Color variations in an image of the cosmic microwave background radiation depict temperature fluctuations caused by seeds of matter that eventually became galaxies. Black lines show polarization of the radiation caused by gravity waves, a sign that an inflationary burst of expansion occurred immediately after the birth of the universe.
BICEP2 Collaboration
Sandra Bullock now has the perfect title for a sequel to Gravity. Gravity Waves.
She won’t have to dodge space debris in this one. She’ll just have to brush up on cosmic inflation and perhaps laser interferometry. She can start by reading the wave of publicity in the wake of Monday’s announcement of gravity wave detection. Within the glow of cosmic microwaves left over from the Big Bang, astronomers have spotted subtle patterns reflecting the wriggles of gravity waves during the universe’s infancy.
OK, skeptics will demand confirmation before awarding the Nobel prizes. But here’s a prediction: This report will turn out to be right. And that would mean that Einstein’s greatest theory will once again have been verified. And that the leading modern theory describing the birth of the universe, so often derided in recent years, will have passed its defining test.
It will become a landmark in the history of physics. Gravity waves rank right up there with the Higgs boson as evidence that scientists pursuing the secrets of the universe are at least in the right ballpark.
True, evidence for gravity waves has been previously discovered. And their existence was suspected even before Einstein composed his general theory of relativity, the basis for the belief that they were worth searching for. But they have not always been taken for granted, and whether they’d show up in the cosmic microwave radiation was certainly uncertain.
But first, apologies for questionable nomenclature. “Gravity waves,” technically, are disturbances in fluids related to density differences where different fluids (or layers within a fluid) meet. You can have gravity waves in the atmosphere or on the surface of the ocean. Gravity waves in space, on the other hand, are ripples in the fabric of spacetime. Purists refer to them as gravitational radiation. But as that would make a terrible movie title, we’ll stick with gravity waves.
Some commentators trace the idea of gravity waves back to the 18th century. In the 19th century, British physicist James Clerk Maxwell also hinted at the gravity wave possibility. Maxwell figured out that light waves were just one of several types of electromagnetic radiation, consisting of vibrations in (he thought) the ether. He suspected that similar waves, or “pressure radiating out in straight lines” from massive bodies, might transmit gravity. He could not figure out any way to make the math for that work, though.
In 1908, the French mathematician Henri Poincaré raised the gravity wave possibility a little more specifically. He wondered whether the orbits of the planets might lose energy over time by emitting waves into the gravitational field that their mass created. Perhaps something like that might explain why Mercury’s orbit didn’t quite match the predictions of Newtonian gravity. But Poincare concluded that if they existed, gravity waves would be much too weak to explain Mercury’s orbit.
That explanation came in 1915 when Einstein worked out the equations for general relativity. Einstein’s theory corrected Newton’s by exactly the right amount to account for Mercury’s orbit. And Einstein’s theory differed from Newton’s in another essential way. For Newton, gravity would be transmitted across space instantaneously. For Einstein, gravity would travel at the speed of light.
In Einstein’s view, gravity was not really a force. Rather it was the result of distortions in the geometry of space (or more correctly, since it’s relativity, spacetime). Mass warps space; objects moving in space follow the resulting curves, so that nearby masses orbit each other or smash into each other. As described by the legendary physicist John Archibald Wheeler, “Mass grips spacetime, telling it how to curve. Spacetime grips mass, telling it how to move.”
When masses abruptly change their rate or direction of motion, they should send ripples through spacetime. Or at least that is what you would expect if gravity behaved like electromagnetism. As Maxwell figured out, an electric charge changing its state of motion would emit electromagnetic waves (i.e., blame him for talk radio).
At first, Einstein wasn’t so sure that gravity would work the same way. But in 1916, in his paper showing that gravity would travel at the speed of light, he concluded that gravity waves would therefore exist and carry energy away from matter in motion. (He later found some errors and published a better paper in 1918.) Others suspected that gravity waves would not transmit energy and that it therefore didn’t matter whether they existed or not.
Then in the 1930s, Einstein himself decided that gravity waves didn’t really exist. With a collaborator, Nathan Rosen, he wrote a paper about it — that turned out to be wrong. When they received the proofs of the journal paper they had to make a lot of last-minute corrections, violating a standard publishing rule against editing on the proofs.
