AS HOLY grails go, a glimpse of the first moments of creation seems like a cut above the rest. And unlike Indiana Jones, King Arthur and countless other real and fictional explorers, the cosmologists searching for this one know where to look.
Ripples in space-time from a fraction of a second after the big bang should have left an imprint on the cosmic microwave background, the radiation left over from the big bang. But unfortunately, the very age of the CMB has also rendered that imprint somewhat illegible.
Now Asantha Cooray of the University of California at Irvine and Kris Sigurdson at the Institute for Advanced Study in Princeton, New Jersey, have devised a method to undo the ravages of time and reveal the 鈥渋nflationary gravitational waves鈥� (IGWs). Gravitational waves are distortions in space-time predicted by Einstein鈥檚 theory of general relativity, and would have been created mere instants after the big bang. During this dizzying episode known as inflation, the universe was expanding faster than the speed of light.
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Theorists say the gravitational waves should have subtly altered the CMB, which dates from about 300,000 years after the big bang 鈥� the point when the universe cooled enough to become transparent, as the opaque soup of particles combined to form the first hydrogen atoms. 鈥淯nfortunately, the [IGW] signal is very small and there are many effects that can swamp it,鈥� says cosmologist Robert Caldwell at Dartmouth College in Hanover, New Hampshire.
One way the imprint has been swamped is to do with the fact that it has travelled across most of the observable universe to reach us today. Along the way, the radiation can get deflected by the matter it encounters 鈥� a process known as gravitational lensing 鈥� which can distort the CMB and even produce effects that mimic the expected signature of IGWs.
Cooray and Sigurdson have identified a way around this problem, and it depends on another form of radiation that permeates the universe. Neutral hydrogen atoms sometimes absorb or spontaneously emit a pulse of radiation at a wavelength of 21 centimetres, and have been doing so almost since the time the CMB began its journey. 鈥淭he distant light from the CMB and from the cosmic 21-centimetre radiation must pass through the same patch of the universe on its way to us and so must be lensed in a similar way,鈥� says Sigurdson. 鈥淲e exploit this fact to help 鈥榙e-lens鈥� the CMB light.鈥� This should allow cosmologists to distinguish between the effects of lensing and the imprint of the inflationary gravitational waves (Physical Review Letters, vol 95, p 211303).
鈥淭he detection [of IGWs] has been called the holy grail of cosmology,鈥� says Ben Wandelt of the University of Illinois at Urbana-Champaign. 鈥淎ny work that promises to help us find these is exciting and a welcome addition to the conceptual toolbox.鈥�
鈥淭he method might undo the ravages of time and reveal the distortions in space-time created instants after the big bang鈥�
Experiments to probe the early 21-centimetre radiation are only now getting started, such as LOFAR, a large radio telescope array being built in the Netherlands. Sigurdson is hopeful that such experiments will help uncover the signature of the gravitational waves. 鈥淭he universe is the ultimate physics experiment,鈥� he says. 鈥淚f we can tune into the cosmic radio station and see all the information sitting out there, we may finally be able to answer questions about the very beginning of time.鈥�
