Primordial fluctuations

Primordial fluctuations are tiny variations in density and spacetime curvature in the early universe. Their gravitational growth produced the galaxies and large-scale structure observed today.

Cosmic inflation: A hypothesized period of extremely rapid expansion in the early universe. Inflation can stretch quantum fluctuations to cosmic scales and leave them as primordial seeds.
Density contrast: The fractional difference between a local density and the mean density. It quantifies the overdensities and underdensities that make up primordial density fluctuations.
Planck (spacecraft): A European Space Agency observatory that mapped the cosmic microwave background from 2009 to 2013. Its measurements tightly constrained the amplitude and scale dependence of primordial fluctuations.
Adiabatic perturbation: A perturbation in which the relative abundances of particle species remain unchanged across space. Adiabatic initial conditions fit observations better than many alternatives.
Inflationary model: A theory specifying the fields and dynamics responsible for cosmic inflation. Different models predict distinct fluctuation amplitudes, scale dependence, and statistical signatures.
Quantum fluctuations: Temporary variations in quantum fields arising from their quantum state. Inflationary models trace the initial perturbations to fluctuations in quantum fields.
Curvature perturbation: A gauge-invariant measure of spatial curvature variations on scales larger than the cosmic horizon. It provides a useful description of primordial perturbations while they remain outside the horizon.
WMAP: NASA’s Wilkinson Microwave Anisotropy Probe, which mapped the cosmic microwave background from 2001 to 2010. Its full-sky maps established precise measurements of the microwave background’s primordial pattern.
Isocurvature perturbation: A cosmological perturbation that changes relative particle abundances while initially leaving total energy density unchanged. It is an alternative initial mode to the dominant adiabatic pattern.
Tensor-to-scalar ratio: The ratio of primordial tensor-mode power to scalar-mode power at a specified scale. Its value could constrain inflationary energy scales and distinguish models of fluctuation generation.

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