Kosminė foninė mikrobangė spinduliuotė (KFMS)

Space background microwave radiation (KFMS)

Residual radiation from when the Universe became transparent about 380,000 years after the Big Bang

The cosmic microwave background (CMB) is often described as the oldest light we can observe in the Universe – a faint, nearly uniform glow that permeates all of space. It was formed during a fateful epoch around 380 thousand years after the Big Bang, when the primordial plasma of electrons and protons coalesced into neutral atoms. Until then, photons were often scattered by free electrons, making the Universe opaque. When a sufficient number of neutral atoms were formed, scattering became less frequent, and photons could travel freely - this moment is called recombinationSince then, those photons have been traveling through space, gradually cooling and lengthening their wavelength as the Universe expands.

Today, these photons are detected as microwave radiation, which almost perfectly matches the spectrum of blackbody radiation and has a wavelength of about 2.725 K temperature. CMB studies have revolutionized cosmology, revealing insights into the composition, geometry, and evolution of the Universe – from the early density perturbations that led to the formation of galaxies to precise estimates of fundamental cosmological parameters.

In this article we will discuss:

  1. A historic discovery
  2. The universe before and during recombination
  3. Main properties of the CMB
  4. Anisotropies and power spectrum
  5. Key CMB experiments
  6. Cosmological constraints from the CMB
  7. Current and future missions
  8. Conclusion

2. Historical discovery

2.1 Theoretical assumptions

The idea that the early Universe was hot and dense goes back George Gamow, Ralph Alpher and Robert Herman work in the 1940s. They realized that if the Universe began with a "hot Big Bang," the primordial radiation emitted at that time should survive, but be cooled and stretched out into the microwave range. They predicted a blackbody spectrum with temperatures of a few kelvins, but this idea did not receive much experimental attention for a long time.

2.2 Discovery of observations

1964–1965 m. Arno Penzias and Robert Wilson from Bell Labs investigated noise sources in a highly sensitive, horn-shaped radio antenna receiver. They found a constant background noise that was isotropic (equal in all directions) and did not subside despite all calibration attempts. At the same time, a group from Princeton University (led by Robert Dicke and Jim Peebles) were preparing to search for "remnant radiation" from the early Universe, which was a theoretical assumption. When the two groups began communicating, it became clear that Penzias and Wilson discovered the CMB (Penzias & Wilson, 1965 [1]). This discovery in 1978 won them the Nobel Prize in Physics and established the Big Bang model as the prevailing theory of cosmic origins.

3. The Universe Before and During Recombination

3.1 Primary plasma

For the first few hundred thousand years after the Big Bang, the Universe was filled with a hot plasma of protons, electrons, photons, and (to a lesser extent) helium nuclei. Photons were constantly scattered by free electrons (Thomson scattering), so the Universe was effectively opaque, similar to how light has difficulty penetrating through the Sun's plasma.

3.2 Recombination

As the universe expands, it cools. About 380 thousand years After the Big Bang, the temperature dropped to about 3 thousand Q. At this energy level, electrons could combine with protons to form neutral hydrogen – we call this process recombination.As free electrons "bound" into neutral atoms, the scattering of photons decreased significantly, and The universe has become transparent radiation. The CMB photons we observe today are the same photons emitted at that moment, only 13 times older. traveling for billions of years and "stretched" by redshift.

3.3 Final scattering surface

The epoch when photons last significantly scattered is called on the surface of the last scattering. In fact, recombination was not an instantaneous event; it took some time (and redshift) for most of the electrons to combine with the protons. However, for practical purposes, we can roughly treat this process as a rather thin "time shell" - the region of origin of the CMB.

4. Main features of CMB

4.1 Blackbody spectrum

One of the striking results of observing the CMB is that its radiation almost perfectly matches the spectrum of a black body with a temperature of about 2.72548 K (precisely measured by the COBE-FIRAS instrument [2]). This is the most precisely measured blackbody spectrum. The nature of a near-perfect blackbody strongly supports the Big Bang model: a highly thermally balanced early Universe that cooled adiabatically as it expanded.

