2023-06-15 20:10:04
Guadalupe Cañas is a postdoctoral researcher in Cosmology at the European Space Agency (ESA). She is part of the mission Euclid, which will be launched on July 1 to create the largest and most accurate three-dimensional map of the cosmos ever made by observing some 2 billion galaxies located up to 10 billion light-years away, covering a third of the sky. . It will help to clarify the true nature of dark matter, which no one has been able to see but which is believed to make up 25% of the universe, and of the even more enigmatic dark energy, which makes up 70%. We only know about the remaining 5% of the cosmos.
“Why haven’t we found dark matter and dark energy yet?”
—The reason is that modern cosmology is very young, barely 30 or 40 years old. We believe that dark energy drives the accelerating expansion of the universe. Dark matter behaves like matter we know, it has gravity, but it doesn’t interact with light, so it can’t be seen. To understand its nature, we would have to assume that it is composed of some kind of particle that has not yet been discovered.
What candidates do we have?
—When you study dark matter, you have to make a decision and that is to assume either that all the matter we call dark is the same or that there are different components. We know that there are astronomical objects that can contribute to this dark matter, for example, black holes produced in the early universe or white dwarfs that are already at the end of their lives and become black dwarfs. But they do not serve to predict everything. There are also candidate particles to form this matter such as axions or wimps (Weakly Interacting Massive Particles).
“Could primordial black holes be a source?”
—Yes, that theory is very fashionable and in fact it is one that we would like to test. By giving us this huge catalog of galaxies and being able to create this map in 3D, Euclid is also going to tell us what the initial conditions were in the universe and if it is possible for those holes to exist.
It may happen that gravity behaves differently on a large scale
—So, the dark matter was already in the so-called cosmic soup, the universe shortly after the Big Bang?
—Yes, we believe that it was there when the universe was very small and very very hot, and that it had to play a significant role. During that cosmic soup, the first atomic nuclei tried to collapse gravitationally, but it was all so hot that they had enough energy to reverse that gravity. Dark matter allowed gravity to begin to clump matter together.
“Can there be stars made of dark matter?”
“We don’t rule it out. In the 2000s there was a rather interesting theory of something called males (MAssive Compact Halo Objects), objects that we know don’t emit light but have to be there. Black dwarfs are white dwarfs that have already stopped shining. We think they have to exist, but since they don’t emit light and are small compared to galaxies, it’s hard to detect them gravitationally. What happens is that our universe is not old enough yet for there to be enough of them. It is also possible that there is something that we are missing, which is why Euclid focuses on obtaining measurements of at least 2,000 million galaxies between now and 10,000 million years ago.
“Are there other possibilities?”
—That dark energy may be associated with empty spaces in the universe, although quantum physics disagrees. It may also be that our understanding of gravity is incomplete. We have not tested Einstein’s theory of relativity beyond the solar system. On a large scale, with much greater distances, it could happen that it behaves differently.
“What if we find out that dark matter doesn’t exist?”
“It is highly unlikely. It would be very strange if Euclid did not confirm it. In addition to the cosmic microwave background, the radiation left behind by the Big Bang, there is much current evidence indicating its existence. If you watch a galaxy rotate, the only way to explain the speed at which it rotates is to add more mass. In fact, that was the first proof that there was more matter than we could actually see. And poor Vera Rubin, she never got a Nobel for discovering it.
“What if Euclid doesn’t confirm the standard model?”
It’s very difficult to refute. It only has six parameters and it works very well. But I would be absolutely happy if we could find another one that would better explain the observations. If Euclid doesn’t confirm it, he leaves us a fantastic window open to test alternative models and look for a particle type that fits.
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