Ancient people thought that light spreads instantly, but now we know that its speed is approximately 300,000 km / s. For example, the Earth is located about 150 billion km from the Sun. That is, when a ray of light leaves the Sun, it needs a little more than 8 minutes to reach us. Looking at the Sun, we see it as it was 8 minutes ago!
Proxima Centauri, on the other hand, is about 4.25 light years distant. When we look at it through a telescope, we see it as this Star was 4 years and three months ago.
The further we look, the further back into the past our gaze rushes. And the question arises. The Big Bang was just under 14 billion years ago. If we look this far, shouldn’t we see the big bang in all its glory? But we don’t see anything. Maybe the big bang theory is wrong? Let’s figure it out together.
The big bang theory says that at the very beginning, our universe was in an extremely compressed state. The density of matter and the temperature were so enormous that even particles such as protons and neutrons could not form.
We can only guess what processes were taking place at that time. The laws of physics, as we understand it, are inapplicable to the moment of the big bang.
The earliest moment, about which we have a more or less good idea, came when the universe cooled down enough for quarks to combine into protons and neutrons. The universe was still too hot for protons, neutrons and electrons to start forming atoms.
However, time passed and soon the universe cooled down enough to form deuterium nuclei. The universe at this moment was about 10 seconds old. During the next about 20 minutes, active nuclear fusion took place, during which about 30% (by mass) of helium-4 nuclei and 70% of hydrogen nuclei were formed. The nuclei of deuterium, helium-3 and lithium were formed in microscopic quantities. Then the fusion of nuclei stopped, as the temperature dropped too much and there were no conditions for its continuation.
About 377,000 years after that, the universe cooled down enough for electrons to start attaching to nuclei and forming electrically neutral atoms. This moment is called the era of recombination. When an electron joins the nucleus of an atom, there is a small burst of energy in the form of a photon. Those. during recombination, a huge amount of photons were emitted throughout the universe. And it is these photons, emitted at the moment of the formation of atoms, that we are the earliest light that we can consider.
Before that, the universe was opaque to light. The «fog» of free electrons and nuclei obscures from us the earlier history of the universe and the big bang itself. When the recombination happened, the universe became transparent to light and this light went on a journey through the universe in all directions at once. We still see this light in the form of relic radiation. Predicted the presence of relict radiation in 1948 by Georgy Gamow, and discovered it in 1965 by Arno Penzias and Robert Wilson.
But after the era of recombination, the so-called Dark Ages came. There was not a single source of light in the universe — only atoms of hydrogen and helium, which do not emit light by themselves.
It is difficult to say how long it took, but over time it collapsed into gas and the first stars formed. It is very difficult to estimate the time of the beginning of the formation of the first stars, but it is usually estimated at about 400-700 million years after the big bang. The light of these first stars was the first light that illuminated the universe after the big bang.
In theory, in the future, we will be able to see the universe up to 1 second after the big bang, since the big bang theory predicts the existence of not only the relic radiation emitted during recombination, but also the neutrino relic radiation, which appeared about 1 second after the big bang.
Unfortunately, we are not yet able to detect this radiation. According to our calculations, neutrinos emitted immediately after the big bang should have extremely low energies in the range of 10⁻⁴ to 10⁻⁶ eV. At the same time, it is now quite difficult for us to detect even high-energy neutrinos. Therefore, the question of detecting neutrino relic radiation is not a question for the very near future.