Abstract
The standard model of cosmology must not only explain the dynamics of the homogeneous background universe, but also satisfactorily describe the perturbed universe – the generation, evolution and finally, the formation of large-scale structures in the universe. Cosmic microwave background (GlossaryTerm
CMB
) has been by far the most influential cosmological observation driving advances in current cosmology. Exquisite measurements from GlossaryTermCMB
experiments have seen the emergence of a concordant cosmological model. Besides precise determination of various parameters of the standard cosmological model, observations have also established some important basic tenets that underlie models of cosmology and structure formation in the universe. The article reviews this aspect of recent progress in cosmology for a general science reader.Access provided by Autonomous University of Puebla. Download chapter PDF
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Keywords
- Dark Energy
- Cosmic Microwave Background
- Wilkinson Microwave Anisotropy Probe
- Cosmic Microwave Background Anisotropy
- Cosmic Microwave Background Temperature
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
Contemporary View of our Cosmos
The universe is the grandest conceivable scale on which the human mind can strive to understand nature. The amazing aspect of cosmology, the branch of science that attempts to understand the origin and evolution of the universe, is that it is largely comprehensible by applying the same basic laws of physics that we use for other branches of physics. Historically, theoretical developments always preceded observations in cosmology up until the past couple of decades. Recent developments in cosmology have been largely driven by huge improvement in quality, quantity, and the scope of cosmological observations.
We will avoid giving a historical perspective. The theoretical model of cosmology, the Hot Big Bang model (GlossaryTerm
HBBM
), has broadly remained as it was established and widely accepted by the late 1960s. This is readily available in most standard textbooks, as well as, many semipopular books. The perspective would be to review the theoretical model of cosmology in the light of the available data. The main goal is to convey the excitement in cosmology where amazing observations have now concretely verified that the present edifice of the standard cosmological models is robust. A set of foundation and pillars of cosmology have emerged and are each supported by a number of distinct observations:-
Homogeneous, isotropic cosmology, expanding from a hot initial phase due to gravitational dynamics of the Friedmann equations derived from laws of general relativity.
-
The basics constituent of the universe are baryons, photons, neutrinos, dark matter, and dark energy (cosmological constant/vacuum energy).
-
The homogeneous spatial sections of spacetime are nearly geometrically flat (Euclidean space).
-
Evolution of density perturbations under gravitational instability has produced the large-scale structure in the distribution of matter starting from the primordial perturbations in the early universe.
-
The primordial perturbations have correlation on length scales larger than the causal horizon that makes a strong case for an epoch of inflation in the very early universe. The nature of primordial perturbation matches that of the generation of primordial perturbations in the simplest model of inflation.
The cosmic microwave background (GlossaryTerm
CMB
), a nearly uniform, thermal black-body distribution of photons throughout space, at a temperature of , accounts for almost the entire radiation energy density in the universe. Tiny variation of temperature and linear polarization of these black-body photons of the cosmic microwave background arriving from different directions in the sky faithfully encodes information about the early universe and have traveled unimpeded across the observable universe, making them an excellent probe of the universe.There are two distinct aspects to modern day cosmology – the background universe and the perturbed universe. The standard model of cosmology must not only explain the dynamics of the homogeneous background universe, but also satisfactorily describe the perturbed universe – the generation, evolution, and, finally, the formation of large-scale structures in the universe. It is fair to say that cosmology over the past few decade has increasingly been more dominated by the interplay between the theory and observations of the perturbed universe – the origin and evolution of large-scale structures in the matter distribution. The past few years have seen the emergence of a concordant cosmological model that is consistent both with observational constraints from the background evolution of the universe as well that from the formation of large-scale structures (GlossaryTerm
LSS
) in the universe. In particular, the much talked about dawn of precision era of cosmology has been ushered in by the study of the perturbed universe. Measurements of GlossaryTermCMB
anisotropy and polarization have been by far the most influential cosmological observation driving advances in current cosmology in this direction.The Smooth Background Universe
In recent years, vast cosmological surveys have provided a three-dimensional map of the distribution of millions of galaxies extending to a billion light-years around us. If theorists were to start building a model of cosmology today, this would be the cosmos they would need to explain. As shown in Fig. 32.1, there is a rich organized structure in the distribution of galaxies in a region of about . However, this is a typical (statistically speaking) sample of mass distribution. In other words, the mass distribution in the universe averaged over regions of a few hundred mega-parsecs is fairly uniform. A stronger case for the homogeneous cosmology actually comes from the high degree of uniformity in the temperature of the GlossaryTerm
