Astronomers from the BICEP collaboration announced on 17 March that, using a modest-sized telescope near the South Pole, they had detected gravity waves that have been rippling through the cosmos since the Big Bang. This is extraordinary news for our understanding of gravity generally, and for our understanding of how the universe probably evolved during the earliest moments of its history.
According to Einstein’s general theory of relativity, the phenomena of gravity – apples falling from trees, the Moon falling in its orbit – arise because spacetime is curved: space and time bend or distend in the presence of matter and energy. Predictions from the theory have passed every test in a century, accounting for deformations in planetary orbits, bending paths of starlight and much more.
Einstein’s equations imply another type of phenomenon: not only can spacetime warp like a trampoline, it can wiggle as well. Theoretically, at least, gravity waves – produced by such phenomena as collisions between massive stars – can skitter through the fabric of spacetime. Intense efforts to detect gravitational waves have been underway for decades. Now, at last, they appear to have been spotted.
The latest news is particularly exciting for the theory of cosmic inflation. My friend and mentor Alan Guth at MIT introduced the theory more than thirty years ago, and colleagues including Andrei Linde (now at Stanford) quickly found ways to improve the idea. As their work demonstrated, simple forms of matter – very similar to the recently discovered Higgs boson – could have dominated the matter-energy balance of the universe during its earliest moments. Fractions of a second after the Big Bang, the energy trapped in the Higgs-like matter would have driven a rapid, rip-roaring expansion of the universe. In a cosmic blink, space would have stretched by at least a billion billion billion times – a difference in size between a single atom and the Milky Way galaxy.
Models of inflation generically predict several features. Across the largest observable distances, the universe today should be flat, obeying the strict laws of geometry that Euclid formalised, rather than curving in more exotic shapes that are also consistent with Einstein’s relativity. There should be tiny lumps in the primordial distribution of matter through the cosmos: the quantum jitters of Heisenberg’s uncertainty principle, stretched to astrophysical distances by the inflationary expansion. The models also predict that the explosive expansion of space should have generated and amplified gravity waves.
Both the flatness of the universe and the presence of those primordial lumps have been confirmed to high precision by a series of observations, most recently using the European Space Agency’s Planck satellite. Crucial to both of those tests, and to the recent detection of gravity waves, has been a remarkable remnant of the Big Bang known as the cosmic microwave background radiation. First detected fifty years ago, the CMB was produced at a specific moment in time, about 380,000 years after the Big Bang.
The CMB arose from physical processes that are remarkably well understood. Photons (light particles) bounce off electrically charged particles such as electrons and protons. Electrons and protons, meanwhile, attract each other because they have opposite electric charges. Ordinarily, this attraction can bring the particles close enough together to form stable, electrically neutral atoms: in a hydrogen atom, for example, the positive charge of the single proton cancels the negative charge of the single electron. Photons ricochet off charged particles, but pass through neutral hydrogen gas relatively unscathed. A universe filled with neutral matter is transparent to light.
Soon after the Big Bang, however, the universe was so hot that individual particles carried enormous energy. Electrons and protons were unable to form hydrogen atoms because energetic photons blasted them apart. The early, hot universe was filled with a plasma of electric charges, rather than a gas of neutral atoms. Any given photon would have been able to travel only a very short distance before being absorbed or scattered. At its earliest moments, in other words, the universe was opaque rather than transparent.
As the universe expanded and cooled (well after the era of cosmic inflation), the average energy per particle fell. Eventually, individual photons no longer packed the wallop to disrupt electrons and protons from pairing off into hydrogen atoms. In terms of cosmic history, this process of ‘recombination’ (a misnomer, since the particles had never been combined before) happened very quickly. But it was not instantaneous.
I like to think of recombination as something like a school dance. Early on, the DJ plays fast, raucous music, and everyone jostles and dances about in random-looking groups. Later, when the DJ puts on a slow song or two, many of the kids will pair off – but not all at once.
After the universe had been expanding and cooling for roughly 380,000 years, most photons could no longer interrupt the process of recombination. But there were enormous numbers of photons filling the universe, and not all carried the same energy. As with any statistical distribution, some photons were considerably more energetic than the average.
There was thus a brief period of time when most charged particles had paired off into neutral hydrogen atoms and, for the first time in the history of the universe, light could travel substantial distances. Yet there were still a modest number of free electrons and protons around, like the shy kids who hadn’t yet found anyone to dance with.
Twenty years ago, several physicists realised that in the short timespan of recombination, some of the newly freed photons would have scattered off the still-free electrons and protons. Those scatterings would polarise the radiation: the electric fields associated with the lightwaves would oscillate back and forth preferentially in one direction of space rather than another.
But what if, during that brief window of not-yet-complete recombination, powerful gravity waves zoomed through space? Gravity waves produce a characteristic stretching and squeezing of spacetime, which would result in a slight swirling or twisting pattern in the polarisation of the scattered radiation. Cosmic inflation is the only process that has ever been shown to predict gravity waves consistent with the type of polarisation detected in the CMB by the BICEP team.
The team announced their results at Harvard. Alan Guth kindly added my name to the short list of people invited to the briefing and press conference. Like several others, I sat there agape, marvelling at the care and precision with which the BICEP team had collected, analysed and error-checked their data over the years.
But the chills I felt that morning stemmed even more from a sense of history. Subtle features of Einstein’s century-old theory of general relativity might at long last have been glimpsed in the real world. The CMB, that beautiful palimpsest of cosmic history that astronomers have scrutinised for fifty years, seems to have yielded yet another clue to our cosmic origins. And the theory of cosmic inflation – an amazing set of ideas I first encountered in popular books as a child, on which I wrote both my undergraduate and PhD theses, and in which I have been conducting research for years – might well have taken a major leap from blackboard to reality.