The Sachs–Wolfe effect explains how gravitational potential wells and hills imprint large-scale temperature fluctuations on the cosmic microwave background. It connects the first fraction of a second after the Big Bang to the dark-energy-dominated present, offering a key test of inflation and the standard cosmological model.
The sound horizon is the maximum distance a sound wave could have traveled through the hot plasma of the early universe from the Big Bang until recombination. This fixed scale leaves a distinct imprint on the cosmic microwave background and the large-scale structure of the universe, making it a crucial ruler for measuring cosmic expansion and the geometry of space.
Baryogenesis is the hypothetical early-universe process that produced the observed imbalance between matter and antimatter. This article explains the Sakharov conditions, the cosmic epochs involved, and why this asymmetry matters for the existence of stars, galaxies, and life.
Cosmological redshift and Doppler redshift both shift light toward longer wavelengths, but they arise from different physical causes. Cosmological redshift is caused by the expansion of space itself, while Doppler redshift is caused by the relative motion of the source and observer through space. Understanding the distinction is essential for interpreting observations of distant galaxies and the evolution of the universe.
NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) mapped the cosmic microwave background with unprecedented precision, transforming our understanding of the universe’s age, composition, and evolution. Its nine-year dataset confirmed the standard Lambda-CDM model and provided a detailed timeline from the first moments to the present day.
The lithium problem is a major unresolved discrepancy in cosmology: Big Bang nucleosynthesis predicts three to four times more primordial lithium-7 than is observed in the oldest stars. This mismatch challenges our understanding of the early universe and may point to new physics or overlooked astrophysical processes.
Ground-based cosmic microwave background (CMB) experiments such as the Atacama Cosmology Telescope (ACT) and the Simons Observatory measure the oldest light in the universe with extraordinary precision. This article explains how they work, what they reveal about cosmic history, and how they complement space missions like Planck.
The neutron-to-proton ratio in the early universe, set by weak interactions and freeze-out at about 1 MeV, determines the primordial abundance of helium-4 and other light elements. This ratio is a key input to Big Bang nucleosynthesis and a powerful test of the standard cosmological model.
The Epoch of Reionization marks the period when the first stars and galaxies ionized the neutral hydrogen that filled the early universe, transforming it from opaque to transparent. This cosmic milestone bridged the Dark Ages and the universe we see today.
From the Planck epoch to cosmic dawn, this guide traces the universe’s evolution from a hot, dense plasma to the formation of the first stars and galaxies, explaining the key epochs, the role of the cosmic microwave background, and the observations that revealed this history.
