Dark Matter revealed in the early Universe

A signal from the very early Universe has provided a revelation about dark matter...
06 March 2018

Interview with 

Lincoln Greenhill, Harvard University


Artists impression of a black hole at the centre of a galaxy


Scientists have just discovered clues to the behaviour of the enigmatic dark matter, and suggest that it may, contrary to popular belief, be able to interact with normal matter. Two scientific papers in the journal Nature have examined signals from the very early universe, and found that it was colder than expected, suggesting dark matter played a role in cooling it down. Chris heard from Harvard’s Lincoln Greenhill, who has written a commentary summarising the findings...

Lincoln - Before galaxies and stars were around there was just a vast sea of gas left over from the Big Bang. That gas was mainly hydrogen; there was also some helium and there was nothing in it - no stars, no bright objects. The other thing that was around at that time was Dark Matter. Dark Matter is out there because we can see the influence of its gravity on how stars move within galaxies and how galaxies move as they dance about in the cosmos. And we’ve been searching for some time for evidence that would enable us to answer the big question: what is Dark Matter? What are those particles that are causing gravity but don’t seem to interact with radiation? Until now, it has seemed that they only interact with one another.

Chris - How have these researchers managed to get a fresh insight into what Dark Matter might be?

LIncoln - It begins with the first team. They used a very carefully constructed radio antenna which can receive signals at the frequencies that we currently use for television and FM radio. They observed the background, which is left over emission from the Big Bang at radio frequencies. Then they saw a very slight deviation in that signal at frequencies that correspond to what hydrogen atoms would be emitting and absorbing and that very small deviation tells us that the temperature of the hydrogen gas was actually lower than anyone had expected and, in fact, sufficiently low that it can’t be explained. Just normal interactions of matter couldn’t make the hydrogen gas that cold; it has to be losing energy to something else.

Chris - Is it fair to summarise then and say the Big Bang happens, we know there’s a big conversion of energy into material matter and a big chunk of that’s hydrogen? We think we understand what must have happened - the Universe is inflating and growing, a bit like a balloon blowing up and we know how much energy should be there, therefore, we think we know how hot the hydrogen should be but when this group look at how hot the hydrogen is, it’s cooler than the model would predict? So some energy has gone missing and now they’re speculating as to where that heat energy may have gone?

Lincoln - That's correct. At this time, the Universe is relatively simple. We have the hydrogen gas, we have Dark Matter, both left over from the Big Bang, and we have radiation left over from the Big Bang, so there aren’t many places that the energy of the hydrogen could go, apart from leaking into the Dark Matter.

Chris - Why is the hydrogen hotter than the Dark Matter then? If they’re both around at the same time why does the hydrogen lose energy to the Dark Matter?

Lincoln - The great advantage that the hydrogen has is that it can interact with more than just itself. The Dark Matter, so far as we had believed up until this most recent discovery, only interacted with itself. The hydrogen, on the other hand, is able to interact with both itself and with the radiation that surrounds it and, in fact, can interact with the very first stars as they were born approximately 180 million years after the Big Bang. And so the hydrogen can draw upon sources of energy which Dark Matter doesn’t have access to in some sense.

Chris - How do the team think that the hydrogen gives this thermal energy away to the Dark Matter?

Lincoln - We have only the most rudimentary understanding of this at the moment. We think that it’s a process that is like scattering, that it’s billiard balls banging off one another, transferring energy from one to the other.

Chris - What are the implications if they turn out to be right? What are the implications of this discovery?

Lincoln - Potentially, this is revolutionary because Dark Matter makes up most of the matter in the Universe; it outweighs normal matter by at least five times. And we now have a completely new window on a type of physics that no-one had seriously imagined would be the case up until now. Of course, Dark Matter is everywhere: it’s running through our bodies, it’s running through our environment. It does not interact and we don’t see it, but it’s everywhere and so it’s a new universe in some sense.



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