The quiet revolution in astronomy
Around the world, a project looking back 13.8 billion years hopes to solve a mystery from the early Universe. Daniel Clery assesses its progress.
At an observatory in Western Australia, and at others in South Africa and Europe, a quiet revolution in astronomy is taking place. To look at the observatories, though, you might not think so – there are no giant mirrors or huge dishes, just arrays of simple wire antennas, identical in principle to your rooftop TV antenna.
These decidedly lo-fi instruments are peering back more than 13 billion years to the time when the very first stars and galaxies were bursting into light. But they’re not aiming for the stars – they’re after the gas in between them. It is there they hope to fathom a mystery. Our universe was once a calm, endless sea of neutral hydrogen. Then something started ionising it. What?
These “telescopes” are searching for the answer in extremely weak microwave signals that are easily drowned in the noise of the universe. The earliest of these began the search eight years ago, and although they have yet to strike pay dirt, there are hints that something big is in the offing.
“This is the first generation of telescopes that can, in principle, see this type of radiation because it is so feeble and small,” says Saleem Zaroubi, one of the leaders of the Low Frequency Array (LOFAR).
“We’re working up to something big. It’s really exciting,” reveals Aaron Parsons, head of the Precision Array for Probing the Epoch of Re-ionisation (PAPER), the operator of the first arrays to go online.
The three telescopes, the Murchison Widefield Array (MWA) a few hundred kilometres north of Perth, LOFAR in the Netherlands and PAPER in the Karoo semi-desert of South Africa, were all built to solve the puzzle of re-ionisation. The Big Bang 13.8 billion years ago created a soup of superhot, charged particles and energy. Nearly 400,000 years later the Universe had expanded and cooled enough for protons and electrons to combine into hydrogen atoms. Like the calm after the storm, this ocean of neutral hydrogen gas rendered our universe cool, dark and featureless. It was the cosmic “dark ages” and so it remained for hundreds of million years until gravity began pulling the gas into the first stars. But something else was also going on.When astronomers look back to the earliest galaxies they can see, about a billion years after the big bang, that they were surrounded by ionised hydrogen – protons with their electrons stripped away. So some time in between something was re-ionising all the hydrogen in the Universe. What could have blasted all the electrons off the hydrogen atoms? Only something that released staggering amounts of energy – perhaps supergiant stars, quasars (see Another quasar mystery solved), decaying dark matter or even cosmic “strings” left over from the Big Bang.
This sort of astronomy is not about grinding the perfect mirror or
building the biggest dish, it’s all about the signal processing.
Seeing back to this distant “epoch of re-ionisation” (EoR) is at the very limit of what normal telescopes can detect and so far the project has revealed a handful of very bright but not particularly informative objects. But in the late 1990s theorists predicted it might be possible to detect radiation from the neutral hydrogen itself.
A neutral hydrogen atom emits a photon with a wavelength of 21 centimetres. Ionised hydrogen does not. So the theorists predicted, if you can detect this 21-centimetre radiation cloud from the EoR you might also see dark bubbles within, representing the ionised gas. This might or might not directly expose the identity of the “great ionisers” at the centre of those bubbles, but even if not, learning about the size, distribution and growth rate of the bubbles will help astronomers understand how ionisation engulfed the whole universe.
Wind forward 15 years and these three antenna arrays, and other smaller ones, are gathering data, trying to tease out the signal from the EoR. The problem is that the 21-centimetre radiation is very faint and as it has travelled across the millennia to Earth it has been joined by radiation from numerous other sources, including distant radio galaxies, supernova remnants in our Milky Way, accelerating particles, as well as the Sun and the Earth’s own ionosphere.
All of these “foreground” signals combined are between 1,000 and 100,000 times brighter than the expected signal from the EoR. There’s also the problem of FM radio broadcasts which use frequencies very close to those astronomers are interested in. That is why most of these arrays are sited in remote radio-quiet areas – apart from LOFAR, which is in the Netherlands for funding reasons.
So this sort of astronomy is not about grinding the perfect mirror or building the biggest dish, it’s all about the signal processing.
Using supercomputers, the astronomers sift through the flood of data coming from the antenna arrays, pick out those signals coming from a particular patch of sky and then identify and strip out all the foreground noise before they can hope to see the faint 21-centimetre radiation from the EoR. It’s a slow and laborious process and although astronomers have been collecting data for several years they are still months or even years away from making a definitive detection of EoR signals.
“The team is working hard to understand and analyse that data,” says Alan Lonsdale, director of the Massachusetts Institute of Technology’s Haystack Observatory, one of the partners in MWA.
When they do succeed, researchers are hoping they will hit a rich seam of information about this almost unknown transformation in the universe’s early history. The 21-centimetre radiation is “key to understanding the early evolution of galaxies”, says Judd Bowman of Arizona State University, MWA’s chief scientist. “We’ll be seeing the fingerprints of those early galaxies, what they did to their environments.”