What we know about dark energy

How did Adam Riess, Saul Perlmutter, Brian Schmidt and their teams even conceive of the existence of dark matter? Francesca Chadha Day has the timeline...

Francesca - So the team in 1998 were looking at the expansion of the universe and they were observing actually a special kind of supernova called a Type 1A supernova. And this occurs when a white dwarf star eats another star. But the special thing about this kind of supernova is it's what we call a standard candle, which means it's always the same brightness and we know what that brightness is. So because of that, we can use these supernovae to work out how far away the star is, and we can also use an effect called redshift to work out how fast those stars are moving away from us. This happens when the source of light is moving away from us. The wavelength actually changes, it becomes longer, which means an invisible light, it becomes redder and it really allows us to track the expansion of the universe throughout its history. That team got a bit of a surprise because what they found was that the expansion of the universe is actually accelerating. It's speeding up. And it's very puzzling because to explain this accelerated expansion, we need some kind of energy source or effect in our theory. And that's what we call dark energy. And you may notice that this explanation is a bit vaguer than the explanation for dark matter. And that's because the question of what dark energy is, is vaguer. With dark matter, we are looking for a kind of particle that we don't know about yet that doesn't interact with light very much. With dark energy, we don't really know what we're looking for. It's really just the question of why is the expansion of the universe accelerating and dark energy, to my mind, is just the answer to that question.

Will - Some of the most compelling arguments I've seen for universal expansion come from the idea of the cosmological constant and vacuum energy, that there's this ubiquitous energy density that exists throughout the universe and causes its expansion. Is this what a lot of theoretical physicists are running with at the moment?

Francesca - I think most physicists think there is something more fundamental going on. The current theory is really just kind of, 'oh, there's this number and you put it there,' and we'd like something more fundamental than that.

We know that something must be causing our universe to expand at an accelerating rate and, if that is the case, this thing must make up about 68% of our universe. But that’s it. Everything beyond this point is purely hypothetical, we might not have the mathematics yet invented to parse this kind of information. So with that being said, I took a trip down to the University of Cambridge to see theoretical physicist Anne-Christine Davis, to talk us through what dark energy could be…

Anne-Christine - It's difficult to define dark energy. What we know is that there's something mysterious that makes up about 70% of the energy density of the universe and is currently causing the universe to undergo a period of accelerated expansion Today. What it is, is still an open question. There are lots of ideas around. It could be that Einstein's equations are wrong and we need an addition. It could be another particle causing this expansion. We actually really don't know. It could be something like the chameleon theory whereby you have an extra field, an extra particle, scalar particle, and its behaviour is such that its behaviour changes in the environment. So it can have more mass in the solar system and the range of its force is rather short. So it doesn't affect gravity in the solar system, but it could affect the behaviour cosmologically where the density is much lower. It's something that we can work with, but we can't quite make it satisfy everything we want to do with dark energy. It's a bit disturbing. I find it rather disturbing that things we don't know, we dub 'dark'

Will - If though, as you say, we theorised the existence of this dark energy because about 8 billion years ago the expansion of the universe started speeding up, was it always there?

Anne-Christine - Probably it was probably always there in the background, but it only came to dominate later on in the development of the universe. You can imagine the stages in the history of the universe as a bit like a marathon race. Let's say a sprinter, a hundred metre person, a longer distance 400m, then a middle distance runner, and a marathon runner. And they all have to run a marathon when they start out, the sprinters stream on ahead. That's the first period of inflationary expansion that we think the universe underwent. Then we get another period when the runner used to doing 400, 800 metres will take over. She'll continue for a bit and this would be like the period where the universe was dominated by radiation. But she'll tire after a while and then the middle distance runner will take over and will dominate the race for a long time. And this is the period when the universe is dominated by matter, by ordinary matter that we know and love and we can feel the effect of this, we see the galaxies forming the stars. At the very end of the race, the marathon runner will overtake and that's the late period of accelerated expansion or dark energy that we're undergoing now. And that's a bit like dark energy. It was there. It's only dominating today because everything else decreases faster than dark energy does.

Will - Is there a reason that we know of that we can't detect it because we're getting pretty good at finding stuff out in the universe, but if 70% of it is an energy that we can't detect, that seems quite extraordinary.

Anne-Christine - I think it's quite embarrassing actually, <laugh>.

Will - When we have all of this visible, regular, normal energy, I suppose as we dub it. If it's making up 70% of the universe is dark energy in and among us right here, right now?

Anne-Christine - Presumably. It depends precisely what it is. So for example, if it's due to a chameleon-type particle whose behaviour depends on the environment, it would barely interact in the solar system in this room. We wouldn't detect it. But yes, if it is, it should be there. There's no reason why it shouldn't be there. It could just be something like a vacuum energy that we don't know about.

Will - Now this is pure blue sky thinking here, but could the answer be in something, say, a dimension higher than we can perceive and it's not something we will ever be able to know?

Anne-Christine - Oh, it could be. I mean there was this film Interstellar where they went into the extra dimension and it was using theories that were around at the time one of the advisors was Nobel Prize winner Kip Thorne. It could be something like the influence of the extra dimension that is there, but we can't see it, but we feel the effects of it. It could well be, I don't know how we would work that one out, but that could be that we have five dimensions or some theories, 11 dimensions. One of them is sticking out and giving us rise to this mysterious dark energy. I think there's a difference between dark energy and dark matter. A big difference is that we really do sort of feel dark matter in the way that galaxies behave. And dark energy is something much more mysterious than that.