Microwave background radiation
The microwave background radiation (in English: cosmic microwave background or CMB) is a form of radiation electromagnetic field discovered in 1965 that fills the entire universe. Also called cosmic microwave radiation, cosmic background radiation, or cosmic background radiation.
It has characteristics of black body radiation at a temperature of 2.725 K and its frequency belongs to the microwave range with a frequency of 160.2 GHz, corresponding to a wavelength of 1.9 mm. This radiation is one of the main pieces of evidence for the cosmological model of the Big Bang.
Features
The microwave background radiation is isotropic to one part in 105: the variations of the effective value are only 18 µK. The FIRAS spectrophotometer i>The Far-Infrared Absolute Spectrophotometer) on NASA's COBE satellite has carefully measured the spectrum of microwave background radiation. FIRAS compared the CMB with a reference blackbody and no difference could be seen in their spectra. Any deviation from the black body that could remain undetected in the CMB spectrum over the wavelength range from 0.5 to 5 mm would have to have a value of about 50 parts per million of the peak CMB brightness. This made the CMB spectrum the most precisely measured black body in nature.
This radiation is a prediction of the Big Bang model, since according to this model, the early universe was a plasma composed mainly of electrons, photons, and baryons (protons and neutrons). The photons were constantly interacting with the plasma through Thomson scattering. Electrons could not unite with protons and other atomic nuclei to form atoms because the average energy of such a plasma was very high, so electrons constantly interacted with photons through a process known as Compton scattering. As the universe expanded, adiabatic cooling (of which cosmological redshift is a current symptom) caused the plasma to cool until it was possible for electrons to combine with protons to form hydrogen atoms. This occurred when it reached 3,000 K, about 380,000 years after the Big Bang. From that moment on, the photons were able to travel freely through space without rubbing against (without actually joining) the scattered electrons. This phenomenon is known as the age of recombination; microwave background radiation is precisely the result of that period. As the universe expanded, this radiation also decreased in temperature, which explains why it is only about 2.7 K today. Background radiation is the noise the universe makes. The photons have continued to cool ever since, currently dropping to 2,725 K, and their temperature will continue to drop as the universe expands. In the same way, the radiation of the sky that we measure comes from a spherical surface, called the surface of last scattering, in which the photons that decayed in the interaction with matter in the early universe, 13.7 billion years ago, are currently being observed on Earth. The Big Bang suggests that the cosmic radiation background fills all observable space and that much of the radiation in the universe is in the CMB, which is about a 5 10-5 fraction of the total density of the universe.
Two of the great successes of the Big Bang theory are its predictions of this near-perfect blackbody spectrum and its detailed prediction of anisotropies in the cosmic microwave background. The recent WMAP has precisely measured these anisotropies over the entire sky at 0.2° angular scales. These can be used to estimate the parameters of the standard Lambda-CDM Big Bang model. Some information, such as the shape of the universe, can be obtained directly from the CMB, while others, such as the Hubble constant, are unrestricted and have to be inferred from other measurements.
History
This radiation was predicted by George Gamow, Ralph Alpher, and Robert Herman in 1948. Furthermore, Alpher and Herman were able to estimate the temperature of the background microwave radiation to be 5 K, although two years later, they reestimated it at 2.8 K Although there were several previous estimates of the temperature of space (see timeline), they suffered from two flaws. First, they were measurements of the effective temperature of the space and do not suggest that the space was filled with a thermal Planck spectrum. And second, they are dependent on our special location at the edge of the Milky Way and do not suggest that the radiation is isotropic. Furthermore, it would produce very different predictions if the Earth were located anywhere in the universe.
Gamow and Alpher's 1948 results were not widely discussed. However, they were rediscovered by Robert Dicke and Yakov Zel'dovich in the early 1960s. The first appreciation of CMB radiation as a detectable phenomenon appeared in a short article by Soviet astrophysicists A. G. Doroshkevich and Igor Dmitriyevich Novikov, in the spring of 1964. In 1964, David Todd Wilkinson and Peter Roll, and Dicke's colleagues at Princeton University, began building a Dicke radiometer to measure the background microwave radiation. In 1965, Arno Penzias and Robert Woodrow Wilson at Crawford Hill Bell Laboratories near Holmdel Township, New Jersey had built a Dicke radiometer which they intended to use for radio astronomy and satellite communications experiments. Their instrumentation had a noise temperature excess of 3.5 K which they did not count on. After receiving a phone call from Crawford Hill, Dicke gracefully said, "Guys, we've been robbed." A meeting between the Princeton and Crawford Hill groups determined that the antenna temperature was induced due to background microwave radiation.. Penzias and Wilson received the 1978 Nobel Prize in Physics for their discovery.
