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Understanding Our Universe at Levels Too Small to See

Among potential evidence that would further support the Big Bang theory is the cosmic gravitational wave background. Like ripples in a pond, gravitational waves distort the curvature of the spacetime continuum and were first predicted in 1916 by Albert Einstein.

Similar to the Cosmic Microwave Background, a ubiquitous backdrop of gravitational waves permeates space, cosmologists predict. This cosmic gravitational wave background (CGB) should have formed as a result of cosmic inflation, when the universe essentially exploded in size, expanding from smaller than the size of an atom to most of what we see today. All in the time it takes you to blink.

If observed, the cosmic gravitational wave background would be a smoking gun for cosmic inflation and is therefore a popular observing target in cosmology. But gravitational waves are tricky to detect because they do not emit electromagnetic radiation, and the only successful detection so far has been indirect. Most likely, observations of the CGB will also be indirect by observing how they affect light.

In the case of light from the CMB, the oldest and some of the only polarized light in the universe, an interaction with gravitational waves from the CGB will lead to what astrophysicists refer to as B-modes.

Temperature fluctuations in the CMB measured by the Wilkinson Microwave Anisotropy Probe. Credit: NASA.

An extensive team of scientists first presented observational evidence of B-modes last July. While the B-mode observations that the team reported proved that B-modes can be measured and detected, their observations were of B-modes created when the direction of a photon's travel is changed due to gravity from a massive object, like galaxy clusters, and not the CGB. Moreover, the July paper's B-modes were  about seven orders of magnitude more powerful, and therefore easier to detect, than the B-modes discussed here.

What's more is that recent results suggest B-modes from the CGB are even more difficult to detect than originally thought. The reason stems from a type of perturbation that past researchers have neglected in their calculations, some with the assumption that the effect was negligible. But one scientist shows through a series of complex calculations that the effect leads to CGB B-modes four times less powerful than the already small previously predicted values.

Most light waves in the universe vibrate in multiple planes, or dimensions, meaning that they’re unpolarized. Place a filter through which unpolarized light travels, and you can block certain vibrations, thus polarizing the light so that it only vibrates in a single plane. You do this every time you wear polarizing sunglasses. Not all light in space is unpolarized, however. Light from the CMB, for example, is polarized, and, although it’s difficult to picture, the direction that polarized photons vibrate can change under certain perturbations, like gravitational waves.

Liang Dai, of Johns Hopkins University and author of the recent paper that dispatches CGB B-modes to even smaller values, compares this effect to a cannon ball flying through the air. The ball will arc under gravity’s influence and if the ball is spinning, it will spin about its axis. But unlike the direction of the ball’s motion, that axial spin will not change its tilt angle. Due to the geometry of space in our universe, however, the axial tilt of polarized photons will change, and it’s what results from this change that Dai calculated something he did not expect.

Dai first calculated the B-mode that would result from a change in a CMB photon’s direction of vibration, which in scientific circles is referred to as rotation angle. What surprised Dai was that if a polarized CMB photon experiences both a change in its direction of motion and its rotation, the B-modes produced from the change in rotation angle largely cancels the B-modes induced by the change in direction. The equation below is a key equation that Dai presents in his paper that computes a photon's rotation angle in space -- the equation that theorists have previously neglected in their calculations.

This is a key equation Dai offers in his paper. Credit: Liang Dai

Dai’s results lead to B-modes that are four times less powerful than earlier estimates, meaning that even the most sensitive instruments today do not have the capability to detect such B-modes.

The B-mode produced from a change in polarized rotation angle is incredibly small, about seven orders of magnitude smaller than B-modes discussed in the paper published last July. The interesting part is that combined with the second effect, a change in a photon's direction of motion, the two affects together do not simply add up to a larger number.

"Fortunately, the effect is extremely small," said Uros Seljak who is a professor of physics at University of California, Berkeley and University of Zurich and also studies the limits to which experiments could measure these types of B-modes from the CGB.

"This result does not change the bulk pictures of the CMB and the universe in which we live," Dai said. "It highlights an effect that astrophysicists have ignored in their previous calculations. It furthermore implies that the direction of photon vibration also changes slightly due to the nonlinear effects on light propagation from massive objects," he said. A correction that is important for future cosmological surveys with higher precision.

The equations that Dai present in his paper can be used for any polarized light in space, he said, and not just light from the CMB. Other sources of polarized light in space include pulsars, active galaxies and scattered starlight from interstellar dust.

Dai's paper was published in Physical Review Letters on January 28.


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