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Recently, a team of international physicists has determined a subtle characteristic in cosmic microwave polarization background radiation that will eventually allow them to construct the huge structure of the universe, which measure the masses of neutrons and possibly unveil some mysteries of the dark matter and energy.
POLARBEAR consortium, a paper published in the Astrophysical Journal this week, led by UC Berkeley physicist Adrian Lee, explains the first successful isolation of a “B-mode” produced by gravitational lensing in the polarization of the cosmic microwave background radiation.
Certainly, polarization is the point of reference of radiation’s electric field that can be twisted into a “B-mode” pattern as the light passes through the gravitational fields of massive objects, such as clusters of galaxies.
Adrian Lee, the principal investigator of the study and UC Berkeley professor of physics and faculty scientist at Lawrence Berkeley National Laboratory (LBNL) stated that, “We made the first revelation that you can isolate a pure gravitational lensing B-mode on the sky. Moreover, we have shown you can measure the basic signal that will enable very sensitive searches for neutrino mass and the evolution of dark energy.”
The POLARBEAR team consists of around 70 researchers from around the globe who used the microwave detectors mounted on the Huan Tran Telescope in Chile’s Atacama Desert. The team submitted their study to the journal one week before the shocking 17th March announcement by the competitor group, the BICEP2 (Background Imaging of Cosmic Extragalactic Polarization) experiment that it had discovered the holy grail of microwave background research. That team reported finding the signature of cosmic inflation, a rapidly ballooning of the universe when it was a fraction of a fraction of a second old in the polarization pattern of the microwave background radiation.
Later observations like those that were announced by the Planck satellite last month, have since thrown cold water on the BICEP2 results, suggesting that they did not detect what they claimed to detect. Whereas POLARBEAR may eventually confirm or disprove the BICEP2 results, so far it has focused on interpreting the polarization pattern of the microwave background to map the distribution of matter back in time to the universe’s inflationary period, 380,000 years after the Big Bang.
When it comes to the POLARBEAR approach, it’s different from the one used by BICEP2. It might enable the group to measure when dark energy, the mysterious force accelerating the expansion of the universe, began to dominate and overwhelm gravity, which throughout most of cosmic history slowed the expansion.
Early Universe: A High-Energy Lab
Certainly, the POLARBEAR and BICEP2 both were designed to determine the B-mode polarization pattern, which is the angle of polarization at each point in an area of sky. However, the BICEP2 is based at the South Pole, can only measure variation over large angular scales, which is where theorists predicted they would find the signature of gravitational waves created during the early life of the universe. Gravitational waves could only have been created by a brief and very rapid expansion, or inflation, of the universe 10-36 seconds after the Big Bang “when the early universe was a high-energy laboratory, a lot hotter and denser than now, with an energy a trillion times higher than what they are producing at the CERN Collider,” Lee said.
Near Geneva, there is a huge Hadron Collider, which is trying to create that early era by banging together beams of protons to create a hot, dense soup from which researchers hope new particles will emerge, such as the newly discovered Higgs boson. But observing the early universe, as the POLARBEAR group does may also yield evidence that new physics and new particles exist at ultra-high energies.
In comparison to BICEP2, POLARBEAR was designed to determine the polarization at both large and small angular scales. After taking data in 2012, the team focused on small angular scales, and their new paper shows that they can measure B-mode polarization and use it to renovate the total mass lying along the line of sight of each photon.
The team of researchers uses these primitive photon’s light in order to explore large-scale gravitational structures in the universe, such as clusters or walls of galaxies that have grown from what initially were tiny fluctuations in the density of the universe. These structures twist the trajectories of microwave background photons through gravitational lensing, distorting its polarization and converting E-modes into B-modes. POLARBEAR images the lensing-generated B-modes to shed light on the intervening universe.
Last kiss
The microwave polarization background records minute density differences from that early era. After the Big Bang, 13.8 billion years ago, the universe was so hot and dense that light bounced eternally from one particle to another, scattering from and ionizing any atoms that formed. Only when the universe was 380,000 years old was it amply cool to enable an electron and a proton to form a stable hydrogen atom without being instantly broken apart. Out of the blue, all the light particles known as photons were set free.
Lee stated, “The photons go from bouncing around like balls in a pinball machine to flying straight and basically enabling us to take a picture of the universe from only 380,000 years after the Big Bang. The universe was a lot simpler then: mainly hydrogen plasma and dark matter.”
Though, the today’s photons that have cooled to a sheer 3 degrees Kelvin above absolute zero still keep information about their last interaction with matter. Explicitly, the flow of matter due to density fluctuations where the photon last scattered gave that photon a certain polarization dubbed as E-mode polarization.
“Just try to think it this way: the photons are bouncing off the electrons, and there is basically a last kiss, they touch the last electron and then they go for 14 billion years until they get to telescopes on the ground. That last kiss is polarizing,” Lee stated.
When it comes to E-mode polarization it contains some information though, in contrast, B-mode polarization contains more, because photons carry this only if matter around the last point of scattering was unevenly or asymmetrically distributed. Specifically, the gravitational waves created during inflation squeezed space and imparted a B-mode polarization that BICEP2 may have detected. POLARBEAR, on the other hand, has detected B-modes that are produced by the distortion of the E-modes by gravitational lensing.
There are many researchers who assumed that the gravitational-wave B-mode polarization might be too faint to detect easily, the BICEP2 team, led by astronomers at Harvard University’s Center for Astrophysics, reported a large signal that fit predictions of gravitational waves. Current doubt about this result centers on whether or not the astronomers took into account the emission of dust from the galaxy that would alter the polarization pattern.
Additionally, BICEP2’s ability to determine inflation at small angular scales is tainted by the gravitational lensing B-mode signal.
Adrian Lee told, “The strong suit regarding POLARBEAR’s is that it also has high angular resolution where we can image this lensing and subtract it out of the inflationary signal to clean it up.”
