Lambda-CDM model
Overview
The Lambda-CDM model is the cosmological model that combines the cosmological constant (Lambda) and cold dark matter (CDM) to explain the observations of the structure and expansion of the universe. It is currently the simplest model that provides a reasonably accurate description of the universe's properties, including the cosmic microwave background radiation, the distribution of galaxies, and the accelerating expansion of the universe.
Cosmological Constant (Lambda)
The cosmological constant, denoted by the Greek letter Lambda (Λ), was first introduced by Albert Einstein in his field equations of general relativity. Einstein initially introduced the cosmological constant to allow a static solution to his equations. However, after the discovery of the universe's expansion, Einstein reportedly referred to the cosmological constant as his "biggest blunder". In the context of the Lambda-CDM model, the cosmological constant is associated with the energy density of the vacuum, often referred to as dark energy.
Cold Dark Matter (CDM)
Cold dark matter is a hypothetical form of matter that does not interact with electromagnetic radiation, making it invisible to the entire electromagnetic spectrum. Observations of the universe on various scales, from galaxies to the large-scale structure of the universe, suggest that about 85% of the matter in the universe is dark matter. The "cold" in cold dark matter refers to the fact that the dark matter moves slowly compared to the speed of light.
Structure Formation
The Lambda-CDM model successfully explains the large-scale structure of the universe. The model predicts that small initial fluctuations in the density of the universe led to the formation of galaxies and clusters of galaxies we see today. These initial fluctuations are thought to have been caused by quantum fluctuations in the early universe, magnified by cosmic inflation.
Cosmic Microwave Background
The cosmic microwave background (CMB) is the thermal radiation left over from the time of recombination in Big Bang cosmology. Observations of the CMB provide strong support for the Lambda-CDM model. The model accurately predicts the observed large-scale anisotropies and the temperature fluctuations in the CMB.
Accelerating Expansion
One of the key features of the Lambda-CDM model is the prediction of an accelerating universe. This was confirmed by observations of distant supernovae, which showed that the universe's expansion is accelerating. This acceleration is attributed to the energy density of the vacuum, or dark energy, represented by the cosmological constant in the model.
Criticisms and Alternatives
While the Lambda-CDM model is currently the best fit to observational data, it is not without its problems. There are several unresolved issues, such as the nature of dark matter and dark energy, the cosmological constant problem, and the so-called "small-scale crisis". Several alternative models have been proposed, including modified gravity theories and models with additional forms of dark energy.