March 18, 2014 | 25
The shock waves are still reverberating from BICEP2’s bombshell announcement that they’ve discovered the holy grail of cosmology: the telltale signature of gravitational waves from inflation. But what does this discovery really mean, and what impact will it have on cosmology?
About 13.8 billion years ago, merely 400,000 years after our Big Bang, everything in our observable universe was a hot plasma not too different from the surface of the Sun. Photos of this plasma, baby pictures of our universe around its 400,000th birthday, have already revolutionized modern cosmology and triggered two Nobel prizes. Now a team of astronomers has spent three years zooming in on about 1 percent of the sky from a state-of-the-art telescope at the South Pole, taking an even sharper photo of this plasma, including its polarization, and discovered that it’s distorted in a tantalizing way.
At the press conference, I met Alan Guth and Andrei Linde, whose theory of cosmological inflation had predicted this distortion, looking even happier than in the left-hand photo above. If they instead looked distorted as in the right-hand photo, you might wonder whether someone had slipped LSD into your morning coffee. Or whether gravitational waves – distortions in the very fabric of spacetime – were passing between you and them, bending the light rays that you see. BICEP2 has shown that humongous gravitational waves close to a billion light-years long are distorting their cosmic baby picture.
Making such strong gravitational waves requires extreme violence. For example, a cataclysmic collision of two black holes squeezing more than the Sun’s mass into a volume smaller than a city can create gravitational waves that the US-based LIGO experiment hopes to detect – but these waves are only about as big as the pair of objects creating them. So what could possibly have created the vast waves BICEP2 saw, given that our universe seems to contain no objects large enough to make them?
The answer to this question explains why Alan and Andrei were smiling: inflation! Their inflation theory in its simplest form predicts that our universe was once smaller than an atom, repeatedly doubling its size every 0.00000000000000000000000000000000000001 seconds or so, and this rapid doubling was precisely violent enough to create gravitational waves of the strength and length that BICEP2 has observed! Although it sounds like this repeated doubling of the inflating substance would violate the laws of energy physics (specifically energy conservation), it actually doesn’t, thanks to a loophole in Einstein’s theory of general relativity. I explain the physics of inflation and its aftermath in detail in chapter 5 of my new book Our Mathematical Universe in case you’re curious about how this works. When the inflating substance eventually decays into ordinary matter, the resulting hot plasma eventually cooled and clumped into the galaxies, stars and planets that adorn our universe today.
So how seriously should we take inflation? Inflation had emerged as the most successful and popular theory for what happened early on even before BICEP2, as experiments gradually confirmed one of its predictions after another: that our universe should be large, expanding and approximately homogeneous, isotropic and flat, with tiny fluctuations in the cosmic baby pictures that were roughly scale invariant, “adiabatic” and “Gaussian.” To me and many of my cosmology colleagues, the gravitational waves discovered by BICEP2 provide the smoking-gun evidence that really clinches it, because we lack any other compelling explanations for them. For example, the ekpyrotic and cyclic models of the universe that had emerged as the most popular alternatives to inflation are now suddenly ruled out because they cannot explain BICEP2’s gravitational wave detection.
This means that if the BICEP2 results hold up and we take inflation seriously, then we need to understand and take seriously also everything that inflation predicts – and these predictions form quite a long list! First of all, inflationary cosmology (“IC”) radically changes the answers to key questions given in the traditional cosmology (“TC”) textbooks I once studied:
Q: What caused our Big Bang?
TC: There’s no explanation – the equations simply assume it happened.
IC: The repeated doubling in size of an explosive subatomic speck of inflating material.
Q: Did our Big Bang happen at a single point?
IC: Almost: it began in a region of space much smaller than an atom.
Q: Where in space did our Big Bang explosion happen?
TC: It happened everywhere, at an infinite number of points, all at once, with no explanation for the synchronization.
IC: In that tiny region – but inflation stretched it out to about the size of a grapefruit growing so fast that the subsequent expansion made it larger than all the space that we see today.
Q: How could an infinite space get created in a finite time?
