‘No evidence for or against gravitational waves’           

Two analyses suggest signal of Big Bang ripples announced in March was too weak to be significant.

Ron Cowen, 05/2914

Preliminary data from the Planck probe on how galactic dust scatters microwave radiation, presented at an April 2013 meeting, are now being used to evaluate the strength of signals from the primordial Universe.

The astronomers who this spring announced that they had evidence of primordial gravitational waves jumped the gun because they did not take into proper account a confounding effect of galactic dust, two new analyses suggest. Although further observations may yet find the signal to emerge from the noise, independent experts now say they no longer believe that the original data constituted significant evidence.

Researchers said in March that they had found a faint twisting pattern in the polarization of the cosmic microwave background (CMB), the Big Bang’s afterglow, using a South Pole-based radio telescope called BICEP2. This pattern, they said, was evidence for primordial gravitational waves, ripples in the fabric of space-time generated in the early Universe (see 'Telescope captures view of gravitational waves'). The announcement caused a sensation because it seemed to confirm the theory of cosmic inflation, which holds that the cosmos mushroomed in size during the first fraction of a second after the Big Bang.

However two independent analyses now suggest that those twisting patterns in the CMB polarization could just as easily be accounted for by dust in the Milky Way Galaxy^{1, 2}.

“Based on what we know right now… we have no evidence for or against gravitational waves,” says Uroš Seljak, an astrophysicist at the University of California, Berkeley, a co-author of one of the latest studies¹.

But James Bock, a physicist at the California Institute of Technology in Pasadena and a co-leader of the BICEP2 experiment, says that although his group’s main paper “has been revised based on many referee comments and resubmitted” for publication, the evidence for gravitational waves “is certainly not being retracted”.

The BICEP2 results “are basically unchanged”, Bock says.

The critique comes on the heels of a presentation two weeks ago by Raphael Flauger, a theoretical physicist at New York University, who re-examined a map of dust polarization that the BICEP2 team had used in their analysis (see 'Gravitational wave discovery faces scrutiny'). Flauger concluded that the researchers had probably underestimated the fraction of polarization due to dust in the map, which was compiled from data gathered between 2009 and 2013 by the European Space Agency’s Planck spacecraft. Flauger says that when the dust is fully accounted for, the signal that can be attributed to gravitational waves either vanishes or is greatly diminished.

“I had thought that the result was very secure,” Alan Guth, the cosmologist who first proposed the concept of cosmic inflation in 1980, and who is at the Massachusetts Institute of Technology in Cambridge, told Nature after learning about Flauger's talk. “Now the situation has changed.”

That map figured in only one of the six models that the BICEP2 team used to examine the role of dust in their results.

But in a study² posted on 28 May on the arXiv preprint server, Flauger and his co-authors David Spergel and Colin Hill, both of Princeton University in New Jersey, cast doubt on the remaining models, saying that all are based on a low estimate -- between 3.5% and 5% -- of the fraction of total polarization caused by galactic dust.

New information, based on more detailed Planck maps released after the BICEP2 team did their analysis, suggest that the fraction is closer to 8–15%, Spergel says.The higher percentage is an extrapolation, he acknowledges, because the newest Planck maps exclude the south polar region of sky that BICEP2 examined (see 'Milky Way map skirts question of gravitational waves').

The difference between 3.5% and 8% may seem small, notes Spergel, but it becomes significant because the signal detected by BICEP2 depends on the square of the polarization fraction, he says.

With those updated numbers, “there’s no evidence for the detection of gravitational waves,” Spergel declares. “It’s consistent with dust.” But a final determination cannot be made until more a precise dust map, expected to be released by the Planck team in October, is available, he adds.

“Dust could account for all or most of the signal” seen by BICEP2, says Paul Steinhardt, a cosmologist at Princeton University, who was not involved with any of the studies.

At the time of press, Bock told Nature he had not had time to read the paper by Flauger, Spergel and Hill, which the authors gave to him and other members of the BICEP2 team only a few hours before it appeared online.

