What Is The Cosmic Microwave Background, And Why Is It Significan

Interstellar Matter

Donald G. York , in Encyclopedia of Concrete Science and Technology (3rd Edition), 2003

IV.D Atomic and Molecular Absorption

Absorption lines, already explained, are used to derive column densities of many species. Because of collisional de-excitation mechanisms, H₂CO is seen in assimilation against the microwave catholic background of only 3 K. 20-one-centimeter radiations is captivated by H atoms in the lowest hyperfine state against background radio continuum sources. Resonance assimilation lines of molecules such as C₂, H_ii, CN, and CH⁺ are seen in the optical and ultraviolet spectral regions. Resonance (basis-state) transitions of well-nigh of the starting time xxx elements in diminutive or ionic form are seen as optical or ultraviolet assimilation in the spectra of afar stars. In special circumstances, the degree of absorption is related linearly to the number of atoms leading to derived column densities (particles per square centimeter) to be compared with respective values of N for hydrogen. The ratio North(X)/North(How-do-you-do) gives the abundance of the species X, though in practice several ionization states of the species X must be accounted for. Conversely, some ions of heavy elements may be detected when hydrogen is ionized (HI unobservable), and the number of ionized hydrogen atoms must be accounted for (using, for instance, knowledge of the electron density and the intensity of the Balmer recombination radiations).

When ratios N(X)/N(HI) are bachelor for clouds, they are oft referred to solar abundance ratios. If there are 1/ten equally many atoms of a sure kind per H cantlet as institute in the dominicus, the chemical element is said to exist depleted by a cistron of 10 in interstellar space.

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Galactic Structure and Evolution

John P. Huchra , in Encyclopedia of Physical Science and Technology (Third Edition), 2003

5 Summary

Equally of this writing, no individual objects take been observed at redshifts greater than vi, then there is a large unexplored region of time and space between the time of recombination (the formation of the Cosmic microwave background at a redshift of ∼yard or an age of a few 100,000 years) and the starting time observable objects. These "dark ages" will be explored with a new ready of infinite-borne telescopes such as SIRTF (the Space InfraRed Telescope Facility) and the NGST (Side by side generation Space Telescope).

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Cherenkov counting

Michael F. L'Annunziata , ... Nataša Todorović , in Handbook of Radioactivity Analysis: Book 2 (Fourth Edition), 2020

E Radio Cherenkov counting

There is an ongoing search for ultra-high energy (UHE) neutrinos with energies above ten¹⁸ eV to beyond the decade of ten^xx eV and to notice answers to their astrophysical origin. Varner (2010) underscores that protons of such high energy cannot travel far through the cosmic microwave background (CMB) without interacting, which would hateful that such UHE neutrinos should exist produced nearby; although at that place is no show for nearby point sources. The origin and backdrop of the cosmic microwave background are reviewed past the writer in a previous text ( L'Annunziata, 2016). The loss or degradation of the UHE proton flux is due to the interaction of protons exceeding ∼4 × 10¹⁹ eV with the cosmic microwave groundwork photons forming a Δ⁺ resonance (Gerhardt et al., 2010; Varner, 2010). The Δ⁺ decay results in a chain of decays leading to ultra-high energy (UHE) neutrinos according to the GZK process; and these UHE neutrinos are as well called GZK neutrinos. The GZK process is derived from the findings of Greisen (1966) and Zatsepin and Kuz'min (1966) where UHE protons would interact with CMB photons according to the process:

(vi.160) $p + γ \to Δ^{+} \to n + π^{+}$

where the subsequent π ⁺ decay leads to a flux of UHE neutrinos according to

(six.161) $π^{+} \to μ^{+} + ν_{μ}$

(vi.162) $μ^{+} \to e^{+} + ν_{e} + {\bar{ν}}_{μ}$

UHE neutrinos, which interact in matter, produce a shower of particles. The particle shower produces ultimately a radio pulse of brusque duration (∼1 ns) via a process known as the Askaryan Effect (Askaryan, 1962, 1965). As described by Gerhardt et al. (2010), Varner (2010) and Kravchenko (2012), the Askaryan Effect is the resultant radio emission due to the production of an excess of negative charge following the UHE neutrino interactions in matter. The particle shower that follows the interaction of a UHE neutrino with thing progresses with the Compton scattering of electrons and the anything of positrons resulting in a relativistic internet negative charge excess of the lodge of magnitude of 20%–30% of the total number of charged particles in the shower. The charged particles are relativistic, and in a transparent medium such as water ice or salt, they volition emit Cherenkov radiation. Gerhardt et al. (2010) elucidate, that long wavelengths compared to the lateral spread of the shower add coherently; and the Cherenkov radiation is proportional to the square of the charge excess, i.e., the intensity is a function of the square of the neutrino energy. In ice, the coherence yields radio waves with frequencies up to a few GHz.

Numerous detector designs accept been implemented to measure out the coherent Cherenkov emission at radio frequencies resulting from UHE neutrino interactions in massive transparent media. A thorough review of the numerous methods of detection of radio Cherenkov emissions is provided past Schröder (2017), and only a few examples will exist described briefly hither. One of these, is RICE (Radio Water ice Cherenkov Experiment), described by Kravchenko (2012). The RICE experiment was installed in the South Pole with radio dipole antennas submerged deep inside the ice in a 3D grid. The experiment included sensor arrays co-deployed with the IceCube experiment described previously in Part D of this department. The extension of the project was named NARC for Neutrino Array Radio Calibration. The dipole antennas are tuned to 200–500 MHz bandwidth in ice at a depth of 100–300 chiliad below the water ice surface. The RICE hardware has been modified for its implementation for the detection of radio Cherenkov emissions in boreholes of the IceCube experiment described preciously.

