A window opens on to the Big Bang
Discovery of gravitational waves sheds light on the birth of the universe and provides a final validation of Einstein. Philip Dooley reports.
Astronomers peering out into the clear Antarctic sky appear to have found a key missing piece of the puzzle to our universe.
Embedded in radiation from the Big Bang they found the signature of gravitational waves, first predicted by Einstein nearly a century ago. The discovery also validates an extraordinary model for the birth of the universe known as “inflation”, which was proposed by Massachusetts Institute of Technology physicist Alan Guth in 1980. The team headed by John Kovac from the Harvard-Smithsonian Centre for Astrophysics delivered their news at a press conference at Harvard University on 16 March.
“The significance of these new findings is enormous,” Guth told MIT News. “As far as we know, there is nothing other than inflation that can produce these waves.”
Inflation is a refinement of the Big Bang theory, which says that the universe underwent a period of prodigious growth in the first fractions of a second after its birth. Belgian Catholic priest and cosmologist George Lemaître proposed the Big Bang theory in 1931 to explain how all the galaxies in the sky seem to be moving away from us. His theory gave the universe an original birth moment at which its density was infinitely large and its size infinitely small, conditions that couldn’t help but cause a very big bang. Refinements of the theory in the 1970s added to the picture.
Many variants of inflation theory emerged, some with exotic
predictions such as multiple universes.
For its first 380,000 years, the universe was a hot, seething soup of particles impenetrable to light. As it cooled, several things happened. The particles coalesced to form atoms, the soup cleared, and for the first time light was able to travel out through the universe. That ancient light is still echoing around the cosmos 13.8 billion years later, stretched and cooled a thousand-fold by the universe’s expansion. Today it is a faint glow that pervades the entire sky, known as the cosmic microwave background (CMB). One characteristic of that ancient afterglow is that wherever you look in the cosmos its intensity appears to be the same. And according to the Big Bang theorists' calculations that ought not to be the case.
Guth’s solution was to propose that during the Big Bang, the universe went through a faster-than-light expansion he dubbed “inflation”. He calculated it began a trillionth of a trillionth of a trillionth (10-36) of a second after the birth of the universe when it momentarily fell into a weird state dominated by a repulsive form of gravity which, instead of pulling things together, pushed everything apart exponentially.
“At some point … the repulsive gravity turns off but the region continues to expand in a coasting pattern for billions of years to come,” Guth explained.
Guth knew his inflation model had shortcomings so he and other theoretical physicists of the day set about trying to refine it. Many variants of the theory emerged, some with exotic predictions such as multiple universes. One of the most robust models was by Andrei Linde, then at the Lebedev Physical Institute in Moscow. It predicted that the inflation period might have left a fingerprint in the CMB. In the early 1990s, the Cosmic Background Explorer Satellite (COBE) satellite found hints of this fingerprint in temperature fluctuations in the CMB.
Now Kovac and his team seem to have found the definitive evidence using a telescope at the Amundsen-Scott polar base with the acronym BICEP2, for Background Imaging of Cosmic Extragalactic Polarisation.
BICEP2 detected the fingerprint of gravitational waves in the pattern of polarised CMB light. Similar to a wave travelling along a rope, light waves can vibrate in any plane, left to right, up and down or any angle in between. But polarised light vibrates only in a single plane. The astronomers found that the CMB is polarised, but not uniformly. It shows a swirling pattern referred to as “B mode” exactly as inflation theory predicted. “They’re gravitational waves that we are seeing,” says James Bock, from NASA’s Jet Propulsion Laboratories, one of the team’s leaders.
The finding is not just a vindication of Guth’s and Linde’s theory of inflation. Gravitational waves were first predicted by Einstein as part of his general theory of relativity. But because gravity is weak compared with the other forces, only stupendously energetic events such as inflation could reveal their existence.
Thanks to the muscular efforts of BICEP2, the last element of Einstein’s theory of relativity has now been vindicated. The historic result comes after 13 years’ work for the team. A succession of custom-designed telescopes have hunted for B modes: BICEP1, which operated from 2006 to 2008, BICEP2, which ran from 2010 to 2012 and the current Keck Array of five telescopes. All were located in Antarctica to take advantage of the dry, thin atmosphere, which gives a clear view of the sky.
When the signal was first detected by BICEP2, the team thought it was just noise in the data, says Bock. But then the newer, more sensitive Keck Array came online, and the same signal could be seen.
“The Keck Array data agreed exactly,” says Bock. “At that point I was starting to think ‘Oh my God, this signal could be real!’.”
'The ability to measure primordial gravitation waves provides an amazing window back to the first moments of the universe.'
Although the pattern seemed to match, the signal seemed almost too strong to be true. So the astronomers drew up a list of possible sources of false signals and worked their way through the list, discounting them one by one, said Bock. “For the last three months, we compared the data from the three experiments, and they agreed, so we were pretty confident.”
The ability to measure primordial gravitation waves provides an amazing window back to the first moments of the universe, says Bock. “The fact that we have seen this signal is going to usher in a new era that we can mine for all it’s worth by measuring the fluctuations on the whole sky. So, that’ll be the work for the next decade.”
Further observations will be needed, but physics department tearooms are buzzing.
So far physics has been able to find a single theory to explain the three forces that reign over small scales: electromagnetism, that holds atoms together, and the two nuclear mechanisms, the so-called strong and weak forces. The unification of the fourth force, gravity, with the forces of the small world has remained elusive. If these gravitational waves, measurable at the cosmic scale in the CMB, are born of the quantum world they offer the hope of reconciling these forces, leading to the long sought “Theory of Everything".
"That would be a big deal,” says Katie Mack, a theoretical astrophysicist at University of Melbourne.