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https://www.quantamagazine.org/big-bang-signal-could-all-be-caused-by-dust-planck-team-says-20140921/ September 21, 2014

‘Big Bang Signal’ Could All Be DustBy Natalie Wolchover

The BICEP2 telescope detected a swirl pattern resembling an expected signal from the Big Bang, butwas it all cosmic dust? Click here for the full-size graphic.

There was little need, before, to know exactly how much dust peppers outer space, far from theplane of the Milky Way. Scientists understood that the dimly radiating grains aligned with ourgalaxy’s magnetic field and that the field’s twists and turns gave a subtle swirl to the dust glow. Butthose swirls were too faint to see. Only since March, when researchers claimed to have glimpsed theedge of space and time with a fantastically sensitive telescope, has the dust demanded a reckoning.For, like a cuckoo egg masquerading in a warbler’s nest, its pattern mimics a predicted signal fromthe Big Bang.

Now, scientists have shown that the swirl pattern touted as evidence of primordial gravitationalwaves — ripples in space and time dating to the universe’s explosive birth — could instead all comefrom magnetically aligned dust. A new analysis of data from the Planck space telescope hasconcluded that the tiny silicate and carbonate particles spewed into interstellar space by dying starscould account for as much as 100 percent of the signal detected by the BICEP2 telescope andannounced to great fanfare this spring.

The Planck analysis is “relatively definitive in that we can’t exclude that the entirety of our signal is

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from dust,” said Brian Keating, an astrophysicist at the University of California, San Diego, and amember of the BICEP2 collaboration.

“We were, of course, disappointed,” said Planck team member Jonathan Aumont of the UniversitéParis-Sud.

The new dust analysis leaves open the possibility that part of the BICEP2 signal comes fromprimordial gravitational waves, which are the long-sought fingerprints of a leading Big Bang theorycalled “inflation.” If the universe began with this brief period of exponential expansion, as thecosmologist Alan Guth proposed in 1980, then quantum-size ripples would have stretched into huge,permanent undulations in the fabric of the universe. These gravitational waves would have stampeda swirl pattern, called “B-mode” polarization, in the cosmic microwave background, the oldest lightnow detectable in the sky.

But beware the cuckoo.

At a much-publicized March 17 news conference, BICEP2 team leader John Kovac of HarvardUniversity announced that the group’s South Pole-based telescope had found evidence of B-modesthat “matched very closely the predicted pattern” of primordial gravitational waves. After probing aregion of space far from the dusty plane of the galaxy — “the cleanest patch of sky we can train ourtelescope on,” Kovac said — and measuring the polarization of incoming microwaves with 12 timesthe sensitivity of any previous experiment, he and his colleagues were convinced that they haddetected proof of inflation.

BICEP2 principal investigators, from left, Clem Pryke, Jamie Bock, Chao-Lin Kuo and John Kovacduring a March 17 news conference at the Harvard-Smithsonian Center for Astrophysics inCambridge, Mass.

But in the months following the announcement, outside experts cried foul, arguing that the scientistshad used highly uncertain models of galactic dust emission that now appear to have underestimated

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dust contamination in the BICEP2 region. “The community as a whole underestimated it,” said AviLoeb, a theorist at Harvard who is not affiliated with BICEP2.

Had the BICEP2 telescope been capable of detecting B-modes at multiple microwave frequencies,the scientists could easily have distinguished between light from interstellar dust grains and themore ancient light they sought. Both light sources become brighter at higher frequencies, but dustemissions brightens more dramatically. By plotting the strength of the B-mode signal as a function offrequency, the scientists could have determined whether the curve resembled the shallow rise of thecosmic microwave background or the steeper rise of dust light.

Instead, the team opted for maximum sensitivity and designed their detectors to receive a singlefrequency: 150 gigahertz. “This was the Achilles’ heel of the experiment,” Loeb said.

With higher frequencies swamped by dust emission and lower frequencies by another “foreground”called synchrotron radiation, 150 gigahertz sat at a sweet spot with minimal contamination. But asingle data point can lie on any curve.

Unable to directly determine the fraction of their signal that came from dust, the scientists relied onexisting models of contamination in their patch of the sky — including data incorrectly extractedfrom a preliminary dust map in a Planck scientist’s PowerPoint slide — and concluded that dustcould account for no more than one-fifth of their signal. After a group led by Raphael Flauger, nowof Carnegie Mellon University, pointed out errors and the Planck team released better (though stillpreliminary) dust estimates, Kovac and his team revised their paper and hedged on their claim of amajor discovery.

“They should have been much more cautious in their initial presentation,” said Lyman Page, acosmologist at Princeton University. “They should not have claimed measuring a primordial B-modebecause the uncertainty on foregrounds is and was simply too large.”