CMB
. These provide observational support for the cosmological principle that postulates a homogeneous universe invoked by theorists in the 1920 to 1930s to develop the first physical models of cosmology.The evolution of the universe is an initial value problem in general relativity that governs Einstein’s theory of gravitation – the dynamical evolution in time of the three-dimensional spatial sections in the foliation of spacetime. The, now observationally confirmed, large-scale homogeneity and isotropy of the matter distribution implies that the spatial sections of the universe are homogeneous (i. e., 3-D spaces of constant curvature). This reduces the problem to one of the simplest applications of general relativity formulated as a dynamical system. The dynamics of the spatial sections reduces to the time evolution of the scale factor of the spatial section. Averaged on large scales, the spatial sections at any time t are simply a scaled version of the present universe at time t 0 – i. e., the physical distance between two points in the universe at time t is given by , where d is the present distance. It is convenient to define with no loss of generality. Observationally, the expansion of the universe causes a redshift , of the spectrum light from a (cosmologically) distant astrophysical object (galaxy or quasar) emitted at a time t, when the universe had a scale factor a. The observation that all galaxies (on the average) appear to have a redshift in the spectra proportional to their distance confirms the expansion of the universe. The cosmic time t, the scale factor , and the redshift z can be used interchangeably to label spatial hyper-surfaces of the evolving universe.
The dynamics of the universe is encoded in the simple Friedmann equation
deduced from the Einstein equations. It relates the Hubble parameter , that measures the expansion rate of the universe, to the matter density in the universe. Here we use the conventional dimensionless density parameter in terms of the critical density at that time. The key components of the universe are radiation , pressure-less gravitating matter , and cosmological vacuum (dark) energy . The departure of the total matter density parameter from unity contributes to the curvature of the space and can, hence, be represented by an effective curvature energy density that determines the effect of curvature on the expansion of the universe. (Note that is only a convenient notation and not a physical energy density, in particular, the curvature density is negative when the spatial section of uniform positive curvature.) Dividing out (32.1) by H 2 on both sides leads to a simple sum rule that summarizes the evolution of the universe
Since the expansion rate evolves with time, are time dependent. Further, the components (species) of matter are assumed to be noninteracting (on cosmological scales), ideal, hydrodynamic fluids, specified by their energy/mass density and the pressure p i (equivalently, by the equation of state w i, where ). For given species, the evolution of the density is governed by the conservation equation of the energy–momentum tensor. In a volume V 0 in the current universe, the conservation equation implies
where in arriving at the second equation we use the fact that the physical volume . The second equation resembles the first law of thermodynamic for an isentropic system with energy E and work done under pressure p (recall, ). It is straightforward to derive the scaling of the energy density with the evolution of the universe as relative to its present value as
The equation of the state characterizes the ideal cosmological fluid, e. g., for radiation (relativistic matter); w = 0 for pressure-less (nonrelativistic matter), curvature density can be expressed as an ideal fluid with , and for a cosmological constant (in general, for the dark energy component ).
The entire dynamics of the universe is then completely determined by the present matter composition. Explicitly, (32.1) and (32.4) lead to the more commonly seen version of the Friedmann equation
Equation (32.5) shows that the energy in an expanding universe is dominated successively by matter with a smaller value of w – i. e., first a radiation dominated phase , followed by matter-dominated , then curvature-dominated and finally a cosmological vacuum (dark) energy .
The relativistic mater density is almost entirely dominated by the GlossaryTerm
CMB
and the relic background of three species of light neutrinos (expected to have a density of that of the GlossaryTermCMB
). The isentropic expansion dictated by the Friedmann equations implies that although, at present (given by the temperature of the GlossaryTermCMB
), is negligible, at an early epoch the universe was dominated by relativistic matter density. The pressure-less matter density minimally consists of three distinct components, the baryonic matter, cold dark matter, and a possibly minor contribution from massive neutrino species. The constraint on the Baryon density from the predicted abundances of light elements from Big-Bang nucleosynthesis (GlossaryTermBBN
) is consistent with that recently obtained from considerations of structure formation.The present state of the universe in terms the three dominant components can be neatly summarized on the cosmic triangle shown in Fig. 32.2 [2]. The three axes address fundamental issues regarding background cosmology – Does space have positive, negative or zero curvature ()? Is the expansion accelerating, or decelerating (determined by )?, and, what is the fraction of the nonrelativistic matter, ()?
Historically, the focus has shifted between different sectors of the cosmic triangle depending on which of the three is the dominant player , , or, . The canonical standard cold dark matter (GlossaryTerm
SCDM
) is a model where the present universe is a flat universe () dominated by nonrelativistic matter density (). This is also theoretically the simplest since it avoids the fine tuning problem of having a curved universe by invoking inflation and was the favorite in the 1980s. The nonrelativistic matter had to be mostly nonbaryonic dark matter (i. e., matter than does not interact with light), since Big-Bang nucleosynthesis and the absence of GlossaryTermCMB
temperature fluctuations at the power level of limit the baryonic fraction to a much smaller value than that inferred for . (Nonbaryonic dark matter component has to be nonrelativistic to satisfy power spectrum measurements of the GlossaryTermLSS
.)At the end of the 1980s and early 1990s, observations of GlossaryTerm
LSS
made it clear that was much smaller than unity. The sum rule, (32.2), then implies that either , or , or both had to be non zero. The theoretical discomfort with a nonzero (that still persists today) led to the era of open cold dark matter (GlossaryTermOCDM
) models, where . The conflict with a robust prediction of inflation promptly development of open models of inflationary scenarios that could avoid this problem.Toward the end of the 1990s, the observation of a high-redshift supernova indicated an acceleration in the present expansion universe. Very soon after, GlossaryTerm
CMB
anisotropy observations revealed a flat universe (). This leads to the currently favored ΛCDM model in cosmology. The energy density of the cosmological constant (or, more broadly quintessence) can be inferred from the measurement of luminosity distance as a function of redshift using the high-redshift supernova SN Ia as standard candles. In this chapter, we limit our attention to the simplest case of a cosmological constant that has a constant equation of state , which is also completely consistent with all observations to date. Alternative propositions for the nature of the dark energy are discussed in the chapter by Tsujikawa in this volume.The key program of the Hubble space telescope (GlossaryTerm
HST
) mission measured the expansion rate of the universe in 2001. Recently, new Spitzer calibration, has allowed the systematic uncertainty in H 0 from the GlossaryTermHST
key project to be decreased by over a factor of 3. Also optical and infrared observations of over 600 Cepheid variables in the host galaxies of eight recent Type Ia supernovae (SNe Ia) determines . This is broadly consistent with the constraints from the GlossaryTermCMB
anisotropy and large-scale structure observations and combined constraints are remarkably tight. Cosmological observations have definitively determined the present universe to be located in the Λ-CDM sector. (The above improvement in H 0, combined with Wilkinson microwave anisotropy probe (GlossaryTermWMAP
)-7yr data, results in a strong constraint on the nature of dark energy , close to a cosmological constant.) The expansion rate and age estimates of the present universe measured from GlossaryTermCMB
data are again consistent with, and considerably improved in precision by including structure formation consideration.One the most crucial observational pillars that support the GlossaryTerm
HBBM
of the background universe is the GlossaryTermCMB
discussed in the next section.The Cosmic Microwave Background
The GlossaryTerm
CMB
, a nearly uniform, thermal black-body (Planck) distribution of photons throughout space, at a temperature of , accounts for almost the entire radiation energy density in the universe. The GlossaryTermHBBM
ascribes cosmic significance to this microwave radiation background, and hence GlossaryTermCMB
measurements play a role of great importance. In this widely accepted view, the GlossaryTermCMB
comprises the oldest photons that last interacted when the universe was only old (compared to the present age of about 14 billion years). The photons have freely traveled right from the edge of the observable universe a distance of about 43 billion light years () as explained in Fig. 32.3.The prediction of the Planck distribution of the GlossaryTerm
CMB
in the GlossaryTermHBBM
dates from the early nucleosynthesis calculations of Gamow and collaborators in 1948. Thermal equilibrium in the early universe establishes a Planck energy distribution for the photons. In the GlossaryTermHBBM
the universe expands adiabatically conserving the photon entropy per comoving volume. (The observed GlossaryTermCMB
accounts for almost all the entropy.) The adiabatic Hubble expansion conserves the Planck distribution. However, the energy density of photons in an expanding universe (see (32.4) for radiation ). Recalling, that the energy density of a black body is proportional to the fourth power of the temperature, it is clear that the temperature of the GlossaryTermCMB
photons scales as the inverse of the expansion of universe. At a redshift of , the temperature of GlossaryTermCMB
falls below the threshold required to keep the hydrogen atoms in the universe ionized. At this epoch of recombination at around , the protons and electrons form a neutral hydrogen atom and lose their coupling to the GlossaryTermCMB
. (This happens a bit earlier for the helium fraction). The baryonic matter in the universe transits from an ionized plasma state to neutral one where GlossaryTermCMB
photons can freely travel over cosmic distances.The serendipitous discovery of this extra galactic microwave background Penzias and Wilson in 1965 provided a big boost to the GlossaryTerm
HBBM
. This was followed up by numerous measurements of the GlossaryTermCMB
flux at other wavelengths that were broadly consistent with a Planck distribution of GlossaryTermCMB
photons. The Nobel prize in Physics in 2006 was awarded to John Mather (NASA Goddard Flight Center, USA) and George Smoot (University of Berkeley, USA), who led experimental teams of the pioneering Cosmic Background Explorer (GlossaryTermCOBE
) mission – a US space Administration, NASA, satellite launched in 1989 to measure the cosmic microwave background radiation with unprecedented accuracy over the full sky. The satellite operated for in a circumpolar orbit at an altitude of . GlossaryTermCOBE
carried three different instruments: far-infrared absolute spectrophotometer (GlossaryTermFIRAS
), differential microwave radiometer (GlossaryTermDMR
), and diffuse infrared background experiment (GlossaryTermDIRBE
). John Mather was the principle investigator (GlossaryTermPI
of the GlossaryTermFIRAS
experiment that measured the energy distribution of GlossaryTermCMB
photons to unprecedented accuracy. The GlossaryTermFIRAS
instrument measurements of the radiation flux in the frequency band shown in Fig. 32.4 confirmed the Planck distribution of GlossaryTermCMB
photons beyond reasonable doubt. The flux measurement at a given wavelength can be converted into an equivalent thermodynamic temperature T 0 for the GlossaryTermCMB
. Recent results derived from the GlossaryTermFIRAS
data combined with GlossaryTermWMAP
in 2009 find that the energy spectrum of GlossaryTermCMB
photons is accurately described by a Planck distribution at the precise temperatureOver the frequency band used to deduce the above GlossaryTerm
FIRAS
result, the maximum 1-σ deviation of the GlossaryTermCMB
spectrum from a Planck is constrained to be of the peak brightness. The observationally established Planck distribution of the energy spectrum of the GlossaryTermCMB
is naturally explained as arising from the thermal equilibrium the baryons and photons set up at very high temperatures and densities, that is expected to exist in the early universe. The present temperature T 0, of the GlossaryTermCMB
sets the total entropy of the universe (given the number of relativistic neutrino species). The origin of this entropy is not explained within the classical Big Bang model (inflation scenarios do provide an explanation but not a prediction). Working backward in time, adiabatic expansion implies a smaller and hotter universe expected in the GlossaryTermHBBM
.Perturbed Universe: Structure Formation
The standard model of cosmology must not only explain the dynamics of the homogeneous background universe, but also describe the perturbed universe – the generation, evolution, and the formation of large-scale structures in the universe. There is a well understood (if not rigorously defined) notion of a standard model of cosmology that includes the formation of a large-scale structure. It is fair to say that much of the recent progress in cosmology has come from the interplay between refinements of the theories of structure formation and the improvements in observations.
Although the simple homogeneous and isotropic cosmological model does fit the dynamics of the background universe averaged on large scales, the rich structure in the distribution of galaxies shown in Fig. 32.1 suggests that there is more information to be gleaned about the universe from the large-scale structure of mass distribution (GlossaryTerm
LSS
). It has been a well-accepted notion that the large-scale structure in the distribution of matter in the present universe arose gradually due to gravitational instability from tiny primordial perturbation in the early universe. Although explosive mechanisms for structure formation in a relatively recent epoch had been proposed, the limits of input into the radiation budget in the recent past due to the tight adherence of the GlossaryTermCMB
to the Planck form seen in the GlossaryTermCOBE
-GlossaryTermFIRAS
data make them nonviable. Also, the tiny level of fluctuations in the temperature of the GlossaryTermCMB
implies that the level of inhomogeneity in the universe at a redshift of is at most few . A recent exciting success of observational cosmology has been in detecting the baryon acoustic oscillations that establish the gravitational instability mechanism beyond reasonable doubt.As schematically summarized in Fig. 32.5, cosmological observations have placed the theory of structure formation in an enviable position for any branch of physics where the initial and final states as well as the dynamical mechanism are known:
-
The exquisite maps of GlossaryTerm
CMB
anisotropy provide a snap shot of perturbation in the universe at a redshift of when the universe is only about old. -
In the past decade an extensive survey of galaxies has mapped out the distribution of matter in the present -old universe.
