What Gravitational-Wave Astronomy May Reveal
Early last year, the field of astrophysics announced one of its biggest discoveries. A cosmic phenomenon that Albert Einstein had predicted a century earlier was at last detected directly. Two massive black holes collided, their spectacular merger generating huge ripples—gravitational waves—in the very fabric of space-time. After spreading out and traveling more than a billion light-years, some of those waves passed through the Earth and were recorded by two gravity-wave “telescopes”—one in Louisiana, the other in Washington state. In the midst of a hotly contested presidential campaign, science won out over politics for a day. The name Einstein dominated the headlines, wielding its power once again.
But the story doesn’t end there. Over the succeeding year, two additional black-hole collisions were firmly sighted, the latest reported in early June. In each case, though, the news was pushed to the back pages. But I would argue that these later detections should be even more celebrated. The first detection was like the magnum of champagne hitting the proverbial ship, launching a new vehicle for astronomy to explore the cosmos. But now we know that ship is finally gaining its sea legs and advancing its mission.
The data gathered so far, sparse as it is, has already revealed a new class of black holes that astronomers were never quite sure had existed; they are heavier than the stellar black holes forged directly from dying stars. These newly discovered holes possibly formed in dense star clusters where they collided and built up mass. A technical issue, yes, but surely a strong hint that more surprises await us as the two U.S. gravity-wave detectors gain sensitivity, and instruments in Italy, Japan, and India join them in a worldwide network. Soon these observatories will be recording signals weekly, possibly even daily.
It’s almost guaranteed these observatories will register the resounding crash as two city-sized neutron stars (paired together in a binary system) spiral into each other as their orbital dance decays. A solitary tsunami of a wave may also hit our shores whenever a star within the Milky Way explodes as a brilliant supernova. This happens when the star’s nuclear core runs out of fuel, collapses, and sends out a shock wave that blows the rest of the star apart. Examining the gravitational waveform from such a spectacular blast will allow astronomers to see, for the first time, the very birth of a neutron star or black hole.
And beneath all those varied gravity wave “songs” astronomers expect an underlying murmur—constant, unvarying, and as delicate as a whisper. This buzz would be the faint reverberation of our universe’s creation, its remnant thunder echoing down the passages of time. That detection cannot occur until gravity-wave instruments are sent into space, but when it happens those primordial waves would bring us the closest we have ever come to our origins, possibly verifying that the universe emerged as a quantum fluctuation out of nothingness.
But even more tantalizing is the prospect of encountering something never before imagined. Some theorists already wonder whether there might be relics from the early universe—highly energetic “defects” that were generated as the cosmos cooled down over its first second of existence. These include one-dimensional cosmic strings, extremely thin tubes of space-time in which the energetic conditions of the primeval fireball still prevail. Wiggling around like rubber bands, they would produce plenty of gravity waves.
Not until astronomers scanned the heavens with radio telescopes did they discover pulsars and quasars. What else might be inhabiting the darkness of space, awaiting detection by gravity-wave astronomers? Their voyage has only just begun.
Marcia Bartusiak is professor of the practice, graduate program in science writing, Massachusetts Institute of Technology, and the award-winning author of six previous books, including most recently Black Hole.
Featured Image: “Black-Hole Merger” by NASA, licensed for use on the public domain by Wikimedia Commons.