Tag: work (page 56 of 129)

Cosmic dust may have distorted cosmic inflation breakthrough


The 10-meter South Pole Telescope and the BICEP (Background Imaging of Cosmic Extragalactic Polarization) Telescope at Amundsen-Scott South Pole Station, which detected evidence of gravitational waves, is seen against the night sky with the Milky Way in this National Science Foundation picture taken in August 2008.

By Ben P. Stein, Inside Science

Harvard researchers rocked the science community last March with an apparent discovery of gravitational ripples that gave credence to cosmic inflation theory – a finding that met as much skepticism as enthusiasm. Now, further analysis raises more doubts.


"Extraordinary claims require extraordinary evidence." This phrase, popularized by the late Carl Sagan, kept going through my head on March 17, the day that researchers involved with BICEP2, a telescope in Antarctica, made a big announcement at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

The researchers reported that BICEP2 detected gravitational waves from the first moments after the big bang, a feat, which if confirmed, would open up a new field of study and would surely be recognized in a future Nobel Prize.

Gravitational waves are ripples in space and time. They're created when any object with mass accelerates. However, they're extremely weak, making them very hard to detect directly. Even for the most massive and cataclysmic events, such as the collision of two black holes, their effects, observed from Earth, are very hard to detect.

If you're looking for a detectable gravitational wave signal, what bigger event can there be than cosmic inflation? According to inflation theory, the universe multiplied its size by as much as 10 trillion trillion trillion times in the first fractions of a second after the big bang.  Inflation would have generated lots of gravitational waves. In turn, gravitational waves can subtly change the properties of light that they pass through. Specifically, they can slightly affect the polarization of light, the direction in which light's electric fields vibrate. The universe's rapid expansion during inflation would have amplified the waves' imprint on the early light in the universe.

The state-of-the-art BICEP2 experiment, which uses super-sensitive superconducting sensors, could detect tiny changes in polarization in the cosmic microwave background, the very first light released in the universe, which is still reaching us today. The BICEP2 researchers reported a very high polarization signal, known as B-mode polarization after its characteristics, in the cosmic microwave background, which they interpreted as a strong gravitational wave signal in the early universe.

Detecting this polarization signal was a striking result, announced in a series of scientific talks and a press conference shortly after a preprint of the paper was posted online. Notice these last two points: announced at a press conference, and a preprint posted online. A preprint is a written paper that has not been formally reviewed by independent peers or published in a scientific journal.

Nonetheless, scientists and reporters alike reported excitement over the results. If true, they would provide the greatest experimental support yet of cosmic inflation, and the first direct detection of gravitational waves. Previously, gravitational waves have been detected indirectly, such as in observations of pairs of stars falling towards each other: they were losing energy in the form of gravitational waves.

On the day of the BICEP2 announcement, and for many days afterward, people were largely accepting the results as correct and already jumping to the implications of the BICEP2 results for what appeared to be a new era of gravitational-wave cosmology.
In writing my story for Inside Science News Service, I was fortunate to get an early voice of skepticism from David Spergel, a theoretical cosmologist at Princeton University in New Jersey. He commented:

"Given the importance of this result, my starting point is to be skeptical. Most importantly, there are several independent experimental groups that will test this result in the next year."
Spergel explained that the new gravitational wave measurements did not appear to agree with those of previous experiments, known as WMAP and Planck, unless the simplest models of inflation were replaced by more complicated ones. On the first day and week of coverage, I became very disappointed with the many commentators who disregarded or underemphasized that the earlier measurements from instruments on WMAP and Planck, which had been reported and covered for years.

Sure enough, in the weeks that followed, other researchers pointed out that the signal that BICEP2 detected may have been attributable to the polarization of light caused by dust in our galaxy. The BICEP2 team certainly knew that dust could also polarize light in a similar way to gravitational waves, but they used a model, based on the data that was available from the Planck satellite, that, the other researchers pointed out, may have underestimated the amount of dust in the part of the sky they were studying.

