A CENTURY ago, the universe was a calm and stable place – at least in the minds of cosmologists. Even as the guns rumbled on the Western Front and the world convulsed in total war, Albert Einstein was putting the finishing touches to his vision of a perfectly balanced cosmos. In a paper he presented to the Prussian Academy of Sciences in February 1917, he added a new ingredient to his freshly baked equations of general relativity – one designed to guarantee that the universe could ride out eternity unchanged.
Today Europe is at peace, but there is uproar in the universe. We have long since abandoned Einstein’s idea of a static, unchanging cosmos in favour of a universe that is not only constantly expanding, but whose expansion is continually accelerating, at the mercies of the mysterious agent called dark energy. No one knows what this dark energy might be, beyond the fact that it must make up two-thirds of everything there is.
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Except that even this intruder might not be enough. New measurements of how fast the universe is speeding apart suggest that the one thing we thought we knew about dark energy is wrong. A new phantom could be stalking the universe, and the prospect is giving cosmologists chills. It would not only cause the universe to speed away from itself faster and faster until it rips apart, but also imply that none of us should even be here in the first place. “You take it literally, and it has weird ramifications,” says of Johns Hopkins University in Baltimore, Maryland.
When Einstein put the finishing touches to his theory of general relativity in 1915, he was left with a dilemma. His equations provided an elegant description of the workings of a cosmos dominated by gravity, aside from one thing: they offered nothing to keep it static, as it was presumed to be, rather than ballooning wildly or collapsing in on itself. His solution was a classic fudge – a new term crowbarred into the equations, a “cosmological constant” to provide the extra energy required to stabilise the universe.
Einstein was never a fan of his own invention, and it wasn’t long before astronomers Edwin Hubble and Milton Humason handed him a reason to disown it. In the 1920s, they showed that distant galaxies were speeding away from our own, and therefore that the universe was expanding. Einstein reputedly decried the constant as his greatest blunder.
A shifting constant
Dismissing it may have been a greater one. In the late 1990s, the cosmological constant made a triumphant return to the stage, when two groups of astronomers, one led by Riess, used light from distant exploding stars called supernovae to show that the universe’s expansion was accelerating. The existence of a repulsive dark energy that counteracts the gravitational pull of all the matter in the universe seemed all but incontrovertible. There were all sorts of ways to account for this interloper, but oddly the simplest fix was to reinstate the cosmological constant. Although it was originally intended to provide stability, when given just the right value it could produce the desired dark energy effect. The latest observations suggest that whatever is described by the cosmological constant accounts for 68 per cent of the universe.
What could it be? The explanation favoured by most physicists is that it represents the energy density of space, a manifestation of fundamental quantum effects active even in the vast swathes of nothingness between galaxies and star systems. But according to our best estimates, the strength of these quantum fluctuations would be 10120 times higher than is necessary to account for the rate at which the universe is ballooning. “The cosmological constant is extremely problematic,” says at the University of Rome. “Theoretically, we don’t have any explanation.”
“Cracks have appeared that could bring the pillars of cosmology crashing down”
Even so, dark energy and the cosmological constant are now firmly established as pillars of the standard model of cosmology, known as Lambda-CDM. The Greek letter lambda denotes the cosmological constant; the CDM is cold dark matter, an equally mysterious invisible and inert form of matter needed to hold galaxies together.
The Lambda-CDM model has proven itself remarkably robust over the past couple of decades, capable of explaining all facets of our evolving view of the cosmos. But in the past few years, cracks have begun to appear that threaten to bring the whole edifice crashing down.
The cracks come in the form of discrepancies between observations of the universe today and what Lambda-CDM says it should look like based on extrapolations of the early universe. The most significant is to do with the rate at which the universe is currently expanding, a figure known as the Hubble constant. Until recently, the accepted figure came from the European Space Agency’s Planck satellite, which measured the light of the big bang as it looked 380,000 years after the event. These most accurate measurements of the cosmic microwave background (CMB) to date have given us unprecedented insight into the universe’s earliest visible moments.