The interpretation of the microwave background radiation was a controversial issue in the 1960s among proponents of the steady state theory arguing that the microwave background was the result of scattered starlight from distant galaxies. Using this model and based on the study of the reduced absorption lines that characterize the spectrum of stars, astronomer Andrew McKellar wrote in 1941: "The temperature curl of interstellar space can be calculated to be 2 K.", during the 1970s the consensus was that the microwave background radiation is a remnant of the Big Bang. This was largely because new measurements over a range of frequencies showed that the spectrum was a thermal, black body, a result that the steady-state model could not reproduce.
Harrison, Peebles and Yu, and on the other hand Zel'dovich realized that the early Universe would have to have inhomogeneities at a level of 10-4 or 10− 5. Rashid Siunyaev then calculated the observable imprint these inhomogeneities would have on the microwave radiation background. Increasingly strict limits on the anisotropy of the microwave radiation background were established by ground-based experiments, but the anisotropy was first detected by the Differential Microwave Radiometer on the COBE satellite.
Inspired by the COBE results, a series of ground-based and balloon-based experiments measured the anisotropies of the microwave radiation background on small angular scales over the next decade. The primary objective of these experiments was to measure the scale of the first acoustic peak, for which the COBE did not have sufficient resolution to resolve it. The first spike in anisotropy was tentatively detected by the Toco experiment and the result was confirmed by the BOOMERanG and MAXIMA experiments. These measurements showed that the universe was roughly flat and could rule out cosmic strings as a large component in the formation of structures. cosmic and suggests that cosmic inflation was the correct theory for structure formation.
The second peak was tentatively detected by several experts before being definitively detected by WMAP, which has also hesitantly detected the third peak. Several experiments to improve measurements of polarization and the microwave background on small angular scales are ongoing. These are the DASI, WMAP, BOOMERanG and the Cosmic Background Imager. Future experiments in this field are the Planck satellite, the Atacama Cosmological Telescope and the South Pole Telescope.
Timeline of the Microwave Background
- 1940. Andrew McKellar: The observational detection of an average bolometric temperature of 2.3 K based on the study of interstellar absorption lines is informed from the Dominion Observatory, British Columbia.
- 1946. Robert Dicke discovers "the radiation of cosmic matter" to ≥ 20 K, does not refer to background radiation.
- 1948. George Gamow calculates a temperature of 50 K (assuming a universe of three billion years), comenting it. He agrees reasonably with the current temperature of interstellar space, but does not mention the radiation background.
- 1948. Ralph Alpher and Robert Herman estimate "the temperature in the universe" in 5 K. Although they do not specifically mention the microwave radiation background, it can be inferred.
- 1950. Ralph Alpher and Robert Herman re-estimate the temperature at 2 K.
- 1953. George Gamow estimates 7 K.
- 1956. George Gamow estimates 6 K.
- Years 1960. Robert Dicke reestimates a microwave background radiation temperature of 40 K.
- 1964. A. G. Doroshkevich and Igor Novikov publish a brief article, where they say that the phenomenon of microwave background radiation is detectable.
- 1964-65. Arno Penzias and Robert Woodrow Wilson measure the temperature as approximately 3 K. Robert Dicke, P. J. E. Peebles, P. G. Roll and D. T. Wilkinson interpret radiation as a Big Bang signature.
- 1983. The Soviet RELIKT-1 experiment on CMB anthropy begins.
- 1990. FIRAS measurements are obtained from the black body shape of the CMB spectrum with exquisite precision.
- 1992. The discovery of anthropy by the RELIKT-1 spacecraft was officially reported in January 1992 at the Moscow Astrophysics Seminar.
- 1992. COBE DMR reveals the temperature of primary anthropy for the first time.
