goopy dark matter Goopy dark matter could slow down inflation of the universe

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A strangely goopy form of the dark stuff that makes up the majority of the universe’s matter could have had a surprising effect on its early evolution – and it could make ripples from the big bang easier to spot.

Dark matter is the mysterious substance that makes up 80 per cent of the universe’s matter, yet it only interacts with ordinary matter through gravity. The most popular candidate for this stuff is the WIMP, or weakly interacting massive particle, but decades of searches for this particle have come up empty. WIMPs also predict certain things that we don’t actually see in the universe, such as a swarm of mini-galaxies around the Milky Way.

There are other dark matter candidates. For example, Paul Shapiro at the University of Texas at Austin and his colleagues have previously investigated one alternative form of dark matter that invokes particles called bosons, which – unlike WIMPs and ordinary matter – can share the same quantum state. This property could also let them clump together in a strange, goopy state of matter called a Bose-Einstein condensate (BEC) in which a population of particles all behave as a single quantum object.

Now, Shapiro and his graduate student Bouha Li have worked out how this form of dark matter would affect the early universe.

Growth spurt

Cosmologists’ favourite view of the universe’s earliest moments involves the universe going through an exponential growth spurt in the first slivers of a second after the big bang. This expansion, called inflation, would send relativistic ripples through space-time called primordial gravitational waves.

Physicists thought they had seen evidence of these waves using the BICEP2 telescope in 2013, but it turned out to be a mistake. Earlier this year, though, the LIGO experiment saw gravitational waves from colliding black holes, proving that such waves really exist.

In the standard picture, those primordial gravitational waves would be so small that LIGO will never see them.

“In our model, something drastically different happens,” Shapiro says. “The [dark matter] changes its stripes as you go back in time.”

Although goopy dark matter behaves exactly the same as WIMPs in modern times, their calculations suggest that in an earlier phase, it changed its behaviour from acting like matter to acting like radiation. Going back even further, the dark matter becomes stiff, and behaves like a fluid, resisting compression.

Clump and push it

“There’s a pressure associated with trying to disturb it,” Shapiro says. “When you clump it, it wants to push back. It’s like we filled the universe with a fluid.”

“This is not intuitive – we did not expect it,” adds Li.

That stiffness means that this strange, goopy dark matter would have held back the expansion rate of the universe at that time. Starting from just after the end of inflation, the universe would expand less quickly with this dark matter in it, than without it.

But the primordial gravitational waves would shoot through the infant universe at the same speed as before. That means that they would show up more easily against the background more, making them easier to spot.

Primordial waves

In a talk at the American Physical Society meeting in Salt Lake City, Utah, last month, the pair claimed that such dark matter could suppress expansion enough for primordial gravitational waves to be visible to LIGO.

“In the standard story, without our dark matter, it’s way below the limit of all gravitational wave detectors we have or will have,” Li says. “But our model says there’s still hope.”

Tania Regimbau of the LIGO team points out that, because we don’t know a lot about what the early universe was actually like, it can be easy to tweak.

“There is no guarantee these extra contributions exist or can be seen by our future detectors,” she says. “But this work is certainly interesting because it shows that this could be the case.”

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