Nope, We Have Not Detected Gravitational Waves (Yet)

When it comes to big bang ripples, all we have is dust in the wind. In March of last year, a team of astronomers working with the BICEP2 telescope at the South Pole caused a flurry of excitement when they claimed to have discovered evidence for primordial gravitational waves, ripples in space-time triggered by a growth spurt in the universe’s early days. However, a leaked press release has teased the results from a long-awaited joint analysis between BICEP2 and a European space telescope team, the Planck collaboration. As many had feared, the release says that the signal was caused by something much more mundane: dust.

(Update: ESA has now posted a news release confirming that the joint analysis has found no conclusive evidence for gravitational waves.)

Gravitational waves are thought to have been produced when the universe went through an incredibly rapid period of inflation in the fractions of a second after the big bang. Discovering them, and thus proving inflation to be true, is central to many of our theories about the early universe. Some cosmologists even argue that finding the primordial waves would be indirect evidence that parallel universes exist.

Using powerful telescopes like BICEP2 and Planck, astronomers have been hunting for signs of these waves in the cosmic microwave background (CMB), ancient light that was emitted just 380,000 years after the big bang and that now permeates the cosmos. Theory says that the waves would have created a distinct swirly pattern in the CMB known as B-mode polarization.

This is what BICEP2 reportedly discovered last year. Their analysis, based on three years of observing a single patch of sky, showed a B-mode pattern that was even stronger than expected—almost double the strength it should be based on preliminary studies carried out by Planck in 2013. However, this polarization signal can be caused by other phenomena, such as charged particles moving around in our galaxy’s magnetic field and, most notably, emissions from intergalactic dust. The BICEP2 researchers did correct for possible contamination from other sources, but it was unclear if the values used were accurate.

“A number of papers have been written over the last year taking a closer look at the data and trying alternative methods of doing the analysis,” says Phil Bull of the University of Oslo, Norway. “Many of these suggested that polarized dust emission from our own galaxy could be significantly more important than the BICEP2 team originally thought.”

A cross-correlation of data from Planck, BICEP2 and the Keck Array has been eagerly anticipated by astronomers for months. BICEP2 could only study a small part of the sky in a small wavelength range. Planck was able to look at more of the sky in other parts of the spectrum known to be dominated by dust emission, enabling the collaborations to combine forces to identify and isolate the dust within the signal.

Now comes the killer blow for BICEP2. According to the leaked release, which has since been taken offline, the new analysis of polarized dust emission within our galaxy by Planck, BICEP2 and Keck confirms that BICEP2 “significantly underestimated” the amount of dust contributing to their data.

“To be blunt, the BICEP2 measurement is a null result for primordial gravitational waves,” writes Peter Coles of the University of Sussex, UK, in a blog post today. “It’s by no means a proof that there are no gravitational waves at all, but it isn’t a detection.”

The data now show that the BICEP2 signal is only very slightly larger than the contribution from intergalactic dust itself. Once the polarized emissions from dust have been subtracted from the B-mode signal, the remainder is too small to be considered a detection, the Planck team says in the release. The document appeared on an official Planck website in French, but according to a translation, the team says the gravitational wave signal is at most half as strong as previously estimated. A full paper on the results of the joint analysis has been submitted to the journal Physical Review Letters, and a preprint is now online.

“The sad thing is that the more data you add in, the more the gravitational wave signal seems to fade,” says Andrew Pontzen of University College London, UK. “But it’s possible they’re homing in on a signal, just at a lower intensity than originally thought. This search is far from over.”

Nicola Jenner | | READ MORE

Nicola Jenner is a freelance science writer based in London.