The quick answer to your question could be nothing, or it is also unknown. But let's get to what we do know. What we do know is that observations indicate that the expansion of the universe is accelerating. And this is very surprising because gravity (which is the force that counts at great distances like astronomical and cosmological) is attractive. Newton's famous apple fell to the ground, it didn't shoot skyward
So, if gravity works as we expect, if Einstein's theory of general relativity works (and, by the way, if our GPS navigator gets us right it is because it does work), the expansion of the universe would have to slow down and not accelerate.
But let's go back a bit. When Einstein formulated his theory of general relativity, all his calculations said that the universe was not stable, it had to expand or contract. But that idea did not convince Einstein and so he added a term to his equations that would make the cosmos static. This Einstein addition is known as the cosmological constant. Looking at his equations, Einstein quickly realized that by adding the cosmological constant to the other side of the = sign, it could be interpreted as a vacuum energy. And there it left off, for the moment.
A few years later, a study program on the distances of galaxies culminated in Edwin Hubble reaching the conclusion that the universe is expanding. The universe is not static, it is expanding. This was also the first step in arriving at the Big Bang theory and led Einstein to recognize that the cosmological constant was an error.
Dark energy means everything that can explain the observations that we interpret as an accelerated expansion of the universe
We come to the decade of the nineties of the last century. Then, three researchers, Adam G. Riess, Saul Perlmutter and Brian P. Schmidt (and their collaborators and collaborators), saw that their observations indicated that the expansion of the universe was accelerated, and that this unexpected acceleration could be explained very well with a cosmological constant
But of course, the observations could also be explained in other ways. So dark energy means everything that can explain the observations that we interpret as an accelerating expansion of the universe.
Could it be something other than a cosmological constant? Yes. It could be a cosmological non-constant, which means that it could change over time. Observations say not much, but they can never tell with infinite precision what exactly is constant. Could it change in space? Yes, again the observations say not much, but…
Could it be that we don't understand gravity at these large scales? The scientific community has made an enormous effort and has not found any sign that would force us to modify Einstein's theory of gravity in that sense, but the fact that we have not found it does not mean that it does not exist.
In short, we do not know What is dark energy. And this is one of the most important unsolved mysteries in physics today. And if an accelerated expansion of the universe reminds readers of something other than inflation, it is because perhaps the two phenomena are not, after all, fundamentally so different.
Licia Verde is a cosmologist and theoretical physicist, professor ICREA at the Institut de Ciències del Cosmos of the University of Barcelona.
Question sent via email by Rómulo Armando Albán Villena
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