Gravitational wave discovery still clouded by galactic dust
Monday 5 January 2015
- Analysis for The Conversation by Alan Duffy, Swinburne University of Technology
One of this century’s greatest potential discoveries concerning the origins of the universe has now fallen to galactic dust. That’s according to a new joint-analysis of all the existing data – including by the team that made the original claim.
The new results have been provisionally accepted by the peer review journal, Physical Review Letters, but are already generating interest among scientists.
In March 2014 scientists from the Harvard-Smithsonian Centre for Astrophysics – the BICEP2 team using a specialised radio telescope based near the South Pole – rocked the science world with an amazing announcement.
They claimed to have found swirls in the polarised light coming from the Cosmic Microwave Background, which formed about 300,000 years after the universe began.
The spectacular claim was that these swirls were the imprint of gravitational waves created from an even earlier age of the universe, which took place a trillionth of a trillionth of a trillionth of second after the Big Bang.
The gravitational waves are thought to have arisen from quantum mechanical effects on scales much smaller than an atom, which were then increased in size until they stretched across the universe by an early period of expansion called inflation.
But rather than waiting for the findings to undergo the usual peer review process, giving independent experts outside the group the chance to check their results, the BICEP2 team announced their findings at a televised press conference.
This was an unusual move for such a major finding, even if they thought they were right, and potentially a major embarrassment if they were wrong.
Dust clouds the issue
Almost immediately after the BICEP2 announcement, scientists in the field started to question the findings.
One of the biggest concerns was that the swirls of polarised light (also called B-modes) in the cosmic microwave background could also be from dust shining in our own galaxy twisted along magnetic fields, clouding the original result, as it were.
Unsettlingly, the team had used a digitised image from a preliminary talk by another research team, the European Space Agency’s Planck survey, to estimate the all important potential for dust contamination in their maps.
This was dropped in the final peer-reviewed version of the paper. Other estimates of the dust contamination also seem to have been too low, with our galaxy now appearing to be dustier than first thought.
To settle the debate, in a good example of the scientific method, both teams – BICEP2 and Planck – worked together to perform a combined analysis to best use the strengths of both facilities.
Greater than the sum of their parts
The BICEP2 data (combined with its successor, the Keck Array) is an exquisitely sensitive observation over a 1% patch of the sky at one frequency (colour) of light, effectively giving a black and white image.
The Planck telescope observed the entire sky with lower sensitivity but at many frequencies of light, akin to creating a colour photograph that was much better at determining the dust outline of our galaxy.
The joint analysis now reveals that all of the previously claimed signal of swirls of polarised light can be entirely attributed to dust within our galaxy.
Back to the beginning
When the original press announcement was made last March it rightly made a huge impact on the astronomical community. It appeared to offer a window to the early universe at energies far in excess of anything that could be accessed by Earth-based experiments such as the Large Hadron Collider.
If the dust contamination was small, as was assumed last year, that would mean that the imprints in the radiation measured by BICEP2 were signatures of gravitational waves produced in the energetic event of inflation.
There was a lot of excitement among scientists because inflation provides an explanation for many observational features of our universe that would otherwise be very puzzling.
Currently we do not have any direct confirmation that inflation occurred. Thus, detection of gravitational waves in the early universe would be strong evidence that inflation is correct.
As a result of this new combined analysis by BICEP and Planck, we now know that if gravitational waves are there, then they can’t be too strong. Or even might not be there at all.
In science a null result can often be as important as a positive detection.
This alone will begin to put pressure on certain models of how the universe might have behaved early in its history.
More importantly, the new combined data sets the gold standard in quality of data and careful analysis for all future telescopes and teams.
The lesson learned?
Questions still remain as to whether the BICEP2 team was premature in making its original announcement.
But peer review of any results pending publication in a journal can be a slow process, and usually involves the research being reviewed by just a few experts in the relevant field.
It could be argued that by widely publishing the result in a press-conference, the BICEP2 team ensured many scientists around the world would check their results, not just the experts selected by a journal.
What was so unusual about this saga was just how visible the scientific method was to the general public, with plenty of coverage in the media.
Usually the highly competitive world of science, with diligent (perhaps even belligerent) testing of the claims of others, are hidden from sight.
We hope that scientists arguing publicly has allowed the public to see what goes into making a final published statement, rather than increase the scepticism with which science is often perceived by the public.
Looking to the future
The latest results show that the dust is more a problem than was previously thought. Without understanding the properties of our galactic environment we cannot advance our knowledge of what happened in the earliest moments of the universe.
Thanks to Planck, other telescopes can try to avoid the most dust-contaminated areas of sky to hunt for the elusive signal of gravitational waves.
Also, new telescopes – including the successor to BICEP2/Keck Array, called BICEP3 – will be more powerful and able to look at different frequencies of light than BICEP2. Others are employing different strategies to scan larger areas of sky but less intensely, like the South Pole Telescope or POLARBEAR, to maximise any gravitational wave signal.
One thing’s for sure: the excitement over the potential discovery will only have encouraged astronomers in their efforts to uncover the illusive gravitational waves and a window into our universe’s earliest moments.