Cosmology Cheat Sheet

The core ideas of Cosmology distilled into a single, scannable reference — perfect for review or quick lookup.

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Quick Reference

The Big Bang

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.

Cosmic Microwave Background (CMB)

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.

Dark Matter

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.

Dark Energy

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.

Cosmic Inflation

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.

Hubble's Law

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.

General Relativity and Spacetime

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.

Nucleosynthesis (Big Bang Nucleosynthesis)

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.

Redshift

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.

Large-Scale Structure

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.

Key Terms at a Glance

Big Bang:The cosmological model describing the universe's origin from an extremely hot, dense state approximately 13.8 billion years ago, followed by continuous expansion and cooling.

Cosmic Microwave Background (CMB):Thermal radiation filling the observable universe nearly uniformly, released about 380,000 years after the Big Bang when the universe became transparent to photons.

Dark Matter:A form of matter that does not interact electromagnetically and is detectable only through its gravitational effects on visible matter, radiation, and the large-scale structure of the universe.

Dark Energy:A form of energy with negative pressure that permeates all of space and drives the accelerating expansion of the universe, constituting roughly 68% of the total energy density.

Hubble Constant:The rate of expansion of the universe at the present time, expressed in kilometers per second per megaparsec, with current measurements yielding values between approximately 67 and 73 km/s/Mpc.

Redshift:The stretching of electromagnetic radiation to longer wavelengths, caused in cosmology by the expansion of space. Higher redshifts correspond to greater distances and earlier cosmic times.

Cosmic Inflation:A hypothesized phase of exponentially rapid expansion in the very early universe, lasting a tiny fraction of a second, that explains the large-scale uniformity and flatness of the observable universe.

General Relativity:Einstein's theory of gravitation that describes gravity as the curvature of spacetime caused by mass and energy, providing the foundational equations for cosmological models.

Friedmann Equations:A set of equations derived from general relativity for a homogeneous, isotropic universe that govern the expansion dynamics, relating the scale factor to the energy density and pressure of the universe's contents.

Nucleosynthesis:The production of light atomic nuclei (hydrogen, helium, lithium) during the first few minutes after the Big Bang, when temperatures and densities were sufficient for nuclear fusion reactions.

Cosmological Constant:A constant term (Lambda) in Einstein's field equations representing a uniform energy density of the vacuum, which produces a repulsive gravitational effect and is the simplest model for dark energy.

Scale Factor:A dimensionless quantity in cosmological models that describes how the distances between objects in the universe change over time due to expansion. It is conventionally set to 1 at the present epoch.

Recombination:The epoch approximately 380,000 years after the Big Bang when the universe cooled enough for free electrons and protons to combine into neutral hydrogen atoms, allowing photons to travel freely for the first time.

Gravitational Lensing:The bending of light from distant sources by the gravitational field of intervening massive objects, as predicted by general relativity. It is used to detect dark matter and measure mass distributions.

Observable Universe:The spherical region of the universe from which light has had time to reach Earth since the Big Bang, with a comoving radius of about 46.5 billion light-years.

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