An interactive guide to the fundamental concepts underlying the origin and evolution of the universe.
Dark matter is the invisible substance that makes up about 27% of the universe, shaping galaxies and cosmic structure. This guide explains its discovery, its role in the universe’s evolution from the Big Bang to today, and the evidence that confirms its existence.
The cosmic scale factor a(t) is a dimensionless parameter that tracks how distances in the universe grow with time. It connects redshift to expansion and underlies the standard model of cosmology, from the Planck epoch to the dark-energy era.
The baryon-to-photon ratio (η) is one of the most precisely measured numbers in cosmology, quantifying the tiny excess of matter over radiation in the early universe. It determines the abundances of light elements created during Big Bang nucleosynthesis and shapes the temperature fluctuations we observe in the cosmic microwave background, making it a cornerstone of the ΛCDM model.
Explore the shape and evolution of the cosmos from the Planck epoch to today. Learn how the cosmic microwave background, inflation, and large-scale structure reveal a universe that is flat to within 0.4 percent—yet whose global topology remains unknown.
A cosmic horizon marks the boundary of what we can observe in an expanding universe. The particle horizon defines the edge of the observable universe, while the event horizon sets the limit of future observations. This guide explains the concept and traces the cosmic epochs from the Planck era to structure formation.
Critical density is the precise average matter density that separates a universe that expands forever from one that eventually recollapses. This article explains the concept, its equation, its role in the Friedmann equations, and how it connects to the major epochs of cosmic history—from the Planck epoch to the formation of stars and galaxies.
Omega (Ω) is the ratio of the universe’s actual density to the critical density, determining whether space is flat, open, or closed. This guide explains its meaning, how it governs cosmic evolution, and its role in the standard Lambda-CDM model.
Baryon acoustic oscillations (BAO) are subtle ripples in the distribution of matter across the cosmos, frozen in when the universe was only 380,000 years old. They serve as a standard ruler for measuring cosmic distances and probing the nature of dark energy.
The observable universe is the spherical region of space that we can see from Earth, bounded by the cosmic horizon set by the speed of light and the age of the universe. This guide explores its origin, the key epochs of cosmic evolution, and the evidence that shapes our understanding of everything from the Big Bang to the present day.
The scale factor is a dimensionless number that describes how the universe expands over time. Normalized to 1 today, it allows cosmologists to track the evolution of the cosmos from the Planck epoch to the present, revealing the sequence of radiation, matter, and dark-energy-dominated eras.
