This landmark result contradicts the now-infamous BICEP2 announcement of the discovery of gravitational waves last March — but this is not the end of gravitational waves, nor the theories behind inflation. In fact, according to cosmologists, we can expect the search to intensify over the coming months and years.
After the details behind the Planck observations were revealed this week, Discovery News was able to speak with cosmologist Kendrick Smith, of the Perimeter Institute for Theoretical Physics in Ontario, Canada, to find out what impact these Planck data will have on our quest to understand what happened when the universe was born.
To recap, in March 2014, researchers of the BICEP2 telescope made a very public announcement that they had discovered the fingerprint of gravitational waves in the most ancient radiation observed in the distant universe — the cosmic microwave background, or simply, the CMB. This radiation is the remnants of the Big Bang and therefore originates from the genesis of our universe.
By studying the CMB, cosmologists are looking into a cosmic time capsule of sorts — the features etched into this radiation were created moments after the Big Bang, so their structure can reveal the conditions (and therefore the physics) of our universe back in the beginning of time.
How the universe began is “one of the biggest open questions cosmologists are trying to answer,” said Smith. “There are several different theories on what happened shortly after the Big Bang … the problem isn’t that we don’t have a successful theory, it’s that we have too many successful theories! We’re trying to narrow down the possibilities.” Although Smith isn’t directly involved in this week’s joint BICEP2/Planck publication, he is a member of the international Planck Collaboration.
One theory is that the universe underwent a rapid expansion immediately after the Big Bang and one possible way to detect whether that inflationary period occurred is to look for gravitational waves etched into the CMB.
The BICEP2 telescope, based near the South Pole, is designed to specifically seek out a type of polarization in the ancient CMB radiation called “B-mode polarization.” Should B-modes be detected, it’s a sign that gravitational waves are present, proving certain inflationary universe theories.
“By searching for these gravitational waves in the cosmic microwave background, we can narrow down the physics of the Big Bang,” said Smith.
And that’s what the BICEP2 team thought they’d found in their observations of a small patch of sky. There appeared to be a very strong B-mode signal that couldn’t (at the time) be explained by any other phenomena.
“In the original BICEP2 result, they saw B-mode polarization in the CMB at a level that roughly corresponded to the largest gravitational wave signal that would be consistent with our observations so far,” Smith added.
But just because the signal looked like evidence for gravitational waves, the BICEP2 researchers had underestimated the impact of the magnetized dust that fills our galaxy.
When observing any radiation from beyond the Milky Way, we have to stare through a thin fog of interstellar dust and it just so happens that this dust can also generate B-mode polarized radiation. To compensate for this interference, the European space-based Planck telescope, which is sensitive to frequencies generated by the CMB and galactic dust, was tasked to map the magnetic fingerprint of galactic dust. Planck’s mission was completed in 2013, but its huge database is still being processed and interpreted.
Last March, the BICEP2 team only had access to a preliminary Planck dataset and concluded that, in the BICEP2 field of view, the impact of galactic dust was minimal and the B-mode signal they’d detected originated in the CMB.
“They thought it had to be cosmological gravitational waves rather than dust based on a number of statistical analyses, which were mainly driven by very approximate measurements of dust emission in our galaxy,” said Smith.
Although no one in the cosmological community disputed the fact that BICEP2 had detected B-mode polarization, they argued that too little was known about the galactic dust and that the signal was just as likely gravitational waves as it was dusty interference, urging caution against concluding that it had to be gravitational waves. And this week’s paper, a collaborative effort between Planck and BICEP2 scientists, has shown that there are no detectable traces of gravitational waves in the BICEP2 data.
“What this recent joint Planck/BICEP paper did is take Planck measurements, that are at a different frequency to BICEP2, combine them with the BICEP measurements, so that now with multiple observing frequencies, one can make a clear statistical separation between the cosmological gravitational wave signal and the dust signal,” said Smith.
In other words, the B-mode polarization that BICEP2 originally detected was caused by dust and not gravitational waves.
“That’s interesting to comment on as often data analysis is very subtle; a paper may have multiple interpretations or loopholes. But this paper is not one of them,” he said. “The conclusion is very clear: when you combine the observing frequencies of BICEP and Planck, all of the B-mode (polarization) in the sky can be accounted for by dust and there’s no evidence that any of it is gravitational waves.”
So what now? Although the March announcement may have been premature, the search for gravitational waves continues. Cosmologists are now armed with a comprehensive map of the obscuring dust in our galaxy. Smith hopes that, over the next 5 years, extensive multifrequency observations may start to root out the illusive B-mode polarization generated by gravitational waves. But they may not.
“There are working models for how the Big Bang might’ve worked that produce large levels of gravitational waves and there are working models for how the Big Bang might’ve worked that produced gravitational waves at such tiny levels that they’ll never be measured. Either one is a possibility, so it’s hard to speculate.”
As for the massive interest that surrounded the gravitational wave drama that unfolded in a very public arena, Smith isn’t surprised that this particular cosmological study has garnered such public excitement.
Read more at Discovery News
No comments:
Post a Comment