In the second analysis, Seljak and his Berkeley colleague, Michael Mortonson, took a more conservative approach to scrutinizing the BICEP2 results. With the amount of polarization due to dust in the south polar region as yet unmeasured, Seljak and Mortonson restricted their analysis to a known quantity -- the intensity of microwaves emitted by dust over different spatial scales on the sky. In assuming that the intensity of dust varies in the same way over all parts of the sky, including the South Pole, the researchers found no clear evidence that the BICEP2 signal should be attributed to gravitational waves.

Seljak and Mortonson also re-examined data on how the strength of the signal detected by BICEP2 varied with the frequency of the microwaves. The BICEP2 team argued that the intensity of the signal recorded at 150 gigahertz, when compared with data recorded by an older telescope, BICEP1, at 100 gigahertz, did not match the intensity pattern expected from dust. That finding seemed to favour gravitational waves over dust by an 11-to-1 margin.

But Seljak and Mortonson say that the BICEP2 team did not exclude data on small spatial scales in their frequency analysis. That is a problem, Seljak says, because on small scales, gravitational lensing -- the bending of light due to massive objects -- exactly mimics the twisting polarization pattern that gravitational waves imprint on larger spatial scales.

Accounting for the lensing signal, “the primordial gravity-wave signal is preferred to dust with odds of less than 2 to 1 -- in other words, not significant odds at all,” says Seljak.

The BICEP2 team did not respond to several requests for comment on the paper by Seljak and Mortonson.

Seljak says that new data expected soon from the Keck Array, a telescope at the South Pole built by the BICEP2 team, could provide a litmus test for the true nature of the BICEP2 signal.

Bock says that he and his BICEP2 colleagues eagerly await the Planck map this autumn, and already have in hand better 95- and 150-gigahertz data from the Keck Array.

Nature

DOI: doi:10.1038/nature.2014.15322

References

1.     Mortonson, M. M. & Seljak, U. preprint at http://arxiv.org/abs/1405.5857 (2014).

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2.     Flauger, R., Hill, J. C. & Spergel, D. N. preprint at http://arxiv.org/abs/1405.7351 (2014).

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Have Cosmologists Lost Minds in Multiverse?

Luke Barnes & Geraint Lewis, 05/15/14

Editor's Note: This article was provided by The Conversation. The original is here.

The recent BICEP2 observations – of swirls in the polarisation of the cosmic microwave background – have been proclaimed as many things, from evidence of the Big Bang and gravitational waves to something strange called the multiverse.

The multiverse theory is that our universe is but one of a vast, variegated ensemble of other universes. We don’t know how many pieces there are to the multiverse but estimates suggest there many be squillions of them.

But (if they exist) there has not been enough time since our cosmic beginning for light from these other universes to reach us. They are beyond our cosmic horizon and thus in principle unobservable.

How, then, can cosmologists say they have seen evidence of them?

Seeing the Unobservable

Unobservable entities aren’t necessarily out-of-bounds for science. For example, protons and neutrons are made of subatomic particles called quarks. While they cannot be observed directly, their existence and properties are inferred from the way particles behave when smashed together.

But there is no such luxury with the multiverse. No signals from other universes have or will ever bother our telescopes.

While there is some debate about what actually makes a scientific theory, we should at least ask if the multiverse theory is testable? Does it make predictions that we can test in a laboratory or with our telescopes?

The answer is yes, but perhaps not as you’d expect. And the exploration of the multiverse theory involves some very complex, and very controversial, ideas.

The Mark of the Generator

If your multiverse theory generates its universes via some physical process, then that process may leave its fingerprints on this universe. This is what BICEP2 might have seen.

Cosmologists think that in its earliest stages, the universe underwent an extraordinarily rapid expansion, known as inflation. In many versions of inflation, gravitational waves leave an imprint in fossil radiation, recently observed as characteristic swirls in this ancient light; a successful prediction of inflation.

