Testing Einstein's Theory Of Relativity Through Cosmic Microwave Measurements

April Flowers for redOrbit.com – Your Universe Online

What do you get when you combine polarized radiation with Einstein’s theory of general relativity? According to a group of astrophysicists at UC San Diego, you just might get more accurate estimates for the mass of ghostly subatomic particles known as neutrinos.

The study, published in Physical Review Letters, measures the distortions in polarized radiation from the early universe and demonstrates that these ancient microwaves can provide an important cosmological test of Einstein’s theory.

The team, led by UCSD physics graduate student Chang Feng, might even provide researchers with clues to another puzzling element of our universe: how the invisible “dark matter” and “dark energy,” which has been undetectable through modern telescopes, may be distributed throughout the universe.

The scientists used microwaves emanating from the Cosmic Microwave Background (CMB) of the early universe to measure the variations in the polarization. Polarized light, produced by the scattering of visible light—off the surface of the ocean, for example, vibrates in one direction. Likewise, the newly discovered “B-mode” polarized microwaves are produced when CMB radiation from the early universe scatters off electrons 380,000 years after the Big Bang. This is the approximate time that the universe cooled enough to allow protons and electrons to combine into atoms.

The scientific team had two goals for this study: to use the B-mode polarization signature to effectively “see” portions of the universe that are invisible to optical telescopes as gravity from denser portions of the universe tug on the polarized light, slightly deflecting its passage through the cosmos during its 13.8 billion year trip to Earth; and to use “weak gravitational lensing” to map regions of the universe filled with invisible “dark matter” and “dark energy,” as well as provide a test for general relativity on cosmological scales through distortions in the B-mode polarizations.

Both goals were realized by the recent discovery.

POLARBEAR, a collaboration of astronomers working on a telescope in the high-altitude desert of northern Chile designed specifically to detect “B-mode” polarization, provided CMB polarization data that the team measured to discover weak gravitational lensing. This lensing will allow astronomers to make detailed maps of the structure of the universe, constrain estimates of neutrino mass and provide a firm test for general relativity.

“This is the first time we’ve made these kinds of measurements using CMB polarization data,” said Feng, who conducted his study with Brian Keating, an associate professor of physics at the university and a co-leader of the POLARBEAR experiment. “This was the first direct measurement of CMB polarization lensing. And the amazing thing is that the amount of lensing that we found through these calculations is consistent with what Einstein’s general relativity theory predicted. So we now have a way to verify general relativity on cosmological scales.”

A relatively small portion of the sky—a 30-degree square—was examined by the POLARBEAR team to produce high resolution maps of B-mode polarization. The maps allowed the scientists to determine that the amplitude of gravitational fluctuations they measured was consistent with the leading theoretical model of the universe, known as the Lambda Cold Dark Matter cosmological model. BICEP2, a Harvard University-Smithsonian Center for Astrophysics-based group that Keating’s team also collaborated with, used a telescope at the South Pole to examine B-mode polarization across wide swaths of the sky. The group found evidence for a brief, and very rapid, expansion of the early universe called inflation.

Feng notes that these datasets address one of the most important questions in physics—the mass of the weakly interacting neutrino. The neutrino was originally thought to have no mass, but current limits indicate that neutrinos have masses below 1.5 electron volts. Although the B-mode polarization data in this study is consistent with the predictions of general relativity, the data is not statistically significant enough yet to make any firm claims about neutrino masses. Keating and Feng hope to analyze enough data over the coming year, however, to provide more certainty about the masses of neutrinos.

“This study is a first step toward using polarization lensing as a probe to measure the mass of neutrinos, using the whole universe as a laboratory,” Feng said.

“Eventually we will be able to put enough neutrinos on a ‘scale’ to weigh them—precisely measuring their mass,” Keating says. “Using the tools Chang has developed, it’s only a matter of time before we can weigh the neutrino, the only fundamental elementary particle whose mass is unknown. That would be an astounding achievement for astronomy, cosmology and physics itself.”

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Image 2 (below): The UC San Diego astrophysicists employed the HuanTran Telescope in Chile to measure the polarization of the cosmic microwave background. Credit: POLARBEAR