An illustrative example of radio Cherenkov measurements is the design of the ANITA (ANtarctic Impulsive Transient Antenna) experiment, which makes use of the unabridged Antarctic ice sheet equally the neutrino target volume, is described past Varner (2010). The ANITA concept is illustrated in Fig. 6.77. A balloon capable of maintaining a 30- to 40-mean solar day flight with a large antenna payload at an distance of ∼37 km above the Antarctic ice sheet records events in the 200–1200 MHz frequency range. At the altitude of ∼37 km, Allison et al., 2017 notes that the ANITA tin can monitor an extremely large book of Antarctic water ice equivalent to ∼ane.6 × 10⁶ km^three.

The ANITA balloon payload is equipped with upwards to 48 horn antennas, for the measurement of radio emissions at high frequencies of 200–1200 MHz. Schröder (2017) reports that at this frequency range detection is expected only for showers with favorable geometry: The Cherenkov cone of the shower must hit the antenna, which limits the detection to (i) most-horizontal air showers and (2) neutrino-induced cascades in the overflown ice. ANITA has measured dozens of cosmic-ray air showers of energies of ∼ 10¹⁹ eV (Schoorlemmer et al., 2016; Hoover et al., 2010). For nearly of these high-free energy air showers ANITA measured the radio signal afterward it was reflected off the ice rather than measure out the radio bespeak directly (Alvarez-Muñiz et al., 2015).

An experimental design similar to ANITA is TAROGE (Taiwan Astroparticle Radiowave Observatory for Geo-synchrotron Emissions) described by Chen et al. (2015) and Schröder (2017). The TAROGE experimental design is similar to that of ANITA with the exception that the radio Cherenkov emissions are detected by an antenna receiver on a mountain top instead of a high-distance airship. The TAROGE receiver is designed to detect the Cherenkov radio emissions from UHE cosmic-ray showers either directly in the atmosphere or reflected off the bounding main floor also as those produced by CC interactions of neutrinos in the body of water flooring.

Numerous radio Cherenkov detection designs are described in a very comprehensive review by Schröder (2017); and the reader is invited to peruse this work. Among these are the ARA (Askaryan Radio Array) described past Allison et al. (2017), which include antennas on the water ice surface nigh the IceTop assortment illustrated earlier in this affiliate. ARIANNA (Antarctic Ross Water ice Shelf Antenna Neutrino Array) on the ross Ice Shelf at the Antarctic coast (Barwick et al., 2017), and Trend (Tianshan Radio Experiment for Neutrino Detection) in the Xianjiang Province, Prc, which is a radio-quiet zone permitting the design of a self-triggering detection of cosmic-ray showers. There are several other radio Cherenkov detector arrangements reviewed past Schröder (2017).

A unique detector arrangement is the Table salt Sensor Array (SalSA) described by Connolly (2012). SalSa entails the deployment of an antenna array into one of many naturally occurring salt formations called diapirs that are institute throughout the world. These consist of ∼10 km deep salt beds originating from 100 to 200 million yr sometime dried body of water salt. As described by Connolly (2012), these salt beds have purities of ∼95% and showroom long attenuation lengths in the radio microwave frequency range. The volumes of these salt formations extend into the 10'south-of-km³, which tin can provide an excellent target textile for UHE neutrinos. Arrays of antenna inserted vertically deep inside the table salt deposits tin can capture the radio Cherenkov bespeak created by particle cascades from UHE neutrino interactions in the salt. Withal, Schröder (2017) points out that due to the larger attenuation length of radio waves in water ice, and because of the large available ice volumes at Antarctica and on Greenland, ice seems to exist the medium of selection for future big-scale detectors of several ten or 100 kmⁱⁱⁱ.

Other methods, such as the Lunar Cherenkov technique described by Bray et al. (2012, 2015), McFadden et al. (2012), Mevius et al. (2012), and Schröder (2017), are aimed at detecting a nanosecond pulse of Cherenkov emissions, which are produced during UHE catholic ray and neutrino interactions in the Moon's regolith. Earth-based radio telescopes would notice the coherent Cerenkov radiation emitted when the UHE neutrinos collaborate in the outer layers of the Moon. The maximum intensity of the coherent Cherenkov emission is reached at a frequency of about 3 GHz, where the radiation is full-bodied in a narrow cone around the Cherenkov bending. Bray et al. (2012) describe the Parkes radio telescope, which is a single dish of 64 m diameter and 20 cm multi-axle receiver. They describe the technique used for aiming of the telescope whereby a radio pulse from a lunar Cherenkov issue is expected to come from the limb of the Moon with radial polarization.

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Cosmic Aggrandizement

Edward P. Tryon , in Encyclopedia of Concrete Science and Technology (3rd Edition), 2003

VIII.D Successes of Inflation

Our universe lies within a domain with a uniform vacuum state, corresponding to a item mode in which any (at present) subconscious symmetry of the underlying particle theory was broken. Standard aggrandizement predicts that this domain inflated past a factor much greater than 10³⁰, in which case our universe (almost certainly) lies deeply inside. Neither domain walls nor magnetic monopoles would so exist found within the observable universe (except for a few that might accept resulted from extreme thermal fluctuations after aggrandizement ended). Standard inflation therefore solves the monopole problem. The horizon problem is solved besides, every bit illustrated in Fig. two.

