Earlier this year, a stunning announcement about the birth of the universe was made: scientists had observed gravity waves that seemed to prove a key part of the Big Bang theory.
However, it has now become clear that it will take at least until October before scientists have a chance to amass enough evidence to prove out their extraordinary theory. Only then will we find out if that cosmic bombshell truly is worthy of a clutch of Nobel prizes, as global headlines claimed a few months ago, or if it was a damp squib. Whatever happens, however, the saga has revealed a profound challenge facing theorists attempting to chart the origin, evolution and future of the universe.
The story dates back to March, when Harvard University astrophysicists announced that a telescope near the South Pole, known as the Background Imaging of Cosmic Extragalactic Polarization 2 (BICEP2), had observed a subtle twisting pattern in a map of the polarization of the faint afterglow of the Big Bang of creation, known as the cosmic microwave background. If light is polarized it means a majority of its component electromagnetic waves have an electric field that is oscillating along a certain direction as they travel towards the Earth. While mapping the polarization direction over a patch of sky, the scientists looked for a special twist pattern in the ancient light called “B-modes” — and then found it. John Kovac, Jamie Bock, Clem Pryke, Chao-Lin Kuo and colleagues quickly announced the swirly patterns during a news conference at the Harvard-Smithsonian Centre for Astrophysics in Cambridge, Massachusetts, and then presented the discovery in a draft paper online.
The swirls immediately caused a sensation. They were hailed as being bigger than the discovery of the Higgs particle, and were said to herald a new era in physics. As Max Tegmark of the Massachusetts Institute of Technology, remarked at the time, “if it holds up, it will go down as one of the greatest discoveries in the history of science.”
One reason for the global headlines was that the twisting pattern seen by the BICEP2 team marked the first time that astrophysicists had spotted direct evidence of gravitational waves. First predicted by Albert Einstein, who spoke about gravitational fields that propagate with the speed of light in 1916, only indirect evidence had previously emerged (from the slowing down of the rotation of binary pulsars, superdense neutron stars). To generate gravitational waves requires a cataclysmic event, even by the cosmic standards of the universe. For example, they can be made in the collision between black holes but, even so, the wavelengths in this case would be tiny compared with those seen by BICEP2.
They relatively massive size of the waves seen in 2014 gave support to an idea known as inflation. At its most basic, the theory of inflation says that at its birth some 13.8 billion years ago our universe was once smaller than an atom, then grew so mind-bogglingly rapidly in size that it would produce and stretch out gravitational waves of the kind that BICEP2 had observed. In other words, only primordial gravitational waves can imprint this pattern, and only then if they have been amplified by inflation.
But there was a problem, one that has been raised repeatedly since the original announcement: the signals detected by BICEP2 are masked by distortions caused by the light’s passage through clusters of galaxies, and through dust clouds in the Milky Way. Carlos Frenk, Professor of Fundamental Physics at Durham University, UK, says he has been “suspicious from day one because the signal seemed implausibly large.”
The BICEP2 team attempted to account for the role of distortion-causing dust in their results by using data gathered by the European Space Agency (ESA), which the latter had been using to normalize the findings of their Planck telescope. The Planck telescope has been mapping tiny differences in microwave radiation leftover from the Big Bang at its spot on a “LaGrange point,” a gravitational stable position about 1 million miles from Earth.
But in recent weeks others have claimed that the Harvard scientists missed a key point: the ESA had averaged the effects of dust in the Milky Way along with dust far beyond our own galaxy, leading them to underestimate how much this cosmic lint naturally twists light. Another problem is that synchrotron radiation, a glow generated by electrons moving around galactic magnetic fields within our own galaxy, can also produce these polarized twists. “The BICEP2 team claimed to have done the foreground subtraction very carefully but perhaps they were not careful enough,” says Frenk.
A reanalysis by a team of researchers from Princeton University and the Institute for Advanced Study concluded that the seductive swirls could be the result mostly or entirely of foreground effects, without any contribution from gravitational waves. Alan Guth, the cosmologist who first proposed the concept of cosmic inflation in 1980, had thought the initial BICEP2 results were secure but, in the light of the kerfuffle, now says “the situation has changed.” Paul Steinhardt of Princeton University, who did early work on inflation, went further, remarking that “serious flaws in the analysis have been revealed that transform the sure detection into no detection. The search for gravitational waves must begin anew.”
There are many other research projects able to spot ancient gravitational waves among noisy signals, such as the South Pole Telescope, POLARBEAR experiment in the Atacama Desert of northern Chile, and the Keck Array, a suite of telescopes at the South Pole. Observations from any of these projects could help clear up the controversy.
The Planck satellite team is gearing up to release their more complete analysis of dust in the region probed by BICEP2 and later this year will present their own analysis of B-mode polarization. If their team corroborates the Harvard findings, then the blockbuster discovery will stand. “I am confident that these Planck data will resolve the issue conclusively,” says Frenk. “Until then, one needs to be cautious and skeptical.”
Even if this matter is settled, a deeper issue remains — one that cuts to the core of the meaning of the universe.
What is normally thought of as the universe — all the stuff that can be observed with telescopes – is just one pocket, where inflation happened to grind to a halt. That allowed matter to condense, galaxies and stars to form, and humans and other life to evolve.
Elsewhere, beyond what we can see, space and time may still be inflating, so that, according to some models of inflation, other “bubble” universes are popping into being with completely different properties. There are an infinite number of bubbles, in which the cosmic and physical properties vary locally from bubble to bubble and where everything that can physically happen does indeed happen, and an infinite number of times.
Paul Steinhardt, a proponent of the cyclic universe, wrote an opinion piece recently in Nature, in which he argues that “the inflationary paradigm is so flexible that it is immune to experimental and observational tests.” In fact, some proponents of inflation insist that the theory remains equally valid, whether or not gravitational waves are detected. In other words, the theory lies beyond the reach of evidence, no matter how extensive and extraordinary.
By potentially backing the radical idea of a multiverse, BICEP2’s results could mark the death knell for alternative cosmic visions, cyclic “big bounce” models of the universe where time has no beginning or end and the cosmos endlessly dies and rises from its ashes.