When scientists jubilantly announced last week that a telescope at the South Pole had detected ripples in space from the very beginning of time, the reverberations went far beyond the potential validation of astronomers’ most cherished model of the Big Bang
It was the second time in less than two years that ideas thought to be radical just decades ago had been confirmed (at least so the optimists think) by experiment.
The first was the discovery of the Higgs boson, associated with an energy field that gives mass to other particles, announced in July 2012; physicists have said they will be studying the Higgs for the next 20 years at the Large Hadron Collider in Europe and perhaps at successor machines, hoping for a clue that will lead them beyond the Standard Model, which has ruled physics for the last half-century.
Now the South Pole telescope team, led by John M Kovac of the Harvard-Smithsonian Centre for Astrophysics, has presented physicists with another clue from what the Russian cosmologist Yakov B Zeldovich once called the poor man’s particle accelerator — the universe itself.
The ripples detected by the telescope, Bicep2, were faint spiral patterns from the polarisation of microwave radiation left over from the Big Bang. They are relics from when energies were a trillion times greater than the Large Hadron Collider can produce. These gravitational waves are the long-sought markers for a theory called inflation, the force that put the bang in the Big Bang: an antigravitational swelling that began a trillionth of a trillionth of a trillionth of a second after the cosmic clock started ticking. Scientists have long incorporated inflation into their standard model of the cosmos, but as with the existence of the Higgs, proving it had long been just a pipe dream.
Astronomers say they expect to be studying the gravitational waves from mountaintops, balloons and perhaps satellites for the next 20 years, hoping to gain insight into mysteries like dark matter and dark energy. The cosmic Kahuna that now dangles before astronomers and physicists is understanding what caused inflation. What is it that “turns gravity on its head” — as Alan Guth of MIT, a founder of inflation theory, has put it — and blew up the universe?
Antigravity may sound like a crazy science-fiction idea, but Einstein himself introduced the notion into physics. For him it was a fudge factor, known as the cosmological constant, that he inserted into his equations to account for the fact that the universe didn’t collapse.
He later abandoned the cosmological constant, calling it a mistake, but it was resurrected 15 years ago when astronomers discovered that the expansion of the universe was speeding up because of the mysterious force called dark energy. As with inflation, the repulsion is part of space itself: The bigger the universe gets, the more powerfully it pushes apart, resulting in an exponential runaway expansion.
The recently discovered Higgs field could also behave in this way. It was by playing mathematically with a version of the Higgs field in 1979 that Dr Guth stumbled on the concept of inflation. In the years since, dozens of versions of inflation have been proposed, like chaotic inflation, eternal inflation, slow-roll inflation, hybrid inflation, supersymmetric inflation and natural inflation, based on various kinds of fluctuating hypothetical fields.
Assuming they are confirmed (yet to be published in a peer-reviewed journal), the Bicep2 results eliminate most of these versions, including the Higgs, according to the Stanford physicist and inflation theorist Andrei Linde. But the winnowing could go on for decades.
Knowing inflation’s identity could be crucial if scientists are ever to unwind cosmic history back to the beginning, when they suspect the universe was ruled by a single unified force instead of the four distinct forces we know today: gravity, electromagnetism and the strong and weak nuclear forces.
The Bicep2 waves seem to date from the time that theorists suspect electromagnetism and the weak force divorced the strong force, gravity having already gone its own way, but Dr Linde says that could be a coincidence.
If the chain of evidence and reasoning holds up, however, the Bicep2 waves do bear witness to the most fervently hoped-for unification of all, or what John A Wheeler of Princeton once called “the fiery marriage” of Einstein’s gravity, which shapes the universe, and quantum theory, which governs the behaviour of atoms inside it. The discovery suggests that gravity, too, might ultimately be described by the same weird quantum rules as those that describe the other forces.
According to inflation theory, the Bicep2 waves are magnified images of the hypothetical particles called gravitons that would transmit gravity in quantum theory. Imprinted on the cosmos when it was a subatomic quantum speck, they have been blown up a trillion trillion times and spread across the sky for inspection. It would take a thousand trillion Large Hadron Colliders to make one in the lab, but the poor man’s particle accelerator has done it free of charge.
“The universe does what we can’t do,” said the physicist Lawrence M Krauss of Arizona State, who recently wrote a paper on this with Frank Wilczek, a Nobel laureate and physics professor at MIT. While some theorists have questioned whether quantum theory can ever be applied to gravity, Wilczek said last week that the new discovery “means gravity is quantised.”
This telegram from the past does not give any details about what might be the right quantum gravity theory. One well-known effort, verging on old age and lacking experimental proof, is string theory. The gravitons themselves, theory says, are produced by the same process by which black holes leak. It is known as Hawking radiation, after Stephen Hawking of Cambridge University, the renowned black hole theorist, best-selling author and avatar of cosmic mystery who discovered it in a prodigious calculation in 1973.
Shortly thereafter, William Unruh, now at the University of British Columbia, showed that you didn’t need black holes to see this radiation, just acceleration in space. In this case, the role of the black hole you can’t get out of is played by the rapidly retreating horizon you can’t reach in the inflating universe.
Hawking radiation has been part of the physics firmament for decades; it’s the best-known prediction of quantum gravity. “Now it seems that Hawking and Unruh were right!” said Max Tegmark, a cosmologist at MIT, noting that some physicists had wondered whether gravity obeyed the dice-playing quantum principles that Einstein had disdained.
If the Bicep2 results are confirmed, and if astronomers agree that the ripples were gravitational waves from inflation, the discovery of Hawking radiation could win a Nobel Prize for Hawking.