The Big Bang theory is now considered the only one able to explain the observational data we have, including the existence of the cosmic background radiation, the mutual recession of the galaxies, and the abundance of hydrogen, helium, and lithium in the Universe. But first, it had to overcome competition from another cosmological model, the steady-state theory
The observations of the deep sky over the past century have taught us that the observable Universe extends over billions of light-years and is made up of countless billions of galaxies, distributed more or less evenly throughout the sky. We also discovered that the Universe is continually expanding and becoming colder on average.
But what is its origin? Where do all the matter and radiation that pervade it come from? In short, how did the Universe come about? Science’s answers to this formidable question have not always been in agreement. There was a time, for example, around the middle of the last century, when scientists were divided into two opposing fields: the steady-state advocates, led by Fred Hoyle, and the Big Bang advocates, led by George Gamow.
For steady-state advocates, the Universe is infinite in time and space. It has always existed and will exist forever, keeping its general characteristics of homogeneity and density unchanged. But Hubble had already shown in the late 1920s that galaxies move away from each other. How can density remain constant if the intergalactic spaces expand? Hoyle and the other steady-state advocates responded with the theory of continuous creation; the expansion of space is balanced by a constant creation of matter, which causes the average density to remain constant. To this end, a very low rate would be sufficient, the creation of 1 hydrogen atom per cubic meter of space every billion years.
The theory embraced by Gamow proposed instead a completely different vision, developed starting from an idea formulated in 1927 by the Belgian priest and astronomer Georges Lemaître. According to this theory, the expansion and cooling of the Universe is the trace of an evolution lasting billions of years, which, traced back, brings to an initial condition in which all the matter and radiation that fill the cosmos today were enclosed in a “primeval atom” inconceivably hot and dense. From that sort of cosmic egg, the Universe originated. Over a very long time, space expanded in all directions, and the temperature and density of matter decreased proportionally. Countless galaxies gradually formed under the push of gravity, which, due to successive mergers and aggregations, finally reached the evolutionary stage that we can observe today in the local Universe. It was the “Big Bang” hypothesis, as steady-state advocate Fred Hoyle had sarcastically labeled it in 1949.
How to decide which of the two theories was the best? Until the 1960s, there was no strong enough evidence to declare the success of one of the two positions and the defeat of the other. But things changed suddenly in 1964, the year in which Arno Penzias and Robert W. Wilson, two Bell Laboratories radio astronomers, accidentally came across cosmic microwave background or CMB, a discovery that brought them the Nobel prize for physics in 1978.
The existence of this cosmic background, detectable in all areas of the sky in the microwave region, was predicted in 1948 by two American scientists, Ralph Alpher and Robert Herman, who had calculated what temperature and spectrum this radiation should have had.
But what exactly is the CMB? We can consider it as the light echo of the Big Bang. In the beginning, the temperature was too high for protons, neutrons, and electrons to combine to form neutral atoms. All matter existed in the state of plasma, i.e., ionized particles, and light remained trapped in that plasma; photons — the quanta of light, mediators of the electromagnetic force — were continuously absorbed and re-emitted by free electrons. It was a Universe potentially full of light, but paradoxically dark, because the light did not have the possibility of freely propagating in space.
The situation changed entirely around 380,000 years after the Big Bang, an epoch that cosmologists call the era of recombination. Space had continued to expand from the Big Bang onwards continuously, and, as a result of this, the global temperature had dropped to the point where atomic nuclei and electrons could bind to each other forming neutral atoms. It allowed the photons to propagate in space without being continuously absorbed and re-emitted. In fact, unlike the free electrons diffused in the primordial plasma, the neutral atoms absorb only photons of particular wavelengths, leaving all the others to pass undisturbed. After the phase transition of the primeval plasma into a gas of neutral atoms, the collisions of photons with subatomic particles drastically decreased. The space filled in every direction with photons bearing the imprint of the last interactions with matter, occurred before it cooled beyond the critical threshold that caused the phase transition from the plasma state to the neutral gas state.
Those photons have traveled the space for nearly 14 billion years and today form the CMB, the distant echo of the turmoil of that primordial era in the history of the Universe. Although not having interacted with other matter throughout the very long time elapsed since their freeing, the photons of the CMB have suffered a significant loss of energy, caused by the uninterrupted expansion of space happened in the meantime; they have moved to the red end of the electromagnetic spectrum. That’s why today they are only detectable in the microwave region, with wavelengths that correspond to a temperature of fewer than three degrees above absolute zero.
With the discovery of the CMB, the cosmological model based on the Big Bang hypothesis became, in fact, the most plausible explanation for the origin of the Universe. But the steady-state theorists were not discouraged. They conjectured that the background radiation was not the echo of a hypothetical Big Bang, but only the light of distant stars absorbed and re-emitted in the microwave region by dust diffused in the intergalactic space.
According to the predictions of the Big Bang theorists, the background radiation should have had the spectrum of a black body, that is, a particular energy distribution curve determined solely by temperature. But stars also emit radiation with a spectrum that is a good approximation of a black body. The peak intensity of the flow from the CMB had been measured in 400 Megajansky per steradian (a measure of the amount of radiation received per unit of the celestial surface observed). If the CMB was starlight absorbed and re-emitted by dust, then deviations of the order of 10 Megajansky per steradian should have been found concerning the spectral distribution of the radiation emitted by an ideal black body.
At the time of the discovery of the CMB and in the years immediately following, instruments capable of such precise observations were not yet available. But they became available later. Thanks to the launch of three artificial satellites (COBE in 1989, WMAP in 2001, and Planck in 2009), it was finally possible to record the tiny variations of the CMB with the highest level of detail, without suffering the blurring caused by the filter of the Earth’s atmosphere.
The measurements made by the three satellites showed that the cosmic background radiation perfectly matches the characteristics predicted by the Big Bang hypothesis:
- it comes from all directions of the sky;
- has an almost identical temperature everywhere, equal to 2.725 K, with an uncertainty of only 470 microkelvins;
- has the spectrum of a black body.
As for the latter item, as early as 1992, the results of the observations made by the COBE satellite indicated that the energy distribution of the CMB was that of an almost perfect blackbody, with variations of no more than 0.01 megajansky per steradian. It was equivalent to a funeral prayer for the hypothesis of the steady-state.
The theoretical model of the Big Bang can also boast two other important successes: the prediction of the expansion of the Universe, confirmed by the Hubble-Lemaître law, and the prediction of the abundances of the various chemical elements produced during the so-called primordial nucleosynthesis. In the first minutes after the Big Bang, and only for a short time, the temperature was so high as to allow the formation by nuclear fusion of hydrogen, helium and lithium isotopes, but of no other heavier element (oxygen, iron, gold, etc. were created only much later, inside the first stars and during multiple supernovae explosions). Observations made in 2011 spectacularly confirmed this prediction; the analysis of the footprint left in the spectra of distant quasars by the gas of primordial intergalactic clouds crossed by their light made it possible to establish that the Universe began (in terms of mass) with 76% hydrogen, 24% helium-4 and minuscule percentages of deuterium, helium-3, and lithium-7.
Almost all physicists and cosmologists agree today that the Big Bang cosmological model is the only hypothesis that can make sense of the observational data available.