Astronomers Hedge on Big Bang Detection Claim

leave a comment »


A group of astronomers who announced in March that they had detected space-time disturbances — gravity waves — from the beginning of the Big Bang repeated that claim Thursday but conceded that dust from the Milky Way galaxy might have interfered with their observations.

The original announcement, apparently heralding what they said was “a new era” in cosmology, astounded and exhilarated scientists around the world. At a splashy news conference on March 17 at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., the talk quickly turned to multiple universes and Nobel Prizes.

But even as reporters and scientists were gathering there, others convened on Facebook and elsewhere to begin picking apart the findings. What ensued was a rare example of the scientific process — sharp elbows, egos and all — that has played out over the last three months.

If indeed true, the detection of those gravity waves would confirm a theory that the universe began with a violent outward anti-gravitational swoosh known as inflation — a notion that would explain the uniformity of the heavens, among other mysteries, and put physicists in touch with quantum forces that prevailed when the universe was only a trillionth of a trillionth of a second old. The idea once seemed like science fiction, but the astronomers’ findings put it almost tangibly in reach.

As everyone involved said at the time, however, the results needed to be confirmed; it was far too soon to book travel to those other universes.

Now, after weeks of public and private wrangling, discussion and debate with peer reviewers and other astrophysicists, the group, which goes by the name Bicep, has published its official paper in the journal Physical Review Letters. The authors, led by John Kovac of the Harvard-Smithsonian Center for Astrophysics, write that they stand by their original claim — but they also now acknowledge it is possible that dust in the Milky Way galaxy might have interfered with their observations, producing much or even all of their signal.

Nobody knows for sure – yet. New data expected within weeks from the European Space Agency’s Planck satellite, which spent two years scanning and mapping the sky, and other experiments now underway should help clarify the situation, they say.

“The basic takeaway has not changed; we have high confidence in our results,” Dr. Kovac said in a phone call from a conference in Moscow. But, he added, “new information from Planck makes it look like pre-Planckian predictions of dust were too low.”

Many of the Bicep team’s critics had said the astronomers might have spoken too soon, before all the uncertainties in the observations were nailed down.

“We’re not saying it’s not there,” Raphael Flauger, a theorist who splits his time between the Institute for Advanced Study in Princeton and New York University and has worked on inflation, said of the gravity wave signal. But because of uncertainty about the dust, he said in a recent paper, “no cosmological inference can be drawn at this time.”

The dissection of the Bicep report has been conducted in a storm of talks, workshops, papers posted on the Internet, jokes and whispers. Rumors have circulated that the astronomers would withdraw their claim. Some of those speaking out were early universe theorists looking for confirmation that their dreams had come true. Some were members of the dozen or so groups around the world doing competing experiments. Still others were advocates of alternative theories that would die if this one was right.

But nobody really knows. It would take a factor-of-three increase in the amount of polarized dust to account for the entire B-mode signal, the Bicep authors write. Whether that is a big increase or not depends on whom you talk to.

In an email, Avi Loeb, who as chairman of the Harvard astronomy department helped organize the March media event but is not part of the Bicep group, compared the ensuing ruckus to ancient Romans watching gladiators fight.

For the most part, the Bicep group has maintained radio silence. “It’s our job to take the heat and respond as we can,” said Jamie Bock of Caltech and the Jet Propulsion Laboratory, another of the group’s leaders, during the Caltech workshop.

Dr. Kovac said, “We’ve been trying not to seed the dialogue, but rather focus on the scientific process.”

The putative gravitational waves are the subtlest and most profoundly mysterious features discovered yet in a thin haze of microwaves, known as the cosmic background radiation, left over from end of the Big Bang, when the cosmos was some 380,000 years old.

According to the prevailing theory, the violent onset of inflation, which happened less than a trillionth of a second after time began, should have left ripples in space-time. These ripples, known as gravitational waves, would manifest as corkscrew patterns in the direction of polarization of the cosmic microwaves — “B-modes,” in the physics vernacular.

