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161-597: The CMB is landmark evidence of the Big Bang theory for the origin of the universe. In the Big Bang cosmological models , during the earliest periods, the universe was filled with an opaque fog of dense, hot plasma of sub-atomic particles . As the universe expanded, this plasma cooled to the point where protons and electrons combined to form neutral atoms of mostly hydrogen. Unlike the plasma, these atoms could not scatter thermal radiation by Thomson scattering , and so

322-566: A μ {\displaystyle \mu } -distortion with μ ∼ 2 × 10 − 8 {\displaystyle \mu \sim 2\times 10^{-8}} . This signal can be used as a powerful test for inflation, as it is sensitive to the amplitude of density fluctuations at scales corresponding to physical scales of λ ∼ 0.6 k p c {\displaystyle \lambda \sim 0.6\,{\rm {kpc}}} (i.e., dwarf galaxies). By combining COBE’s measurements of

483-441: A Belgian physicist and Roman Catholic priest , proposed that the recession of the nebulae was due to the expansion of the universe. He inferred the relation that Hubble would later observe, given the cosmological principle. In 1931, Lemaître went further and suggested that the evident expansion of the universe, if projected back in time, meant that the further in the past the smaller the universe was, until at some finite time in

644-638: A Russian cosmologist and mathematician , derived the Friedmann equations from the Einstein field equations, showing that the universe might be expanding in contrast to the static universe model advocated by Albert Einstein at that time. In 1924, American astronomer Edwin Hubble 's measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. Starting that same year, Hubble painstakingly developed

805-484: A future horizon , which limits the events in the future that we will be able to influence. The presence of either type of horizon depends on the details of the Friedmann–Lemaître–Robertson–Walker (FLRW) metric that describes the expansion of the universe. Our understanding of the universe back to very early times suggests that there is a past horizon, though in practice our view is also limited by

966-456: A blackbody temperature. The radiation is remarkably uniform across the sky, very unlike the almost point-like structure of stars or clumps of stars in galaxies. The radiation is isotropic to roughly one part in 25,000: the root mean square variations are just over 100 μK, after subtracting a dipole anisotropy from the Doppler shift of the background radiation. The latter is caused by

1127-417: A characteristic lumpy pattern that varies with angular scale. The distribution of the anisotropy across the sky has frequency components that can be represented by a power spectrum displaying a sequence of peaks and valleys. The peak values of this spectrum hold important information about the physical properties of the early universe: the first peak determines the overall curvature of the universe , while

1288-491: A few keV. In this case, the scattering electrons can have speeds of v ∼ 0.1 c {\displaystyle v\sim 0.1c} , such that relativistic corrections to the Compton process become relevant. These relativistic corrections carry information of electron temperatures which can be used as a measure for the cluster energetics. The classical studies mainly considered energy release (i.e., heating) as

1449-597: A heterogeneous plasma. E-modes were first seen in 2002 by the Degree Angular Scale Interferometer (DASI). B-modes are expected to be an order of magnitude weaker than the E-modes. The former are not produced by standard scalar type perturbations, but are generated by gravitational waves during cosmic inflation shortly after the big bang. However, gravitational lensing of the stronger E-modes can also produce B-mode polarization. Detecting

1610-1271: A measure for the total amount of energy that was injected into the CMB. CMB spectral distortions therefore provide a powerful probe of early-universe physics and even deliver crude estimates for the epoch at which the injection occurred. The current best observational limits set in the 1990s by COBE-satellite/FIRAS-instrument (COBE/FIRAS) are | μ | < 9 × 10 − 5 {\displaystyle |\mu |<9\times 10^{-5}} and | y | < 1.5 × 10 − 5 {\displaystyle |y|<1.5\times 10^{-5}} at 95% confidence level. Within Λ {\displaystyle \Lambda } CDM we expect μ ∼ 2 × 10 − 8 {\displaystyle \mu \sim 2\times 10^{-8}} and y ∼ f e w × 10 − 6 {\displaystyle y\sim {\rm {few}}\times 10^{-6}} , signals that have come into reach of current-day technology (see § Experimental and observational challenges ). Richer distortion signals, going beyond

1771-494: A more generic early hot, dense phase of the universe. In either case, "the Big Bang" as an event is also colloquially referred to as the "birth" of our universe since it represents the point in history where the universe can be verified to have entered into a regime where the laws of physics as we understand them (specifically general relativity and the Standard Model of particle physics ) work. Based on measurements of

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1932-504: A process in the very early universe has reached thermal equilibrium is the ratio between the rate of the process (usually rate of collisions between particles) and the Hubble parameter . The larger the ratio, the more time particles had to thermalize before they were too far away from each other. According to the Big Bang models, the universe at the beginning was very hot and very compact, and since then it has been expanding and cooling. In

2093-425: A renaissance of CMB spectral distortions with numerous theoretical studies and the design of novel experimental concepts In the cosmological 'thermalization problem', three main eras are distinguished: the thermalization or temperature-era, the μ {\displaystyle \mu } -era and the y {\displaystyle y} -era, each with slightly different physical conditions due to

2254-467: A series of distance indicators, the forerunner of the cosmic distance ladder , using the 100-inch (2.5 m) Hooker telescope at Mount Wilson Observatory . This allowed him to estimate distances to galaxies whose redshifts had already been measured, mostly by Slipher. In 1929, Hubble discovered a correlation between distance and recessional velocity —now known as Hubble's law. Independently deriving Friedmann's equations in 1927, Georges Lemaître ,

2415-514: A series of peaks whose angular scales ( ℓ values of the peaks) are roughly in the ratio 1 : 3 : 5 : ..., while adiabatic density perturbations produce peaks whose locations are in the ratio 1 : 2 : 3 : ... Observations are consistent with the primordial density perturbations being entirely adiabatic, providing key support for inflation, and ruling out many models of structure formation involving, for example, cosmic strings. Collisionless damping

2576-414: A single release of energy. In 1982, the importance of double Compton emission as a source of photons at high redshifts was recognized by Danese and de Zotti. Modern considerations of CMB spectral distortions started with the works of Burigana, Danese and de Zotti and Hu, Silk and Scott in the early 1990s. After COBE/FIRAS provided stringent limits on the CMB spectrum, essentially ruling out distortions at

2737-469: A singularity in which space and time lose meaning (typically named "the Big Bang singularity"). Physics lacks a widely accepted theory of quantum gravity that can model the earliest conditions of the Big Bang. In 1964 the CMB was discovered, which convinced many cosmologists that the competing steady-state model of cosmic evolution was falsified , since the Big Bang models predict a uniform background radiation caused by high temperatures and densities in

2898-412: A source of distortions. However, recent work has shown that richer signals can be created by direct photon injection and non-thermal electron populations, both processes that appear in connection with decaying or annihilating particles. Similarly, it was demonstrated that the transition between the μ {\displaystyle \mu } and y {\displaystyle y} -eras

3059-452: A surrounding space, the Big Bang only describes the intrinsic expansion of the contents of the universe. Another issue pointed out by Santhosh Mathew is that bang implies sound, which is not an important feature of the model. An attempt to find a more suitable alternative was not successful. The Big Bang models developed from observations of the structure of the universe and from theoretical considerations. In 1912, Vesto Slipher measured

3220-598: A temperature of T < 10 5 K {\displaystyle T<10^{5}{\rm {K}}} , such that CMB photons are boosted via non-relativistic Compton scattering, giving rise to a y {\displaystyle y} -distortion. Again, by considering the total energetics of the problem and using photon number conservation, one can obtain the estimate y ∼ 1 4 Δ ρ ρ . {\displaystyle y\sim {\frac {1}{4}}\;{\frac {\Delta \rho }{\rho }}.} The name for

3381-430: A temperature of approximately 10 degrees Celsius. Even the very concept of a particle breaks down in these conditions. A proper understanding of this period awaits the development of a theory of quantum gravity . The Planck epoch was succeeded by the grand unification epoch beginning at 10 seconds, where gravitation separated from the other forces as the universe's temperature fell. At approximately 10 seconds into

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3542-407: A unique probe of the pre-recombination Universe that allows us to peek behind the last scattering surface that we observe using the CMB anisotropies. It gives us a unique way to constrain the primordial amount of helium in the early Universe, before recombination, and measure the early expansion rate. The expected Lambda-CDM (LCDM) distortion signals are small – The largest distortion, arising from

3703-610: Is flat . A number of ground-based interferometers provided measurements of the fluctuations with higher accuracy over the next three years, including the Very Small Array , Degree Angular Scale Interferometer (DASI), and the Cosmic Background Imager (CBI). DASI made the first detection of the polarization of the CMB and the CBI provided the first E-mode polarization spectrum with compelling evidence that it

3864-399: Is a prime example within Λ {\displaystyle \Lambda } CDM that is created by photon injection from the recombining hydrogen and helium plasma around redshifts of z ∼ 10 3 − 10 4 {\displaystyle z\sim 10^{3}-10^{4}} . The first considerations of spectral distortions to the CMB go back to

4025-495: Is accelerating , an observation attributed to an unexplained phenomenon known as dark energy . The Big Bang models offer a comprehensive explanation for a broad range of observed phenomena, including the abundances of the light elements , the CMB , large-scale structure , and Hubble's law . The models depend on two major assumptions: the universality of physical laws and the cosmological principle . The universality of physical laws

4186-422: Is assumed to be cold. (Warm dark matter is ruled out by early reionization .) This CDM is estimated to make up about 23% of the matter/energy of the universe, while baryonic matter makes up about 4.6%. In an "extended model" which includes hot dark matter in the form of neutrinos, then the "physical baryon density" Ω b h 2 {\displaystyle \Omega _{\text{b}}h^{2}}

4347-525: Is caused by two effects, when the treatment of the primordial plasma as fluid begins to break down: These effects contribute about equally to the suppression of anisotropies at small scales and give rise to the characteristic exponential damping tail seen in the very small angular scale anisotropies. The depth of the LSS refers to the fact that the decoupling of the photons and baryons does not happen instantaneously, but instead requires an appreciable fraction of

4508-403: Is estimated at 0.023. (This is different from the 'baryon density' Ω b {\displaystyle \Omega _{\text{b}}} expressed as a fraction of the total matter/energy density, which is about 0.046.) The corresponding cold dark matter density Ω c h 2 {\displaystyle \Omega _{\text{c}}h^{2}} is about 0.11, and

4669-447: Is expected because in the early Universe matter and radiation are in thermal equilibrium . However, at redshifts z < 2 × 10 6 {\displaystyle z<2\times 10^{6}} , several mechanisms, both standard and non-standard, can modify the CMB spectrum and introduce departures from a blackbody spectrum. These departures are commonly referred to as CMB spectral distortions and mostly concern

4830-604: Is frequently referred to as the thermalization or temperature era and ends at redshift z ∼ 2 × 10 6 {\displaystyle z\sim 2\times 10^{6}} . At redshifts between 5 × 10 4 {\displaystyle 5\times 10^{4}} and 2 × 10 6 {\displaystyle 2\times 10^{6}} , efficient energy exchange through Compton scattering continues to establish kinetic equilibrium between matter and radiation, but photon number changing processes stop being efficient. Since

4991-425: Is interpreted as a Doppler shift, the recessional velocity of the object can be calculated. For some galaxies, it is possible to estimate distances via the cosmic distance ladder . When the recessional velocities are plotted against these distances, a linear relationship known as Hubble's law is observed: v = H 0 D {\displaystyle v=H_{0}D} where Hubble's law implies that

Cosmic microwave background - Misplaced Pages Continue

5152-588: Is known quite precisely. The first-year WMAP results put the time at which P ( t ) has a maximum as 372,000 years. This is often taken as the "time" at which the CMB formed. However, to figure out how long it took the photons and baryons to decouple, we need a measure of the width of the PVF. The WMAP team finds that the PVF is greater than half of its maximal value (the "full width at half maximum", or FWHM) over an interval of 115,000 years. By this measure, decoupling took place over roughly 115,000 years, and thus when it

5313-548: Is modeled by a cosmological constant term in Einstein field equations of general relativity, but its composition and mechanism are unknown. More generally, the details of its equation of state and relationship with the Standard Model of particle physics continue to be investigated both through observation and theory. All of this cosmic evolution after the inflationary epoch can be rigorously described and modeled by

5474-458: Is more gradual and that the distortion shape is not simply given by a sum of μ {\displaystyle \mu } - and y {\displaystyle y} . All these effects could allow us to differentiate observationally between a wide range of scenarios, as additional time-dependent information can be extracted. About 280,000 years after the Big Bang, electrons and protons became bound into electrically neutral atoms as

5635-427: Is no preferred (or special) observer or vantage point. To this end, the cosmological principle has been confirmed to a level of 10 via observations of the temperature of the CMB. At the scale of the CMB horizon, the universe has been measured to be homogeneous with an upper bound on the order of 10% inhomogeneity, as of 1995. An important feature of the Big Bang spacetime is the presence of particle horizons . Since

5796-410: Is one of the underlying principles of the theory of relativity . The cosmological principle states that on large scales the universe is homogeneous and isotropic —appearing the same in all directions regardless of location. These ideas were initially taken as postulates, but later efforts were made to test each of them. For example, the first assumption has been tested by observations showing that

5957-465: Is out of phase with the T-mode spectrum. In June 2001, NASA launched a second CMB space mission, WMAP , to make much more precise measurements of the large scale anisotropies over the full sky. WMAP used symmetric, rapid-multi-modulated scanning, rapid switching radiometers at five frequencies to minimize non-sky signal noise. The data from the mission was released in five installments, the last being

6118-424: Is related to physical origin of the polarization. Excitation of an electron by linear polarized light generates polarized light at 90 degrees to the incident direction. If the incoming radiation is isotropic, different incoming directions create polarizations that cancel out. If the incoming radiation has quadrupole anisotropy, residual polarization will be seen. Other than the temperature and polarization anisotropy,

6279-590: Is similar in design to the Cosmic Background Imager (CBI) and the Very Small Array (VSA). A third space mission, the ESA (European Space Agency) Planck Surveyor , was launched in May 2009 and performed an even more detailed investigation until it was shut down in October 2013. Planck employed both HEMT radiometers and bolometer technology and measured the CMB at a smaller scale than WMAP. Its detectors were trialled in

6440-449: Is still a matter of scientific debate. It may have included starlight from the very first population of stars ( population III stars), supernovae when these first stars reached the end of their lives, or the ionizing radiation produced by the accretion disks of massive black holes. The time following the emission of the cosmic microwave background—and before the observation of the first stars—is semi-humorously referred to by cosmologists as

6601-432: Is the proper distance, v {\displaystyle v} is the recessional velocity, and v {\displaystyle v} , H {\displaystyle H} , and D {\displaystyle D} vary as the universe expands (hence we write H 0 {\displaystyle H_{0}} to denote the present-day Hubble "constant"). For distances much smaller than

Cosmic microwave background - Misplaced Pages Continue

6762-595: The y {\displaystyle y} -distortion simply stems from the choice of dimensionless variables in the seminal paper of Zeldovich and Sunyaev, 1969. There, the energy injection caused by the hot electrons residing inside clusters of galaxies was considered and the associated effect is more commonly referred to as the thermal Sunyaev-Zeldovich (SZ) effect . Like for the μ {\displaystyle \mu } -distortion, in principle many non-standard physics examples can cause y {\displaystyle y} -type distortions. However,

6923-460: The y {\displaystyle y} -type distortion. μ {\displaystyle \mu } -distortion signals can be created by decaying particles, evaporating primordial black holes, primordial magnetic fields and other non-standard physics examples. Within Λ {\displaystyle \Lambda } CDM cosmology, the adiabatic cooling of matter and dissipation of acoustic waves set up by inflation cause

7084-530: The Dark Age , and is a period which is under intense study by astronomers (see 21 centimeter radiation ). Two other effects which occurred between reionization and our observations of the cosmic microwave background, and which appear to cause anisotropies, are the Sunyaev–Zeldovich effect , where a cloud of high-energy electrons scatters the radiation, transferring some of its energy to the CMB photons, and

7245-533: The Hubble Space Telescope and WMAP. Cosmologists now have fairly precise and accurate measurements of many of the parameters of the Big Bang model, and have made the unexpected discovery that the expansion of the universe appears to be accelerating. "[The] big bang picture is too firmly grounded in data from every area to be proved invalid in its general features." — Lawrence Krauss The earliest and most direct observational evidence of

7406-491: The Milne model , the oscillatory universe (originally suggested by Friedmann, but advocated by Albert Einstein and Richard C. Tolman ) and Fritz Zwicky 's tired light hypothesis. After World War II , two distinct possibilities emerged. One was Fred Hoyle's steady-state model, whereby new matter would be created as the universe seemed to expand. In this model the universe is roughly the same at any point in time. The other

7567-674: The Sachs–Wolfe effect , which causes photons from the Cosmic Microwave Background to be gravitationally redshifted or blueshifted due to changing gravitational fields. The standard cosmology that includes the Big Bang "enjoys considerable popularity among the practicing cosmologists" However, there are challenges to the standard big bang framework for explaining CMB data. In particular standard cosmology requires fine-tuning of some free parameters, with different values supported by different experimental data. As an example of

7728-417: The cosmic microwave background (CMB) radiation , and large-scale structure . The uniformity of the universe, known as the flatness problem , is explained through cosmic inflation : a sudden and very rapid expansion of space during the earliest moments. Extrapolating this cosmic expansion backward in time using the known laws of physics , the models describe an increasingly concentrated cosmos preceded by

7889-488: The dwarf galaxy problem of cold dark matter. Dark energy is also an area of intense interest for scientists, but it is not clear whether direct detection of dark energy will be possible. Inflation and baryogenesis remain more speculative features of current Big Bang models. Viable, quantitative explanations for such phenomena are still being sought. These are unsolved problems in physics. Observations of distant galaxies and quasars show that these objects are redshifted:

8050-549: The inflaton field that caused the inflation event. Long before the formation of stars and planets, the early universe was more compact, much hotter and, starting 10 seconds after the Big Bang, filled with a uniform glow from its white-hot fog of interacting plasma of photons , electrons , and baryons . As the universe expanded , adiabatic cooling caused the energy density of the plasma to decrease until it became favorable for electrons to combine with protons , forming hydrogen atoms. This recombination event happened when

8211-573: The peculiar velocity of the Sun relative to the comoving cosmic rest frame as it moves at 369.82 ± 0.11 km/s towards the constellation Crater near its boundary with the constellation Leo The CMB dipole and aberration at higher multipoles have been measured, consistent with galactic motion. Despite the very small degree of anisotropy in the CMB, many aspects can be measured with high precision and such measurements are critical for cosmological theories. In addition to temperature anisotropy,

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8372-437: The photon – baryon plasma in the early universe. The pressure of the photons tends to erase anisotropies, whereas the gravitational attraction of the baryons, moving at speeds much slower than light, makes them tend to collapse to form overdensities. These two effects compete to create acoustic oscillations, which give the microwave background its characteristic peak structure. The peaks correspond, roughly, to resonances in which

8533-410: The "four pillars" of the Big Bang models. Precise modern models of the Big Bang appeal to various exotic physical phenomena that have not been observed in terrestrial laboratory experiments or incorporated into the Standard Model of particle physics. Of these features, dark matter is currently the subject of most active laboratory investigations. Remaining issues include the cuspy halo problem and

8694-477: The 1978 Nobel Prize in Physics . Cosmic microwave background spectral distortions CMB spectral distortions are tiny departures of the average cosmic microwave background (CMB) frequency spectrum from the predictions given by a perfect black body . They can be produced by a number of standard and non-standard processes occurring at the early stages of cosmic history , and therefore allow us to probe

8855-502: The 2013 data, the universe contains 4.9% ordinary matter , 26.8% dark matter and 68.3% dark energy . On 5 February 2015, new data was released by the Planck mission, according to which the age of the universe is 13.799 ± 0.021 billion years old and the Hubble constant was measured to be 67.74 ± 0.46 (km/s)/Mpc . The cosmic microwave background radiation and the cosmological redshift -distance relation are together regarded as

9016-665: The Antarctic Viper telescope as ACBAR ( Arcminute Cosmology Bolometer Array Receiver ) experiment—which has produced the most precise measurements at small angular scales to date—and in the Archeops balloon telescope. On 21 March 2013, the European-led research team behind the Planck cosmology probe released the mission's all-sky map ( 565x318 jpeg , 3600x1800 jpeg ) of the cosmic microwave background. The map suggests

9177-451: The Big Bang models. After its initial expansion, the universe cooled sufficiently to allow the formation of subatomic particles , and later atoms . The unequal abundances of matter and antimatter that allowed this to occur is an unexplained effect known as baryon asymmetry . These primordial elements—mostly hydrogen , with some helium and lithium —later coalesced through gravity , forming early stars and galaxies. Astronomers observe

9338-533: The Big Bang. Then, from the 1970s to the 1990s, cosmologists worked on characterizing the features of the Big Bang universe and resolving outstanding problems. In 1981, Alan Guth made a breakthrough in theoretical work on resolving certain outstanding theoretical problems in the Big Bang models with the introduction of an epoch of rapid expansion in the early universe he called "inflation". Meanwhile, during these decades, two questions in observational cosmology that generated much discussion and disagreement were over

9499-462: The Big Bang. Since the early universe did not immediately collapse into a multitude of black holes, matter at that time must have been very evenly distributed with a negligible density gradient . The earliest phases of the Big Bang are subject to much speculation, given the lack of available data. In the most common models the universe was filled homogeneously and isotropically with a very high energy density and huge temperatures and pressures , and

9660-408: The CMB as a function of redshift, z , can be shown to be proportional to the color temperature of the CMB as observed in the present day (2.725 K or 0.2348 meV): The high degree of uniformity throughout the observable universe and its faint but measured anisotropy lend strong support for the Big Bang model in general and the ΛCDM ("Lambda Cold Dark Matter") model in particular. Moreover,

9821-415: The CMB frequency spectrum is expected to feature tiny departures from the black-body law known as spectral distortions . These are also at the focus of an active research effort with the hope of a first measurement within the forthcoming decades, as they contain a wealth of information about the primordial universe and the formation of structures at late time. The CMB contains the vast majority of photons in

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9982-452: The CMB is expressed in kelvin (K), the SI unit of temperature. The CMB has a thermal black body spectrum at a temperature of 2.725 48 ± 0.000 57  K . Variations in intensity are expressed as variations in temperature. The blackbody temperature uniquely characterizes the intensity of the radiation at all wavelengths; a measured brightness temperature at any wavelength can be converted to

10143-415: The CMB should have an angular variation in polarization . The polarization at each direction in the sky has an orientation described in terms of E-mode and B-mode polarization. The E-mode signal is a factor of 10 less strong than the temperature anisotropy; it supplements the temperature data as they are correlated. The B-mode signal is even weaker but may contain additional cosmological data. The anisotropy

10304-419: The CMB, showing as small distortions to the CMB blackbody commonly referred to as the cosmological recombination radiation (CRR). The specific spectral shape of this distortion is directly related to the redshift at which this emission takes place, freezing the distortion in time over the microwave frequency bands. Since the distortion signal arises from the hydrogen and two helium recombination eras, this gives us

10465-531: The Earth to another. On 20 May 1964 they made their first measurement clearly showing the presence of the microwave background, with their instrument having an excess 4.2K antenna temperature which they could not account for. After receiving a telephone call from Crawford Hill, Dicke said "Boys, we've been scooped." A meeting between the Princeton and Crawford Hill groups determined that the antenna temperature

10626-620: The Prognoz 9 satellite (launched 1 July 1983), gave the first upper limits on the large-scale anisotropy. The other key event in the 1980s was the proposal by Alan Guth for cosmic inflation . This theory of rapid spatial expansion gave an explanation for large-scale isotropy by allowing causal connection just before the epoch of last scattering. With this and similar theories, detailed prediction encouraged larger and more ambitious experiments. The NASA Cosmic Background Explorer ( COBE ) satellite orbited Earth in 1989–1996 detected and quantified

10787-457: The Universe expanded. In cosmology, this is known as recombination and preludes the decoupling of the CMB photons from matter before they free stream throughout the Universe around 380,000 years after the Big Bang. Within the energy levels of hydrogen and helium atoms, various interactions take place, both collisional and radiative. The line emission arising from these processes is injected into

10948-464: The absence of a perfect cosmological principle , extrapolation of the expansion of the universe backwards in time using general relativity yields an infinite density and temperature at a finite time in the past. This irregular behavior, known as the gravitational singularity , indicates that general relativity is not an adequate description of the laws of physics in this regime. Models based on general relativity alone cannot fully extrapolate toward

11109-508: The age measured today). This issue was later resolved when new computer simulations, which included the effects of mass loss due to stellar winds , indicated a much younger age for globular clusters. Significant progress in Big Bang cosmology has been made since the late 1990s as a result of advances in telescope technology as well as the analysis of data from satellites such as the Cosmic Background Explorer (COBE),

11270-417: The age of the universe up to that era. One method of quantifying how long this process took uses the photon visibility function (PVF). This function is defined so that, denoting the PVF by P ( t ), the probability that a CMB photon last scattered between time t and t + dt is given by P ( t )   dt . The maximum of the PVF (the time when it is most likely that a given CMB photon last scattered)

11431-483: The anisotropies in the cosmic microwave background. The CMB spectrum has become the most precisely measured black body spectrum in nature. In the late 1940s Alpher and Herman reasoned that if there was a Big Bang, the expansion of the universe would have stretched the high-energy radiation of the very early universe into the microwave region of the electromagnetic spectrum , and down to a temperature of about 5 K. They were slightly off with their estimate, but they had

11592-409: The average CMB spectrum across the full sky (i.e., the CMB monopole spectrum). Spectral distortions are created by processes that drive matter and radiation out of equilibrium. One important scenario relates to spectral distortions from early energy injection, for instance, by decaying particles, primordial black hole evaporation or the dissipation of acoustic waves set up by inflation. In this process,

11753-408: The background radiation with intervening hot gas or gravitational potentials, which occur between the last scattering surface and the observer. The structure of the cosmic microwave background anisotropies is principally determined by two effects: acoustic oscillations and diffusion damping (also called collisionless damping or Silk damping). The acoustic oscillations arise because of a conflict in

11914-497: The baryons heat up and transfer some of their excess energy to the ambient CMB photon bath via Compton scattering . Depending on the moment of injection, this causes a distortion, which can be characterized using so-called μ {\displaystyle \mu } - and y {\displaystyle y} -type distortion spectra. The dimensionless μ {\displaystyle \mu } and y {\displaystyle y} -parameters are

12075-638: The best available evidence for the Big Bang event. Measurements of the CMB have made the inflationary Big Bang model the Standard Cosmological Model . The discovery of the CMB in the mid-1960s curtailed interest in alternatives such as the steady state theory . In the Big Bang model for the formation of the universe , inflationary cosmology predicts that after about 10 seconds the nascent universe underwent exponential growth that smoothed out nearly all irregularities. The remaining irregularities were caused by quantum fluctuations in

12236-411: The big-bang predictions by Alpher, Herman and Gamow around 1950. Through the 1970s, the radiation was found to be approximately consistent with a blackbody spectrum in all directions; this spectrum has been redshifted by the expansion of the universe, and today corresponds to approximately 2.725 K. This tipped the balance of evidence in favor of the Big Bang model, and Penzias and Wilson were awarded

12397-473: The blackbody nature of the CMB. However, as shown by Zeldovich and Sunyaev, energy exchange with moving electrons can cause spectral distortions. The pioneering analytical studies of Zeldovich and Sunyaev were later complemented by the numerical investigations of Illarionov and Sunyaev in the 1970s. These treated the thermalization problem including Compton scattering and the Bremsstrahlung process for

12558-421: The change in the density and temperature of particles caused by the Hubble expansion. In the very early stages of cosmic history (up until a few months after the Big Bang ), photons and baryons are efficiently coupled by scattering processes and, therefore, are in full thermodynamic equilibrium. Energy that is injected into the medium is rapidly redistributed among the photons, mainly by Compton scattering, while

12719-432: The classical μ {\displaystyle \mu } and y {\displaystyle y} distortions, can be created by photon injection processes, relativistic electron distributions and during the gradual transition between the μ {\displaystyle \mu } and y {\displaystyle y} -distortion eras. The cosmological recombination radiation (CRR)