-
As mentioned above, the well-understood gravitationally instability is the underlying mechanism for amplifying the tiny perturbations at a redshift of to give rise to the observed GlossaryTerm
LSS
now.
The recent era of precision cosmology arises from the sensitivity of a consistent picture on the cosmological parameters. The parameters have to be dialed to precise values to make a consistent description of the perturbed universe starting with the mildly perturbed universe at seen in the GlossaryTerm
CMB
to the present universe with a well-developed GlossaryTermLSS
.CMB Anisotropy and Polarization
The GlossaryTerm
CMB
photons arriving from different directions in the sky show tiny variations in temperature, at a level of ten parts per million, i. e., tens of micro-Kelvin, referred to as the GlossaryTermCMB
anisotropy, and a net linear polarization pattern at micro-Kelvin to tens of nano-Kelvin level. The tiny variation of temperature and linear polarization of these black-body photons of the cosmic microwave background arriving from different directions in the sky faithfully encodes information about the early universe and have traveled unimpeded across the observable universe making them an excellent probe of the universe. As illustrated in the cartoon in Fig. 32.3, the cosmic microwave background radiation sky is essentially a giant, cosmic super IMAX theater screen surrounding us at a distance of 43 billion light-years displaying a snapshot of the universe at a time very close to its origin.The GlossaryTerm
CMB
anisotropy is imprints of the perturbed universe in the radiation when the universe was only old. On the large angular scales, the GlossaryTermCMB
anisotropy directly probes the primordial power spectrum on scales enormously larger than the causal horizon. On smaller angular scales, the GlossaryTermCMB
temperature fluctuations probe the physics of the coupled baryon–photon fluid through the imprint of the acoustic oscillations in the ionized plasma sourced by the same primordial fluctuations. The physics of GlossaryTermCMB
anisotropy is well understood, and the predictions of the linear primary anisotropy and their connection to observables are, by and large, unambiguous [3] [4] [5].It is convenient to express the sky map of the GlossaryTerm
CMB
temperature anisotropy in the direction as a spherical harmonic expansionTheory predicts that the primary GlossaryTerm
CMB
anisotropy is a Gaussian field (of zero mean), and current observations remain fully consistent with this expectation. The anisotropy can then be characterized solely in terms an angular spectrumThe spectra for a wide variety of models share a generic set of features clearly related to basics physics of primary GlossaryTerm
CMB
anisotropy.The acoustic peaks occur because the cosmological perturbations excite acoustic waves in the relativistic plasma of the early universe. The recombination of baryons at redshift effectively decouples the baryon and photons in the plasma abruptly switching off the wave propagation. In the time between the excitation of the perturbations and the epoch of recombination, a sound wave could have traveled a fixed distance. Modes of different wavelengths can complete different numbers of oscillation periods. This translates the characteristic time into a characteristic length scale and produces a harmonic series of maxima and minima in the GlossaryTerm
CMB
anisotropy power spectrum. The acoustic oscillations have a characteristic scale known as the sound horizon, which is the comoving distance that a sound wave could have traveled up to the epoch of recombination. This is a well-determined physical scale imprinted on the GlossaryTermCMB
fluctuations on the surface of last scattering – the cosmic super-IMAX screen.The angle subtended by this physical scale in the GlossaryTerm
CMB
anisotropy sky at the distance of allows a sensitive determination of the geometry () of the background universe. This is determined by the location of the harmonic peaks series of seen in Fig. 32.6 a,b. The amplitude of baryon–photon oscillations consequently, the height of the peaks in the sensitively determine the baryon density . The are sensitive to other important cosmological parameters, such as, the relative density of matter ; cosmological constant ; Hubble constant H 0, and deviation from flatness (curvature) . Implicit in is the hypothesized nature of random primordial/initial metric perturbations – (Gaussian) statistics , (nearly scale invariant) power spectrum, (largely) adiabatic versus isocurvature, and (largely) scalar versus tensor component. The default settings in bracket are motivated by inflation.The transition to precision cosmology has been spearheaded by the measurements of GlossaryTerm
CMB
anisotropy and, more recently, polarization. The GlossaryTermCOBE
