Proposed evidence of universe’s expansion could just be galactic dust, UC Berkeley researchers say

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Two UC Berkeley researchers recently published a paper challenging the results of a March study that claimed to have found evidence indicating that the universe exponentially expanded immediately after the Big Bang.

The paper, published on May 22, investigated the March study’s conclusions, which were based on data from a telescope at the South Pole called BICEP2. Co-authors Uros Seljak, a professor of physics, and Michael Mortonson, a postdoctoral scholar working with Seljak, found that the authors of the March study underestimated the interference caused by galactic dust. Their findings call into question the paper’s discovery of signals from primordial gravitational waves — waves that would support the theory of the universe’s initial rapid expansion.

The study is one of two recent analyses that challenged the March findings. Colin Hill, a co-author of the second analysis and a graduate student in the astrophysics department at Princeton University, noted the significance of the March study.

“It was a very important claim for physics as a whole,” Hill said. “It would have profound implications for our understanding of physics and cosmology. So we wanted to revisit the analysis.”

Mortonson, who re-examined the March study with Seljak, said the original findings were not necessarily incorrect. Their paper used data from the Planck space observatory and BICEP2 to conclude that the March results could also be explained by patterns caused by galactic dust.

Polarized light in the universe, which moves primarily in one direction, can be separated into two components called B-mode and E-mode polarization. Like hands, B-mode patterns are asymmetric and differ from their mirror image, while E-mode patterns are entirely symmetric.

The simplest theory of the universe assumes that there exists only E-mode polarization, according to Adrian Lee, a campus professor of astrophysics. But scientists predict gravitational waves traveling through the cosmic microwave background — often called the earliest light we can see from the Big Bang — would have a pattern indicative of B-mode polarization.

Seeing B-mode signals such as the ones detected by BICEP2 would be consistent with the theory of expansion, Lee said. He said galactic dust, however, could also produce B-mode polarization that would interfere with the data.

While the BICEP2 team acknowledged some interference from polarized dust, its estimate came from data presented at a conference by a member of a different research team, yielding results that did not seem to correct for the effects of the emission from dust in other galaxies, Hill said.

Additionally, BICEP2 only looks at a single frequency of light, unlike the Planck satellite, which collects data on a greater frequency to help distinguish galactic dust.

Both studies came to the same conclusion that the BICEP2 team’s findings could be explained by galactic dust interference. Mortonson, however, emphasized the uncertainty of data surrounding the behavior of galactic dust.

“The uncertainties are large enough that it’s still possible that gravitational waves from inflation are responsible for some of the signal,” Mortonson said in an email.

More data from the Planck satellite, expected to be released later this year, may reduce that uncertainty.

“I’d say it’s a really exciting time,” Hill said. “There will soon be enough data within the next couple of years to see if this is a primordial gravitational wave signal or just some dust from our own galaxy.”

Contact Katy Abbott at [email protected] and follow her on Twitter @katyeabbott.

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