Armed with this data set, the researchers took the patterns they spotted in the CMB and projected them 13 billion years forward to arrive at a figure for the universe’s rate of accelerated expansion today: 67.3 kilometres per second per megaparsec.
The numbers extrapolated from Planck fit nicely with recent observations of the cosmos. “The data are more or less in agreement with a cosmological constant,” says Melchiorri. But projecting from the CMB is not the only way to calculate the Hubble constant.
“The latest measurements suggest the cosmological constant is anything but”
Ever since Hubble’s day, astronomers have measured it directly by using the light from distant stars and galaxies. Certain stars, known as standard candles, give off a predictable amount of energy, which allows for the careful measurement of distances in the nearby universe. Looking at entire galaxies lets us take those measurements further out, in a technique known as the cosmic distance ladder.
Riess has spent the last decade leading a collaboration adjusting and extending this ladder. When he published his first results in 2011, the uncertainties on his measurement and those derived from the Planck data were sufficiently large to make them consistent. Since then, however, those uncertainties have been reduced, and the two figures have grown apart (see diagram). Riess’s latest measurement puts the Hubble constant at 73.2 kilometres per second per megaparsec, with a 99.9 per cent confidence that the figure is not consistent with the Planck data. “That starts to get pretty serious, that level of conflict,” says Riess.
There is a chance it could disappear when the researchers clean up their data further, or it could still be a statistical fluke. But it’s not the only thing that’s got cosmologists worried.
Phantom menace
at the University of Edinburgh, UK, works on the Kilo-Degree Survey (KiDS), an attempt to map a broad swathe of the sky using a technique called weak lensing, which measures the way light bends around concentrations of mass. Their observations place limits on another key prediction of the Lambda-CDM model: the way dark matter clumps together across the cosmos. Their latest analysis, published this year, reveals a significantly different value to that predicted by Planck, with dark matter distributed a lot more smoothly than expected.
Heymans recalls that when she presented the conflicting results in 2012, it seemed that George Efstathiou, a senior figure on the Planck team, was always sitting in the front row, hand raised. “He would say, ‘Catherine, can you tell the audience what you’ve done wrong?’,” she says. “I didn’t have the balls to say ‘George, can you tell the audience what your team’s done wrong?'”.
Perhaps no one has done anything wrong. Over the past century, we have gone from seeing the universe as stable to imagining it expanding at a constant velocity to assuming that this velocity increases at a constant rate. The simplest way to reconcile the tensions with Lambda-CDM, says Shahab Joudaki at the University of Oxford, another member of the KiDS collaboration, is to go a step further and relax the requirement that dark energy’s density must remain constant over the lifetime of the universe. The acceleration attributed to the cosmological constant is itself increasing – and so the cosmological constant is anything but.
The idea that dark energy can evolve is not new. The most popular variant, known as quintessence, was first proposed in the 1980s. It treated dark energy as an all-pervading field, akin to a fifth force of nature, whose strength can shift over time. But old-fashioned quintessence doesn’t quite cut the mustard. To explain the latest results, you need a dark energy that evolves in some surprising ways.
The way dark energy behaves can be summed up by the relationship between its energy density and the outward pressure it exerts. Under Lambda-CDM, the ratio of those two properties is a constant with a value at or just above -1. Under traditional quintessence models, it is allowed to grow as high as it likes, ensuring the energy density fades away with time. For the latest data to make sense, however, the ratio has to go into a forbidden zone below -1, which means the strength of dark energy in the universe has to increase with time. Negative energy – the kind pulling the universe apart – has to be generated ceaselessly and out of nowhere.
This is a deeply odd scenario, says Heymans. Energy of any kind isn’t usually generated from nothing. Models that predict such seemingly impossible behaviour were traditionally ignored, acquiring the dismissive moniker of phantom dark energy. But the ghosts are now at the feast.
