One of the hypotheses defended by the Big Bang model is precisely that what we observe today is what was there at the beginning. I remember again that precisely the initial moment in which the universe began to expand is not described by the Big Bang because we do not yet have a model that describes the gravitational interactions in that regime of quantum gravity. But a little later, when the cosmos begins to expand, the content of matter and energy that was at that beginning is what there is today and what there will be at the end.
At the beginning, the volume was very small, but the universe is expanding and, simply, what happens is that the density, that is, the energy per unit of volume, is diluted.
This also causes the universe to cool. Today, outer space is a very cold place. Cosmological radiation, not that which reaches us directly from the stars, but rather the background radiation, has a very low temperature of 2.7 kelvin (-270.5 °C). And yet, at the origin, as you go back in the history of the universe, that radiation is much hotter. The effect of expansion causes it to cool.
So the answer to your question is that what the Big Bang model predicts, which at the moment is the one that best explains the universe, is that all the matter and energy were already at the beginning. What may have been changing is the character of what we call matter and energy. So that you understand, what we rely on to explain it is the particle physics model. And according to this model, particles can have mass. What we have to compare is the mass of these particles at the temperature of each moment in the evolution of the universe. And that is what has been changing. How much radiation did we have and how much is left now?
But if we calculate the energy that was at the beginning, it is the same as that at the end. Although that energy, as the universe expands and cools, changes. When there is a lot of thermal agitation, as we think it must have occurred at the origin, although we are not sure either, but we imagine that it must have been like that, due to that thermal agitation everything must behave as radiation, as energy. But, as it cools, part of these particles stop behaving as relativistic, the thermal agitation decreases and they begin to behave like our concept of matter, something with mass and with small speeds, such as dark matter. You have to remember that in special relativity, matter and energy are interchangeable concepts. It is Einstein’s famous equation, we can convert something with mass into energy.
What we must take into account is its speed, whether it is something that moves at speeds close to that of light or whether it is something that would have much smaller and therefore non-relativistic speeds. When we say non-relativistic it means that they are speeds of thermal agitation much smaller than the speed of light. And what we call matter are, precisely, particles with small speeds compared to the speed of light.
Mar Bastero GilShe is a professor and researcher in the High Energy Theoretical Physics Group (FTAE) at the University of Granada.
Question sent via email byDavid Robinson
Coordination and writing:Victoria Toro
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