The BICEP2 paper underwent peer review and was published in Physical Review Letters. As a result of the peer-review process, the researchers made revisions, including removing the model that contained the lower estimates of dust based on the earlier Planck data, and thereby reducing the certainty with which they could state that they accounted for signals from interstellar dust.

During the summer, the BICEP2 and Planck collaborations agreed to work together to analyze their data, to help determine if gravitational waves had really been detected.

This week, the Planck team issued a preprint, based on an analysis of much additional data, showing a comprehensive map of dust in the sky. According to their analysis, the signal in the part of sky that BICEP2 analyzed could be completely attributable to dust and not to gravitational waves.

But, the story is not over. For starters, keep in mind the new preprint, like all newly posted publications, still needs to undergo formal peer review.

And the latest data do not completely rule out the possibility that the BICEP2 group detected a gravitational wave signal. If the evidence holds up at all, it would likely be a weaker signal, after accounting for the dust. Or, the gravitational-wave signal may completely turn to dust.

It may be possible to detect primordial gravitational waves in a different, less dusty part of the sky, or with new measurements by BICEP2, Planck or the many other experiments that are looking for them.  Just as the first reported detections of exoplanets turned out to be false, perhaps this is a prelude to an actual detection of gravitational waves.

"You cannot ignore dust," he quotes from Planck scientist Charles Lawrence of NASA’s Jet Propulsion Laboratory in Pasadena, California.

The biggest lesson, to me, is that no one should rush to make announcements and pronouncements, whether big or small, even in the face of intense competition and the alluring prospects of launching a new field of study and winning a Nobel Prize. 

Scientists, and the rest of the public, should follow the time-tested scientific practice of subjecting claims to sufficient levels of scrutiny, and waiting for other groups to validate results, before making bold statements. At the very least, there have been major caveats and qualifiers in announcing new data with potentially huge implications.

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Criticism of Study Detecting Ripples From Big Bang Continues to Expand

The lab housing the Bicep2 telescope near the South Pole. Credit Steffen Richter, Harvard University
nytimes.com

Stardust got in their eyes.
In the spring a group of astronomers who go by the name of Bicep announced that they had detected ripples in the sky, gravitational waves that were the opening notes of the Big Bang. The finding was heralded as potentially the greatest discovery of the admittedly young century, but some outside astronomers said the group had underestimated the extent to which interstellar dust could have contaminated the results — a possibility that the group conceded in its official report in June.

Now a long-awaited report by astronomers using data from the European Space Agency’s Planck satellite has confirmed that criticism, concluding that there was enough dust in Bicep’s view of the sky to produce the swirly patterns without recourse to primordial gravitational waves.
“We show that even in the faintest dust-emitting regions there are no ‘clean’ windows in the sky,” the Planck collaboration, led by Jean-Loup Puget of the Astrophysical Institute in Paris, wrote in a paper submitted to the journal Astronomy & Astrophysics and posted online Monday.
As a result, cosmologists like the Bicep crew cannot ignore dust in their calculations. “However,” said Jonathan Aumont, another of the Planck authors, also from the Paris institute, “our work does not imply that they did not measure at all a cosmological signal.

Moreover, due to the very different observation techniques and signal processing in the Bicep2 and Planck experiments, we cannot say how much of the signal they measured is due to dust” and how much to gravitational waves.

So this is not the end of the story, both the Planck scientists and the Bicep group agree. But the original euphoria that the secrets of inflation and quantum gravity might be at hand has evaporated. Planck and Bicep are now collaborating on a detailed comparison of their results.

John M. Kovac of the Harvard-Smithsonian Center for Astrophysics, lead author of the Bicep paper, said the new report confirmed in greater detail the trend suggested by the first Planck papers in the spring, which indicated there is more dust even in the cleanest parts of the galaxy than anyone had thought.

Raphael Flauger of the Institute for Advanced Study in Princeton, N. J., who first raised the issue of dust in the Bicep report, said it confirmed what he had thought. “It doesn’t leave a lot of wiggle room,” he wrote in an email, “and it seems clear that at least the majority of the signal is caused by dust.”

