Galactic cosmic noise primarily arises from diffuse produced by relativistic electrons, typically with energies around 1 GeV, spiraling in the interstellar of the . These electrons, accelerated by shocks and other processes, interact with the galactic strengths of approximately 3–6 μG, generating non-thermal radio emission throughout the . This diffuse component accounts for the majority of the galactic radio continuum at frequencies below about 1 GHz. In addition to synchrotron emission, thermal bremsstrahlung from ionized hydrogen (H II) regions contributes to galactic noise, particularly at higher frequencies where it becomes comparable to non-thermal sources. These regions, excited by young massive stars, consist of hot plasma (T ≈ 10^4 K) where free electrons are decelerated by ions, producing continuum emission with a nearly flat spectrum. Discrete structures like supernova remnants, such as —the brightest extrasolar radio source—also emit strong synchrotron radiation from shocked cosmic rays, with a shell-like morphology extending over several arcminutes. The intensity of galactic synchrotron emission follows a power-law , Sν∝ν−αS_\nu \propto \nu^{-\alpha}, where the α\alpha typically ranges from 0.7 to 1.0, reflecting the underlying energy distribution. This results in a scaling as Tb≈104(ν10 MHz)−2.5 KT_b \approx 10^4 \left( \frac{\nu}{10 \, \mathrm{MHz}} \right)^{-2.5} \, \mathrm{K}, which steeply decreases with increasing frequency and highlights the dominance of low-frequency observations. Extragalactic emissions include from radio galaxies, such as , where extended lobes powered by supermassive black holes produce powerful jets and diffuse structures spanning hundreds of kiloparsecs. Quasars similarly contribute through lobe emissions, often exhibiting double-lobed morphologies with hotspots where relativistic plasma interacts with intergalactic medium, yielding steep spectra similar to galactic . The (CMB) provides an isotropic thermal component at 2.725 K, manifesting as blackbody noise primarily below 10 GHz, though it is spectrally distinct from non-thermal sources. At low radio frequencies, the overwhelmingly dominates the sky background due to its high filling factor of emitting regions and partial absorption by interstellar gas, which scatters and attenuates emission from distant sources. Early mapping efforts, such as those by in the 1940s, first revealed this concentration along the galactic equator.

Solar and Planetary Contributions

Solar and planetary contributions to cosmic noise arise primarily from dynamic, localized processes within the solar system, producing episodic radio emissions that contrast sharply with more diffuse backgrounds. The Sun generates intense radio bursts through mechanisms involving accelerated in its atmosphere and corona. Type I, II, and III bursts are the principal solar radio emissions, each linked to distinct solar activity. Type III bursts result from electron beams accelerated during flares or coronal mass ejections (CMEs), which excite Langmuir waves that convert into radio emission via plasma processes. These bursts drift rapidly from high to low frequencies as the electrons propagate outward through decreasing plasma density. Type II bursts originate from MHD shocks driven by CMEs, producing similar plasma emissions but with slower frequency drifts. Type I bursts, part of longer noise storms, emerge from electron or processes in magnetic loops above regions. interactions contribute to decametric noise, particularly through low-frequency extensions of Type III bursts in the . These emissions span a broad frequency range from 10 kHz to 10 GHz, with Type III bursts often observed down to kilohertz levels near 1 AU. The variability of solar radio noise is pronounced and tied to the 11-year solar cycle, peaking during maximum sunspot activity when flares and CMEs are more frequent. Burst durations vary by type: Type III events last from 1 to 3 seconds at higher frequencies, reflecting the transient nature of electron beams, while Type II bursts can extend to 10-30 minutes or longer as shocks propagate. Flux densities reach extreme levels during intense flares, up to approximately 10^{11} Jy at decametric wavelengths, overwhelming receivers and causing temporary blackouts in radio communications. In the solar context, synchrotron radiation from relativistic electrons in coronal loops provides a brief link to broader emission mechanisms, though plasma processes dominate at lower frequencies. Planetary sources, though less intense than solar ones, add variable noise from magnetospheric interactions. Jupiter's decameter emissions, observed between 10 and 40 MHz, are modulated by the Io-Jupiter interaction, where the moon's motion through the planetary generates Alfvén waves that accelerate electrons via the cyclotron instability. These emissions are highly beamed and occur in specific orbital phases of Io, producing bursts lasting seconds to minutes with flux densities up to 10^7 Jy. Saturn exhibits weaker radio aurorae at kilometric to hectometric wavelengths (around 100 kHz to 10 MHz), driven by modulation of its , which energizes electrons along auroral field lines through similar instabilities. These planetary signals are sporadic, influenced by dynamic pressure, and typically orders of magnitude fainter than solar bursts. Unlike the steady, isotropic galactic background, solar and planetary contributions are highly directional, often confined to narrow beams aligned with , and exhibit rapid time variability on scales from seconds to hours, making them distinguishable in dynamic spectra.

Detection and Measurement

Observational Techniques

for cosmic noise in emphasize methods to isolate extraterrestrial radio emissions from local interferences, leveraging antenna configurations, calibration protocols, and post-processing to achieve precise measurements. arrays, such as interferometers, are employed to resolve the angular structure of cosmic noise sources, providing high that single-dish telescopes cannot match; for instance, aperture synthesis synthesizes a large effective aperture from multiple smaller antennas to map extended emissions like those from the . Frequency switching techniques further enhance signal isolation by rapidly alternating the receiver between the target frequency and an offset reference frequency, effectively subtracting time-variable atmospheric and instrumental noise contributions while preserving the stable cosmic signal. Calibration of cosmic noise observations relies on standard procedures to scale flux densities and measure system performance accurately. Known strong sources, such as (Cas A), serve as primary flux calibrators due to their well-characterized, bright emission, allowing observers to establish absolute flux scales across frequencies by comparing measured intensities against historical models that account for the source's secular fading rate of approximately 0.67% per year at L-band. measurements, which quantify the total system noise including cosmic contributions, are typically performed using a Dicke radiometer; this instrument switches between the antenna signal and a matched reference load to differentially measure temperature differences, mitigating gain fluctuations and yielding sensitivities on the order of σT=2Ts/Δντ\sigma_T = 2T_s / \sqrt{\Delta \nu \tau}
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