DISCOVERING that the universe’s expansion is speeding up garnered three cosmologists a Nobel prize last week, even though the force apparently responsible – dark energy – remains deeply mysterious. Now there’s a way to measure this cosmic acceleration that should be able to probe a much earlier period in the universe’s history. This primordial era may be key to revealing what dark energy is.
Cosmologists probe the early universe by looking at distant objects. Light speed is constant, so the further away something is, the further back in time a viewed event actually happened. The advantage of the new method, which uses glowing discs around black holes, is that it should work at larger distances than any other.
“We can fill in the puzzle in regions where no other method can see,” says of the Dark Cosmology Centre at the University of Copenhagen in Denmark, one of the researchers proposing the method.
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In principle, measuring cosmic distances should be simple: objects look dimmer when farther away so comparing a star’s apparent brightness with how bright it actually is should tell you its distance. The trouble is that there’s no way to tell the intrinsic brightness of most stars and galaxies, and so no way to know their true distance. That’s why astronomers use “standard candles”, bodies of known brightness, to figure out how far away other objects are.
“There’s no way to tell the intrinsic brightness of most galaxies, so no way to know their distance”
The Nobel laureates used exploding stars called type Ia supernovae, which seem to all have the same intrinsic brightness (see interview with Adam Riess). By comparing supernovae of different apparent brightnesses – and therefore at different distances – they worked out that the universe’s expansion has been faster in more recent times than it was in the distant past.
This was a surprise, as it had been assumed that over time gravity would slow the expansion. Theorists’ answer is dark energy, a repulsive force driving space-time apart. But is this energy constant or has it been changing with time? The answer could be key to working out its true nature, but has remained elusive – partly because supernovae are rare, unpredictable and fade quickly. So far, they have only allowed astronomers to look back to about 9.6 billion years ago – seven-tenths of the age of the universe.
A team led by at the University of Copenhagen offers an alternative that will let astronomers look further back than any other technique: bright beacons generated by supermassive black holes at the hearts of distant galaxies.
Some black holes that live in galactic cores gather huge discs of gas around them, which glow white-hot as they are slowly devoured. Called , they can send energy out into the galaxy which ionises clouds of gas that orbit the discs. The brighter the AGN, the greater its reach, so the more distant the ionised clouds can be. Denney and her colleagues realised that if they could measure the distance between the AGN and the ionised gas, they could calculate the nucleus’s absolute brightness. Then they could measure its apparent brightness to find its distance from Earth. “This measurement of the ionised cloud tells us how bright the object is intrinsically,” says Watson. “That gives you your standard candle.”
The galaxies are too far away to measure the distance between the AGNs and the ionised gas clouds directly, but the team reasoned that as the light from a given cloud originally came from the AGN that is ionising it, it should take longer to reach Earth than the light coming directly from the AGN. So by measuring this time delay, the researchers could determine the distances from the AGNs to their respective clouds.
They used this method to figure out the distances from Earth to 38 AGNs, some of which had already been measured by other means. Comparing their results with the known values showed the technique worked pretty well, though measuring distances with supernovae is still three times as accurate, on average ().
AGNs have a number of advantages over supernovae, though, that suggest they might one day provide much more precise measurements of distance: they’re bright, they’re persistent, they’re everywhere and we understand how they ionise their surroundings better than we do how supernovae explode. As measurements tend to get less accurate the further away supernovae are, the new approach could allow more distant times to be probed. The researchers estimate that AGNs could reveal the rate of the universe’s acceleration as far back as 11.6 billion years ago, further than any other technique.
“I’m quite impressed. I think this is a very promising method,” says of Ohio State University in Columbus, who was not involved in the study. “There are good physical reasons why this should work.”
Peering back to epochs when the universe was much younger can help determine whether or not dark energy has changed with time, which could give us more of a handle on what it actually is.
“Observing epochs when the universe was younger could give us a handle on what dark energy is”
One theory has it that the amount of repulsive energy in every unit volume of space stays the same over time. As the universe expands, more space exists, which accelerates the expansion. This fits with Einstein’s cosmological constant hypothesis and with the supernova measurements.
However, an alternative model suggests that the repulsive force is a varying field called quintessence: in some versions of quintessence, the field can become dormant and so behaves like the cosmological constant, but merely over the recent past. Yet another theory says that dark energy is not an entirely different force but just gravity behaving weirdly over very large scales. Studying dark energy over long distances would test these ideas.
Looking back in time might, alternatively, reveal some surprises, says cosmologist of Princeton University. Perhaps dark energy isn’t driving the accelerating universe after all, and something even stranger is going on.
Accelerating expansion, in juicy detail
A technique based on glowing discs around black holes could allow us to measure the effects of dark energy – the enigmatic entity accelerating the expansion of space-time – at earlier points in the universe’s history than ever before. But some physicists say that the juiciest insights into dark energy will come from studying its more recent past more accurately.
Astronomer at Harvard University says the “best bang for the buck” will come from looking at the last 7 billion years with much more precision. While dark energy was there all along, it wasn’t until about 5 billion years ago that the universe’s expansion meant matter and radiation were diffuse enough for dark energy to dominate over gravity.
Here are some ways to probe this more recent period in greater detail:
Galaxy clusters Dark energy acts against the clumping force of gravity, so studying the mass of galaxy clusters at various times after the big bang can reveal the strength of dark energy back to 7.7 billion years ago.
Deep Hum Sound waves filled the universe with a deep hum in the aftermath of the big bang, squeezing matter and causing more galaxies to form in the regions compressed by the waves. The apparent distance between these clumps in different epochs reveals the changing rate of cosmic expansion – and dark energy’s strength.
Currently, this method can look back 6 billion years but the European Space Agency’s , planned to launch in 2019, will look back 10 billion years.
Constant constants A less well-known idea is to constrain the possibilities for dark energy via other physical constants. Paul Steinhardt at Princeton University says that if dark energy changes with time, so must they, according to most models.
But so far, observations show that the constants associated with gravity and electromagnetism are constant to very high accuracy, placing constraints on dark energy that are 1000 times as tight as those derived from supernovae. Maggie McKee
