Primordial gravitational waves

Primordial gravitational waves are spacetime ripples generated in the early universe, including a possible background produced during cosmic inflation.

Cosmic inflation: A hypothesized era of extremely rapid expansion in the universe's earliest moments. Inflation can amplify quantum tensor fluctuations into a background of primordial gravitational waves.
BICEP2: A South Pole telescope experiment that measured polarization of the cosmic microwave background. Its 2014 claim of a primordial B-mode signal was later explained largely by polarized Galactic dust.
Cosmic strings: Hypothetical one-dimensional topological defects that may have formed during symmetry-breaking transitions in the early universe. A cosmic-string network could generate a stochastic gravitational-wave background distinct from inflation's signal.
Astrophysical gravitational-wave background: A stochastic gravitational-wave signal produced by many unresolved astrophysical sources. Unlike a primordial background, it arises from later populations such as compact-object binaries.
Tensor perturbation: A transverse, traceless perturbation of spacetime, including gravitational waves in cosmology. Primordial gravitational waves are the tensor perturbations whose early-universe origins are being tested.
Planck satellite: A European Space Agency mission that mapped the cosmic microwave background across the sky. Planck's measurements of polarized dust helped reassess claims of an inflationary B-mode detection.
First-order phase transition: A transition between physical phases involving nucleation and expansion of bubbles of the new phase. Colliding bubbles and related fluid motion could produce gravitational waves in the early universe.
Galactic foreground: Emission from sources within the Milky Way that contaminates observations of more distant signals. Polarized Galactic dust can mimic the CMB B-modes sought as evidence of primordial waves.
Quantum fluctuations: Temporary variations in quantum fields arising from their quantum nature. Inflationary expansion could stretch quantum fluctuations into macroscopic gravitational waves.
BICEP/Keck Array: A series of South Pole experiments measuring cosmic microwave background polarization on degree scales. Their joint limits with Planck constrain the strength of inflationary gravitational waves.

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