12880-438: The color temperature of the background radiation has dropped by an average factor of 1,089 due to the expansion of the universe. As the universe expands, the CMB photons are redshifted , causing them to decrease in energy. The color temperature of this radiation stays inversely proportional to a parameter that describes the relative expansion of the universe over time, known as the scale length . The color temperature T r of

13041-432: The corresponding neutrino density Ω v h 2 {\displaystyle \Omega _{\text{v}}h^{2}} is estimated to be less than 0.0062. Independent lines of evidence from Type Ia supernovae and the CMB imply that the universe today is dominated by a mysterious form of energy known as dark energy , which appears to homogeneously permeate all of space. Observations suggest that 73% of

13202-705: The cosmic microwave background. In 1964, Arno Penzias and Robert Woodrow Wilson at the Crawford Hill location of Bell Telephone Laboratories in nearby Holmdel Township, New Jersey had built a Dicke radiometer that they intended to use for radio astronomy and satellite communication experiments. The antenna was constructed in 1959 to support Project Echo —the National Aeronautics and Space Administration's passive communications satellites, which used large earth orbiting aluminized plastic balloons as reflectors to bounce radio signals from one point on

13363-591: The cumulative flux of all hot gas in the Universe, has an amplitude that is about one order of magnitude below the limits of COBE/FIRAS. While this is considered to be an ‘easy’ target, the cosmological recombination radiation (CRR), as the smallest expected signal, has an amplitude that is another factor of 10 3 {\displaystyle 10^{3}} smaller. All LCDM distortions are furthermore obscured by large Galactic and extragalactic foreground emissions (e.g., dust, synchrotron and free-free emission, cosmic infrared background), and for observations from

13524-453: The decoupling event is estimated to have occurred and at a point in time such that the photons from that distance have just reached observers. Most of the radiation energy in the universe is in the cosmic microwave background, making up a fraction of roughly 6 × 10 of the total density of the universe. Two of the greatest successes of the Big Bang theory are its prediction of the almost perfect black body spectrum and its detailed prediction of

13685-452: The determination of the Hubble constant is known as Hubble tension . Techniques based on observation of the CMB suggest a lower value of this constant compared to the quantity derived from measurements based on the cosmic distance ladder. In 1964, Arno Penzias and Robert Wilson serendipitously discovered the cosmic background radiation, an omnidirectional signal in the microwave band. Their discovery provided substantial confirmation of

13846-426: The distant past. A wide range of empirical evidence strongly favors the Big Bang event, which is now essentially universally accepted. Detailed measurements of the expansion rate of the universe place the Big Bang singularity at an estimated 13.787 ± 0.020   billion years ago, which is considered the age of the universe . There remain aspects of the observed universe that are not yet adequately explained by

14007-450: The early days of CMB cosmology starting with the seminal papers of Yakov B. Zeldovich and Rashid Sunyaev in 1969 and 1970. These works appeared just a few years after the first detection of the CMB by Arno Allan Penzias and Robert Woodrow Wilson and its interpretation as the echo of the Big Bang by Robert H. Dicke and his team in 1965. These findings constitute one of the most important pillars of Big Bang cosmology, which predicts

14168-443: The early universe may be observable as radiation, but his candidate was cosmic rays . Richard C. Tolman showed in 1934 that expansion of the universe would cool blackbody radiation while maintaining a thermal spectrum. The cosmic microwave background was first predicted in 1948 by Ralph Alpher and Robert Herman , in a correction they prepared for a paper by Alpher's PhD advisor George Gamow . Alpher and Herman were able to estimate

14329-591: The expansion using Type Ia supernovae and measurements of temperature fluctuations in the cosmic microwave background, the time that has passed since that event—known as the " age of the universe "—is 13.8 billion years. Despite being extremely dense at this time—far denser than is usually required to form a black hole —the universe did not re-collapse into a singularity. Commonly used calculations and limits for explaining gravitational collapse are usually based upon objects of relatively constant size, such as stars, and do not apply to rapidly expanding space such as

14490-400: The expansion, a phase transition caused a cosmic inflation , during which the universe grew exponentially , unconstrained by the light speed invariance , and temperatures dropped by a factor of 100,000. This concept is motivated by the flatness problem , where the density of matter and energy is very close to the critical density needed to produce a flat universe . That is, the shape of

14651-589: The fine-tuning issue, standard cosmology cannot predict the present temperature of the relic radiation, T 0 {\displaystyle T_{0}} . This value of T 0 {\displaystyle T_{0}} is one of the best results of experimental cosmology and the steady state model can predict it. However, alternative models have their own set of problems and they have only made post-facto explanations of existing observations. Nevertheless, these alternatives have played an important historic role in providing ideas for and challenges to

14812-415: The first Doppler shift of a " spiral nebula " (spiral nebula is the obsolete term for spiral galaxies), and soon discovered that almost all such nebulae were receding from Earth. He did not grasp the cosmological implications of this fact, and indeed at the time it was highly controversial whether or not these nebulae were "island universes" outside our Milky Way . Ten years later, Alexander Friedmann ,

14973-471: The fluctuations are coherent on angular scales that are larger than the apparent cosmological horizon at recombination. Either such coherence is acausally fine-tuned , or cosmic inflation occurred. The anisotropy , or directional dependency, of the cosmic microwave background is divided into two types: primary anisotropy, due to effects that occur at the surface of last scattering and before; and secondary anisotropy, due to effects such as interactions of

15134-404: The gravitational effects of an unknown dark matter surrounding galaxies. Most of the gravitational potential in the universe seems to be in this form, and the Big Bang models and various observations indicate that this excess gravitational potential is not created by baryonic matter , such as normal atoms. Measurements of the redshifts of supernovae indicate that the expansion of the universe

15295-413: The ground (APSERa and Cosmo at Dome-C, TMS at Teide Observatory ). These are all designed to reach important milestones towards a detection of CMB distortions. As an ultimate frontier, a full characterization and exploitation of the cosmological recombination signal could be achieved by using a coordinated international experimental campaign, potentially including an observatory on the moon In June 2021,

15456-559: The ground or balloons, atmospheric emission poses another hurdle to overcome. A detection of the LCDM distortions therefore requires novel experimental approaches that provide unprecedented sensitivity, spectral coverage, control of systematics and the capabilities to accurately remove foregrounds. Building on the design of FIRAS and experience with ARCADE , this has led to several spectrometer concepts to observe from space (PIXIE, PRISM, PRISTINE, SuperPIXIE and Voyage2050), balloon (BISOU) and

15617-411: The lambda-CDM model of cosmology, which uses the independent frameworks of quantum mechanics and general relativity. There are no easily testable models that would describe the situation prior to approximately 10 seconds. Understanding this earliest of eras in the history of the universe is one of the greatest unsolved problems in physics . English astronomer Fred Hoyle is credited with coining

15778-666: The large scale anisotropies at the limit of its detection capabilities. The NASA COBE mission clearly confirmed the primary anisotropy with the Differential Microwave Radiometer instrument, publishing their findings in 1992. The team received the Nobel Prize in physics for 2006 for this discovery. Inspired by the COBE results, a series of ground and balloon-based experiments measured cosmic microwave background anisotropies on smaller angular scales over

