Cosmologists Puzzle Over the Mystery of Dark Energy

Scientists ponder the nature of dark energy - and wonder if there might be more than one kind.

Back in 1998, astronomers discovered something puzzling about the universe. It was, the motion of distant supernova seemed to show, not only expanding – something well established for decades – but accelerating, growing bigger at an ever faster rate. The discovery was utterly unexpected; something quite at odds with what we thought we knew about the universe.

Until that time, astronomers believed the expansion of the universe, set in motion by the big bang, should be slowing over time. After all, the universe is filled with stuff – planets, stars, galaxies – that all exert a gravitational pull on each other. This pull should act as a kind of drag on the expansion, slowing the rate at which things move apart. Eventually it may even bring everything back together again.

For it to be expanding instead seemed to fly in the face of all logic. Gravity - the only force known to act on cosmic scales - is purely attractive. It can pull stars and planets together, weave them into galaxies and thus shape the night sky - but it can never force things apart.

To explain the acceleration, then, science needed a counterpart to gravity. Some unknown fifth force that, over vast distances, acts to push things apart. Even weirder, this mysterious force must actually be the dominant power in the universe. That is, it is both widespread and powerful enough to overcome the gravitational attraction of every star, planet and galaxy in creation.

With no obvious candidates as to what this could be, scientists labelled it dark energy, and started hunting for clues. Two decades later, however, and a name is still pretty much all we’ve got. No trace of another force has ever been found on Earth, no hint of what could be ripping the universe apart.

In one area, though, we have made progress. Analysis of the cosmic microwave background – the left over radiation from the Big Bang – show that matter, both dark and normal, makes up just 30% of the energy in the universe. The rest, we presume, is made of dark energy. That, at least, puts a number on what we are looking for.

Even so, the findings are troubling. A large fraction of the universe, the majority of it in fact, is utterly unknown. Astronomers can hardly pretend to understand the cosmos, to speak so grandly of black holes and grand theories of everything, if three-quarters of it remains utterly mysterious.

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Most attempts to explain dark energy invoke a new force of nature, one that has so far passed undetected on Earth. Its effects on small scales – say over the range of half the galaxy or so – must be tiny, otherwise we should surely have seen them by now. Experimental tests of Einstein’s relativity have revealed no unexpected discrepancies, and thus show that dark energy, whatever it is, must only act on larger scales.

To account for this, theories often invoke ways to reduce the effects of dark energy over small distances. One possibility is the so-called chameleon particle, a hypothetical particle that is weakened in places where lots of matter gathers – like galaxies – but more powerful in places where matter and energy are sparse, like the vast voids lying between galaxy clusters.

Such particles could even be detected on Earth, according to one recent paper. The authors suggest that chameleon particles could be created in the Sun, from which they stream outwards across the Solar System. A recent signal detected in an Italian experiment, they speculate, could be a possible sighting of these particles.

The experiment in question is known as XENON1T, and was actually intended to look for dark matter, and not dark energy at all. The signal, which still needs more experimental work to fully confirm, was first found last year. So far there is no clear answer to what it might come from. Still, the result needs to be treated with caution.

The simplest explanation is that it is only a fluctuation, a random statistical quirk that will disappear under closer inspection. It could also, the XENON1T team admit, be caused by a contamination in the experiment, or by neutrinos behaving in an unexpected way. But it could also be something real – something new.

That could, the team originally suggested, be a theoretical particle called an axion. If so, that would be exciting, as axions are a hypothetical particle of dark matter. But, as the more recent paper speculates, it could also be something else, even a particle of dark energy.

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There is another, more fundamental, question. Astronomers still aren’t sure how fast the universe is expanding. This is more than just a measurement error: the two different approaches used to calculate it show incompatible results. That somehow suggests that how fast the universe expands depends on how you look at it.

Traditionally astronomers rely on observations of distant galaxies to do this. The further away a galaxy is, the faster it is moving away from us. This result, which is seen in almost every galaxy, in every direction, gives astronomers an idea of how fast the universe is expanding, both now and over the billions of years since the Big Bang.

A second approach relies on measurements of the Big Bang, and the amounts of matter and dark energy we think make up the cosmos. By plugging these values into the standard model of cosmology - known as lambda-CDM - they can calculate how the universe should have evolved.

The results, generally, are good. Cosmologists can build a virtual universe that looks much like our own; scattered with galaxies and clusters, laced with filaments and walls. The rate of expansion it predicts, however, is wrong; at least compared to measurements of galaxies.

Scientists have thus wondered if the model is the source of the problem. If a modification to the model could somehow be found, one that perhaps refines the behaviour of dark energy, then maybe the two approaches can be brought into alignment.

One idea, proposed by researchers at the University of Baltimore, suggests that there is more than one type of dark energy. One kind, known as early dark energy could have dominated in the early years of the universe. Over time it dissipated, leaving the dark energy we now see to take over.

If that early dark energy had the right kind of properties, it could have caused the heat of the big bang to fade more quickly than lambda-CDM predicts. That, recent studies have found, would imply the universe is perhaps a billion years younger than we think. With less time to reach its current size, then, the universe must be expanding faster than the model currently predicts. That would bring it into alignment with the observations of galaxies.

Though some possible signs of this early dark energy may have been spotted, the theory seems inefficient. We already have two “dark” substances in the universe - do we really need to add a third? And what exactly happened to all that early dark energy? It must long ago have vanished, according to the model, but making it do so is somewhat of a hack.

None of the answers to the questions around dark energy, then, are yet satisfying. Indeed, cosmology is currently in a strange and difficult position; one where we know the answers we have are wrong, but where the way forward seems blocked. Until a crack in the hidden side of the universe is revealed, cosmologists will be left standing in the dark.

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