dark matter

dark matter is a form of matter hypothesized to account for approximately 85% of the matter in the universe and about a quarter of its total mass–energy density. Its presence is inferred from gravitational effects on visible matter, radiation, and the large-scale structure of the cosmos. Unlike ordinary matter, it does not interact with the electromagnetic force, rendering it invisible to the entire electromagnetic spectrum and detectable only through its gravitational influence.

Overview

The concept arose from discrepancies between the calculated mass of large astronomical objects and their observed gravitational effects. Early work by astronomers like Jan Oort on the Milky Way and Fritz Zwicky on the Coma Cluster suggested missing mass. Definitive evidence came from the rotation curve measurements of spiral galaxies by Vera Rubin and W. Kent Ford using instruments at the Carnegie Institution for Science. These observations, consistent with earlier predictions by Horace W. Babcock, showed stars orbiting at speeds impossible unless enveloped by a massive, unseen component. This component, pervasive in galaxy clusters and evident in gravitational lensing studies like the Bullet Cluster, forms the cosmological framework of the Lambda-CDM model.

Observational evidence

Multiple independent lines of evidence strongly support its existence. Galaxy rotation curves, first conclusively mapped by Rubin and Ford at the Kitt Peak National Observatory, remain a primary indicator. The dynamics of galaxy clusters, initially studied by Zwicky at the Mount Wilson Observatory, require far more mass than is luminous. Observations of gravitational lensing, such as those conducted with the Hubble Space Telescope on systems like the Bullet Cluster, directly map its distribution. The cosmic microwave background anisotropy data from missions like WMAP and the Planck spacecraft precisely constrain its total density. Additionally, baryon acoustic oscillations measured by surveys like the Sloan Digital Sky Survey and the large-scale structure of the universe align with simulations requiring a dominant cold component.

Composition and properties

Its fundamental nature remains one of the greatest puzzles in modern physics. It is non-baryonic, distinct from the protons and neutrons described by the Standard Model. Leading candidates include Weakly Interacting Massive Particles (WIMPs), which would be relics from the Big Bang and could be detected via their weak-scale interactions. Other proposals are axions, hypothetical light particles postulated to solve the strong CP problem in quantum chromodynamics, and sterile neutrinos. Alternatives within particle physics include theories involving supersymmetry. Its key properties are stability over cosmological timescales, lack of electromagnetic interaction, and it is "cold," meaning it moved slowly in the early universe, crucial for structure formation.

Detection methods

Experiments worldwide aim to identify its particle nature through direct detection, indirect detection, and collider production. Direct detection experiments like XENON at the Gran Sasso National Laboratory, LUX at the Sanford Underground Research Facility, and PandaX search for rare nuclear recoils from WIMP interactions. Indirect detection efforts, using instruments like the Fermi Gamma-ray Space Telescope and the IceCube Neutrino Observatory, look for anomalous products from self-annihilation in regions like the Galactic Center. Particle colliders, notably the Large Hadron Collider at CERN, attempt to create it in high-energy proton–proton collisions. Astronomical surveys, including those by the Vera C. Rubin Observatory, will further constrain its distribution through weak lensing.

Theoretical models and alternatives

While the Lambda-CDM model is the standard cosmological framework, alternative explanations for the observational phenomena exist. Modified Newtonian dynamics (MOND), proposed by Mordehai Milgrom, modifies Newton's laws of motion at low accelerations. Other theories involve modifying general relativity, such as Tensor–vector–scalar gravity. However, these alternatives struggle to explain all evidence simultaneously, particularly data from the Bullet Cluster and the cosmic microwave background. Particle physics models beyond the Standard Model, including those from string theory, provide numerous candidate particles. The interplay between cosmology and particle theory continues to drive research at institutions like the Institute for Advanced Study.

Role in structure formation

It is the gravitational scaffolding for the cosmic web. In the early universe, its density perturbations, imprinted from cosmic inflation, grew under gravity. Because it does not interact with radiation, these clumps could begin coalescing before recombination, unlike ordinary matter which was coupled to the photon bath. After recombination, baryons fell into these pre-existing gravitational wells, forming the first stars, galaxies, and clusters. Supercomputer simulations, such as the Millennium Run and those performed on systems at the NASA Ames Research Center, show that a universe dominated by cold material reproduces the observed large-scale structure, including filaments and voids. Its distribution continues to influence the evolution of galaxies and clusters, a key focus of missions like the James Webb Space Telescope. Category:Cosmology Category:Hypothetical particles Category:Physical cosmology