15939-435: The large-scale CMB anisotropies with the μ {\displaystyle \mu } -distortion constraint, the first limits on the small-scale power spectrum could be obtained well-before direct measurements became possible At redshifts z ≲ 5 × 10 4 {\displaystyle z\lesssim 5\times 10^{4}} , also Compton scattering becomes inefficient. The plasma has

16100-418: The largest contribution to the all-sky y {\displaystyle y} -distortion stems from the cumulative cluster SZ signal, which provides a way to constrain the amount of hot gas in the Universe. While at z < 10 4 {\displaystyle z<10^{4}} , the cosmic plasma on average has a low temperature, electrons inside galaxy clusters can reach temperatures of

16261-493: The largest possible deviation of the fine-structure constant over much of the age of the universe is of order 10 . Also, general relativity has passed stringent tests on the scale of the Solar System and binary stars . The large-scale universe appears isotropic as viewed from Earth. If it is indeed isotropic, the cosmological principle can be derived from the simpler Copernican principle , which states that there

16422-460: The leading theory of cosmic structure formation, and suggested cosmic inflation was the right theory. During the 1990s, the first peak was measured with increasing sensitivity and by 2000 the BOOMERanG experiment reported that the highest power fluctuations occur at scales of approximately one degree. Together with other cosmological data, these results implied that the geometry of the universe

16583-496: The level Δ I I ∼ 10 − 5 − 10 − 4 {\displaystyle {\tfrac {\Delta I}{I}}\sim 10^{-5}-10^{-4}} , the interest in CMB spectral distortions decreased. In 2011, PIXIE was proposed to NASA as a mid-Ex satellite mission, providing first strong motivation to revisit the theory of spectral distortions. Although no successor of COBE/FIRAS has been funded so far, this led to

16744-405: The light emitted from them has been shifted to longer wavelengths. This can be seen by taking a frequency spectrum of an object and matching the spectroscopic pattern of emission or absorption lines corresponding to atoms of the chemical elements interacting with the light. These redshifts are uniformly isotropic, distributed evenly among the observed objects in all directions. If the redshift

16905-452: The mathematical derivation of the Friedmann equations . The earliest empirical observation of the notion of an expanding universe is known as Hubble's Law , published in work by physicist Edwin Hubble in 1929, which discerned that galaxies are moving away from Earth at a rate that accelerates proportionally with distance. Independent of Friedmann's work, and independent of Hubble's observations, physicist Georges Lemaître proposed that

17066-423: The microwave radiation was truly "cosmic". First, the intensity vs frequency or spectrum needed to be shown to match a thermal or blackbody source. This was accomplished by 1968 in a series of measurements of the radiation temperature at higher and lower wavelengths. Second, the radiation needed be shown to be isotropic, the same from all directions. This was also accomplished by 1970, demonstrating that this radiation

17227-513: The nine year summary. The results are broadly consistent Lambda CDM models based on 6 free parameters and fitting in to Big Bang cosmology with cosmic inflation . The Degree Angular Scale Interferometer (DASI) was a telescope installed at the U.S. National Science Foundation 's Amundsen–Scott South Pole Station in Antarctica . It was a 13-element interferometer operating between 26 and 36 GHz ( Ka band ) in ten bands. The instrument

17388-453: The notions of space and time would altogether fail to have any meaning at the beginning; they would only begin to have a sensible meaning when the original quantum had been divided into a sufficient number of quanta. If this suggestion is correct, the beginning of the world happened a little before the beginning of space and time. During the 1930s, other ideas were proposed as non-standard cosmologies to explain Hubble's observations, including

17549-431: The observable imprint that these inhomogeneities would have on the cosmic microwave background. After a lull in the 1970s caused in part by the many experimental difficulties in measuring CMB at high precision, increasingly stringent limits on the anisotropy of the cosmic microwave background were set by ground-based experiments during the 1980s. RELIKT-1 , a Soviet cosmic microwave background anisotropy experiment on board

17710-460: The observational evidence, most notably from radio source counts , began to favor Big Bang over steady state. The discovery and confirmation of the CMB in 1964 secured the Big Bang as the best theory of the origin and evolution of the universe. In 1968 and 1970, Roger Penrose , Stephen Hawking , and George F. R. Ellis published papers where they showed that mathematical singularities were an inevitable initial condition of relativistic models of

17871-482: The opacity of the universe at early times. So our view cannot extend further backward in time, though the horizon recedes in space. If the expansion of the universe continues to accelerate, there is a future horizon as well. Some processes in the early universe occurred too slowly, compared to the expansion rate of the universe, to reach approximate thermodynamic equilibrium . Others were fast enough to reach thermalization . The parameter usually used to find out whether

18032-411: The original B-modes signal requires analysis of the contamination caused by lensing of the relatively strong E-mode signal. Big Bang The Big Bang is a physical theory that describes how the universe expanded from an initial state of high density and temperature . The notion of an expanding universe was first scientifically originated by physicist Alexander Friedmann in 1922 with

18193-404: The original matter particles and none of their antiparticles . A similar process happened at about 1 second for electrons and positrons. After these annihilations, the remaining protons, neutrons and electrons were no longer moving relativistically and the energy density of the universe was dominated by photons (with a minor contribution from neutrinos ). A few minutes into the expansion, when

18354-529: The other astronomical structures observable today. The details of this process depend on the amount and type of matter in the universe. The four possible types of matter are known as cold dark matter (CDM), warm dark matter , hot dark matter , and baryonic matter . The best measurements available, from the Wilkinson Microwave Anisotropy Probe (WMAP), show that the data is well-fit by a Lambda-CDM model in which dark matter

18515-469: The other forces, with only the electromagnetic force and weak nuclear force remaining unified. Inflation stopped locally at around 10 to 10 seconds, with the observable universe's volume having increased by a factor of at least 10 . Reheating followed as the inflaton field decayed, until the universe obtained the temperatures required for the production of a quark–gluon plasma as well as all other elementary particles . Temperatures were so high that

18676-427: The past all the mass of the universe was concentrated into a single point, a "primeval atom" where and when the fabric of time and space came into existence. In the 1920s and 1930s, almost every major cosmologist preferred an eternal steady-state universe, and several complained that the beginning of time implied by the Big Bang imported religious concepts into physics; this objection was later repeated by supporters of

18837-534: The peaks give important information about the nature of the primordial density perturbations. There are two fundamental types of density perturbations called adiabatic and isocurvature . A general density perturbation is a mixture of both, and different theories that purport to explain the primordial density perturbation spectrum predict different mixtures. The CMB spectrum can distinguish between these two because these two types of perturbations produce different peak locations. Isocurvature density perturbations produce

18998-428: The photon number density is adjusted by photon non-conserving processes, such as double Compton and thermal Bremsstrahlung. This allows the photon field to quickly relax back to a Planckian distribution , even if for a very short phase a spectral distortion appears. Observations today cannot tell the difference in this case, as there is no independent cosmological prediction for the CMB monopole temperature. This regime

19159-512: The photon radiation . The recombination epoch began after about 379,000 years, when the electrons and nuclei combined into atoms (mostly hydrogen ), which were able to emit radiation. This relic radiation, which continued through space largely unimpeded, is known as the cosmic microwave background. After the recombination epoch, the slightly denser regions of the uniformly distributed matter gravitationally attracted nearby matter and thus grew even denser, forming gas clouds, stars, galaxies, and

