Cosmology Key Terms and Concepts

The theory suggests that the Universe originated from an extremely hot and dense state and has been expanding ever since.

The theory predicted that leftover radiation from the early Universe would still exist today, and would have stretched (redshifted) due to the expansion of the Universe. This radiation should be observable as low-energy microwaves uniformly present in all directions.

The CMB should have a nearly perfect blackbody spectrum because the early universe was in thermal equilibrium. This means the intensity of radiation at each wavelength should follow a specific curve described by Planck’s law.

The CMB should be nearly the same in all directions (isotropic) because the early universe was homogeneous and isotropic on large scales.

The CMB should have small temperature fluctuations (anisotropies) corresponding to tiny density variations in the early universe that later gave rise to galaxies and cosmic structure.

A comprehensive ancient treatise on astronomy written by Ptolemy in the 2nd century AD, it was the most important source of information on ancient Greek astronomy for centuries.

A theory that explains the apparent motions of celestial objects by assuming that the Earth is stationary at the center of the universe, while all other objects revolve around it.

A theory that proposes that celestial objects, including the planets, move in circular paths called epicycles, which in turn revolve around a larger circle called the deferent.

A theory that proposes that the Sun is the center of the universe, with Earth and other planets revolving around it.

Three laws that describe the motion of planets around the Sun, first published by Johannes Kepler in the 17th century.

A constant term in Einstein's field equations that represents the energy density of the vacuum, influencing the expansion of the universe.

A Belgian priest and physicist who proposed the Big Bang theory, suggesting the universe started from a hot, dense state and has been expanding ever since.

A crucial factor in General Relativity that describes how the universe expands or contracts over time. It is the scale factor of the universe.

A model of the universe in cosmology that assumes a universe that is homogeneous, isotropic, and dynamically expanding. It is based on the Friedmann-Lemaître-Robertson-Walker (FRWL) metric.

Defines that the universe appears the same in all directions and is uniform on large scales.

Defines that the universe looks the same regardless of the direction it is observed from.

A fundamental principle in cosmology stating that the universe is homogeneous and isotropic on a large scale.

A key concept in cosmology that describes how the density of the universe changes with its expansion.

An English physicist and mathematician known for his laws of motion, universal gravitation, and calculus.

A German astronomer, mathematician, and astrologer who formulated the laws of planetary motion.

The elliptical path that planets follow as they orbit the sun. Kepler inferred this by observing Mars's movement.

A collection of observations of Mars when it is in a specific position relative to the Sun and Earth. This data helped Kepler understand Mars's orbit.

He revolutionized how we understand the universe, shifting the focus from Earth to the Sun. He also used telescopes to make observations, starting the field of observational astronomy.

Venus displays phases like the Moon (new, crescent, full, etc.). This supports the idea that Venus orbits the Sun, not the Earth.

Tycho Brahe was famous for meticulously recording the positions of stars for a long period. He discovered a supernova, which proved the universe wasn't static, contradicting Aristotle's ideas.

Tycho Brahe proposed a model where the planets orbit the Sun, but the Sun still orbits the Earth. This was a compromise between the heliocentric (Sun-centered) and geocentric (Earth-centered) views.

Copernicus believed the Sun was the center of the universe and not the Earth. His model challenged traditional thinking and paved the way for a new understanding of the cosmos.

Copernicus's ideas were originally outlined in his book, 'Little Commentary'. This work laid the foundation for his later, more extensive heliocentric model.

A dense, hot mixture of quarks, gluons, and other particles, existing in the early universe, approximately 10⁻⁶ seconds after the Big Bang.

Tiny, nearly massless particles that decoupled from matter around 1 second after the Big Bang, leaving a detectable 'cosmic neutrino background.'

The process by which protons and neutrons combined to form the first atomic nuclei (mostly hydrogen, helium, and lithium), occurring approximately 3 minutes after the Big Bang.

The observed brightness of a celestial object, as seen from Earth.

The actual brightness of a celestial object, regardless of its distance from Earth.

Celestial objects with known intrinsic brightness, used to determine distances in the universe.

A type of variable star with a consistent relationship between its pulsation period and its luminosity, used to measure distances in the universe.

The change in the frequency of light (or sound) due to the relative motion of the source and the observer. In astronomy, it helps determine the velocity of celestial objects.

The energy distribution of light across different wavelengths, with red representing lower energy and blue representing higher energy.

Dark lines appearing in a continuous spectrum of light, caused by the absorption of specific wavelengths by atoms in a gas or cloud. These lines can tell us about the composition and temperature of the absorbing material.

The combined light of billions of stars within a galaxy, creating a unique 'fingerprint' of the galaxy's composition and motion.

The shift of light towards longer wavelengths (redshift) as objects move away from us due to the expansion of the universe.

The linear relationship between the recessional velocity of a galaxy and its distance from Earth, indicating that galaxies are moving away from us at a rate proportional to their distance.

The observed expansion of the universe, where galaxies move away from each other without a central point, supported by the increasing redshift of distant galaxies.

The stretching of wavelengths of light due to the expansion of the universe, making light appear redder than it actually is. Redshift is directly related to the scale factor of the universe, representing the expansion of space itself.

A period in the early universe when photons and baryonic matter decoupled, allowing photons to freely travel and create the Cosmic Microwave Background (CMB).