The gravitational waves may exist, although they would be weaker than the Bicep analysis indicated, causing theorists to reshuffle their ideas. As Richard Bond, an early universe expert at the University of Toronto and a Planck team member, put it: “Planck showed that dust could possibly be the entire Bicep2 signal, but Planck alone cannot decide. We have to do this in combination with Bicep2.”

The joint comparison and Planck’s own polarization maps are due at the end of the year.

If true, Bicep’s detection of gravitational waves would confirm a theory that the universe began with a violent outward antigravitational swoosh known as inflation, the mainspring of Big Bang theorizing for the last three decades.

The disagreement over the Bicep finding will not mean the end of inflation theory; it just means it will be harder for cosmologists to find out how it worked. The Bicep group and an alphabet soup of competitors are soldiering on with new telescopes and experiments aimed at peeling away the secrets of the sky.

Michael S. Turner, a cosmologist at the University of Chicago, said: “This is going to be a long march, but the goal of probing the earliest moments of the universe makes it well worth the effort. Dust is the bane of the existence of astrophysicists — and cosmologists. It is everywhere, and yet our understanding of it is very poor.”

Others are less optimistic. Paul J. Steinhardt of Princeton University, a critic of the Bicep paper — and of inflation theory — said in an email that the Bicep paper should be retracted, “and we should return to good scientific practice.”

The Bicep observations are the deepest look yet into a thin haze of microwaves, known as cosmic background radiation, left over from end of the Big Bang, when the cosmos was about 380,000 years old.

According to theory, the onset of inflation, less than a trillionth of a second after time began, should have left ripples in space-time known as gravitational waves. They would manifest as corkscrew patterns in the direction of polarization of the cosmic microwaves.
The Bicep group — its name is an acronym for Background Imaging of Cosmic Extragalactic Polarization — is led by Dr. Kovac; Jamie Bock of Caltech; Clement Pryke of the University of Minnesota; and Chao-Lin Kuo of Stanford. They have deployed a series of radio telescopes at the South Pole in search of the swirl pattern. Their most recent, Bicep2, detected a signal in the sweet spot for some of the most popular models of inflation, leading to a splashy news conference and a summer of controversy and gossip.
As the critics pointed out, things besides quantum ripples from the beginning of time could produce those swirls, including light from interstellar dust polarized by magnetic fields in space.
Planck, launched in 2008 to survey the cosmic microwave sky, can distinguish the characteristic signature of dust by comparing the sky brightness in several radio frequencies, as well as measuring its direction of polarization. Bicep2, in contrast, looked at only one frequency, 150 gigahertz.

The Bicep astronomers asked for Planck data on their patch of sky, but it was not available until now because of suspected instrument problems, Dr. Aumont said. So they extrapolated from existing data to conclude that there was little dust interfering with their observations.

The new Planck report has knocked the pins out from under that. But there are still large uncertainties that leave room for primordial gravitational waves at some level. For example, the Planck team had to extrapolate some of its own measurements.

As the Planck report says, “This result emphasizes the need for a dedicated joint Planck-Bicep2 analysis.”

The group hopes this analysis will include data from the latest Bicep telescope, called the Keck Array, which has been gathering data for several months. In an interview this summer, Dr. Kovac said, “It’s been a funny year to be in the spotlight like this.” He said the group stood behind its work, even if the ultimate interpretation of the measurements is up for grabs.

Acknowledging that dust would not be as sexy a discovery as ripples from inflation, Dr. Kovac said, “It’s really important as an experimentalist that you can divorce yourself from an investment in what the answer is.”

He went on: “One thing that would distress me bitterly is if a major mistake in the measurement or of the analysis would come to light. The most pressing question is, what are the dust contributions to the signal?”

Stay tuned.

Lyman Page, an astrophysicist at Princeton, said the episode illustrated the messy progress of science.

“Taking a step back,” he said by email, “it is amazing that a precise measurement of the cosmos can be made, discussed in fullness, and refuted by another measurement in such a short amount of time. It is testament to a healthy field.”

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