In some versions of inflation, the process that causes our universe to inflate is expected to produce huge numbers of other universes. Evidence for inflation isn’t exactly direct evidence for the multiverse, but it’s a start.

A Known Generator

We cannot see the creation of other universes, but if we have evidence for the physics that powers the universe generator then we have another piece of the puzzle.

In particular, a multiverse theory that requires only well-tested physics such as gravity and quantum fields is preferable to one that requires new physics, or requires extrapolating known physics to situations where we expect them to break down.

Inflation’s scorecard is mixed: some of the underlying physics is known, some is hypothetical, and some worry that it skirts close to (or perhaps into) the quantum gravity regime, where all tested physical theories break down.

Observing Our Universe in the Ensemble

Let’s think about prediction with a simple example. Alice predicts that a certain factory makes 99% red widgets, 1% blue. Bob predicts the opposite: 99% blue and 1% red.

A packet arrives from the factory and they open it to find a red widget – whose theory is correct? Neither theory is certainly false, but the evidence clearly favours Alice.

A multiverse theory will (by definition) predict the statistical properties of its universes. We can then ask whether our universe is the kind of universe one would expect to observe.

The more unusual our universe is, the more likely it is that a different multiverse theory would better explain our universe. And if our universe is just too weird for the vast majority of multiverse theories, then the whole idea of a multiverse comes under question.

It is thus relevant to ask: how typical is our universe of the set of possible universes?

There is one way in which our universe is highly unusual: it contains life. If our laws of nature were only slightly different then our universe would look and behave quite differently: atoms would fall apart, or the universe would have expanded so fast that stars and galaxies could not form.

Most cosmological scenarios would have left our universe stone-cold dead, devoid of life.

The multiverse can handle this. The probability of observing a particular type of universe depends on that universe first creating observers. We are not just passive observers, setting up our equipment and taking measurements of the universe at our leisure. We are products of this universe.

While universes with observers may be highly unusual in the entire multiverse, they will obviously be the norm for observed universes. And so, the life-permitting nature of our universe can be counted as a successful prediction of the multiverse. (Prediction in the logical, rather than chronological sense.)

Revenge of the Boltzmann Brains

Or can it? We’ve assumed that the most likely way for a universe to make observers is via suitable laws and biological evolution, as in our universe. Such a universe is probably extremely unusual in the multiverse. But what if just any old universe could get lucky and fluke a few observers?

Quantum mechanics, the same physics that predicts the inflationary fluctuations in the cosmic microwave background, seen by BICEP2, also predicts that there is an extremely tiny probability of a fully-formed brain spontaneously popping out of “empty” space. Given enough time and space this vanishingly improbable event will occur.

While such freak observers, known as Boltzmann Brains, would be massively outnumbered by biological observers in our universe, they could be common in the almost unending time and space of the entire multiverse.

In that case, the fact that we are not that kind of observer is like seeing the red widget – it is evidence against a multiverse theory that says we should expect to be freaky observers. The multiverse is not just testable; it might even fail.

Ifs and Buts

At the moment, there are too many ifs and maybes in this story.

Observations do not uniquely favour inflation though the BICEP2 results are an impressive step in this direction. It is a matter of some debate whether inflation naturally generates a multiverse.

Further, many multiverse theories struggle to predict anything, so clearly there is much much more to be done.

But positing the multiverse is not, as claimed by some, the end of science. It may be the start of the biggest scientific adventure of all.

Luke Barnes is a Super Science Research Fellow at University of Sydney. He receives funding from the Australian Research Council. Geraint Lewis is Professor of Astrophysics at University of Sydney. He receives funding from the Australian Research Council, including Discovery Projects and a Future Fellowship.

http://www.realclearscience.com/articles/2014/05/15/have_cosmologists_lost_minds_in_multiverse_108656.html

Gravitational-wave finding causes 'spring cleaning' in physics           

Big Bang findings would strengthen case for multiverse and all but rule out a 'cyclic Universe'.