TC: There’s no explanation — the equations simply assume that as soon as there was any space at all, it was infinite in size.
IC: By exploiting a clever loophole in Einstein’s general relativity theory, inflation produces an infinite number of galaxies by continuing forever, and an observer in one of these galaxies will view space and time differently, perceiving space as having been infinite already when inflation ended.
Q: How big is space?
TC: There’s no prediction.
IC: Probably infinite.
Because of this last prediction, the BICEP2 discovery should cause dismay among multiverse skeptics – at least in this particular universe. This is because Alex Vilenkin, Andrei Linde, Alan Guth and others have shown that the space that inflation generically creates is not merely infinite, but uniformly filled with matter that forms infinitely many galaxies. This in turn means that no matter how unlikely it is that a galaxy will be indistinguishable from ours, containing someone whose life has so far been identical to yours, the probability is not zero since it clearly happened here. Which means that there must be duplicate copies of you far away in space, and indeed also similar versions of you living out countless variations of your life. Now it’s harder for skeptics to dismiss this by saying “inflation is just a theory” – first they need to come up with another compelling explanation for BICEP2’s gravitational waves.
I think that if the BICEP2 discovery holds up, it will go down as one of the greatest discoveries in the history of science. It has pushed our knowledge frontier back 38 orders of magnitude in time in a single giant leap, from the creation of Helium seconds after our Big Bang to inflation during the first few trillionths of a trillionth of a quadrillionth of that first second. This teaches us about physics at energies a trillion times greater than those produced in the Large Hadron Collider, of great relevance to string theory and other quests to unify the four fundamental forces into a single consistent theory.
Moreover, it’s a sensational breakthrough involving not only our cosmic origins, but also the nature of space: by producing the first-ever detection of Hawking/Unruh radiation (the process by which inflation’s rapid doubling generates these gravitational waves), the BICEP2 team has found the first experimental evidence for quantum gravity.
So what lies ahead? Once the celebrations are over, we’ll look forward to seeing whether today’s announcement stands the test of time. The wait won’t be long since many other experiments have been racing against BICEP2 and will soon have the data to confirm or refute their findings. Many questions should get cleared up already in October, when the Planck satellite experiment is due to release its first polarized images of the cosmic plasma. Planck will produce the first good maps of polarized dust and establish whether its contaminating effect was as small as BICEP2 assumed. It will also produce an improved estimate of a parameter known as “tau” related to when the first stars ionized our universe. My personal guess is that they’ll find these stars to have formed later than currently assumed, which (for complex but well-understood reasons) will lower Plank’s estimate of how clumpy our universe is and will not only help bring the Planck constraints on gravitational waves up into better agreement with BICEP2’s detection, but also bring Planck’s predictions for the number of galaxy clusters down into agreement with what we observe.
If BICEP2 is proven correct, it should lead to at least one Nobel Prize. Further down the road, I bet that a first-ever satellite designed specifically to measure inflationary gravitational waves will get funded (now that we know there’s a signal for it to measure), which can determine how the cosmic doubling rate during inflation changed with time. This provides a great way to distinguish between specific inflation models and also to test any inflation competitors that may have gained credibility by then (for example, string gas models predict an increase whereas all inflation models predict a decrease).
But first, let’s celebrate one of the most exciting moments in the history of science! Above all, this feels like a great triumph for Occam’s razor: although countless complicated models for inflation emerged over the years, the BICEP2 data is beautifully fit by simple “classic’’ inflation, known in geek-speak as a single slow-rolling scalar field. And Andrei Linde looked particularly happy at the press conference, perhaps because two numbers have now been measured that act as a sort of fingerprint of inflation models: n=0.9608±0.0054 (reported by Planck, quantifying the ratio of small to large spots in the baby pictures) and r=0.16±0.05 (reported by BICEP2, quantifying the gravitational wave amplitude after correcting for galactic radio noise). These measurements agree tantalizingly well with the specific predictions of what’s arguably the simplest model of all: Andrei’s own favorite, whose potential energy curve is a simple parabola, which predicts n=0.96 and r=0.15. I think William of Occam would have been impressed!