An interactive guide to the fundamental concepts underlying the origin and evolution of the universe.
The flatness problem is a cosmological fine-tuning issue: the universe appears almost perfectly flat, requiring extreme fine-tuning in the early universe. Inflation explains this by driving spatial curvature to zero.
The density of the universe determines its geometry and fate. Current measurements show it is composed of 5% normal matter, 27% dark matter, and 68% dark energy, with a total density very close to the critical value. This article explores the cosmic density budget, the epochs that shaped the universe, and the evidence from missions like Planck and WMAP.
The tensor-to-scalar ratio (r) measures the relative strength of primordial gravitational waves versus density fluctuations in the early universe. It is a key observable for testing cosmic inflation and is probed through the B-mode polarization of the cosmic microwave background.
The deceleration parameter q measures how the expansion of the universe changes over time. A negative q today indicates cosmic acceleration, a discovery that reshaped modern cosmology.
Conformal time is a coordinate that factors out cosmic expansion, letting light travel in straight lines on a comoving grid. It underlies the particle horizon and provides a natural timeline for the universe’s major epochs, from the Planck era to structure formation.
The Friedmann equation is a set of equations in physical cosmology that govern the expansion of a homogeneous, isotropic universe. Derived from Einstein’s field equations, it forms the backbone of the standard Big Bang model and the ΛCDM paradigm, describing how the scale factor evolves with time.
The Hubble tension is a growing discrepancy between two precise measurements of the universe’s expansion rate. This article explains the conflict, its implications for cosmology, and places it within the full story of cosmic evolution from the Planck epoch to today.
Comoving distance is a fundamental measure in cosmology that factors out the expansion of the universe, providing a stable ruler for cosmic scales. This guide explains how comoving distance relates to redshift, traces the major epochs of cosmic evolution, and shows how modern missions have mapped our universe’s history.
The observable universe spans 46.5 billion light-years in radius despite being 13.8 billion years old because space itself has expanded. This article explains the cosmic epochs, the role of inflation, and the evidence from the cosmic microwave background.
Dark matter provided the gravitational scaffolding for the first stars, gathering gas into halos where molecular hydrogen cooling could trigger collapse. Without dark matter, the first stars could not have formed so early in cosmic history, and their properties would be drastically different.
