The ΛCDM (Λ [Greek sign for Lambda] Cold Dark Matter), is a model definition of the Big Bang cosmological theory. In the ΛCDM model definition, the universe is made of three major components. There is a cosmological constant, denoted by Λ (Lambda, a symbol associated with dark energy in physics), the energy density of space, or vacuum energy, from Einstein’s field equations of general relativity. The second component is the hypothesized cold dark matter (CDM). The third major component is ordinary matter. The ΛCDM is often referred to as the Standard Model of Big Bang cosmology because it is the simplest explanation that accounts for the following properties.
- It posits the existence of a Cosmic Microwave Background. The Cosmic Microwave Background is electromagnetic radiation that is a remnant from the early stage of the universe, known as relic radiation. It is the oldest electromagnetic radiation in the universe and is a faint filler of all space. It cannot be detected with a regular telescope. However, when looking through a sensitive radio telescope, the space between any celestial objects is not black, but filled with a faint, near isotropic glow, unassociated with any ordinary matter.
- It accounts for the accelerating expansion of the universe, which can be observed in the light of supernovae and distant galaxies.
- It gives a good account of the large scale structure in the distribution of galaxies.
- It accounts for the abundances hydrogen, helium, and lithium in the universe.
The ΛCDM assumes that Einstein’s general relativity is the correct theory of gravity on a cosmic scale. In this model, the total mass–energy in the universe contains 5% ordinary matter and energy, 27% dark matter, and 68% of a form of energy known as dark energy. Therefore, dark matter constitutes 85% of total universal mass, while dark energy plus dark matter constitute 95% of its total mass–energy content.
Dark matter is thought to be completely uniform across space. Though dark matter has never been observed, it is thought to just barely interact with ordinary matter and only through gravity and weak force, therefore it is collisionless. It may be composed of yet undiscovered subatomic particles. Dark matter can be classified according to its velocity as cold, warm, and hot. Cold dark matter moves slowly compared to the speed of light. It is referred to as cold because of its weak interaction with ordinary matter and electromagnetic radiation. It is hypothesized that ordinary matter, observable cosmological structures, emerge in this way through a gradual accumulation of particles.