- 2002. DASI discovers the polarization of the CMB.
- 2004. The CBI gets the CMB's polarization spectrum.
Relation to the Big Bang
The hot standard Big Bang model of the Universe requires that the initial conditions for the Universe be a nearly invariant Gaussian field or Harrison-Zel'dovich spectrum. This is, for example, a prediction of the cosmic inflation model. This means that the initial state of the Universe is random, but in a clearly specified way in which the amplitude of virgin inhomogeneities is 10-5. Therefore, postulates about inhomogeneities in the Universe need to be statistical in nature. This leads to cosmic variance in that uncertainties in the variance of large-scale fluctuations observed in the Universe have difficulty in accurately comparing to theory.
Temperature
The cosmic microwave background radiation and the cosmological redshift are jointly considered the best available evidence for the Big Bang theory. The discovery of the CMB in the mid-1960s reduced interest in alternatives such as the Steady State Theory. The CMB provides a picture of the Universe when, according to conventional cosmology, the temperature dropped enough to allow electrons and protons to form hydrogen atoms, thus making the Universe transparent to radiation. When it originated about 379,000 years after the Big Bang, this period is generally known as the "period of last scattering" or the period of recombination or decoupling, the temperature of the Universe was about 3,000 K. This corresponds to an energy of about 0.25 eV, which is much less than the 13.6 eV ionization energy of hydrogen. Since then, the radiation temperature has dropped by a factor of about 1100 due to the expansion of the Universe. As the Universe expands, the cosmic microwave background photons are redshifted, making the radiation temperature inversely proportional to the scale factor of the Universe.
Study of anisotropies
Background radiation appears at first glance isotropic, that is, independent of the direction in which it is measured. This fact was difficult to explain according to the original Big Bang model and was one of the causes that led to the formulation of the Big Bang inflationary model. One of the predictions of this model is the existence of small variations in the temperature of the cosmic microwave background. These anisotropies or inhomogeneities were finally detected in the 1990s by various experiments, especially by the NASA satellite COBE (Cosmic Background Explorer) between 1989 and 1996, which was the first experience capable of detecting irregularities and anisotropies in this radiation. The irregularities are considered density variations of the early universe and their discovery provides clues to the formation of the first large-scale structures and the distribution of galaxies in the current universe. In 2001, the American space agency NASA launched the WMAP (Wilkinson Microwave Anisotropy Probe), a new satellite capable of studying the cosmic background radiation in great detail, which achieved the most complete map of anisotropies in microwave background radiation. Other instruments have detected the CMB anisotropies with even more detail and at a higher angular resolution, such as the Cosmic Background Imager but in only a few areas of the sky. Data provided by the WMAP in 2003 and 2006 reveal an expanding universe made up of 4% baryonic matter, 22% dark matter and 74% dark energy. In 2009, ESA launched Planck, a satellite with much greater capabilities than WMAP.
The anisotropy of the microwave radiation background is divided into two types: primary anisotropy – due to effects that occur on the last scattering surface and on the previous one – and secondary anisotropy – which is due to effects, such as interactions with hot gases or gravitational potentials, between the last scattering surface and the observer.
Primary Anisotropy
The structure of the anisotropies of the microwave radiation background is mainly determined by two effects: acoustic oscillations and wet diffusion (also called collisionless moisture or wet silk). Acoustic oscillations arise from competition in the photon-baryon plasma in the early Universe. The pressure of the photons tends to eliminate the anisotropies, while the gravitational attraction of the baryons —which move at speeds much less than the speed of light— makes them tend to collapse to form dense halos. These two effects compete to create acoustic oscillations that give the background of microwave radiation its characteristic peak structure. The peaks roughly correspond to resonances where photons become decoupled when a particular mode is at its peak amplitude.
The spikes 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 second peak—actually the ratio of odd to even peaks—determines the reduced baryonic density. The third peak can be used to extract information about the density of dark matter.
The locations of the peaks also give important information about the nature of the primordial perturbation density. There are two fundamental types of perturbation density – called “adiabat” and “isocurvature”. A general perturbation density is a mixture of these two types and there are different theories that seem to explain the primordial perturbation density spectrum that predicts different mixtures.
