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Stars emit energy in different ways:
1. In the form of mass-free electromagnetic radiation photons, from the more energetic gamma rays to the less energetic radio waves (even cold matter radiates photons; the colder the matter is, the weaker the photons are). Visible light is part of this kind of radiation.
2. In the form of other particles without mass, such as neutrinos and gravitons.
3. In the form of high energy charged particles, mainly protons, but also smaller amounts of various atomic nuclei and other kinds of particles. They are the cosmic rays.
All these emitted particles (photons, neutrinos, gravitons, protons, etc.) are stable as long as they are isolated in space. They can travel billions of years without suffering any changes, at least as far as we know.
Thus, all these radiated particles survive so far (however distant) when they collide with some form of matter that absorbs them. In the case of photons, almost any kind of matter is used. Energy protons are already harder to stop and absorb, and even more difficult neutrinos. As for the gravitons, little is known so far.
Suppose now that the universe consisted only of stars placed in an invariable configuration. Any particle emitted by a star would travel through space until it hit something (another star) and be absorbed. The particles would travel from one star to another and, after all, each one would recover all the energy it had radiated. It seems then that the universe should continue immutable forever.
The fact that it is not so is a consequence of three things:
1. The universe does not consist only of stars, but also contains a significant amount of cold matter, from large planets to interstellar dust. When this cold matter slows down a particle, it absorbs it and emits less energetic particles in return. Which means that ultimately the temperature of the cold matter increases with time, while the energy content of the stars decreases.
2. Some of the particles (neutrinos and gravitons, for example) emitted by the stars and also by other forms of matter have such a small tendency to be absorbed by them that since the universe exists only a tiny percentage of them have been absorbed . Which is equivalent to saying that the fraction of the total energy of the stars that it swarms through space is increasing and that the energy content of the stars decreases.
3. The universe is expanding. Each year the space between the galaxies is greater, so that even absorbable particles, such as protons and photons, can travel on average longer distances before hitting matter and being absorbed. This is another reason that each year the energy absorbed by the stars is lower compared to that emitted, because it takes an extra amount of energy to fill that additional space, produced by the expansion, with energy particles and until then not absorbed.
This last reason is sufficient by itself. As long as the universe continues to expand, it will continue to cool. Naturally, when the universe begins to contract again (assuming it does) the situation will be the reverse and will begin to heat up again.
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