Cosmology

The prevailing cosmological model describing the universe's origin from an extremely hot, dense state approximately 13.8 billion years ago. It is not an explosion in space but rather the rapid expansion of space itself, during which matter, energy, and the fundamental forces emerged.

The faint thermal radiation left over from the era of recombination, about 380,000 years after the Big Bang, when the universe cooled enough for electrons and protons to form neutral hydrogen atoms and photons to travel freely. It has a nearly perfect blackbody spectrum at a temperature of approximately 2.725 Kelvin.

A hypothetical form of matter that does not emit, absorb, or reflect electromagnetic radiation, making it invisible to telescopes. Its existence is inferred from gravitational effects on visible matter, such as galaxy rotation curves that spin faster than predicted by visible mass alone.

A mysterious form of energy that permeates all of space and is responsible for the observed accelerating expansion of the universe. It constitutes roughly sixty-eight percent of the total energy density of the universe and behaves like a cosmological constant with negative pressure.

A theoretical period of exponentially rapid expansion of space in the first tiny fraction of a second after the Big Bang, proposed by Alan Guth in 1981. Inflation solves several problems of the standard Big Bang model, including the horizon problem and the flatness problem.

The observation that the recessional velocity of a galaxy is proportional to its distance from the observer, expressed as v = H0 * d, where H0 is the Hubble constant. This relationship provides direct evidence for the expansion of the universe.

Einstein's theory describing gravity not as a force but as the curvature of spacetime caused by mass and energy. In cosmology, general relativity provides the Friedmann equations that govern how the scale factor of the universe evolves over time.

The process by which light elements such as hydrogen, helium, and trace amounts of lithium and beryllium were produced during the first few minutes after the Big Bang, when temperatures were high enough for nuclear fusion. The predicted abundances match observations closely.

The increase in wavelength of electromagnetic radiation as it travels through expanding space or moves away from an observer. Cosmological redshift is caused by the stretching of space itself and serves as a key tool for measuring distances and the expansion rate of the universe.

The organization of matter in the universe on scales of hundreds of millions of light-years, forming a cosmic web of galaxy filaments, walls, and clusters separated by vast voids. This structure grew from tiny primordial density fluctuations amplified by gravity over billions of years.