4.2 Isotropy and homogeneity

Early observations showed that the CMB is almost isotropic (i.e., of equal intensity in all directions) even down to 1 part in 105This nearly uniform distribution means that the Universe was very homogeneous and in thermal equilibrium at the time of recombination. However, small deviations from isotropy – the so-called anisotropy – are essential because they reflect the early beginnings of structure formation.

5. Anisotropy and power spectrum

5.1 Temperature fluctuations

1992 m. COBE-DMR (Differential Microwave Radiometer) experiment detected small fluctuations in the CMB temperature – about 10−5 These fluctuations are depicted in a "temperature map" of the sky, showing faint "hot" and "cold" spots corresponding to slightly denser or rarer regions in the early Universe.

5.2 Acoustic oscillations

Before recombination, photons and baryons (protons, neutrons) were strongly bound, forming photon-baryon fluid. The density waves (acoustic oscillations) that propagated in this fluid were caused by gravity pulling matter inward and radiation pressure pushing it outward. When the Universe became transparent, these oscillations "locked in", leaving characteristic traces. In the CMB power spectrum – measures how temperature fluctuations depend on angular scale. Important features:

  • First acoustic peak: related to the largest scale that has managed to complete a half-period oscillation before recombination; allows us to estimate the geometry of the Universe.
  • Other peaks: provides information about baryon density, dark matter density, and other cosmological parameters.
  • Damping tail: at very small angular scales, fluctuations are suppressed due to photon diffusion (Silk suppression).

5.3 Polarization

In addition to temperature fluctuations, the CMB is partly polarized due to Thomson scattering in the presence of an anisotropic radiation field. Two main polarization modes are distinguished:

  • E-mode polarization: formed by scalar density perturbations; first detected by the DASI experiment in 2002 and precisely measured by WMAP and Planck data.
  • B-mode polarization: may originate from primordial gravitational waves (e.g., which arose during inflation) or due to E-type polarization lensing. The signal of B-type polarization primordials would be a direct trace of inflation. Although B-modes of gravitational lensing origin have already been detected (e.g. by the POLARBEAR, SPT and Planck collaborations), the search for B-modes primordials is still ongoing.

6. Key CMB experiments

6.1 COBE (Cosmic Background Explorer)

  • Released 1989 NASA.
  • FIRAS device confirmed with great precision the blackbody nature of the CMB spectrum.
  • DMR device was the first to detect large-scale temperature anisotropies.
  • Strongly strengthened the Big Bang theory, removing fundamental doubts.
  • Researchers John Mather and George Smoot for their work with COBE in 2006 won the Nobel Prize in Physics in 2001.

6.2 WMAP (Wilkinson Microwave Anisotropy Probe)

  • Launched in 2001 NASA.
  • Provided detailed maps of the CMB temperature (and later polarization) over the entire sky with an angular resolution of ~13 arcminutes.
  • Precisely refined key cosmological parameters, such as the age of the universe, the Hubble constant, the density of dark matter, and the proportion of dark energy.

6.3 Planck (ESA mission)

  • Operated from 2009 to 2013 m.
  • It had better angular resolution (~5 arcminutes) and sensitivity in temperature measurements compared to WMAP.
  • Measured all-sky temperature and polarization anisotropies at several frequencies (30–857 GHz).
  • They produced the most detailed CMB maps to date, further refining cosmological parameters and firmly confirming the ΛCDM model.

7. Cosmological constraints from the CMB

Thanks to the efforts of these and other missions, the CMB has become one of the cornerstones in determining cosmological parameters:

  1. Geometry of the Universe: The position of the first acoustic peak indicates that the Universe is almost spatially flat (Ωtotal ≈ 1).
  2. Dark matter: The relative heights of the acoustic peaks allow us to determine the dark matter (Ωc) and baryonic matter (Ωb) density.
  3. Dark energy: By combining CMB data with other observations (e.g. supernova distances or baryonic acoustic oscillations), it is possible to determine the fraction of dark energy (ΩL) In the universe.
  4. Hubble's constant (H0): The angular scale of the acoustic peaks allows for an indirect determination of H0Current CMB data (from Planck) show H0 ≈ 67.4 ± 0.5 km/s−1 Mpc−1, but this result conflicts with local measurements ("distance staircase") showing ~73. This discrepancy, called Hubble voltage, current cosmological research is trying to solve.
  5. Inflation parameters: CMB anisotropies allow us to constrain the amplitude and spectral index (As, ns), which is important for evaluating inflation models.

