An interactive guide to the fundamental concepts underlying the origin and evolution of the universe.
The Friedmann–Lemaître–Robertson–Walker (FLRW) metric is the exact solution of Einstein’s field equations that describes a homogeneous, isotropic expanding universe. It underpins the ΛCDM concordance model, providing the framework for cosmic epochs, the cosmic microwave background, and the large-scale structure we observe today.
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.
The Hubble parameter (H) describes the expansion rate of the universe at any given time. Its present-day value, the Hubble constant (H₀), is about 70 km/s/Mpc and is a cornerstone of modern cosmology, linking the age, composition, and fate of the cosmos.
The flatness problem is a cosmological fine-tuning puzzle: why is the universe so precisely flat when standard Big Bang theory predicts it should curve over time? Inflation offers a compelling solution, and measurements from COBE, WMAP, and Planck confirm the near-perfect flatness of space.
Dive into the geometry and topology of the cosmos—from local curvature to global shape—and trace the universe’s evolution from the Planck epoch to the present, with insights from the cosmic microwave background and missions like COBE, WMAP, and Planck.
Dark energy makes up about 70% of the universe and drives the accelerating expansion of space. Whether it is constant or changing will determine if the cosmos ends in a Big Freeze, Big Rip, or Big Crunch. Recent surveys hint that dark energy may be weakening, but the standard Lambda-CDM model remains the consensus.
The two-point correlation function quantifies how galaxies and matter are clustered, encoding the universe’s history from quantum fluctuations to the cosmic web. This guide explains its definition, measurement, and role in tracing cosmic epochs from the Planck era to today.
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.
