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In March 2014, at Harvard, physicists announced they'd made one of the most significant scientific discoveries of the 21st century.
Using BICEP2, a telescope located near the South Pole, they'd found evidence of gravitational waves — variations in the strength of gravity throughout space that would serve as crucial evidence for part of the Big Bang theory. Physicists everywhere rejoiced, and many predicted that the work would lead to a Nobel Prize.
Three months later, a lot has changed. Many scientists have questioned the discovery — and last week, when the data was finally published in a peer-reviewed journal, the physicists behind it subtly qualified their claim, saying it was impossible to rule out the chance of an error.
It's still uncertain whether the discovery is right or wrong. Future data from other telescopes — including data from an orbiting space telescope to be released in October — will hopefully help clarify the issue.
So what exactly was the discovery — and how has it been thrown into doubt so quickly? Here's a basic primer.
What the physicists originally found
The Big Bang theory is a model that describes the beginning of the universe. Essentially, it states that about 13.82 billion years ago, all matter that we see around us was contained in a single, extremely dense point. Suddenly, it began expanding faster than the speed of light, eventually growing into the universe we know today.
But according to the theory, this expansion didn't happen at a uniform rate: a tiny fraction of a second after the Big Bang began, the universe expanded at an exponentially fast rate — growing from smaller than an atom to roughly the size of a golf ball in a particularly pivotal instant — then slowed gradually. This part of the theory is called cosmic inflation.
A timeline of the Big Bang, with cosmic inflation on the far left. NASA
Inflation, calculations predict, should have led to something called gravitational waves. Basically, the sheer strength of gravity would have varied slightly from one part of the early microscopic universe to another — and as the universe expanded, these variations would have been stretched out, producing fluctuations in gravity on a much larger scale. These gravitational waves are often referred to as ripples in the fabric of spacetime.
Until recently, we had physical evidence for the Big Bang theory in general, but not cosmic inflation: the main reason we had to believe in it was a series of theoretical calculations, originally made in the late 1970s.
But scientists have long been searching for physical evidence, and in March, a four person team — made up of John Kovac, Clement Pryke, Jamie Bock and Chao-Lin Kuo — announced they'd found it, in the form of indirect evidence for gravitational waves.
How they made this discovery
The BICEP2 telescope. Steffen Richter, Harvard University
Using the BICEP2 telescope in Antarctica (where cold, dry air limits interference from the Earth's atmosphere), the scientists looked at a faint form of light called the Cosmic Microwave Background, which was emitted shortly after the Big Bang and is still permeating through the universe.
They found a distinct twisting pattern in the light (formally called B-mode polarization). This type of polarization could be caused by the light crossing through gravitational waves as it travelled through the early universe billions of years ago — so the discovery, if accurate, would have confirmed the theory of cosmic inflation.
Why the discovery may have been wrong
A graphic showing the twisting B-mode polarization detected in the light. BICEP2 Collaboration
The physicists publicly announced their discovery and released their data on March 17, noting that they'd spent years analyzing it to eliminate the chance of an error.
Normally, scientific discoveries are publicly announced after they've been peer-reviewed — a process in which other scientists in the field critically analyze the work, looking for any weaknesses present. But in this case, the discovery was announced prior to peer review, causing this sort of normal skeptical analysis to occur in the public eye.
And, over the past couple of months, several independent scientists have suggested the detection of this form of polarization may have been an error. Initially, the researchers responded confidently, but their stance has now subtly changed.
The peer-reviewed version of their work, published in journal Physical Review Letters last week, notes that they can't "exclude the possibility of dust emission bright enough to explain the entire excess signal."
In other words, dust may be to blame. The basic problem is that in looking for the polarization, the researchers are analyzing an extremely faint form of light in a tiny slice of the sky, so any kind of interference can throw off the results.
In this case, the light may have been polarized by dust scattered throughout our galaxy just before it reached us. As Joel Achenbach of the Washington Post put it, "rather than seeing the aftershock of the birth of the universe, the scientists may have seen only some schmutz in the foreground, as if they needed to clean their eyeglasses."
The physicists tried to take this dust into account to eliminate its effects from their analysis, but they did so with an unpublished map of the Milky Way's dust, and may have misinterpreted what exactly it showed. That could have misled them into thinking less dust was present than there actually is — so the polarization they found, rather than being caused by gravitational waves, may have simply been due to extra dust in the galaxy.
What happens next
The Planck satellite, currently in Earth's orbit, which has collected data on the density of dust in the sky. ESA
A number of different instruments are currently collecting data that might resolve this dispute. It could happen as soon as October, when data from the European Space Agency's orbiting Planck satellite will be released.
This satellite is analyzing the faint cosmic microwave background, and among other things, it should give us a more complete map of how much dust is scattered throughout the sky. Its resolution might not be fine enough to say exactly how much dust is in the tiny slice of sky the BICEP2 telescope looked at, but if it indicates there is much more dust throughout the sky than the researchers estimated, it'd make their conclusion look less likely.
If the Planck data doesn't rule out the BICEP2 findings, subsequent data from several other telescopes (including the adjacent South Pole Telescope and the POLARBEAR experiment in Chile) could confirm or refute the gravitational wave discovery over the next few years.
But one thing to note is that even if this finding does turn out to be an error, caused by galactic dust, it doesn't rule out the cosmic inflation theory — any more than digging into the ground and not finding any dinosaur fossils would mean that dinosaurs never existed. It's entirely possible, for instance, that cosmic inflation occurred but the gravitational waves it generated are too small for us to detect. But the vast majority of theoretical physicists believe in cosmic inflation, based on calculations — the main thing that's uncertain right now is whether we have physical evidence of the theory yet or not.
Further reading:
- Joel Achenbach's rundown of the BICEP2 debate
- New Scientist's detailed coverage of the potential error
- My coverage of the original discovery