- For disturbance densities adiabatics, fractional overdensity in each component of matter (barions, photons...) is the same. That is, if there is 1% more power in bars than the average at one point, then with a pure adiabatic disturbance density there is also 1% more energy in photons and 1% more energy in neutrinos, than the average. Cosmic inflation predicts that the primordial disturbances are adiabatic.
- With the density of disturbances isocurvatura, the sum of the fractional overdensities is zero. That is, a disturbance where at some point there is 1 % more energy in bars than the average, 1 % more energy in photons than the average and 2 % more less energy in neutrinos that the average would be a disturbance of pure isocurvature. Cosmic strings would be produced mainly by disturbances of the primordial isocurvature.
In the CMB spectrum, these two types of disturbances can be distinguished because the peaks occur at different locations. The density of isocurvature disturbances produce a series of peaks whose angular scales (l-values of the peaks) are approximately in the ratios 1: 3: 5..., while the density of adiabatic disturbances they produce peaks whose locations are in the ratios 1: 2: 3. Observations are consistent with the primal perturbation density being fully adiabatic, providing the key to support for inflation and rule out many models of structure formation including, for example the cosmic strings.
Collisionless moisture is caused by two effects, when the treatment of primal plasma as a fluid begins to break down:
- the increase of the medium free path of photons in the primordial plasma becomes incrementally bound in an expanding Universe.
- the thickness of the last surface of dispersion, which causes the increase of the medium free path during the unfolding, even while the Compton dispersion continues to occur.
These effects contribute equally to the suppression of anisotropies at small scales and give rise to the characteristic exponential wet tail seen in anisotropies at very small angular scales.
The thinness of the last scattering surface refers to the fact that the decoupling of photons and baryons does not occur instantaneously, but requires an appreciable fraction of the age of the Universe above that epoch. One method to quantify exactly how long this process took uses the Photon Visibility Function. This function is defined such that, defining it as P(t), the probability that a photon from the last scattering of the CMB between t and t+dt is given by P(t)dt.
The maximum of the visibility function (the time at which a given photon is most likely to last scatter from the CMB) is known very precisely. Results from the first year of WMAP say the time when P(t) is maximum within 372,000 years (±14,000). It is often considered the "time" when the microwave radiation background formed. However, to understand how long it took for photon and baryon decoupling, a measure of the width of the visibility function is needed. The WMAP team finds that P(t) is greater than half its maximum value (the entire width at half maximum) in the interval 115,000 years (±5,000). By this measure, the decoupling lasted about 115,000 years, and when it was complete, the Universe was about 487,000 years old.
Late Anisotropy
After the CMB is created, it is modified by various physical processes collectively known as late anisotropy or secondary anisotropy. After the CMB emission, ordinary matter in the Universe consisted mainly of neutral hydrogen and helium atoms, but from observations of galaxies it appears that much of the volume of the intergalactic medium (IGM) now consists of ionized material (already that there are some absorption lines due to hydrogen atoms). This implies a period of reionization in which the material of the Universe collapses into hydrogen ions.
CMB photons scatter in free charges as electrons that are not bound to atoms. In an ionized Universe, such electrons have been released from neutral atoms by ionizing (ultraviolet) radiation. Today these free charges are of a sufficiently low density in much of the volume of the Universe that they do not affect CMB measurements. However, if the IGM was ionized at very early times when the Universe was very dense, then there would be two main effects on the CMB:
- Small-scale anthropies are eliminated (just as when an object is seen through the fog, the details of the object are blurred).
- The physics of how photons spread into free electrons (Thomson Dissemination) induces the polarization of anthropies in large angular scales. This wide-angle polarization is correlated with a wide-angle temperature disturbance.
Both of these effects have been observed by the WMAP satellite, providing evidence that the Universe was ionized at very early times, with a redshift of more than 17. The detailed provenance of this early ionizing radiation continues to be debated by scholars. scientists. This can include starlight from the first population of stars (population III), the supernovae these stars became at the end of their lives, or the ionizing radiation produced by the addition of massive black hole disks.
The period after the emission of the background microwave radiation and before the observation of the first stars is known almost comically by cosmologists as the dark ages, and is a period that is under intense study by scientists. astronomers (See the radiation of 21 centimeters).
