CMBR: The oldest lights of the Universe.

Parth Sarda
9 min readOct 19, 2021

Cosmic microwave background radiation, CMBR is the radiation produced shortly after the big bang.

Overview

You may have seen the effect of Cosmic Microwave background when the old television screen’s pixels get blurred as the passing CMBR disrupts the electric field of the television. Cosmic microwave background radiation is the radiation produced right after the big bang at the time of recombination. It is sort of a map of the Big Bang as it went through. It is found everywhere around the universe in a pure isotropic state. CMBR, in simple words, is electromagnetic radiation formed from the remnant of the big bang when the hot plasma produced high energetic oscillations which created the sound wave which leads to the formation of light which is spread all across the universe. The spaces between the star and the dark are completely empty however, a sufficiently sensitive radio telescope shows a faint background noise or glow, that is not associated with any star, galaxy, or another object. This glow is the strongest in the microwave part of the spectrum, thus adding the microwave to their name. The accidental discovery of the CMB in 1965 by American radio astronomers Arno Penzias and Robert Wilson, was the culmination of work initiated in the 1940s and earned the discoverers the 1978 Nobel Prize in Physics.

Through the medium of this article, I would tell you the importance of CMBR to astronomy and all of physics and how does it help us in our understanding of the universe.

Measurement of CMBR

Microwave spectrum of CMBR

CMBR is generally detected by a microwave radiometer, as an extra noise equivalent to a black body radiating at a temperature of 2.73 K. A radiometer is a radio telescope whose response is calibrated with known temperature sources. The temperature differences that are found in the map as tiny blotches and a deviation of 1 in 10,000 from the average temperature of 2.7 K. It represents the varied density in the early universe which further lead to the formation of the galaxy as it was refueled by dark matter and gravity stabilizing at its place as the fluctuations flowed outwards. The numerous wave oscillations that spread as the big bang progressed by the models that developed them. Many of these simple models of simple density fluctuations from the highest to the lowest, and get a map of the universe by using spherical harmonics, like the sine wave on the 2-D surface of a sphere. The detailed analysis of CMBR data to produce maps, an angular power spectrum, and ultimately cosmological parameters is a complicated, computationally difficult problem. Although computing a power spectrum from a map is in principle a simple Fourier Transform, decomposing the map of the sky into spherical harmonics.

Precise measurements of the CMB are critical to cosmology since any proposed model of the universe must explain this radiation. The CMB has a thermal black spectrum at a temperature of 2.72548±0.00057 K. The spectral radiance dEν/dν peaks at 160.23 GHz, in the microwave range of frequencies, corresponding to a photon energy of about 6.626 ⋅ 10−4 eV. They have been measured in detail, and match what would be expected if small thermal variations, generated by the quantum fluctuations of matter in a very tiny space, had expanded to the size of the observable universe we see today.

Anomalies

Raw CMBR data, even from space vehicles such as WMAP or Planck, contain foreground effects that completely obscure the fine-scale structure of the cosmic microwave background. The most prominent of the foreground effects is the dipole anisotropy caused by the Sun’s motion relative to the CMBR background. The dipole anisotropy and others due to Earth’s annual motion relative to the Sun and numerous microwave sources in the galactic plane and elsewhere must be subtracted out to reveal the extremely tiny variations characterizing the fine-scale structure of the CMBR background.

In practice, it is hard to take the effects of noise and foreground sources into account. In particular, these foregrounds are dominated by galactic emissions such as Bremsstrahlung, synchrotron, and dust that emit in the microwave band; in practice, the galaxy has to be removed, resulting in a CMB map that is not a full-sky map. In addition, point sources like galaxies and clusters represent another source of foreground which must be removed so as not to distort the short-scale structure of the CMB power spectrum.

Features

Recombinations

Recombination refers to the time when the universe froze as the temperature reduced to a critical point in which proton and electron froze to form Hydrogen and Helium atoms. The electrons settling to the state of an atom was very inefficient due to the less energy of the orbitals, they settled to their high energy state and transitioning to the lower energy state releasing a photon. This production of photons is known as decoupling, which leads to recombination sometimes being called photon decoupling, but recombination and photon decoupling are distinct events. It also refers to when the earliest light of the universe was released in the form of cosmic microwave background radiation. The sound oscillations as they were moving in the form of hot plasma froze interacting with each other and even at their maximum and minimum state. It is a period when the stabilizing of the universe as the temperature reached a critical of 3300 K.

Baryon Acoustic oscillations(BAO)

This state before the recombination is also called the baryon acoustic oscillation state. It is associated with when the universe was in the form of hot dense plasma, and it used to oscillate back and forth creating sound waves which used to travel at high speeds equivalent to more than half the speed of light (0.58c). It also represents the early black body state of the universe as the baryons oscillated back and forth. BAO matter clustering provides a “standard ruler” for length scale in cosmology. BAO measurements help cosmologists understand more about the nature of dark energy by constraining cosmological parameters.