The flatness problem has an interesting history. When aggrandizement was proposed in 1980, the matter yet constitute by astronomers corresponded to a density of thing ρ_M that was ∼ten% of the critical density ρ_c. Astronomers had past no means completed their search for thing, nevertheless. When inflation was proposed, information technology seemed possible that hereafter discoveries would bring ρ₁₀₀₀ upwardly to ρ_c, equally required by standard inflation'due south prediction of flatness. With the passage of time, the presence of additional (dark) matter was indicated past observations, but there has never been show that ρ_M is even half of ρ_c. The currently observed value is ρ_M = (0.31 ± 0.06)ρ_c, and it seems virtually sure that ρ_M ≤ ρ_c/2 (Section V).

By the early on 1990s, the credible discrepancy between ρ_K and ρ_c led to a serious consideration of "open" inflationary models, wherein ρ_G < ρ_c and space has negative curvature. Such models are ingenious and technically viable, only require special assumptions that seem rather artificial.

As prove mounted that ρ_Thou < ρ_c, withal, standard inflation acquired a bizarre feature of its own. Its prediction of flatness implied the being of positive vacuum energy, in social club to bring the total density up to ρ_c. This would exist equivalent to a positive cosmological constant (Section IV.F and 1000), a concept that Einstein abandoned long ago and few had been tempted to resuscitate. Such vacuum energy would have acquired an dispatch of the cosmic expansion, beginning when the universe was roughly half its nowadays historic period (Section Five.F).

In 1998 two teams of astronomers, one led past Saul Perlmutter and the other by Brian P. Schmidt, reported an astonishing discovery. They had observed supernovae of type Ia (SNe Ia) over a wide range of distances out to ∼12 billion light-years (redshift parameter z ≈ 1, see Department Vi.A). Their data strongly indicated that the cosmic expansion is, indeed, accelerating. This determination has been confirmed past subsequent studies of SNe Ia. Furthermore, the amount of vacuum energy required to explain the acceleration appears to exist just the amount required (±10%) to make the universe flat: ρ_V ≈ 0.7ρ_c, hence (ρ_M + ρ_V) ≈ ρ_c (details in Section V.E).

The vacuum energy implied by supernovae data has been strikingly confirmed by studies of the CMB, which has been traveling toward us since the time of concluding scattering: t _LS ≈ 300,000 yr. The CMB is extremely uniform, but contains pocket-sized regions with deviations from the average temperature on the order of δT/T ∼ 10⁻⁵. These provide a snapshot of temperature perturbations at t _LS, which were strongly correlated with density perturbations.

Prior to t _LS, baryonic matter was strongly ionized. The costless electrons and nuclei were frequently scattered by abundant photons, which pressurized the medium. Density waves (ofttimes called audio-visual waves) volition be excited by perturbations in any pressurized medium, and the speed of such waves is adamant (in a known way) by the medium's backdrop. At any given time before t _LS, in that location was a maximum distance that such waves could still have traveled, called the audio-visual horizon distance.

Every bit the acoustic horizon expanded with the passage of time, it encompassed a growing number of density perturbations, which excited standing waves in the medium. The fundamental mode caused the greatest aamplitude, spanning a altitude comparable to the acoustic horizon distance. This is chosen the first acoustic tiptop. Perturbations in the CMB should contain this peak, with an angular bore reflecting the acoustic horizon distance at t _LS.

The angular diameters of perturbed regions in the CMB depend not only on the linear dimensions of their sources, merely also on the curvature (if any) of infinite. Sources of the CMB are presently a distance r _p(z _CMB) ≈ 47 × x⁹ ly from usa (Section VI.A). If space is curved, angular diameters of perturbations in the CMB would be noticeably affected unless the cosmic scale factor a ₀ satisfied (roughly) a ₀ ≥ r _p(z _CMB) (Section III.B). From Eqs. (48) and (69), nosotros see that this would require ∣Ω₀ − one∣≤0.11.

BOOMERanG (Balloon Observations of Millimetric Extragalactic Radiation and Geomagnetics) results published in 2000 are portrayed in Fig. five. The alphabetize ℓ refers to a multipole expansion (spherical harmonics, averaged over k for each ℓ). If infinite were apartment, the angular diameter of the first acoustic peak would be ≈ 0.9°, corresponding to ℓ_peak ≈ 180°/0.ix° ≈ 200.

With ρ_{One thousand} ≈ 0.3ρ_c but no vacuum energy, space would be negatively curved, giving ascension to ℓ_pinnacle ≈ 500. This is clearly ruled out past the data. For Ω₀ near unity, the prediction can be expressed equally $ℓ_{peak} \approx 200 / \sqrt{Ω_{0}}$ , and the data in Fig. 5 indicate ℓ_superlative = (197 ± 6). At the 95% confidence level, the authors concluded that 0.88 ≤ Ω₀ ≤ 1.12. The best-plumbing fixtures Friedmann solution for spacetime (Section IV.A) has ρ_M = 0.31ρ_c and ρ_V = 0.75, reproducing the SNe Ia result for vacuum free energy within the margin of fault. A more than resounding success for standard inflation is non easily imagined.

A sweeping, semiquantitative prediction about density perturbations is made by inflation. In the absenteeism of quantum fluctuations, inflation would rapidly bulldoze the fake vacuum to an extremely uniform state. Breakthrough fluctuations are ordinarily microscopic in size and very short-lived, simply rapid inflation would negate both of these usual backdrop.

Inflation would multiply the diameters of quantum fluctuations, and furthermore, do this so rapidly that opposite sides would lose causal contact with each other before the fluctuation vanished. The resulting density perturbations would be "frozen" into the expanding medium, thereby achieving permanency. Past inflation's end, the sizes of perturbations would span an extremely broad range. The earliest fluctuations would accept been stretched enormously, the latest very trivial, and all those from intermediate times by intermediate amounts. The magnitudes of the resulting δρ/ρ are very sensitive to details of the inflaton potential, but the machinery described higher up results in a distribution of sizes that depends rather trivial on details of the potential.

