Recent observations suggest the universe’s oldest light may have twisted ever so slightly since its inception almost 14 billion years ago. Though subtle, these signals could shift our understanding of physics beyond the Standard Model and shed light on topics like dark matter. However, the same signals could also be the result of a small error in the orientation of the telescope’s polarization detectors.
To rule out the latter, astronomers must first develop a system to determine the relative polarization-angle calibration of the detectors. Writing in The Astrophysical Journal Letters, researchers from UC San Diego describe a new method to do just that.
“The signal we are looking for is incredibly small, so we have to be certain that we are seeing the universe and not our instrument,” Anto I. Lonappan, a Postdoctoral Fellow at UC San Diego, said in a statement.
“Our method gives us a complementary way to check that distinction. Before interpreting a tiny rotation as new physics, we want to know that the calibration itself can be trusted.”
Read More: Light Emitted by a Distant Galaxy Pierces Through the Early Universe's Fog
The Universe’s Oldest Light
The first light in the universe appeared some 380,000 years after the Big Bang. Almost 14 billion years later, astronomers can still make out the remnants of this ancient light — called the cosmic microwave background (CMB) — thanks to highly powerful telescopes armed with extremely sensitive detectors, like the European Space Agency’s Planck satellite.
Explore All OptionsA small proportion of CMB is polarized, meaning it vibrates in a certain direction, and recent observations suggest this polarization may have rotated slightly as it’s traveled through the universe — a phenomenon known as cosmic birefringence. The movement is minimal — just a fraction of a degree — but it could be significant.
Developing a Test for Cosmic Birefringence
To find out if the signals are the result of cosmic birefringence or are simply a calibration error, Lonappan and colleagues compared maps produced by a range of detectors on the basis that true cosmic birefringence would be reflected in each of the maps. As such, their method cancels out any rotation common to all maps, meaning any remaining rotation is most likely caused by differences in polarization calibration.
The team tested their method by applying the technique to eight maps created by the Planck satellite and then compared the results with those obtained from the Minami–Komatsu analysis. The latter is the go-to method for determining what is cosmic birefringence and what is a calibration error.
They found the results produced by the two methods were consistent. Both found a cosmic-birefringence angle of 0.37° ± 0.12°, which suggests the new method could serve as an alternative approach. The researchers stress the results do not confirm the existence of cosmic birefringence. Rather, it offers an independent estimator and modeling route.
Distinguishing B modes from E modes
One way the method could assist future research is in the search for a particular type of CMB polarization that displays curl-like patterns, known as B modes. These can be caused by a process called gravitational lensing, which describes the distortion of light that occurs when it passes a massive object. But they may also be produced by gravitational waves generated in the very early universe (known as primordial gravitational waves). This second group has so far remained undetected but, if discovered, could offer clues to the early universe and topics like cosmic inflation.
The team hopes the method could help avoid errors around the identification of B modes and E modes.
“The detection of primordial B modes would transform our understanding of the early universe, so the measurement must survive rigorous calibration checks,” Chancellor’s Distinguished Professor of Physics Brian Keating, who co-authored the paper, said in a statement.
“Anto has developed a useful new framework for testing relative polarization-angle calibration. This is ultimately about knowing when we can trust a measurement.”
Read More: The Universe Started as a “Hot Soup of Particles and Photons" 13.8 Billion Years Ago
Article Sources
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- This article references information from a study published in The Astrophysical Journal Letters: Differential Polarization Calibration: A Consistency Test for Cosmic Birefringence