These oscillations at the time of recombination can be found in various states, depending on which state they are, maximum or minimum, or a state between the two. These spots of maximum and minimum oscillations are the most prominent in the CMB. It can be inferred that the sizes of the special spots should follow a harmonic series which is exactly what we get in a power spectrum, which is like a histogram that plots all the numbers of spots of all possible sizes and we definitely see that some sizes are common than others. Each of these peaks tells us something unique about the universe. The main value of the first peak is as a measuring tape or a standard ruler as we discussed earlier. Spots of the size had only time to collapse once which means their size has to be equal to the speed of sound(0.58c)multiplied by the years after which it collapsed(380,000 years) into the expansion factor(73.3 ±2.5 km/s) and we get exactly a distance of half a million light-years. When we measure the same distance on our telescope and get the angle it corresponds to a degree which is a good indication that our universe is flat or geometrically regular as it was predicted by our theory. This in turn tells us about the various contents of energy, baryons, dark matter, and energy of the universe.

The second peak represents the maximum rarefaction, fluctuations where the matter has bounced off after its initial collapse. We can think of these oscillations as being heavy masses attached to a spring. These heavy masses can be thought of as baryons and they are falling and they want to fall towards our overdense spot, but the baryons are locked with light which acts like spring. The more baryons the more matter will fall towards the over-density spot. The over-density spots are places where the galaxies were formed as more and more matter gathered inside these spots mostly due to gravity and also due to dark matter which resulted in the galaxy formation. Having more baryons would enhance the odd-numbered peaks, which represents the compressed state. While the even-numbered peaks would not be affected by the number of baryons. This means that if there are more baryons the higher the odd number peaks are compared to the even-numbered peaks. We can measure the height of the first peak relative to the height of the first peak to get the baryon content of the universe. From that, we get to know that the baryon content of the universe is only 5%.

The other smaller peaks tell us about the amount of dark matter in the universe, which doesn’t seem to interact with light. These peaks also tell us about how the radiation epoch gave way to the dark matter-dominated epoch.

Big Bang Theory

The Cosmic Microwave background radiation and the cosmological redshifting of light are the best-regarded evidence for the Big Bang theory. Measurements of the CMB have made the inflationary Big Bang theory the Standard Cosmological Model. The CMB gives a snapshot of the early universe when, according to standard cosmology, the temperature dropped enough to allow electrons and protons to form hydrogen atoms, thereby making the universe nearly transparent to radiation because the light was no longer being scattered off free electrons. Since decoupling, the color temperature of the background radiation has dropped by an average factor of 1090, 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 proportion to a parameter that describes the relative expansion of the universe over time, known as the scale. This also explains how the matter was spread in the early universe and how the harmonic oscillations ruled as plasma traveled at very high speeds of around 0.58 c which we came to know after renormalization through the different spectrums.

Anisotropy

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 the last scattering and before; and secondary anisotropy, due to effects such as interactions of 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. These two effects compete to create acoustic oscillations, which give the microwave background its characteristic peak structure of the power spectrum.

Two other effects which occurred between reionization and our observations of the cosmic microwave background, and which appear to cause anisotropies, are the Sunyaev-Zel’dovich effect, where a cloud of high-energy electrons scatters the radiation, transferring some of its energy to the CMB photons, and the Sachs-Wolfe effect, which causes photons from the Cosmic Microwave Background to be gravitationally redshifted or blueshifted due to changing gravitational fields.

Conclusion

A portion of the CMB.

CMBR after its discovery in 1965 has been an important tool for cosmologists ever since. It has been effectively used to find the science associated with the big bang, also as a standard ruler and a way to precisely measure the distances across the cosmos. It has also been used to find the relative abundance of the normal matter, dark matter, and dark energy in the universe. Its measurements like the thermal black body spectrum, spectral radiance, power spectrum has key insights to our understanding of the universe. With all the developments that have been made since CMBR was discovered, some of it is yet to be found, and work is being done to find the anisotropies in great detail, to find out some more things about fluctuations that happened when recombination hit.

The expansion rate was calculated through CMB by Atacama Cosmology Telescope(ACT), which using the minute variation predicted the rate at which the Universe is currently expanding. The ACT data is similar to the data collected by the Planck’s telescope which using a similar technique calculated its predicted value for Hubble constant. But neither result matches direct measurements of the Hubble constant — a discrepancy that has become known as the Hubble-constant tension. Techniques developed by several teams are aiming to resolve them, but till now ways have been found. Adam Riess, an astronomer from John Hopkins University says, that perhaps it is cosmology’s standard model that is wrong instead.

CMBR, which first seemed to be some random light coming from space, has greatly deepened our understanding of the universe, as it continues to uncover deep insights into the cosmos.

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Parth Sarda

I am just a student looking out in the vast world. I am interested in Science, Humanity. I love to explore new things and to find out the truth behind things.