-GlossaryTermDMR
detection of GlossaryTermCMB
anisotropy provided observational evidence for the origin and mechanism of structure formation in the universe. The following decade has been dominated by high-resolution, full sky, GlossaryTermCMB
anisotropy measurements from the GlossaryTermWMAP
of NASA that has provided observational support for the basic acoustic physics of the baryon–photon plasma.The measured angular power spectrum of the GlossaryTerm
CMB
temperature fluctuations , shown in Fig. 32.6 a,b has become invaluable observables for constraining cosmological models. The position and amplitude of the peaks and dips of the are sensitive to important cosmological parameters. The most robust constraint obtained is that on the spatial curvature of the universe and baryon density. Combining most recent GlossaryTermCMB
observations from GlossaryTermWMAP
9, South Pole Telescope (GlossaryTermSPT
) and Atacama Cosmology Telescope (GlossaryTermACT
) can establish that the space on cosmic scales is geometrically flat () to nearly within 1% precision. From GlossaryTermWMAP
9 alone, the dominant energy content in the present universe is a mysterious matter with negative pressure dubbed, dark energy, or a cosmological constant of about (), followed by cold nonbaryonic dark matter about () and ordinary matter (baryons) account for only about () of the matter budget. Observations of the large-scale structure in the distribution of galaxies, high-redshift supernova, and more recently, GlossaryTermCMB
polarization, have provided valuable complementary information.In addition to the temperature anisotropy, there is also linear polarization information imprinted on the GlossaryTerm
CMB
at the last scattering surface. Thomson scattering generates GlossaryTermCMB
polarization anisotropy at decoupling. The coordinate-free description distinguishes two kinds of polarization patterns on the sky by their different parities. In the spinor approach, the even parity pattern is called the E-mode and the odd parity pattern the B-mode. While the GlossaryTermCMB
temperature anisotropy can also be generated during the propagation of the radiation from the last scattering surface, the GlossaryTermCMB
polarization signal can be generated only at the last scattering surface, where the optical depth transits from large to small values. The polarization information complements the GlossaryTermCMB
temperature anisotropy by isolating the effect at the last scattering surface from effects along the line of sight. Since the GlossaryTermCMB
polarization is sourced by the anisotropy of the GlossaryTermCMB
at the surface of last scattering, the angular power spectra of temperature and polarization are strongly linked to each other. For adiabatic initial perturbations, the acoustic peaks in the polarization spectra are out of phase with that of the temperature.The Degree Angular Scale Interferometer (GlossaryTerm
DASI
) first measured the GlossaryTermCMB
polarization spectrum over a limited band of angular scales (multipole band ) in late 2002. Since then, the polarization power spectrum measurements have been further refined by a number of GlossaryTermCMB
experiments, notably, MAXIMA CBI, QUaD, GlossaryTermBICEP
(background imaging of cosmic extragalactic polarization), etc. The main results indicated by the E-mode polarization measurements is that the acoustic peaks in the polarization spectra are indeed out of phase with that of the temperature. The strong limit on the nonadiabatic contribution to the primordial perturbations constrains the physics of the early universe.The GlossaryTerm
CMB
polarization is a very clean and direct probe of the energy scale of early universe physics that generate the primordial metric perturbations. In the standard model, inflation generates both (scalar) density perturbations and (tensor) gravity wave perturbations. The relative amplitude of inflationary GlossaryTermGW
to scalar density perturbations sets the energy scale for inflation. A measurement of B-mode polarization on large-angular scales would give this amplitude, and hence a direct determination of the energy scale of inflation. Besides being a generic prediction of inflation, the cosmological gravity wave background from inflation would be a fundamental test of GlossaryTermGR
on cosmic scales and the semiclassical behavior of gravity.Conclusion
The remarkable transition to precision cosmology has been spearheaded by the nearly two decade long experimental successes of GlossaryTerm
CMB
measurements. The first results from the GlossaryTermCOBE
team (awarded the Nobel prize in Physics in 2006) provided only a coarse image of infant universe. The data from the Wilkinson Microwave Anisotropy Probe (GlossaryTermWMAP
) refined the image of the infant universe considerably in the following decade. It is the precision of these measurements of the GlossaryTermCMB