The kind of probability distribution described above should be apparent in the CMB, since density perturbations stand for closely to perturbations in the radiations's temperature. The Cosmic Groundwork Explorer satellite, better known as COBE (co-bee), was launched in 1989 to report the CMB with unprecedented precision. Its mission included measuring deviations from uniformity in the CMB's temperature. Over two years were required to gather and analyze the information displayed in Fig. 6.

These data were first presented at a cosmology conference in Irvine, CA, in March of 1992. The long awaited results struck many attendees every bit a historic confirmation of inflation, and none have since had reason to experience otherwise. Many subsequent studies have confirmed and refined the data in Fig. six, but we present the earliest results because of their historical significance.

The density perturbations predicted by inflation are like to those required to explain the evolution of construction germination in our universe. Indeed, i may reasonably hope that some particular inflaton potential volition exist wholly successful in describing further details of the CMB soon to exist measured, and also in explaining details of the structures we observe.

Inflation provides the merely known explanations for several puzzling, quite special features of our universe. Standard inflation has besides fabricated ii quantitive predictions, both of which have been strikingly confirmed (at least inside present uncertainties). Only time can tell, merely the prognosis for inflation seems excellent, in some course broadly similar to that described hither.

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Gravitational Wave Detectors

Rosa Poggiani , in Encyclopedia of Physical Science and Technology (Third Edition), 2003

Two.Eastward Stochastic Background

The stochastic background includes all unresolved sources and cosmological gravitational waves. Primordial gravitational waves are of special involvement since they should have been generated ∼10⁻²⁴ sec after the Big Bang. The gravitational waves could probe the universe at a much earlier life than any other measurement, such as the ane of CMB.

The detectors nether construction have a racket level of the same guild of the signals mentioned above. The dissonance is usually described by the power spectrum S _N(f) or the spectral density $\tilde{N} (f) = \sqrt{{Due south}_{Northward} (f)},$ measured in units of $\frac{racket amplitude}{\sqrt{Hz}}$ . Every bit an example, if a betoken with h ∼ 10⁻²¹ is detected in a bandwidth of 1 kHz, then the spectral density is $\tilde{h} = three \times {ten}^{- 23} \frac{1}{\sqrt{Hz}} .$ All sources of noise in the detectors are causeless to be uncorrelated and thus add in quadrature.

The power to observe signals is given by the characteristic aamplitude $h_{c} = h \sqrt{{Due north}_{cycl}},$ where N _cycl is the number of cycles spent by the waveform close to the maximum aamplitude, i.eastward., N _cycl ∼ f ^* t _o with f ^* typical frequency of the indicate and t _o the ascertainment time.

The intensity of the main astrophysical sources is shown in Fig. 2.

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Cherenkov Counting

Michael F. L'Annunziata , in Handbook of Radioactivity Analysis (Third Edition), 2012

E Radio Cherenkov Counting

At that place is an ongoing search for ultra-high-energy (UHE) neutrinos with energies higher up ten¹⁸ eV to beyond the decade of 10^twenty eV and to discover answers to their astrophysical origin. Varner (2010) underscores that protons of such high-energy cannot travel far through the cosmic microwave background (CMB) without interacting, which would hateful that such UHE neutrinos should be produced nearby, although at that place is no testify for nearby point sources. The origin and properties of the cosmic microwave background are reviewed by the writer in a previous text (L'Annunziata, 2007). The loss or degradation of the UHE proton flux is due to the interaction of protons exceeding ~4 × 10^nineteen eV with the cosmic microwave background photons forming a Δ⁺ resonance (Gerhardt et al., 2010 and Varner, 2010). The Δ⁺ decay results in a chain of decays leading to ultra-high-energy (UHE) neutrinos according to the GZK process, and these UHE neutrinos are also called GZK neutrinos. The GZK process is derived from the findings of Greisen (1966) and Zatsepin and Kuz'min (1966) where UHE protons would interact with CMB photons according to the procedure:

(15.116) $p + γ \to Δ^{+} \to north + π^{+}$

where the subsequent π ⁺ disuse leads to a flux of UHE neutrinos according to

(xv.117) $π^{+} \to μ^{+} + v_{μ}$

(fifteen.118) $μ^{+} \to e^{+} + v_{μ}$

UHE neutrinos, which interact in solid thing, produce a shower of particles. The particle shower produces ultimately a radio pulse of brusque duration (~one ns) via a procedure known every bit the Askaryan issue (Askaryan, 1962, 1965). Every bit described by Gerhardt et al. (2010), Varner (2010), and Kravchenko (2012), the Askaryan consequence is the resultant radio emission due to the production of an excess of negative charge following the UHE neutrino interactions in matter. The particle shower that follows the interaction of an UHE neutrino with matter progresses with the Compton scattering of electrons and the annihilation of positrons resulting in a relativistic net negative charge excess of the guild of magnitude of 20–xxx% of the total number of charged particles in the shower. The charged particles are relativistic, and in a transparent medium such as ice or salt, they will emit Cherenkov radiation. Gerhardt et al. (2010) elucidate that long wavelengths compared to the lateral spread of the shower add together coherently, and the Cherenkov radiation is proportional to the square of the charge excess, i.e., the intensity is a part of the square of the neutrino free energy. In ice, the coherence yields radio waves with frequencies upwards to a few GHz.

