The strange shape of viscous dark matter, which accounts for most of the matter in the universe, could have a surprising effect on its early evolution - and make ripples from the Big Bang easier to detect. It is known that dark matter is a mysterious substance that makes up 80% of the substance in our world, but interacts with ordinary matter only gravitationally. At the moment, the most popular candidate for dark matter is considered to be WIMPs (WIMPs), weakly interacting massive particles, but decades of searching for this particle have led to nothing. WIMPs also predict specific things that we don't see in the Universe, like a swarm of mini-galaxies around the Milky Way.
There are other dark matter candidates. For example, Paul Shapiro of the University of Texas at Austin and his colleagues previously investigated one alternative form of dark matter that includes particles called bosons, which - unlike WIMPs and ordinary matter - can be in the same quantum state. This property could also allow them to coalesce into a strange, viscous state of matter - a Bose-Einstein condensate (BEC), in which a population of a particle behaves like a single quantum object.
Now Shapiro and his graduate student Buha Li are studying how this form of dark matter could have affected the early universe.
Growth spurt
Cosmologists are used to thinking that in the first moments of its existence, the universe experienced an exponential growth spurt. This expansion, which took place in the first few seconds after the Big Bang, is called inflation and was supposed to send relativistic ripples through spacetime - primordial (or primitive, call it what you will) gravitational waves.
Physicists thought they were seeing evidence of these waves when they worked with the BICEP2 telescope in 2013, but this turned out to be not the case. But earlier this year, the LIGO experiment saw gravitational waves of colliding black holes, which proved that such waves actually exist.
In the standard picture, these primordial gravitational waves should be so small that LIGO will never see them. “Something completely different is happening in our model,” says Shapiro. "Dark matter changes its behavior if we go back in time."
Promotional video:
Although viscous dark matter behaves in exactly the same way as WIMPs do today, scientists' calculations show that in the early stages its behavior changed: it acted not like matter, but like radiation. Moving even further back in time, dark matter was denser and behaved like a liquid, resisting compression.
“When we try to break it, we have to keep in mind the pressure,” Shapiro says. - When you collect it in a pile, it wants to swell back. We seem to fill the Universe with liquid."
Scientists did not expect to find this.
This elasticity means that this strange viscous dark matter may have slowed the rate of expansion of the universe at that time. Starting at the very end of inflation, the universe would expand much more slowly with dark matter than without it.
But the primary gravitational waves should have shot through the young Universe at the same speed as before. And because they were easier to print against the background, they might be easier to spot.
Primary waves
In a talk at a meeting of the American Physical Society in Salt Lake City, Utah, last month, a couple of scientists said such dark matter could suppress expansion enough for primordial gravitational waves to be detected by LIGO forces.
“In standard history, without our dark matter, they will be well below the limit at which gravitational wave detectors, current or future, can detect them. But our model shows that there is still hope."
Tanya Rejimbo of the LIGO team points out that since there is much we don’t know about what the early universe was like, we cannot say with certainty about such a possibility. In her opinion, there is no guarantee that these waves exist or that our future detectors will be able to see them. But this work is interesting because it provides such an opportunity.
ILYA KHEL