A relic of the Big Bang: The Cosmological Background

What was hiding among the stars? Space and its beginnings have fascinated researchers investigating and studying the physical mechanisms that have formed the universe as a discipline of astronomy itself: cosmology. 

Nowadays, the scientific consensus accepts the "standard model of cosmology" (a.k.a. the Big Bang) as the origin of the cosmos. For a long time, this model was opposed to that of a stationary space. The validation of the Big Bang theory was largely possible thanks to the discovery, in 1965, of an electromagnetic radiation almost as old as the Big Bang itself, this signal would later be called the cosmological microwave background.

As early as 1930, Georges Lemaître, astrophysicist and Belgian priest, suggested the model of the primitive atom as the origin of the universe. According to Lemaître, "the universe would have begun in the state where the total energy was concentrated in a single quantum" (quantum: primordial particle of matter). 

These writings are the first drafts of the standard model of cosmology. They stem from Lemaître's previous work, which strove to characterize the behavior of space using the position and displacement of galaxies. His conclusions describe an expanding space that grows larger over time.

Georges Lemaître, circa 1930 (Source: Wiki Commons)

These conclusions are strongly opposed by supporters of the theory of a stationary space. At the time, an expanding universe implies a beginning, a "birth" that resembles the idea of a divine intervention. A stationary space was seen to be a more rational model. To end the debate, it was necessary to find evidence of a Big Bang, evidence that could be observed. 

This illustration shows in a simplified way the evolution of a "slice" of space. (Source: Wikicommons)

In 1964, in New Jersey, two physicists from Bell Laboratories, Arno Penzias and Robert Wilson, built a 6-meter high antenna to test satellite communication. In spite of their precautions, a parasitic signal remains. It is difficult to avoid it because it is transmitted from all directions of space (isotropic), regardless of the date or the time of day. 

Robert Wilson, left, and Arno Penzias the Bell Labs antenna in Crawford Hill, N.J. (Source: Wikicommons)

They don't know it yet, but what they consider as a faulty measurement is in reality the Cosmic Microwave Background (CMB), the missing piece to prove the accuracy of the Big Bang. It is important to note that, during the first half of the 20th century, many physicists predicted the discovery of this fossil radiation, originating from the first moments of the Big Bang and that would since travel across the cosmos. 

The CMB recorded by Penzias and Wilson is a microwave emission. Electromagnetic waves are divided into several domains, according to their wavelength. The wavelength corresponds to the distance between two identical maxima of an electromagnetic signal. For microwaves, this distance is between 2 mm and 30 cm. 

Example of two types of waves with their wavelength (Diagram made by A. Herrmann)

It is also possible to determine the radiant temperature using Planck's law ¹. The Wilson and Penzias' calculations measured it at 3.5 K, a value that confirms the predictions of physicists who are in favor of the Big Bang. This fortuitous discovery brought them a Nobel Prize in 1978!

Following a meeting between Bell engineers and two Princeton researchers, Robert Dicke and James Peebles, two articles are published in the same issue of Astrophysical Journal. The first, signed by Penzias and Wilson, describes the discovery, and the second, signed by Dicke and Peebles, explains the link between the cosmological microwave background and the Big Bang. 

In reality, the cosmological microwave background was born 380,000 years after the Big Bang. At this period, called the recombination, the universe expands and cools down to reach about 3000 Kelvin. In these conditions, electrons, negatively charged, pair up with protons, positively charged, to form hydrogen. 

Schematic comparison of hydrogen and helium atoms. (Source: Wikimedia Commons)

Prior to this date, the temperature was too high, constantly destabilizing this union. The universe was but a plasma of particles unable to remain linked for very long. After the recombination, the universe becomes transparent and the photons produced by the Big Bang are allowed to circulate freely. 

Big Bang Theory, Pixabay, Wikimedia, Teresa Gonzalez

Photons are the elementary particles of energy that make up electromagnetic radiation (X-rays, microwaves, light...). In order to imagine their double wave-corpuscular nature, we can compare the photon to a cylinder. When observed from the side, we see a rectangle, but from the front, we see a circle. 

These particles are able to interact with one of the constituents of matter: free electron. During the collision between a free electron and a photon, a phenomenon called the Compton scattering occurs. 

The Compton scattering (Diagram made by A. Herrmann)

However, during the recombination, hydrogen atoms are formed and the electrons are no longer free. Under these conditions, photons can begin to travel throughout the cosmos without bouncing from electron to electron.

Left: before recombination, photons (yellow) bounce off electrons (gray). Right: after recombination, photons can no longer interact with electrons that have mated with protons (red). (Source: Write Science)

Those are the very first photons, true relics of the Big Bang, that constitute the fossil radiation (the nickname of the cosmological microwave background), this electromagnetic signal is almost homogeneous throughout the universe.

But the CMB is not only a legacy of the past, it also has a fundamental role in our understanding of the cosmos of today. Researchers have constructed the electromagnetic spectrum of the CMB (see below) and to this day, the CMB has the closest electronic spectrum to that of a perfect black body. A black body is an object that absorbs all surrounding electromagnetic radiation. As a result of this absorption, it radiates heat in the form of thermal radiation.

Electromagnetic spectrum of the cosmological microwave background (Source: Wikimedia Commons)

Technological advances have made it possible to accurately set the temperature of the CMB at 2.728 ± 0.002 K. The cosmological microwave background is thus a thermal radiation whose emitting black body would be the cosmological origin matter at the center of the universe. This temperature value, much higher at the time of recombination, continues to decrease with time. In the future, we can expect that the CMB signal will eventually be confused in the ocean of cosmic signals.

Researchers have been interested in the subtle thermal variations (anisotropies) of the CMB. Once imaged, these fluctuations reveal the oldest map of the universe. A flash-captured snapshot of the fossil radiation at the time of its birth.

Map of the primordial universe captured by the COBE, WMAP and Planck satellites. (Credits: Brian Koberlein)

Illustration article : Blue and Brown Milky Way Galaxy, Photo by Miriam Espacio from Pexels

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References :

1. R. Brandenberger, Formation of Structure in the Universe, 1995

2. C. Caso et al . (Particle Data  Group), The European Physical Journal C3, 1998

3. Dicke, R. H. ; Peebles, P. J. E.Roll, P. G. Wilkinson, D. T., Cosmic Black-Body Radiation, 1965

4. F. Durham, Frame of the universe : a history of physical cosmology, 1983

5. D. J. Fixsen, The temperature of the Cosmic Microwave Background, 2009