Primordial examples of these phenomena can be inferred from measuring B-mode polarizations in the CMB.
Lawrence Krauss and Frank Wilczek recently argued that the background of these phenomena is proportional to h-bar squared. Schutz pointed out that sources of these entities called "standard sirens" can be used to construct high-redshift distance scales. These phenomena are often studied using a gauge in which h-bar sub alpha-beta is equal to h sub alpha-beta and reduces 10 polarizations to two, called the transverse-traceless gauge. Primordial examples of these phenomena can be inferred from measuring B-mode polarizations in the CMB. These entities must arise from (*) quadrupole or higher multipole sources. The loss of energy of a system named for Hulse and Taylor is indirect evidence for their existence. Methods of detecting them include Weber bars and laser interferometry. Thorne, Drever, and Weiss developed the LIGO detector to search for these entities, which are predicted by general relativity but have not yet been directly observed. For 10 points, name these propagating "ripples" in spacetime, a form of radiation linked to the weakest of four forces.
Lawrence Krauss and Frank Wilczek recently argued that the background of these phenomena is proportional to h-bar squared.
gravitational waves [accept gravitons since wave-particle duality and all that]