Why believe in the Big Bang?

It is often said that the reason why we “know” the Big Bang took place is because the Universe is expanding and if time were played backwards then everything would have started from the same point at the same moment in time. That moment, when everything started moving apart, is what we call the Big Bang.

But things aren’t really quite that simple.

For starters, Edwin Hubble’s 1929 discovery that galaxies are moving away from us was not part of the Big Bang theory but rather was to determine whether certain features, then known as nebulae, were inside our own galaxy (as many astronomers believed) or outside it.

Hubble found that the nebulae were moving away from the Earth and this meant they had to be outside the Milky Way. For the first time, astronomers had direct evidence that the Universe comprises more than just our own galaxy and, as an incidental benefit, that all the galaxies are moving apart.

Hubble’s analysis provided the right data at the right time. The prevailing theory for cosmology was the Steady State model, where the Universe (as a whole) is the same now as it has been in the past and will be in the future. Albert Einstein recognised that a static Universe would collapse under its own gravity so he added an extra term into his equations to prevent this from happening. Hubble’s expansion analysis made Einstein’s extra term redundant but a completely different approach, by Willem de Sitter, predicted that light would be red-shifted as it moved away from the centre of a Universe that contains no matter.

This last point may seem like nonsense but if the Universe has very low density then maybe the effect might still be seen – and when Hubble published his results he mentioned the possibility that they might support de Sitter’s idea.

In separate developments, the Russian mathematician Alexander Friedmann and the Belgian astrophysicist Georges Lemaître both modified Einstein’s equations to define an expanding Universe: Lemaître went on to use the idea of winding time backwards to suggest that everything was created from a single particle, which he named the “primeval atom”. This particle exploded (for want of a better word) to generate the entire Universe. This is strikingly familiar to the Big Bang theory but a set of mathematical equations and the observation of expansion cannot alone prove there was a Big Bang to start everything off.

One of the biggest issues was that initial calculations of elapsed time since the Big Bang gave an age for the Universe that was less than the estimated age of the Solar System.

Significantly, the observed expansion also supported the Steady State theory, which required the Universe to be dynamic in order to be stable. In the Steady State model, expansion is driven by an effect that also creates new matter in the expanding space, so avoiding any change in density and ensuring that the Universe remains both homogeneous (the same everywhere) and isotropic (the same in all directions).

Another issue the Big Bang had to solve was hidden in the “primeval atom” that was supposed to be the birthplace of all the matter in the Universe. Sadly, no theory (of nucleosynthesis) could explain how to create all the elements from one starting point. Building the atoms from their constituent protons and neutrons wouldn’t work either because isolated neutrons are inherently unstable, converting to protons and electrons (and electron anti-neutrinos) in beta-minus decay.

The extent of the difficulty faced by the “primeval atom” theory is nicely expressed in a version of the Periodic Table that was created by Dr Jennifer Johnson in 2017, where only hydrogen, helium and a small amount of lithium can be attributed to the Big Bang event – as shown below.

The instability of neutrons and the proposed change in temperature following a Big Bang event allowed Ukrainian physicist George Gamow to predict the expected proportions of hydrogen (protons without neutrons) and helium (the simplest stable combination of protons with neutrons). The calculated value, equal to approximately 75% hydrogen and 25% helium, is broadly in line with what we see in the Universe today.

Gamov could not predict the formation of other elements so his work simultaneously provided evidence for the Big Bang event and also undermined the idea of Lemaître’s “primeval atom”. Fortunately, later work on nucleosynthesis was able to explain the creation of other elements using subsequent events, such as nuclear fusion and supernovae, as indicated in Johnson’s version of the the Periodic Table.

Final confirmation of the Big Bang theory arrived in 1964 when two scientists working on a high-sensitivity microwave antenna at Bell Laboratories in New Jersey discovered an inexplicable background noise. Despite their best efforts, Arno Penzias and Robert Wilson could not isolate the noise and they were forced to conclude that it was some sort of signal coming from outer space.

Coincidentally, a group of astrophysicists headed by Robert Dicke at nearby Princeton University were preparing to search for a background radio signal that was predicted to remain following the high-temperature Big Bang event originally modelled by George Gamow. The radio signal was expected to be in the microwave region, corresponding to a residual temperature just a few degrees above absolute zero.

When Penzias and Wilson heard about this work they realised they might already have found the evidence that was being sought. Two papers, one containing the Princeton theory and the other containing the findings of Penzias and Wilson, were published on consecutive pages in the July 1965 issue of Astrophysical Journal Letters. With these announcements, the Big Bang theory was finally cemented as our accepted model for the beginning of the Universe.