What is the Universe made of? By Manuel Lozano Leyva

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The Universe is radiation and matter. That is, energy, in the shape of radiation, and matter in the shape of elementary particles, which make up protons, neutrons and matter in general. What happens is that what I described is just the content we detect. There’s still some 90% which remains undetected. The fact that we haven’t detected it doesn’t mean detecting it will radically change our image of the Universe. That ninety something percent which hasn’t been detected is a dark matter component and another component, which makes up seventy something percent, is called dark energy. At first sight, one may ask why there’s some ninety percent of the Universe that we don’t know.

I usually don’t say we don’t know what it is -which is correct- but that it hasn’t been detected. Because, if it’s detected… imagine the neutrino, a kind of particle that’s constantly passing through our body by the millions, because all the stars are emitting it and, since it has no mass or energy it can go through a whole galaxy without even noticing, without even changing direction. If those particles had a mass, however small, we would explain dark matter by changing hardly anything. If there was an overabundance of some heavy elements, like brown dwarfs… anyway, there are plenty of candidates for dark matter. If it turned out to be one of these candidates, it would be a surprise, but it wouldn’t change our paradigm at all. As to dark energy, the remaining seventy percent that makes the Universe expand with an acceleration, against intuition, we could explain it by simply adding a repulsive term to gravitation at large distances, and that wouldn’t change much either. What happens is that our lack of certainty makes us expect some surprises, but in principle we’re pretty sure the Universe is made of matter and energy which, since Einstein’s famous equation e=mc2, where m is matter and e is energy, are known to be the same. They’re in a proportion of one thousand million photons for each material particle, so the Universe is mostly radiation, with very little matter (fortunately we’re a part of it). And, together with the two unknowns which are dark matter and dark energy, that’s the whole of the Universe. When we speak about matter and energy, we’re talking since Einstein, about the geometry of space-time, that is, about space and time. So, in a way, the Universe is simple.

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