The Bicep group — its name is an acronym for Background Imaging of Cosmic Extragalactic Polarization — is led by Dr. Kovac; Dr. Bock; Clement Pryke, now of the University of Minnesota; and Chao-Lin Kuo of Stanford. They have deployed a series of ever more sensitive radio telescopes at the South Pole.

The group said in March that using their second telescope, Bicep2, which was able to peer far deeper into the cosmic background than any previous experiment, they had detected the corkscrew patterns in the sky, about twice the apparent width of a full moon.

But other things besides quantum ripples from the beginning of time could produce those swirls, including polarized light from interstellar dust grains.

Dr. Kovac and this colleagues concluded, however, that there was little or no polarized dust in their patch of sky. One piece of evidence they brandished was their analysis, which has entered the lexicon as “data scraping,” of a preliminary map of polarized dust produced by the Planck satellite and shown at a conference last year.

Dr. Flauger, however, suspected that the Bicep group had misread the Planck map, which is based on data still not published. He did his own analysis under different assumptions and came to the opposite conclusion: The dust had been underestimated and could indeed be a real problem.

That was disappointing, Dr. Flauger said.

“Just like everyone else I was excited,” he said. “If true, it was possibly biggest discovery of my lifetime. I just wanted to know what’s going on.”

After the paper was released on Thursday, Dr. Flauger said the new version was “substantially weaker than before,” noting that it has a number of cautionary comments. The statement that more data are needed to resolve the situation, he said, means, “Strictly speaking this means it was not a detection.”

Dr. Flauger said he worried that the controversy over the interpretation of the Bicep observations would overshadow what a triumph it had been to see so deep into the sky. “The fact that the Bicep team was able to make these measurements is an incredible achievement,” he said.

He reported on his reservations about the Bicep result at a conference in May in Princeton. The next day, his colleague Matias Zaldarriaga arrived at Caltech, the home nest of the Bicep group, to relay the same message to a workshop on primordial waves.

“I was in the promised land a month ago,” Dr. Zaldarriaga told the assembled physicists, “and I’m not there anymore.”

Soon after, another set of astrophysicists, Michael J. Mortonson and Uros Seljak, of the University of California, Berkeley, came to a similar conclusionbased on a different analysis. “It is thus too early to celebrate the Bicep2 results as a definitive proof of inflation,” they wrote.

The notion that such an important discovery hinged on reverse-engineering a PowerPoint slide raised eyebrows. Dr. Bock defended the data scraping by saying that if the Bicep group had not done it, people would have asked why not.

Carlo Contaldi of Imperial College London, whose work on dust figured in the Bicep astronomers’ interpretation, agreed, up to a point.

“I don’t think it was a bad idea to try this,” he said in an email, noting that at the time, the Planck dust map was the best information available. But since the data was unpublished, “you have no handle on the uncertainty in those maps,” he pointed out.

“Relying on this method to underpin a discovery claim is not something most of us would be comfortable with,” he said.

Given the uncertainty, Dr. Zaldarriaga and others have suggested that the Bicep astronomers should have been more modest in their claims, simply reporting an excess of polarization and leaving the interpretation to the future. Had they done so, Dr. Zaldarriaga said at one point during the discussions, “you would not be getting heat; you would be above ground, watching people fire away at each other.”

In their paper published Thursday, they have dropped the disputed Planck map, citing “unquantifiable uncertainty.” But it changes little, they say. The remaining theoretical models all suggest there is not enough dust to be a problem. Based on these, the chance of there being no gravitational waves at all is much less than one in a billion, they say.

Dr. Kovac said the group was in negotiations with the Planck team to collaborate on a more thorough study of the dust.

For now, he and his colleagues contend that the best answer is that gravitational waves are the source of the Bicep signal. If that is true, they write, it does indeed herald a new era of cosmology. But if it is dust, they conclude, “it reveals the scale of the challenges that lie ahead.”


Written by physicsgg

June 19, 2014 at 7:05 pm

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.

%d bloggers like this: