Far out cosmology: using polarization in the CMB (Introduction)

by David Turell @, Friday, March 17, 2023, 20:54 (11 days ago) @ David Turell

Better definition of material parts:


"The team carried out a series of analyses ranging from rock chemistry to microscopic bone structure. “The vertebrae turned out to be from a highly advanced, fast-growing, probably warm-blooded and fully oceanic ichthyosaur,” says [Benjamin] Kear [at Uppsala University].

Now they’re repurposing CMB data to catalog the large-scale structures that developed over "billions of years as the universe matured.

“'That light experienced a bulk of the history of the universe, and by seeing how it’s changed, we can learn about different epochs,” said Kimmy Wu, a cosmologist at SLAC National Accelerator Laboratory.

"Over the course of its nearly 14-billion-year journey, the light from the CMB has been stretched, squeezed and warped by all the matter in its way. Cosmologists are beginning to look beyond the primary fluctuations in the CMB light to the secondary imprints left by interactions with galaxies and other cosmic structures. From these signals, they’re gaining a crisper view of the distribution of both ordinary matter — everything that’s composed of atomic parts — and the mysterious dark matter.


"Standard optical surveys, which track the light emitted by stars, overlook most of the galaxies’ underlying mass. That’s because the vast majority of the universe’s total matter content is invisible to telescopes — tucked out of sight either as clumps of dark matter or as the diffuse ionized gas that bridges galaxies. But both the dark matter and the strewn gas leave detectable imprints on the magnification and color of the incoming CMB light.

“"The universe is really a shadow theater in which the galaxies are the protagonists, and the CMB is the backlight,” Schaan said.


"When light particles, or photons, from the CMB scatter off electrons in the gas between galaxies, they get bumped to higher energies. In addition, if those galaxies are in motion with respect to the expanding universe, the CMB photons get a second energy shift, either up or down depending on the relative motion of the cluster.

"This pair of effects, known respectively as the thermal and kinematic Sunyaev-Zel’dovich (SZ) effects, were first theorized in the late 1960s and have been detected with increasing precision in the past decade. Together, the SZ effects leave a characteristic signature that can be teased out of CMB images, allowing scientists to map the location and temperature of all the ordinary matter in the universe.

"Finally, a third effect known as weak gravitational lensing warps the path of CMB light as it travels near massive objects, distorting the CMB as though it were viewed through the base of a wineglass. Unlike the SZ effects, lensing is sensitive to all matter — dark or otherwise.

"Taken together, these effects allow cosmologists to separate the ordinary matter from the dark matter. Then scientists can overlay these maps with images from galaxy surveys to gauge cosmic distances and even trace star formation.


"The analysis showed that the region’s gas didn’t hug its supporting dark matter network as tightly as many models predicted. Instead, it suggests that blasts from supernovas and accreting supermassive black holes forced the gas away from its dark matter nodes, spreading it out so that it was too thin and cold for conventional telescopes to detect.


"..as theorized three decades ago by Sean Carroll and colleagues, that polarization could be rotated by a field of dark matter, dark energy, or some totally new particle. Such a field would cause photons of different polarizations to travel at different speeds and rotate the net polarization of the light, a property known as “birefringence” that’s shared by certain crystals, such as the ones that enable LCD screens. In 2020, Komatsu’s team reported finding a tiny rotation in the CMB’s polarization — about 0.35 degrees. A follow-up study published last year strengthened that earlier result.

"If the polarization study or another result related to the distribution of galaxies is confirmed, it would imply that the universe does not look the same in all directions to all observers. For Hill and many others, both results are tantalizing but not yet definitive. Follow-up studies are underway to investigate these hints and rule out potential confounding effects. Some have even proposed a dedicated “backlight astronomy” spacecraft that would further inspect the various shadows.
Comment: the CMB is a treasure which is still spilling its treasure.

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