In the 1950s, interest in gravity waves accelerated as physicists realized that a quantum theory of gravity would probably require such waves (just as the original quantum mechanics required particles to be wavelike). Richard Feynman decided that gravity waves should be real, and it was always a bad idea to argue with Feynman. Nevertheless, not everybody agreed.
By the 1970s, some efforts to detect gravity waves claimed success, but without confirmation.
In the 1980s, though, most gravity wave deniers had to capitulate. Joseph Taylor and Russell Hulse analyzed two neutron stars orbiting around each other and found they were getting closer — that is, the system was losing energy, by precisely the amount it would lose if the neutron stars were emitting gravitational waves.
Still, that was just indirect evidence (although good enough to get Taylor and Hulse the 1993 Nobel Prize). Monday’s announcement suggests a much more definitive detection, based on the direct effect of gravity waves on the cosmic microwave background radiation.
As its name suggests, the microwave background consists of microwaves, which are, of course, a form of electromagnetic radiation. They can therefore be polarized, kind of like the way some sunglasses polarize sunlight — orienting the waves along a common direction. That’s good both for reducing glare and for testing theories about the beginning of the universe.
Electromagnetic waves have both electric and magnetic components, and in this case astronomers were looking for the effect of gravity waves on the magnetic part. It’s abbreviated B because B stands for magnetism. (Why B? Because that’s the letter Maxwell used in his equations.) Anyway, to oversimplify, the B part of the polarization of the microwaves could be caused only by gravity waves. Finding the B polarization is therefore conclusive evidence that gravity waves were at work in the first instants of time after the Big Bang. That’s when the patterns of mass density and spacetime ripples were imprinted into matter in the universe. They were “frozen into” temperature patterns when the microwaves were emitted about 380,000 years later. (It’s complicated. Tiny variations in temperature of the microwaves were caused both by matter density differences and by gravity waves; the gravity wave effects are weaker and much more difficult to detect.)
Now here’s the thing. It’s a big deal to find the direct effects of gravity waves from 13 billion years ago, especially since efforts to find them from more recent sources — like black hole formation after supernova explosions — haven’t succeeded. (Scientists have been searching for years with huge detectors, using lasers and mirrors in 2.5-mile-long tubes. Everybody is still waiting for some supernova or other astronomical cataclysm nearby enough for the waves to be detectable.)
Even more spectacular, though, is the fact that physicists figured out in advance that the primordial gravitational waves would be found in the cosmic microwave radiation. They are a consequence of cosmic inflation. Inflation theories propose that immediately after the universe exploded into existence, a sudden burst of ultrarapid expansion stretched everything out smoothly, explaining why the universe today is so uniform (on large scales). Inflation also predicts that quantum fluctuations in matter density would provide the seeds for growing galaxies. And that fluctuations in spacetime — the gravity waves — would polarize radiation emitted from the matter.
Opponents of inflation have contended that everything else can be explained in other ways, or that inflation doesn’t explain what it claims. But even the opponents have conceded that alternative scenarios would not produce the gravity waves as inflation does. The existence of those waves, if confirmed, validates inflation beyond reasonable doubt. So inflation’s pioneers, most notably Alan Guth and Andrei Linde, can join the list of great thinkers who have used mathematical recipes to pry deep secrets from nature long before confirmation by observation. (Even better, gravity waves confirm my contention that Guth and Linde deserve the Nobel Prize.)
One more curiosity. The experiment finding the B polarization signal, a project known as BICEP2 based at the South Pole, recorded a much stronger signal than most cosmologists had expected. To some, that sounds fishy, suggesting that maybe there was a mistake. But if it’s right, that signal could help determine which of many versions of inflation theory best describes the actual universe.
Some versions of inflation predict wild things like a multiplicity (even an infinity) of parallel universes. But most versions of inflation do not predict such a strong B polarization signal. One version that does, called natural inflation, was proposed in 1990 by Katherine Freese (now of the University of Michigan) with collaborators Joshua Frieman and Angela Olinto of Fermilab. It would be nice to know if that version predicts parallel universes. Freese says she doesn’t know. But she’s going to get to work on it right away.
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