8. Current and future missions

8.1 Ground-based and balloon-based observations

Following the work of WMAP and Planck, several extremely sensitive ground-based and balloon-borne telescopes are further refining measurements of the CMB temperature and polarization:

  • Atacama Cosmology Telescope (ACT) and South Pole Telescope (SPT): large aperture telescopes for measuring small angular scale CMB anisotropies and polarization.
  • Balloon experiments: such as BOOMERanG, Archeops and SPIDER, performing high-resolution measurements at near-space altitudes.

8.2 B-mode search

Projects such as BICEPS, POLAR BEAR and CLASS focuses on detecting or constraining B-type polarization. If the primordial B polarization above a certain level were confirmed, this would allow direct evidence of the existence of gravitational waves originating during inflation. Although early claims (e.g., BICEP2 2014 m.) were later explained by Galactic dust contamination, the search for a "clean" discovery of the original B modes continues.

8.3 Next generation missions

  • CMB-S4: A ground-based project is planned that will use a large mass of telescopes to measure the polarization of the CMB with extreme precision, especially at small angular scales.
  • LiteBIRD (planned JAXA mission): A satellite designed to study large-scale CMB polarization, especially searching for traces of primary B polarization.
  • CORE (proposed ESA mission, currently unconfirmed): would have improved the sensitivity of Planck's polarization measurements.

9. Conclusions

The cosmic microwave background provides a unique window into the early Universe, dating back only a few hundred thousand years after the Big Bang. Measurements of its temperature, polarization, and small anisotropy have confirmed the Big Bang model, confirmed the existence of dark matter and dark energy, and formed a precise cosmological framework for ΛCDM. In addition, the CMB continues to push the boundaries of physics, from searches for primordial gravitational waves and testing inflation models to possible clues to new physics related to the Hubble tension and other questions.

As future experiments increase their sensitivity and angular resolution, an even greater harvest of cosmological data awaits. Whether it is refining our understanding of inflation, determining the nature of dark energy, or uncovering clues to new physics, the CMB remains one of the most powerful and significant tools in modern astrophysics and cosmology.

References and further reading

  1. Penzias, AA, & Wilson, RW (1965). "A Measurement of Excess Antenna Temperature at 4080 Mc/s." The Astrophysical Journal, 142, 419–421. [Link]
  2. Mather, J. C., et al. (1994). "Measurement of the Cosmic Microwave Background Spectrum by the COBE FIRAS Instrument." The Astrophysical Journal, 420, 439. [Link]
  3. Smoot, G. F., et al. (1992). "Structure in the COBE DMR First-Year Maps." The Astrophysical Journal Letters, 396, L1–L5. [Link]
  4. Bennett, C. L., et al. (2013). "Nine-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Final Maps and Results." The Astrophysical Journal Supplement Series, 208, 20. [Link]
  5. Planck Collaboration. (2018). "Planck 2018 results. VI. Cosmological parameters." Astronomy & Astrophysics, 641, A6. [arXiv:1807.06209]
  6. Peebles, PJE, Page, LA, & Partridge, RB (eds.). (2009). Finding the Big Bang. Cambridge University Press. – A historical and scientific perspective on the discovery and significance of the CMB.
  7. Kolb, EW, & Turner, MS (1990). The Early Universe. Addison-Wesley.– A detailed description of the physics of the early Universe and the role of the CMB in it.
  8. Mukhanov, V. (2005). Physical Foundations of Cosmology. Cambridge University Press. – Provides a comprehensive look at cosmic inflation, CMB anisotropies, and the theoretical foundations of modern cosmology.
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