When the champagne has been on ice for nigh on a century, the corks pop all the louder. So it was last month, when physicists announced the first direct detection of a gravitational wave. Thatās just the beginning of the story, however.
The existence of gravitational waves was one of the first predictions to emerge from Einsteinās theory of gravity, the general theory of relativity, a century ago ā and it was one of the last to be verified. General relativity tells us that gravity results when massive bodies like Earth, the sun and black holes distort space and time around them. When such bodies move and accelerate in each otherās gravitational fields, the result should be a series of tiny distortions, wave-like ripples in the fabric of the universe that radiate out.
For the past 40 years weāve had indirect evidence of gravitational waves from a pair of neutron stars engaged in a death dance. Since the snappily titled PSR B1913+16 binary was sighted in 1974, the distance between the two stars has been slowly contracting, exactly as general relativity predicts if they were losing energy by radiating gravitational waves.
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Those waves have never been directly detected, perhaps because they are just too slight. But on 15 September last year, Advanced LIGO, a vast facility spread over two sites in Louisiana and Washington state, for the first time sensed a space-time ripple passing directly through Earth. The minute disturbance is thought to have originated from two black holes, each with a mass around 30 times that of our sun, falling into each otherās arms, 1.3 billion light years away.
It is an important proof of principle, says astronomer of Harvard University ā but there is much more to detect. Mergers of bigger black holes such as the supermassive ones thought to form the centres of mature galaxies (see āHow weāll catch a huge black hole on cameraā) will emit gravitational waves at a much lower frequency, beneath LIGOās threshold. āLIGO will not be sensitive to them,ā says Loeb. āWe need different observatories.ā
That probably means waiting for a , expected to launch sometime in the 2030s. A test probe for this project, LISA Pathfinder, blasted off last December.
Even eLISA, though, wonāt detect the most eagerly sought-after gravitational waves. Our most cogent, but untested, model of the universeās birth suggests space-time underwent a period of breakneck expansion known as inflation, sending out extremely low frequency gravitational waves. These should be reverberating through the cosmos even now.
Back in March 2014, researchers at the BICEP2 experiment thought they had seen evidence for these primordial gravitational waves in patterns of light polarisation in the cosmic microwave background, the relic radiation of the big bang. That proved to be a false alarm Ā ā but last monthās detection provides a fresh spur to think how we might detect these echoes of creation, says Loeb. āIt opens a new window into the universe.ā
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