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primordial gravitational waves

(gravitational waves formed in the early universe)

The term primordial gravitational waves indicates the first gravitational waves, formed in the early universe. The term generally is used to refer to gravitational waves leaving an imprint in CMB anisotropies, a reason for studying the anisotropies. The search for primordial gravitational waves is more specifically for B-mode polarization, because of all the effects of the waves on the CMB, the B-modes are expected stand out to the most degree. As of 6/2020, B-modes clearly attributable to primordial gravitational waves have not been detected, though in 2014, observers found a "candidate signal", which turned out to be consistent with known effects of Milky Way dust on the CMB. (One effort toward detecting the CMB B-modes consists of surveys aimed at discovering all possible sources of such B-modes within the galaxy.)

Theories have proposed a number of sources for such primordial gravitational waves: inflation itself is probably the one of most intense interest: such waves could be the most direct evidence to date. Other proposed sources are from soon after inflation including reheating and topological defects. Early black hole mergers of primordial black holes are another possible source. Efforts to detect them include:

(physics,gravity,relativity,wave,early universe,cosmology)
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