The first thing I need is for you to understand temperature in relation to heat. In everyday life we understand that when something is at a very high temperature, for example a pot that is hot, what we are perceiving is the ability for an exchange of energy to occur in the form of heat between the object that is at a higher temperature. and the one that is at the lowest temperature. That is, we understand temperature as a way of knowing what has more heat energy.
All the energy measurements we have from the Big Bang, both mass and pure energy, for example in the form of radiation, we can identify or associate with a temperature based on what statistical physics tells us.
In your question you tell us how the very high temperature of the Big Bang could be reached if there were still no stars. The answer is that you didn’t need the stars to reach it. What was needed was that something existed, which, in this case, was mass and energy. That mass and energy arose from small fluctuations in the vacuum. Due to these fluctuations, particles arise, radiation arises and fields arise. That’s the first thing that happens. Without that background we could not talk about temperature. If there is nothing, as I explained at the beginning, there can be no temperature because there can be no exchange of energy in the form of heat.
From here we have to ask ourselves in what state is everything that has emerged. That matter and that radiation that have appeared by some quantum process can be interpreted in terms of an energy. What we have then are particles with mass, like a quark, and particles without mass, like a photon. What we interpret as temperature is a measure of how much energy is there and how that energy can be exchanged between the different constituent elements of that very early universe.
The dough is very compressed. To give you an idea, it’s like when we squeeze the air inside the bicycle wheels with a piston pump. If we put a lot of air in the same volume, the temperature increases. Initially, before the explosion occurred (the Big Bang), all that mass occupied a very, very small volume. At the same time there were a lot of massless particles, radiation, which is also associated with energy. And that was also in a very compact space, that is, it was very dense, so it also had a lot of energy associated with it, which reached a very high temperature.
The temperature at that moment was so great that there has never been a temperature like that again, because there have never been mass and energy densities as high as those. Since then, since the Big Bang, the universe has continued to cool and expand. The energy that the particles had has been released since then. That energy linked some particles to others, by expanding and breaking those forces that tied some particles to others, the energy has been released and has become thermal energy and that is what makes the universe not as cold as it would have been. came to be due to that progressive cooling process that I was telling you about.
Ruth Lazkoz She is a theoretical physicist, professor and researcher at the University of the Basque Country. Her lines of work are theoretical and observational cosmology, dark energy and modified gravity.
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