The consequences for the ultimate fate of the universe are dramatic. Go forward in time far enough and the accelerating acceleration overtakes the forces holding matter together at every scale. Ultimately, each of the universe’s constituent particles will be torn apart in a scenario known as the big rip.
There would also be some nasty consequences in the here and now that call the whole idea into question. In a universe pulled apart by phantom dark energy, ordinary matter would be too unstable to have persisted for as long as it has. Everything would have decayed into dark energy long before we came into existence. For at Imperial College London, this is a sign that we don’t understand something more fundamental. “I wouldn’t take phantom dark energy as the end of the story,” she says.
So what could be hiding behind the phantom’s mask? One idea is that the universe’s two most antisocial residents – dark matter and dark energy – might be secretly interacting in some way, causing the effects we see. If dark matter is continuously decaying into dark energy, for example, then that could explain the latter’s growth. Another option is that there are several undiscovered energy fields that mingle in such a way as to produce a repulsive force that shifts over time. These multi-field models are not popular, says at Leiden University in the Netherlands, as they would make phantom dark energy behave in ever more baroque ways.
The fact that theorists are even eyeing such uncharted territories makes experimentalists nervous, spurring them on to subject the latest measurements of the Hubble constant to ever more elaborate cross-checks and blind analyses. Heymans has been particularly unsettled. “I flip between waking up in mass panic in the middle of the night going ‘Oh god, there’s all these theoretical physicists coming up with these really mad theories’, and ‘well, if they’re going to come up with these crazy models, then fine, so be it, I’d just better carry on’,” she says.
Many hope the discrepancies grow ever harder to ignore, forcing us to confront new physics. “If it is flat Lambda-CDM, then I might go and do something else with my life,” says , director of the Institute of Cosmology and Gravitation at the University of Portsmouth, UK.
Siren songs
A new generation of observatories should settle the matter once and for all. The Euclid space probe, due to launch in 2020, and the Large Synoptic Survey Telescope, scheduled to start surveying the southern sky as of 2019, will help refine our measurements. And while the uncertainty on gravitational wave measurements, which can also clock the speed of cosmic expansion, is currently too large to discriminate between the data sets, they may yet offer a crucial independent judgement.
Earlier this year, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo detectors sensed faint ripples in space-time resulting from the collision of two neutron stars, and astronomers were able to follow up with telescopes. It was the first time an astronomical event has been observed using both gravitational waves and light. That combination gives us a new way to measure the rate at which the universe’s expansion is accelerating, one that promises enhanced accuracy because it doesn’t rely on the assumptions built in to other methods.
This first “standard siren” is not enough on its own. But, with the few dozen more detections we might expect over the next decade or so, the precision should reach the point at which we will know for sure whether we need to think again about dark energy.
Ultimately, as exciting as it would be to mark the cosmological constant’s centenary with a fatal blow, there are no guarantees for its successors. “I’ve sat through enough talks right now that I don’t think there is a good theory,” says Nichol. “A plague on both your houses. None of it feels right.”
Siren Songs
Earlier this year, our most sensitive ears on the universe heard faint ripples in the fabric of space-time made by a collision between two neutron stars. For many cosmologists, the most important implication of the discovery was what these particular gravitational waves had to say about a dispute threatening to tear apart our standard picture of how the universe evolved.
The rate at which the cosmos is expanding is important because it tells us about what it contains. So when two leading teams of astronomers found that different ways of measuring that rate yielded different results, things got tense. The numbers are far enough apart to suggest we might have missed a crucial ingredient – something that could account for the faster-than-expected acceleration of this cosmic ballooning.
The trouble is that our best measuring sticks are not quite precise enough. We won’t know for sure that something is awry until we get an independent check. And that’s exactly what gravitational waves bring, because they require none of the assumptions used by other methods. The first of these “standard sirens” won’t be enough on its own. But as more detections are made, the precision will quickly get to the point where our standard model of cosmology will face its day of reckoning.
This article appeared in print under the headline “The elephant in the gloom”