fluctuations cosmology that has translated to present day precision cosmology.The past decade has seen the emergence of a concordant cosmological model that is consistent, both, with observational constraints from the background evolution of the universe, and that from the formation of a large-scale structure in the distribution of matter in the universe. Besides precise determination of various parameters of the standard cosmological model, GlossaryTerm
CMB
and related observations have also established some important basic tenets of cosmology and structure formation in the universe – acausally correlated initial perturbations, adiabatic nature primordial density perturbations, gravitational instability as the mechanism for structure formation. We have inferred a spatially flat universe where structures form by the gravitational evolution of nearly scale invariant, adiabatic perturbations, as expected from inflation.The signature of primordial perturbations observed as the GlossaryTerm
CMB
anisotropy and polarization is the most compelling evidence for new, possibly fundamental, physics in the early universe that underlie the scenario of inflation (or related alternatives). Some fundamental assumptions rooted in the paradigm of inflation are still to be observationally established beyond doubt. Besides, there are deeper issues and exotic possibilities that no longer remain theoretical speculations, but have now come well within the grasp of cosmological observations (Chap. 39). These include cosmic topology, extra-dimensions, and violations of basic symmetries such as Lorentz transformations. In order to detect the subtle signatures it is also important to identify and weed out systematic effects such as the noncircularity of the beam in the acquisition and analysis of the GlossaryTermCMB
data.The progress in the field continues unabated, refining the cosmological parameters into increasingly more precise numbers. Numerous ongoing and near future ground and balloon born GlossaryTerm
CMB
experiments at high sensitivity and resolution have sustained a steady pace of progress. The Planck Surveyor mission of GlossaryTermESA
(European Space Agency) launched in May 2009 has acquired considerably more refined GlossaryTermCMB
measurements compared to GlossaryTermWMAP
. In the near future, exquisite results from the Planck satellite are expected. Planck is arguably the most ambitious cosmological space mission till date. It aims to measure GlossaryTermCMB
fluctuations at higher sensitivity and angular resolution to eke out almost all the information expected to be available in the GlossaryTermCMB
sky. Further in the future, dedicated GlossaryTermCMB
polarization space missions are being studied by both NASA and GlossaryTermESA
[6].Abbreviations
- 2dFGRS:
-
2def Galaxy Redshift Survey
- ACT:
-
Atacama Cosmology Telescope
- BBN:
-
big-bang nucleosynthesis
- BICEP:
-
background imaging of cosmic extragalactic polarization
- CMB:
-
cosmic microwave background
- COBE:
-
Cosmic Background Explorer
- DASI:
-
Degree Angular Scale Interferometer
- DIRBE:
-
diffuse infrared background experiment
- DMR:
-
differential microwave radiometer
- ESA:
-
European Space Agency
- FIRAS:
-
far-infrared absolute spectrophotometer
- GR:
-
general relativity
- GW:
-
gravitational wave
- HBBM:
-
hot big bang model
- HST:
-
Hubble space telescope
- LSS:
-
large-scale structure
- OCDM:
-
open cold dark matter
- PI:
-
principle investigator
- SCDM:
-
standard cold dark matter
- SPT:
-
South Pole Telescope
- WMAP:
-
Wilkinson microwave anisotropy probe
- ΛCDM:
-
Lambda-cold dark matter
References
J.P. Ostriker, T. Souradeep: The current status of observational cosmology, Pramana 63, 817 (2004)
N. Bahcall, J.P. Ostriker, S. Perlmutter, P.J. Steinhardt: The cosmic triangle: Revealing the state of the universe, Science 284, 1481 (1999)
W. Hu, S. Dodelson: Cosmic microwave background anisotropies, Annu. Rev. Astron. Astrophys. 40, 171 (2002)
Pedagogical online material on CMB anisotropy and polarization available onlineat http://background.uchicago.edu/
Pedagogical online material on basic cosmology available online athttp://www.astro.ucla.edu/~wright/cosmolog.htm
NASA/DOE/NSF Task Force: Report on Cosmic Microwave Background Research (2005),available online at http://www.nsf.gov/mps/ast/tfcr.jsp; also available at the Legacy Archivefor Microwave Background Data analysis (LAMBDA) site http://lambda.gsfc.nasa.gov/
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Souradeep, T. (2014). Cosmology with the Cosmic Microwave Background. In: Ashtekar, A., Petkov, V. (eds) Springer Handbook of Spacetime. Springer Handbooks. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-41992-8_32
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