Multiple frequencies were needed. From 2009 to 2013, the telescope on board the European SpaceAgency’s Planck spacecraft measured polarization throughout the sky at seven different microwavefrequencies, though in any given patch roughly 100 times less sensitively than BICEP2. In their newanalysis, Planck scientists partitioned the sky into patches the size of the BICEP2 observation regionand calculated the amount of B-mode polarization present in each patch at 353 gigahertz, a highfrequency where dust emission dominates the signal. Some of the other patches gave off only half asmuch dust light as the BICEP2 patch, making it, in Keating’s words, “not squeaky clean.”

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Planck’s full-sky map showing the projected dust contamination at 150 GHz, extrapolated from the353 GHz data, with the cleanest regions shown in blue and the dustiest in red. The northern galactichemisphere appears at left and the southern hemisphere at right, with a black contour outlining theapproximate BICEP2 observation region.

The Planck telescope lacked the sensitivity to detect the faint B-modes at 150 gigahertz as seen byBICEP2, but by knowing roughly how dust emission varies with frequency, the scientistsextrapolated down from its value at 353 gigahertz. They calculated that excess dust emission wouldproduce B-mode polarization as strong as the signal detected by BICEP2, give or take roughly one-third of that strength.

“They more or less assumed that they could find a piece of the sky with low dust emission,” saidDouglas Scott, a cosmologist at the University of British Columbia who was heavily involved in thenew analysis. “And the Planck result shows there is no part of the sky where you can ignore thedust.”

Exactly how much of the total B-mode polarization comes from primordial gravitational waves, ifany, will be a matter of intense ongoing analysis. If there is a primordial signal, its strength,quantified by a parameter called r, will reveal the amount of energy that infused space-time anddrove it apart during inflation. The energy scale of inflation would be a major clue as to why ithappened.

“I can’t overemphasize how interesting this is,” Stanford University inflationary theorist EvaSilverstein said of the possible values of r during a recent talk in Chicago. Theorists like Silversteinmost want to know whether r is greater or less than 0.01, the crossover point between categoriescalled large-field and small-field inflation. The former would reveal details of an all-encompassingtheory of quantum gravity.

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Whilehouse dust is mostly lint or dead cells, cosmic dust is typically composed of carbon, silicates andother minerals. This grain of interplanetary dust caught by a high-flying NASA aircraft measuresonly 10 microns across, or one-tenth the width of a human hair.

Whereas the initial BICEP2 analysis pegged r at 0.2, corresponding to certain large-field inflationarymodels, the Planck study lowers its value much closer to 0. If the waves are detectable at all, a muchmore powerful telescope than BICEP2 will be needed to perceive them behind the swirly haze ofgalactic dust. Already, at least 10 existing or planned experiments have sufficient sensitivity todetect B-modes weaker than r = 0.1. The Atacama Cosmology Telescope, South Pole Telescope, andthe combined BICEP/Keck Array all should be capable of measuring B-modes from gravitationalwaves within two to three years if the signal is larger than r = 0.01. A balloon-borne instrumentcalled SPIDER will eventually achieve similar sensitivity.

To critics of the inflation idea, the heightened sensitivity of these experiments may be of littleconsolation. The theory is flexible enough to survive even if no primordial B-modes are found,making it virtually impossible to falsify.

“There are many models with r so small that you just wouldn’t see it with these experiments,” saidFlauger, who helped develop a testable string theory model of inflation, with r = 0.07, withSilverstein and others.

Inflation will remain the leading Big Bang theory even if the entire BICEP2 signal fades to dust, saidMark Trodden, a professor of physics at the University of Pennsylvania. It explains the smoothnessand uniformity of the universe and gives a mechanism for structure formation, he explained — “butall this evidence is highly circumstantial.”

Confirmation of primordial gravitational waves would have locked the theory down, resolving onceand for all the picture of the beginning of time. Now, “the jury is still out,” Keating said.

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A joint analysis of data from Planck and BICEP2, which is expected to appear in November, coulddetermine whether any primordial B-modes are mixed with the dust swirls in that clean, but notsqueaky clean, patch of sky above the South Pole. By collaborating, Kovac said, the teams should beable to put a new upper limit on the value of r — an assurance that primordial gravitational wavesmust be weaker than a certain strength, if they exist at all — that “relies on data, not models” of dustcontamination.

“I can promise that we are approaching the analysis with a completely unbiased attitude,” Kovacsaid. “We are as eager as everyone else to see the uncertainties reduced here, whatever the finalanswer.”

The success of BICEP2, Loeb said, was in increasing the sensitivity by an order of magnitudecompared to previous experiments. “Definitely they detected something,” he said. “The significanceof that depends on what the interpretation is. If it’s dust, it has no cosmological significancewhatsoever.”

This article was updated on Sept. 22, 2014, with additional details, and was later reprinted onWired.com.