Shedding light on dark energy
Two decades after its postulation, dark energy still provokes more questions than answers - but we could just be making some headway in understanding it
Has a new finding finally solved one of the greatest mysteries in the universe?
Two decades ago, astronomers studying distant stars made a dramatic discovery. They were investigating the future of the universe, which was known to be expanding. Contrary to all expectations they discovered that the rate of expansion was actually speeding up – the universe was getting bigger, and faster. Today the focus is on finding the cause of this unexplained phenomenon, which has been given a suitably mysterious moniker: ‘dark energy’.
The discovery that the universe was expanding ever faster came as a huge surprise. In fact, it was so significant that it led to the award of a Physics Nobel prize in 2011 to Saul Perlmutter, Brian P Schmidt and Adam G Riess.
These were the leading scientists from two competing teams who studied light from a particular type of exploding star – and lots of them. Type Ia supernovae flare up in a predictable way, so their relative brightness reveals how far away they are. Their redshifts – the elongation of wavelengths towards the red end of the spectrum – showed that it was those furthest away that were receding the fastest.
It was the opposite to what most scientists expected, says Robert
Crittenden, professor of cosmology at the University of Portsmouth. “We’d always assumed that because gravity was attractive, the rate of expansion would be slowing down. Distant galaxies would be gravitationally attracted to each other and that would be slowing down the expansion of the universe,” he says.
The discovery that the universe was expanding faster than before begged a question: what was causing it? The simplest solution, which many scientists still believe will ultimately prove to be the correct one, is decades old. It comes from Albert Einstein’s general theory of relativity, which describes how gravity operates.
Before he published it in 1917, Einstein wrestled with one of its predictions. He believed that the universe was static, and yet the general theory of relativity predicted it would either expand or contract. In order to keep things in balance, he added a term to the equations: the famous cosmological constant.
It was effectively a fudge, and it wouldn’t last long. By the 1930s Edwin Hubble’s observations of distant galaxies had shown that the universe wasn’t static at all, but was expanding, and so Einstein abandoned the cosmological constant, reportedly calling it his “biggest blunder”.
In the 21st century, though, the cosmological constant is back as the leading explanation for dark energy. In physical terms it’s a number that describes the energy density of empty space. Even a perfect vacuum, devoid of any particles, is not devoid of energy. As the universe expands and more space is created, there’s more energy to push things outward.
This isn’t hypothetical. There’s a physical mechanism that explains where the energy comes from which can be found in another branch of physics: quantum mechanics. It says that temporary particles pop in and out of existence
– a phenomenon that’s been observed in particle experiments on Earth.
There’s just one very big problem. Quantum calculations have produced a number for the energy density of empty space, but it’s way, way bigger than what’s required. “The cosmological constant is a very simple model, but when you try to relate it to fundamental physics the value you get is arguably 100 orders of magnitude different from what you expect. Apart from that it works very well in explaining what we see, but because of that problem people look at other ways to solve the riddle,” says Crittenden.
One of the other ways of explaining dark energy is an energy field that isn’t constant. Rather, it’s dynamic. Its value changes over space and time, driving the expansion of the universe differently now compared to how it did in the past. The universe may have been born a long time ago – 13.8 billion years ago at the Big Bang – but dark energy has only come to dominate the expansion for the past 5 billion years or so.
There are many variations of this idea, which goes by the name of quintessence, or ‘fifth force’. In physics terminology it would be a ‘scalar field’, similar to the Higgs field. And just as the Higgs field has an associated particle – the Higgs boson – so would the scalar field be responsible for dark energy.
A particle that could be responsible for dark energy has never been detected, but strangely enough this non-detection works in its favour as an explanation. Out in the far-flung reaches of the universe the field would be strong enough to fling galaxies apart, but in the presence of other masses around it, like in laboratories on Earth, the force it exerts would be miniscule.
One such hypothetical particle is called the chameleon. It can change its mass depending on the density of its surroundings, explains Clare Burrage, associate professor at the University of Nottingham. “Chameleons can self-camouflage.
They can learn about their environment and adjust their properties so that the modification of gravity is hidden from experiments. But they can’t hide from everything, so if you do a suitably chosen experiment you might be able to see its effects. We’re using a technique called atom interferometry that’s really sensitive to these forces.”
The chameleon force would be exerted solely by the outer shell of an object rather than, say, gravity on Earth, which is exerted by the entire planet. This idea explains why the force would be so weak and why the experiments, which involve detecting the motion of atoms in free fall, needs to be super-sensitive.
“One of the interesting things about these theories is that you can design an experiment on Earth that might make their effects show up. It doesn’t need to be in a huge particle collider. You just need to do the right kind of really sensitive measurement, using equipment that could fit on a couple of table tops. It’s done on a small scale on a reasonably short timescale – a very different way of studying dark energy than launching a telescope, which can take decades,” says Burrage.
The University of Nottingham team has yet to publish its results, and a group using the same technique at the University of California, Berkeley, has yet to spot anything unusual.
While physicists look for tell-tale signs in the lab, many astronomers have focused on making more precise maps of how the universe has evolved over time. One kind of map that’s particularly important reveals the location of not just ordinary matter, of the kind that constitutes stars and planets, but dark matter too.
Dark matter is another cosmic mystery all by itself – an invisible form of matter that, like dark energy, has evaded detection. But while the particles making up dark matter have yet to be identified, its effects are clearly visible. Crittenden explains: “We’ve known about dark matter for a while longer than dark energy, and we need it on a smaller scale than the universe [as a whole]. Visible matter isn't nearly enough to explain the dynamics of stars around galaxies and galaxies around clusters of galaxies – we need a significant amount of dark matter as well.”
The currently accepted model of the universe holds that it’s geometrically flat. And if this is so then dark matter must account for a staggering 27 per cent of all its mass-energy. Ordinary matter would make up just 5 per cent and the remaining 68 per cent would be dark energy.
However, dark matter and dark energy don’t behave in the same way, says Crittenden. “Dark matter is generally attractive – it's like ordinary matter in how it gravitates. Dark energy has a very different effect – it doesn’t cluster galaxies together and it's difficult to see its effect on individual galaxies.”
Whereas dark matter acts to pull galaxies together, dark energy tries to pull them apart. This
“Visible matter isn't nearly enough to explain the dynamics of galaxies around clusters of galaxies”
Prof Robert Crittenden
A Type Ia supernova pictured by the Hubble Space Telescope in the galaxy M82
The Victor M Blanco telescope in Chile captures light for the Dark Energy Survey
Cooling device for the world’s largest digital camera – part of the Large Synoptic Survey Telescope
The fibres of the DESI can capture 5,000 galaxies simultaneously