In its beginnings the universe was a much smaller, denser and hotter place. If we rewind the movie of the universe, there comes a time when the universe becomes as inhospitable a place as our most fundamental theories allow. If we try to compress or heat it more, our theories stop working. Well, that instant of incredible warmth and smallness and what came immediately after is what we call the “ Big Bang ”.
The name is a bit misleading because that was not really a big explosion . Explosions normally destroy, while the big bang was the beginning, the little seed, of everything we know. Knowing what happened about 13.8 billion years ago is complicated, but luckily an event as significant as the beginning of the universe leaves clues scattered throughout all areas of physics. Let’s look at the four main pieces of evidence that tell us that, regardless of the details of those first moments, the universe started out extremely hot, dense, and tiny.
the universe expands
We have known for decades that all the galaxies we see in the sky are moving away from our own . Not only that, but all the galaxies seem to be moving away from each other. In short, the universe is expanding. In truth, and before the nebangists appeared , not all galaxies move away from the Milky Way, although the vast majority do. The handful of galaxies that don’t stick together thanks to gravity. I am talking about the hundred galaxies (most of them dwarfs) that form the Local Group.
The expansion of the universe was discovered by the American astronomer Edwin Hubble in 1929 , thanks to measurements of variable stars by Henrietta Leavitt and others. Since then we have also discovered that this expansion, instead of being slowed down by the gravitational attraction between galaxy clusters, is being accelerated by the presence of what we call dark energy. It was Hubble’s discovery that the universe was expanding that led several scientists to propose that the universe began its days as a small region, growing ever since to present-day scales.
The exact proportion
The matter in the universe is made up mostly of hydrogen and helium. These two elements make up about 98% of the mass of all the ordinary matter in the universe , which is especially significant considering that they are the two lightest chemical elements. The rest of the elements have been synthesized inside stars or through supernova explosions. These proportions exactly match those predicted by simulating a universe whose beginnings are in the form of a small, dense, hot region. Any other possible start different from the one proposed by the Big Bang theory would give quantities for each element very different from those observed today in our universe.
background radiation
Until about 370,000 years after the Big Bang, the universe was still such a hot place that electrons had too much energy to bond with atomic nuclei (hydrogen and helium, remember) to form neutral atoms. As a consequence, photons of light were not able to travel freely, as they were continuously colliding with these free electrons . At this point, the electrons cooled enough to join the atoms, leaving the light free to travel without being absorbed or reflected. That is, this was the moment when the universe became transparent. That light that was released we can detect today, receiving the unglamorous name of ” cosmic microwave background “. By measuring the energy of the light that reaches us from each point in space, we can make a kind of map of the universe when it was only 370,000 years old. This map is incredibly homogeneous, so homogeneous that it would not be possible if the universe had not been in thermal equilibrium in its early stages . And this, given the enormous scales of the universe, would only be possible if it had been born from a tremendously small region.
perfectly ordered
If we study the universe as a whole, looking not at its details but at its large-scale structure , we can see that the distribution of galaxies and clusters of galaxies, the distribution of dark matter, and the fluctuations of that cosmic microwave background that we have commented, they all agree with each other, giving us a homogeneous and isotropic universe . Homogeneous will mean that any region of the universe will have approximately the same general properties as the rest: the same composition, the same number of galaxies, the same temperature. Isotropic will mean that whichever way we look we will see a fundamentally the same universe. An example of the opposite would be if we were on the edge of a spiral galaxy like the Milky Way. If we look towards the galactic center we will see a large concentration of stars, while if we look outwards we will see hardly any stars. Apparently the universe is not like that. All of this is consistent with a universe that started out much smaller in size and was in equilibrium during its first moments.
As you can see, we have very good reasons to think that the universe began with a Big Bang. The exact details of what this would entail may perhaps change with new discoveries and theoretical advances, but the fundamental pillars seem firm and robust.