19320-461: The photon energy density and number density constraints from before and after the energy injection. This yields the well-known expression, μ ∼ 1.4 Δ ρ ρ , {\displaystyle \mu \sim 1.4\;{\frac {\Delta \rho }{\rho }},} where Δ ρ ρ {\displaystyle {\tfrac {\Delta \rho }{\rho }}} determines

19481-415: The photon number density is conserved but the energy density is modified, photons gain an effective non-zero chemical potential, acquiring a Bose-Einstein distribution. This distinct type of distortion is called μ {\displaystyle \mu } -distortion after the chemical potential known from standard thermodynamics. The value for the chemical potential can be estimated by combining

19642-433: The photons decouple when a particular mode is at its peak amplitude. The peaks contain interesting physical signatures. The angular scale of the first peak determines the curvature of the universe (but not the topology of the universe). The next peak—ratio of the odd peaks to the even peaks—determines the reduced baryon density. The third peak can be used to get information about the dark-matter density. The locations of

19803-506: The picture becomes less speculative, since particle energies drop to values that can be attained in particle accelerators . At about 10 seconds, quarks and gluons combined to form baryons such as protons and neutrons . The small excess of quarks over antiquarks led to a small excess of baryons over antibaryons. The temperature was no longer high enough to create either new proton–antiproton or neutron–antineutron pairs. A mass annihilation immediately followed, leaving just one in 10 of

19964-455: The plasma. The first peak in the anisotropy was tentatively detected by the MAT/TOCO experiment and the result was confirmed by the BOOMERanG and MAXIMA experiments. These measurements demonstrated that the geometry of the universe is approximately flat, rather than curved . They ruled out cosmic strings as a major component of cosmic structure formation and suggested cosmic inflation

20125-488: The precise values of the Hubble Constant and the matter-density of the universe (before the discovery of dark energy, thought to be the key predictor for the eventual fate of the universe ). In the mid-1990s, observations of certain globular clusters appeared to indicate that they were about 15 billion years old, which conflicted with most then-current estimates of the age of the universe (and indeed with

20286-461: The predominance of matter over antimatter in the present universe. The universe continued to decrease in density and fall in temperature, hence the typical energy of each particle was decreasing. Symmetry-breaking phase transitions put the fundamental forces of physics and the parameters of elementary particles into their present form, with the electromagnetic force and weak nuclear force separating at about 10 seconds. After about 10 seconds,

20447-427: The random motions of particles were at relativistic speeds , and particle–antiparticle pairs of all kinds were being continuously created and destroyed in collisions. At some point, an unknown reaction called baryogenesis violated the conservation of baryon number , leading to a very small excess of quarks and leptons over antiquarks and antileptons—of the order of one part in 30 million. This resulted in

20608-430: The right idea. They predicted the CMB. It took another 15 years for Penzias and Wilson to discover that the microwave background was actually there. According to standard cosmology, the CMB gives a snapshot of the hot early universe at the point in time when the temperature dropped enough to allow electrons and protons to form hydrogen atoms. This event made the universe nearly transparent to radiation because light

20769-403: The second and third peak detail the density of normal matter and so-called dark matter , respectively. Extracting fine details from the CMB data can be challenging, since the emission has undergone modification by foreground features such as galaxy clusters . The cosmic microwave background radiation is an emission of uniform black body thermal energy coming from all directions. Intensity of

20930-410: The singularity. In some proposals, such as the emergent Universe models, the singularity is replaced by another cosmological epoch. A different approach identifies the initial singularity as a singularity predicted by some models of the Big Bang theory to have existed before the Big Bang event. This primordial singularity is itself sometimes called "the Big Bang", but the term can also refer to

21091-495: The size of the observable universe , the Hubble redshift can be thought of as the Doppler shift corresponding to the recession velocity v {\displaystyle v} . For distances comparable to the size of the observable universe, the attribution of the cosmological redshift becomes more ambiguous, although its interpretation as a kinematic Doppler shift remains the most natural one. An unexplained discrepancy with

21252-423: The standard explanation. The cosmic microwave background is polarized at the level of a few microkelvin. There are two types of polarization, called E-mode (or gradient-mode) and B-mode (or curl mode). This is in analogy to electrostatics , in which the electric field ( E -field) has a vanishing curl and the magnetic field ( B -field) has a vanishing divergence . The E-modes arise from Thomson scattering in

21413-430: The standard picture of cosmology. Importantly, the CMB frequency spectrum and its distortions should not be confused with the CMB anisotropy power spectrum, which relates to spatial fluctuations of the CMB temperature in different directions of the sky. The energy spectrum of the CMB is extremely close to that of a perfect blackbody with a temperature of 2.7255 K {\displaystyle 2.7255K} . This

21574-406: The steady-state theory. This perception was enhanced by the fact that the originator of the Big Bang concept, Lemaître, was a Roman Catholic priest. Arthur Eddington agreed with Aristotle that the universe did not have a beginning in time, viz ., that matter is eternal . A beginning in time was "repugnant" to him. Lemaître, however, disagreed: If the world has begun with a single quantum ,

21735-411: The temperature of the cosmic microwave background to be 5 K. The first published recognition of the CMB radiation as a detectable phenomenon appeared in a brief paper by Soviet astrophysicists A. G. Doroshkevich and Igor Novikov , in the spring of 1964. In 1964, David Todd Wilkinson and Peter Roll, Dicke's colleagues at Princeton University , began constructing a Dicke radiometer to measure

21896-447: The temperature was about a billion kelvin and the density of matter in the universe was comparable to the current density of Earth's atmosphere, neutrons combined with protons to form the universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis (BBN). Most protons remained uncombined as hydrogen nuclei. As the universe cooled, the rest energy density of matter came to gravitationally dominate that of

22057-441: The temperature was around 3000 K or when the universe was approximately 379,000 years old. As photons did not interact with these electrically neutral atoms, the former began to travel freely through space, resulting in the decoupling of matter and radiation. The color temperature of the ensemble of decoupled photons has continued to diminish ever since; now down to 2.7260 ± 0.0013 K , it will continue to drop as

22218-465: The term "Big Bang" during a talk for a March 1949 BBC Radio broadcast, saying: "These theories were based on the hypothesis that all the matter in the universe was created in one big bang at a particular time in the remote past." However, it did not catch on until the 1970s. It is popularly reported that Hoyle, who favored an alternative " steady-state " cosmological model, intended this to be pejorative, but Hoyle explicitly denied this and said it

22379-408: The time of decoupling. The CMB is not completely smooth and uniform, showing a faint anisotropy that can be mapped by sensitive detectors. Ground and space-based experiments such as COBE , WMAP and Planck have been used to measure these temperature inhomogeneities. The anisotropy structure is determined by various interactions of matter and photons up to the point of decoupling, which results in

22540-452: The total energy density of the present day universe is in this form. When the universe was very young it was likely infused with dark energy, but with everything closer together, gravity predominated, braking the expansion. Eventually, after billions of years of expansion, the declining density of matter relative to the density of dark energy allowed the expansion of the universe to begin to accelerate. Dark energy in its simplest formulation