Ron Cowen, 03/21/14

On 17 March, astronomer John Kovac of the Harvard-Smithsonian Center for Astrophysics presented long-awaited evidence of gravitational waves -- ripples in the fabric of space -- that originated from the Big Bang during a period of dramatic expansion known as inflation.

By the time the Sun set that day in Cambridge, Massachusetts, the first paper detailing some of the discovery’s consequences had already been posted online¹, by cosmologist David Marsh of the Perimeter Institute for Theoretical Physics in Waterloo, Canada, and his colleagues.

The authors wrote that the measurements made by Kovac's team using the BICEP2 telescope at the South Pole all but ruled out a class of models that attempted to explain both inflation and another cosmic mystery -- the nature of dark matter -- based on a hypothetical elementary particle called the axion. The researchers did not rule out all axion models, however, only that “this particular class of axions make up only a tiny fraction of the dark matter”, says Marsh.

Cosmologist Marc Kamionkowski of Johns Hopkins University in Baltimore, Maryland, agrees that some axion models no longer work, “because they require inflation to operate at a lower energy scale than the one indicated by BICEP2”.

Kamionkowski says that the BICEP2 discovery would also rule out a plethora of other theories in one fell swoop, including several other ideas on the properties of the energy field that drove inflation. “The family of acceptable models has been collapsed tremendously,” he says.

“There’s a great spring cleaning, with almost everything ruled out,” says Max Tegmark, a cosmologist at the Massachusetts Institute of Technology (MIT) in Cambridge. “This is really shaking up not just the experimental field but the theoretical world.”

The BICEP2 data would eliminate about 90% of inflationary models, Andrei Linde, a cosmologist at Stanford University in California, told a packed auditorium at MIT the day after the BICEP2 announcement (see picture below). Many of those models do not produce gravitational waves at detectable levels, said Linde, who is one of the founders of inflation theory.

But he said that the findings would agree remarkably well with ‘chaotic inflation’, a simple version of inflation Linde developed 30 years ago. In Linde's model, inflation never completely ends, stopping only in limited pockets of space, while continuing with its exponential expansion elsewhere. Chaotic inflation would produce not just our Universe but a multiverse containing many pocket universes, each with its own laws of physics, an idea that critics say would be untestable.

Linde also says that gravitational waves would seem to rule out an alternative model to inflation known as the cyclic Universe, in which two ‘brane worlds’ -- three-dimensional universes floating inside a higher-dimensional space -- collide to produce the Big Bang. That theory does not produce gravitational radiation.

Paul Steinhardt, a theoretical physicist at Princeton University in New Jersey and an originator of the cyclic theory, agrees that -- if the BICEP2 findings are confirmed -- his theory is now dead, but he says he hasn’t give up hope that a variant of the model might generate the radiation.

The BICEP2 results will also send some string theorists back to the drawing board, says Frank Wilczek, a theoretical physicist and Nobel laureate at MIT. String theory posits that elementary particles are made of tiny vibrating loops of energy. Efforts to combine string theory with cosmology have led to inflationary models that generate gravitational waves with energies much lower than the level detected by BICEP2, he says.

Theoretical physicist Eva Silverstein of Stanford says she disagrees that string theory-based models of inflation are in any sort of trouble. “There is no sense in which we are forced to start over,” she says. She adds that in fact a separate class of theories that involve both axions and strings now look promising.

Linde agrees. “There is no need to discard string theory, it is just a normal process of learning which versions of the theory are better,” he says. “All of us, not just string theorists, should go back to the drawing board, but not because we failed, but because we learned something very important and now we should use this knowledge to make further steps.”

Nature, DOI:10.1038/nature.2014.14910

References

1.     Marsh, D. J. E., Grin, D., Hlozek, R. & Ferreira, P. G. Preprint available at http://arxiv.org/abs/1403.4216 (2014).

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http://www.nature.com/news/gravitational-wave-finding-causes-spring-cleaning-in-physics-1.14910