Several experiments have been designed to measure the coherent Cherenkov emission at radio frequencies resulting from UHE neutrino interactions in massive transparent media. One of these is RICE (Radio Ice Cherenkov Experiment), described by Kravchenko (2012). The RICE experiment is installed in the South Pole with radio dipole antennas submerged deep within the ice in a 3D filigree. The experiment includes sensor arrays co-deployed with the IceCube experiment described previously in Part D of this section. The extension of the projection was named NARC for Neutrino Assortment Radio Calibration. The dipole antennas are tuned to 200–500 MHz bandwidth in ice at a depth of 100–300 m below the ice surface.

Some other experimental pattern is the ANITA (ANtarctic Impulsive Transient Antenna) experiment, which makes utilise of the entire Antarctic ice sheet as the neutrino target book, described by Varner (2010). The ANITA concept is illustrated in Fig. xv.53. A airship capable of maintaining a 30- to 40-day flight with a big antenna payload at an altitude of ~37 km above the Antarctic water ice sheet records events in the 200–1200 MHz frequency range. Varner et al. (2007) reports over 7 one thousand thousand events recorded in a 35-solar day flying with neutrino flux limits.

A projection in the planning stage is the Table salt Sensor Array (SalSA) described past Connolly (2012). SalSa would entail the deployment of an antenna assortment into one of many naturally occurring salt formations chosen diapirs that are constitute throughout the world. These consist of ~10-km-deep common salt beds originating from 100- to 200-1000000-year-sometime dried sea common salt. Every bit described past Connolly (2012) these table salt beds have purities of ~95% and exhibit long attenuation lengths in the radio microwave frequency range. The volumes of these table salt formations extend into the tens of km³, which could provide an splendid target textile for UHE neutrinos. Arrays of antenna inserted vertically deep inside the salt deposits would be employed to capture the radio Cherenkov indicate created by particle cascades from UHE neutrino interactions in the table salt.

Other methods such as the Lunar Cherenkov technique described past Bray et al. (2012), McFadden et al. (2012), and Mevius et al. (2012) are aimed at detecting a nanosecond pulse of Cherenkov emissions, which are produced during UHE cosmic-ray and neutrino interactions in the Moon'southward regolith. World-based radio telescopes would detect the coherent Cerenkov radiation emitted when the UHE neutrinos interact in the outer layers of the Moon. The maximum intensity of the coherent Cherenkov emission is reached at a frequency of near 3 GHz, where the radiation is concentrated in a narrow cone effectually the Cherenkov angle. Bray et al. (2012) describe the Parkes radio telescope, which is a single dish of 64 m diameter and 20 cm multi-beam receiver. They depict the technique used for aiming of the telescope whereby a radio pulse from a lunar Cherenkov event is expected to come from the limb of the Moon with radial polarization.

Additional information on radio Cherenkov detection of UHE neutrinos as well as other Cherenkov detectors practical to studies of astroparticle physics can be obtained from Hallewell (2005), Santangelo (2006), Besson (2008), Cao (2008), Waxman (2009), and Taiuti et al. (2011).

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Laser-Induced Processes in Thin Ices

J.D. Thrower , H. Zacharias , in Encyclopedia of Interfacial Chemistry, 2018

Photon Sources in the ISM

Photons from the interstellar radiation field (ISRF), the background photon field of the galaxy, are abundant in the less dense, diffuse cloud regions, and penetrate to some extent into the outer layers of denser clouds (photon dominated regions, PDRs). The ISRF spans the UV–visible (predominantly from groundwork starlight), IR (dust emission), and microwave (cosmic microwave background) regions. Within dense clouds, the passage of cosmic rays (CRs; typically energetic protons) leads to the excitation and subsequent emission of UV photons from H and H _two, for example, Lyman blastoff (Ly-α) photons at 121.half dozen nm. Higher energy photons are also of interest, both in terms of the γ-ray component of cosmic rays and in the XUV and 10-ray radiation emitted from stars. Light from the central star provides the energetic input for chemical processing within a protoplanetary deejay. In an evolved planetary system, radiation-induced chemical science can influence both the atmospheres and surfaces of planets, moons, and smaller bodies such as dwarf planets. Indeed, contempo missions such equally the New Horizons spacecraft which visited the Pluto system and the Rosetta mission to the comet 67P/Churyumov–Gerasimenko accept highlighted the surprising richness of chemical science in the solar system. The recent appreciation of the likely abundance of planets around other stars in our galaxy, following the success of the Kepler mission, has likewise led to an interest in the chemistry of these exoplanetary systems.

Observational astronomy provides valuable insights into the chemical composition of the interstellar medium, both through the detection of solid-state signatures of molecules within icy mantles and the microwave emission of excited gas-phase molecules. Astrochemical modeling, where the development of an interstellar cloud is simulated using both (a) a physical model of cloud collapse, which determines the physical parameters such equally temperature and gas density, and (b) a chemical model incorporating typically hundreds of species and thousands of reactions. This requires input from fundamental studies of these reactions in the laboratory every bit well equally through computational (e.g., breakthrough chemistry) approaches. Parameters such as reaction rates, barriers, photodissociation cantankerous-sections, and desorption energies (the amount of energy required to return a molecule from the solid state to the gas stage) are essential ingredients for such models. In terms of photon-induced processes, surface science techniques provide the tools with which to probe the products of photon irradiation, quantify desorption yields, and provide insights into the underlying photophysics. Such measurements involve the growth of a thin ice film on a substrate held under ultrahigh vacuum atmospheric condition and subsequent exposure to photons from an advisable source.

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Cosmic Radiation

Peter L. Biermann , Eun-Suk Seo , in Encyclopedia of Physical Science and Technology (Third Edition), 2003

I Introduction and History

Cosmic rays were discovered past Hess and Kohlhörster in the beginning of the 20th century through their ionizing effect on airtight vessels of glass enclosing two electrodes with a high voltage between them. This ionizing issue increased with altitude during airship flights, and therefore the consequence must come from outside the world, so the term cosmic rays was coined. The world's magnetic field acts on energetic particles according to their charge, and hence they are differently affected coming from east and w, and so their accuse was detected, proving that they are charged particles; at high energies near 10¹⁸ eV or 1 EeV, there is observational prove that a small fraction of the particles are neutral and in fact neutrons. From around 1960 onward there has been show of particles at or above ten²⁰eV, with today nigh two dozen such events known. Later the catholic microwave background was discovered in the early 1960s, information technology was noted only a piffling after by Greisen, Zatsepin, and Kuzmin that nigh and above an energy of 5 × ten^xix eV (called the GZK cutoff) the interaction with the microwave background would pb to potent losses if these particles were protons, as is now believed on the basis of detailed air shower data. In such an interaction, protons run into the photon as having an free energy of above the pion mass, and so pions can exist produced in the reference frame of the collision, leading to virtually a xx% free energy loss of the proton in the observer frame. Therefore for an assumed cosmologically homogeneous distribution of sources for protons at extreme energies, a spectrum at globe is predicted which shows a stiff cutoff at 5 × x¹⁹ eV, the GZK cutoff. This cutoff is not seen, leading to many speculations as to the nature of these particles and their origin.

Catholic rays are measured with instruments on airship flights, satellites, the Space Shuttle, the International Space Station, and with ground arrays. The musical instrument chosen depends strongly on what is existence looked for and the free energy of the master particle. One of the most successful campaigns has been with balloon flights in Antarctica, where a airship can float at virtually 40 km altitude and circumnavigate the South Pole one time and mayhap even several times during one Antarctic summer. For very high precision measurements very large instruments on the Space Shuttle or the International Space Station are used, such as for the search for antimatter.

Critical measurements are the verbal spectra of the virtually common elements, hydrogen and helium, the fraction of antiparticles (antiprotons and positrons), isotopic ratios of elements such as neon and iron, the ratio of spallation products such equally boron to primary nuclei such as carbon as a function of free energy, the chemical composition near the knee, at about 5 × 10¹⁵ eV, and beyond, and the spectrum and nature of the particles across the ankle, at 3 × 10^eighteen eV, with special emphasis on the particles beyond the GZK cutoff, at 5 × ten¹⁹ eV.

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Infrared Astronomy

Rodger I. Thompson , in Encyclopedia of Physical Science and Technology (Third Edition), 2003

VI Space Infrared Astronomy

There accept been relatively few infrared space missions, partly considering, for wavelengths longer than well-nigh 2 μm, the whole telescope must be cooled to very low temperatures to reduce infrared emission from the telescope itself. This is quite costly and requires the employ of cryogens that limit the lifetime of the instrument or loftier-ability consumption to provide refrigeration through mechanical or thermoelectric coolers. Techniques for passive radiative cooling are under development. They crave large sunshades and orbits other than depression-earth orbits, where blocking both the solar and terrestrial infrared emission is very difficult.

The advantages of doing infrared astronomy in space are quite large. The absence of the earth'southward atmosphere eliminates the image distortions, termed seeing, that plagues basis-based observations. It also eliminates the infrared emission from the atmosphere, which limits the sensitivity of ground-based telescopes. The opportunity of cooling the entire telescope, although costly as mentioned above, again profoundly improves the sensitivity of space-based telescopes. Finally, since many regions of the infrared spectral range are blocked by absorption in the earth's atmosphere, space missions offer the only way to notice the entire infrared spectral range with no contamination by telluric absorption features.

6.A Limitations of Space-Based Observations

Although vastly superior to footing-based observations, infrared observations from space are not completely background free. The primary source of infrared background radiation in space is reflection and emission by zodiacal dust. This is grit in our solar system that is distributed mainly in aeroplane of the planets or zodiac. As shown in Fig. ix, in that location are two minima in the background radiation, one at 3 μm and another at 400 μm.

The minimum at 3 μm is at the crossover point betwixt reflected and emitted zodiacal radiation. At shorter wavelengths the emission is dominated by reflected sunlight. At wavelengths longer than 3 μm, the principal emission is thermal emission from the dust. The minimum at 400 μm is due to the fall off of the warmer emission from the zodiacal dust and extrasolar grit, termed cirrus , and the brusque wavelength falloff of the cosmic microwave background, which has a much colder temperature. These background minima are very useful regions for the observation of faint sources such as afar galaxies.

Although at that place have been few space infrared missions, the missions carried out to date accept proved quite successful. Due to the advantages of infinite infrared observations, at that place are also several missions in the planning phase. Nosotros will review a few of the major missions here.

6.B InfraRed Astronomical Satellite

The IRAS was the kickoff orbital space mission devoted solely to observations at infrared wavelengths. The advantage of infinite observation are clearly demonstrated by noting that IRAS, with a diameter of but threescore cm, vastly out-performed basis-based observations with even the five-one thousand Mt. Palomar observatory, the largest major observatory at the time. IRAS, launched January 1983, surveyed the heaven for a fiddling less than a year. Observations of approximately 96% of the sky were made in broad photometric bands at 12, 25, 60 and 100 μm, with an array of discrete Ge detectors. Its near polar orbit meant that in the survey mode it covered the sky in strips that overlapped at the equator but had a significantly greater time of coverage almost the poles. IRAS also operated role of the fourth dimension in a pointed set of observations on objects or regions of item interest.

The IRAS focal plane (Fig. 10) contained column detectors bundled so that the centers of the detectors were offset from each other by a little less than the length of the detectors. The detectors were rectangles with the long dimension parallel to the column line, which was perpendicular to the scan direction. Each cavalcade had ii lines of detectors and a filter for one of the four wavelength bands. Equally the satellite scanned along the sky, sources would laissez passer over the detectors producing a transient signal that repeated in each detector at a charge per unit gear up by the rate of scan through the heaven. Since the detector centers were commencement, a source would pass through different parts of each detector as it ran downwards the column. Keeping track of the bespeak strengths and which detectors responded as different strips of sky were observed gave boosted spatial data in the management perpendicular to the scan. IRAS besides contained a low spectral resolution spectrometer that operated in the seven.7- to 22.6-μm region. This spectrometer measured over 5000 sources with bespeak strengths greater than 10 Janskys.

IRAS established a then unprecedented database that is however existence utilized at the present time. Catalogs of both extended and point sources were published from the survey. A very pregnant finding from these catalogs was the realization that many galaxies take the majority of their luminosity at mid- and far infrared wavelengths. IRAS also established that some stars that had been considered quite normal, such as Vega, were really surrounded by large clouds of dust that might be the edifice blocks for planetary systems.

Half-dozen.C Catholic Groundwork Explorer

COBE has produced some of the most dramatic scientific achievements in infrared astronomy. COBE measured the spectral shape of the catholic background and establish it to be a perfect blackbody spectrum at a temperature of ii.37 Yard. COBE also fabricated the kickoff observations of structure in the distribution of cosmic background radiation that are the likely starting time pace in changes that produced the galaxies, stars, and planets we know today from the primordial smooth distribution of matter produced past the Big Bang. (See the discussion in department II.A.v.) COBE had iii instruments for observing the background radiation: the DMR, the FIRAS, and the diffuse infrared background experiment (DIRBE).

The DMR experiment utilized techniques more common to radio astronomy than infrared. The chief function of the musical instrument consisted of sets of opposed antennas looking at different parts of the heaven. These antennas looked for differential signals indicating different temperatures at different locations. FIRAS was a version of the FTS discussed earlier. It compared the spectral shape of the cosmic groundwork radiation against the spectral shape of internal blackbody calibrators. The DIRBE instrument was more like to standard infrared instruments than the other two COBE experiments. DIRBE used a suite of iv different detector types to map out the sky at several infrared wavelengths. This instrument has contributed very important information on the contribution of various types of astronomical objects to the full background radiation in the universe.

Half dozen.D Infrared Space Observatory

The ISO was launched in November 1995. Its diverse instruments operated over the wavelength range betwixt 2.v and 240 μm. With a primary mirror diameter of threescore cm, it was similar in size to IRAS but carried improved detectors and a more than versatile musical instrument complement. ISO differed in its mission from IRAS. Instead of surveys every bit its main mission, ISO was designed primarily for pointed observations of objects of interest. Similar COBE and IRAS, ISO was cooled by cryogens in order to operate in the mid- and far infrared bands. The liquid helium cryogens lasted until Apr 1998, providing an observing period of well-nigh 2 $\frac{1}{ii}$ years. ISO was put into a highly elliptical orbit that provided significant observing time at large distances from the earth with data transmission to two basis stations. ISO was the kickoff infrared space mission to offer observing opportunities to the unabridged community. The iv instruments on the ISO mission were a combination of cameras and spectrometers described beneath. This mission too provided of import data nearly the performance of some classes of infrared detectors in high radiations environments. In spite of the bug with some of the detectors, ISO was a highly successful mission whose database is an of import tool in astrophysics. Its spectrometers demonstrated the richness of the mid- and far infrared spectral region. We will discuss below the ISO instruments.

Half dozen.D.1 ISOPHOT

This musical instrument provided photometric, polarimetric, and spectrophotometry over the entire wavelength range of ISO. Small arrays of Si:Ga, Si:B, and Ge:Ga detectors also provided express imaging capabilities. The primary function of ISOPHOT was to provide accurate photometry of sources. It included an internal chopper and several calibration sources.

VI.D.2 ISOCAM

ISOCAM provided the main imaging capability for the mission. Information technology was dissever into two channels. The short wavelength channel was sensitive between two.5 and 5.5 μm, and the long wavelength aqueduct between 4 and 18 μm. These cameras provided imaging capability in several spectral regions that are inaccessible from the ground. The detector arrays were 32×32 pixels of In:Sb and Si:Ga. The short wavelength In:Sb array was operated in a charge-integrating mode, which has since been superseded by multiplexed readouts for much larger arrays. The long wavelength Si:Ga array was operated as a photoconductor. ISOCAM provided diffraction-limited optical performance with several filter options.

Half-dozen.D.3 Short Wavelength Spectrometer

The short wavelength spectrometer (SWS) covered the spectral range between 2.38 and 42.5 μm, with a spectral resolution ranging from k to 2000. Information technology also carried a Fabry-Perot etalon to enhance the spectral resolution in the 11.4 to 44.5-μm region. Fabry-Perot etalons pass radiation in a narrow wavelength range that is altered past changing the spacing between the optical components. A combination of In:Sb, Si:Ga, Si:As, Si:Sb, and Ge:Exist linear arrays provided the detectors for the large wavelength range covered by the instrument. Most of the arrays were ane×12 pixels, merely the Si:Sb and Ge:Exist arrays were 1×2. The SWS detector arrays were found to be very sensitive to the radiations environment encountered in space missions.