Other effects that occur between reionization and our observation of the CMB that cause the anisotropies are the Siunyaev-Zeldovich effect, in which a high-energy electron cloud scatters radiation, transferring some energy to CMB photons, and the Sachs-Wolfe effect, which causes background photons of microwave radiation to be gravitationally red-shifted or blue-shifted due to changing gravitational fields.
Polarization
The CMB is polarized to a level of a few microkelvins. There are two types of polarization, called E and B modes. This presents an analogy with electrostatics, in that the electric field (E field) has an evanescent curl while the magnetic field (B field) has an evanescent divergence. The E modes appear naturally from Thomson diffusion in a heterogeneous plasma. The B modes, which have not been measured and are thought to have an amplitude of at most 0.1 µK, are not produced solely from plasma. They are a sign of cosmic inflation and are determined from the density of primordial gravitational waves. Detection of the B modes is extremely difficult, particularly since the degree of background contamination is unknown and the signal from gravitational lensing mixes the relative strength of the E mode with the B.
Microwave background observations
Since the discovery of the CMB, hundreds of cosmic microwave background experiments have been performed to measure and characterize the nature of the radiation. The most famous experiment is probably NASA's COBE satellite that orbited between 1989-1996, which detected and quantified large-scale anisotropies at the limit of their detection capabilities. Inspired by initial results from COBE, an extremely isotropic and homogeneous background, a series of balloon- and ground-based experiments quantified CMB anisotropies at small angular scales over the next decade. The main objective of these experiments was to measure at the angular scale the first acoustic peak, for which the COBE did not have sufficient resolution. These measurements could rule out cosmic strings as the main theory for the formation of cosmic structures and suggest that cosmic inflation is the appropriate theory. During the 1980s, the first peak was measured with increasing sensitivity, and in 2000, the BOOMERanG experiment reported that the highest energy fluctuations occurred on scales of approximately one degree. Together with other cosmological data, these results imply that the geometry of the Universe is flat. Several interferometers provided highly accurate fluctuation measurements over the next three years, including the Very Small Array, Degree Angular Scale Interferometer (DASI), and the Cosmic Background Imager (or CBI). The first detection of the DASI was the CMB polarization while the CBI obtained the CMB polarization spectrum.
In June 2001, NASA launched a second space mission for the CMB, WMAP, to make much more precise measurements of large-scale anisotropies across the sky. The first results of this mission, revealed in 2003, were detailed measurements of the angular power spectrum on the lowest scales, constraining various cosmological parameters. The results are broadly consistent with those expected from cosmic inflation as well as other competing theories and are available in detail at NASA's data center for the Cosmic Microwave Background. Although the WMAP provided very accurate measurements of the large-scale angular fluctuations in the CMB (structures that are as large in the sky as the moon), they would not have sufficient angular resolution to measure the small-scale fluctuations that had been observed using ground-based interferometers., such as the Cosmic Background Imager.
A third space mission, Planck, was launched in 2009. Planck will use two HEMT radiometers as well as a bolometer and will measure the CMB at smaller scales than WMAP. Unlike the previous two space missions, Planck is a collaboration between NASA and the European Space Agency (ESA). Its detectors were tested on the Viper Telescope in the ACBAR experiment, which has produced the most precise measurements at small angular scales to date – and on the Archeops balloon telescope.
Additional ground-based instruments such as the South Pole Telescope in Antarctica, the proposed Clover Project, the Atacama Cosmological Telescope, and the Quiet project in Chile will provide additional data not available from satellite observations, possibly including mode polarization. b.
It is possible to "see" the microwave background radiation with something as common as an analog television —that is, the old ones not prepared to receive Digital Terrestrial Television— that tunes to a channel in which there is no station broadcasting; Part (1%) of the "snow" that can be seen on the screen is said background radiation captured by the device's antenna.
The future of the cosmic microwave background
Since as the Universe expands, the redshift suffered by the cosmic background radiation increases, a very distant time will come, assuming an open Universe, in which it will be completely undetectable, ending up being &# 34;covered" by the one caused by the light emitted by the stars and this in turn, as the Cosmos continues to expand, it will suffer the same effect and will be replaced by other processes that take place in the distant future.