22701-499: The total energy that is injected into the CMB photon field. With respect to the equilibrium blackbody spectrum, the μ {\displaystyle \mu } -distortion is characterized by a deficit of photons at low frequencies and an increment at high frequencies. The distortion changes sign at a frequency of ν ∼ 130 G H z {\displaystyle \nu \sim 130{\rm {GHz}}} , allowing us to distinguish it observationally from

22862-459: The two decades. The sensitivity of the new experiments improved dramatically, with a reduction in internal noise by three orders of magnitude. The primary goal of these experiments was to measure the scale of the first acoustic peak, which COBE did not have sufficient resolution to resolve. This peak corresponds to large scale density variations in the early universe that are created by gravitational instabilities, resulting in acoustical oscillations in

23023-416: The universe has no overall geometric curvature due to gravitational influence. Microscopic quantum fluctuations that occurred because of Heisenberg's uncertainty principle were "frozen in" by inflation, becoming amplified into the seeds that would later form the large-scale structure of the universe. At a time around 10 seconds, the electroweak epoch begins when the strong nuclear force separates from

23184-418: The universe became transparent. Known as the recombination epoch, this decoupling event released photons to travel freely through space. However, the photons have grown less energetic due to the cosmological redshift associated with the expansion of the universe . The surface of last scattering refers to a shell at the right distance in space so photons are now received that were originally emitted at

23345-424: The universe by a factor of 400 to 1; the number density of photons in the CMB is one billion times (10) the number density of matter in the universe. Without the expansion of the universe to cause the cooling of the CMB, the night sky would shine as brightly as the Sun. The energy density of the CMB is 0.260 eV/cm (4.17 × 10 J/m), about 411 photons/cm. In 1931, Georges Lemaître speculated that remnants of

23506-411: The universe emerged from a "primeval atom " in 1931, introducing the modern notion of the Big Bang. Various cosmological models of the Big Bang explain the evolution of the observable universe from the earliest known periods through its subsequent large-scale form. These models offer a comprehensive explanation for a broad range of observed phenomena, including the abundance of light elements ,

23667-407: The universe expands. The intensity of the radiation corresponds to black-body radiation at 2.726 K because red-shifted black-body radiation is just like black-body radiation at a lower temperature. According to the Big Bang model, the radiation from the sky we measure today comes from a spherical surface called the surface of last scattering . This represents the set of locations in space at which

23828-422: The universe has a finite age, and light travels at a finite speed, there may be events in the past whose light has not yet had time to reach earth. This places a limit or a past horizon on the most distant objects that can be observed. Conversely, because space is expanding, and more distant objects are receding ever more quickly, light emitted by us today may never "catch up" to very distant objects. This defines

23989-440: The universe is slightly older than researchers expected. According to the map, subtle fluctuations in temperature were imprinted on the deep sky when the cosmos was about 370 000 years old. The imprint reflects ripples that arose as early, in the existence of the universe, as the first nonillionth (10) of a second. Apparently, these ripples gave rise to the present vast cosmic web of galaxy clusters and dark matter . Based on

24150-490: The universe is uniformly expanding everywhere. This cosmic expansion was predicted from general relativity by Friedmann in 1922 and Lemaître in 1927, well before Hubble made his 1929 analysis and observations, and it remains the cornerstone of the Big Bang model as developed by Friedmann, Lemaître, Robertson, and Walker. The theory requires the relation v = H D {\displaystyle v=HD} to hold at all times, where D {\displaystyle D}

24311-449: The validity of the theory are the expansion of the universe according to Hubble's law (as indicated by the redshifts of galaxies), discovery and measurement of the cosmic microwave background and the relative abundances of light elements produced by Big Bang nucleosynthesis (BBN). More recent evidence includes observations of galaxy formation and evolution , and the distribution of large-scale cosmic structures . These are sometimes called

24472-544: The volume of the intergalactic medium (IGM) consists of ionized material (since there are few absorption lines due to hydrogen atoms). This implies a period of reionization during which some of the material of the universe was broken into hydrogen ions. The CMB photons are scattered by free charges such as electrons that are not bound in atoms. In an ionized universe, such charged particles have been liberated from neutral atoms by ionizing (ultraviolet) radiation. Today these free charges are at sufficiently low density in most of

24633-580: The volume of the universe that they do not measurably affect the CMB. However, if the IGM was ionized at very early times when the universe was still denser, then there are two main effects on the CMB: Both of these effects have been observed by the WMAP spacecraft, providing evidence that the universe was ionized at very early times, at a redshift around 10. The detailed provenance of this early ionizing radiation

24794-477: Was Lemaître's Big Bang theory, advocated and developed by George Gamow , who introduced BBN and whose associates, Ralph Alpher and Robert Herman , predicted the CMB. Ironically, it was Hoyle who coined the phrase that came to be applied to Lemaître's theory, referring to it as "this big bang idea" during a BBC Radio broadcast in March 1949. For a while, support was split between these two theories. Eventually,

24955-470: Was complete, the universe was roughly 487,000 years old. Since the CMB came into existence, it has apparently been modified by several subsequent physical processes, which are collectively referred to as late-time anisotropy, or secondary anisotropy. When the CMB photons became free to travel unimpeded, ordinary matter in the universe was mostly in the form of neutral hydrogen and helium atoms. However, observations of galaxies today seem to indicate that most of

25116-419: Was indeed due to the microwave background. Penzias and Wilson received the 1978 Nobel Prize in Physics for their discovery. The interpretation of the cosmic microwave background was a controversial issue in the late 1960s. Alternative explanations included energy from within the solar system, from galaxies, from intergalactic plasma and from multiple extragalactic radio sources. Two requirements would show that

25277-411: Was just a striking image meant to highlight the difference between the two models. Helge Kragh writes that the evidence for the claim that it was meant as a pejorative is "unconvincing", and mentions a number of indications that it was not a pejorative. The term itself has been argued to be a misnomer because it evokes an explosion. The argument is that whereas an explosion suggests expansion into

25438-437: Was no longer being scattered off free electrons. When this occurred some 380,000 years after the Big Bang, the temperature of the universe was about 3,000 K. This corresponds to an ambient energy of about 0.26  eV , which is much less than the 13.6 eV ionization energy of hydrogen. This epoch is generally known as the "time of last scattering" or the period of recombination or decoupling . Since decoupling,

25599-474: Was the right theory of structure formation. Inspired by the initial COBE results of an extremely isotropic and homogeneous background, a series of ground- and balloon-based experiments quantified CMB anisotropies on smaller angular scales over the next decade. The primary goal of these experiments was to measure the angular scale of the first acoustic peak, for which COBE did not have sufficient resolution. These measurements were able to rule out cosmic strings as

25760-418: Was truly cosmic in origin. In the 1970s numerous studies showed that tiny deviations from isotropy in the CMB could result from events in the early universe. Harrison, Peebles and Yu, and Zel'dovich realized that the early universe would require quantum inhomogeneities that would result in temperature anisotropy at the level of 10 or 10. Rashid Sunyaev , using the alternative name relic radiation , calculated

25921-426: Was very rapidly expanding and cooling. The period up to 10 seconds into the expansion, the Planck epoch , was a phase in which the four fundamental forces —the electromagnetic force , the strong nuclear force , the weak nuclear force , and the gravitational force , were unified as one. In this stage, the characteristic scale length of the universe was the Planck length , 1.6 × 10  m , and consequently had

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