VI.D.4 Long Wavelength Spectrometer

The long wavelength spectrometer (LWS) operates between 43.0 and 196.9 μm. Coupled with the SWS it provides continuous spectral coverage from 2.4 to 196.nine μm. This has been a great advantage in studying the emission of objects such equally active galactic nuclei and starburst galaxies. The detector assortment is linear and consists of one Ge:Be, five unstressed Ge:Ga, and iv stressed Ge:Ga photoconductive detectors. Similar SWS, LWS also carried a Fabry-Perot etalon to increase the spectroscopic resolution of the instrument.

Vi.East Nigh Infrared Camera Multiobject Spectrometer on the Hubble Space Telescope

The NICMOS is an instrument on board the HST. Although HST is the largest electric current telescope in space, with a 2.4-m primary mirror, it is not a cooled telescope. Its mirrors are maintained at a temperature of 18 °C or about room temperature. For this reason, NICMOS limits its observations to wavelengths shorter than 2.v μm. Observations at wavelengths longer than 2 μm are somewhat degraded by thermal emission from the telescope's mirrors. At wavelengths shorter than ii μm, the NICMOS sensitivity is greater than the largest ground-based telescopes due to the absence of the bright OH background (discussed in section IV). The absence of the earth'southward atmosphere also ways that NICMOS tin achieve very loftier spatial resolution over its unabridged field of view. This advantage is also enjoyed by the optical instruments on HST. NICMOS has 3 split up cameras for observations at various spatial resolutions. It besides has the adequacy of performing polarimetric and coronagraphic imaging and low-resolution spectroscopy.

The NICMOS detectors were cooled with a big block of solid nitrogen at 65 K, which sublimated away at the end of 1998. Revitalization of NICMOS with a mechanical cooling system is scheduled for belatedly 2001. If this is successful, NICMOS tin be operational for the remaining lifetime of HST. Sensitivity curves for all HST instruments are maintained past the Space Telescope Science Establish and are available on their spider web site: http://www.stsci.edu

6.F Space InfraRed Telescope Facility

The SIRTF is a 85-cm diameter telescope (Fig. eleven) that will exist cooled to five.v Thou and specialize in mid- and far infrared observations. Scheduled to be launched on a Delta rocket in December of 2001, SIRTF will provide an excellent follow-up on the ISO observations and offer unique areas of scientific discipline of its own. At this time SIRTF is expected to provide a significant increase in sensitivity and field of view over ISO, primarily due to an improved detector complement. The SIRTF orbit is a unique world-abaft heliocentric orbit that volition place the spacecraft at a considerable altitude from World. This volition facilitate the cooling of the telescope, which will exist launched warm and so cooled once the proper orbital position is accomplished. SIRTF contains three focal plane instruments described below.

Half dozen.F.ane Multiband Imaging Photometer

The MIPS for SIRTF volition provide diffraction-limited imaging over the wavelengths between xx and 180 μm. This range is covered by three detector arrays optimized for photometric bands centered at 24, lxx, and 160 μm. The respective detector arrays are a 128×128 pixel Si:Every bit BIB array, a 32×32 pixel gallium-doped Germanium (Ge:Ga) photoconductor, and a 2×20 pixel Ge:Ga assortment that has been mechanically stressed to extend its long wavelength coverage to 180 μm. MIPS also has low-resolution (R = 14–25) spectroscopic capabilities in the wavelength region between 52 and 99 μm. R is a measure of resolution and is given by λ/δλ, where λ is the wavelength and δλ is the spectral resolution of two pixels in the Nyquist sampled case. A cryogenic scan mirror system will increase the efficiency of MIPS for mapping regions of sky larger than its field of view.

VI.F.ii InfraRed Array Camera

The InfraRed Array Photographic camera (IRAC) provides imaging at shorter wavelengths with bands centered at three.6, 4.5, 5.eight and 8.0 μm. The 3.vi and iv.five μm bands utilise two 256×256 pixel In:Sb detector arrays while the ii longer bands employ 256×256 pixel Si:As impurity band conduction detectors. Although there are iv photometric bands IRAC has only two entrance apertures. Dichroic beam splitters allow the 3.6 and 5.viii band to share 1 aperture while the 4.5 and 8.0 bands share the other. This doubles the efficiency for multiband observations.

Six.F.iii InfraRed Spectrograph

The InfraRed Spectrograph (IRS) provides the primary spectroscopic capability for SIRTF. It covers the wavelength range of v.3 to xl.0 μm with low-resolution (R =threescore–120) spectroscopy and the range between 10 and 37 μm at college resolution (R =600). IRS utilizes 128×128 Si:Equally and Si:Sb BIB arrays to comprehend its wavelength region.

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IX Conference on Electromagnetic and Light Scattering by Non-Spherical Particles

J.H. Hough , in Journal of Quantitative Spectroscopy and Radiative Transfer, 2007

The first detections of the polarization of the Cosmic Microwave Groundwork (CMB) were made a few years ago by the DASI interferometer at xxx GHz, operating at the Due south Pole [53,54], and from space by the Wilkinson Microwave Anisotropy Probe (WMAP) [55]. CMB photons last interacted with thing 300,000 years after the Big Blindside, virtually 14 billion years ago, through Thomson handful of photons off costless electrons. As the well-measured CMB anisotropy has a quadrupole component, the scattered radiations will exist polarized and the polarization will provide further information on the dynamics of the early universe and the nature of primordial density fluctuations. The polarization is very pocket-sized, being just a few pct of the temperature anisotropies, which in turn are very small. Interestingly, one of the problems in measuring the polarization of the CMB arises from the polarized emission from aligned Galactic dust (see Section three.2). Despite very small signals, the data from DASI and from WMAP are, through polarimetry, providing key information on the early on history of the Universe and current cosmological models.

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