Contents

Hot Dark MatterEdit

Hot dark matter is a class of dark matter composed of light, fast-moving particles that were still relativistic when they decoupled from the primordial plasma in the early universe. The canonical example is the known light neutrinos, whose small masses imply high thermal velocities for a significant period after decoupling. In the standard taxonomy of dark matter, hot dark matter (HDM) is contrasted with cold dark matter (CDM) and warm dark matter (WDM), which differ primarily in their typical velocities and the scales on which they influence structure formation.

In the history of cosmology, HDM played a central role in early model-building. The idea was that fast-moving particles would erase small-scale fluctuations through their free streaming, leading to a “top-down” sequence of structure formation where large objects formed first and fractured into smaller ones. As data accumulated from measurements of the cosmic microwave background and the distribution of galaxies, it became clear that this simple HDM picture could not account for the abundance and formation of small-scale structures like galaxies and dwarf systems. This realization helped steer the cosmology community toward models in which cold dark matter dominates and hot components play only a limited role. Neutrino masses inferred from laboratory experiments and cosmological observations set quantitative limits on how much HDM can contribute to the total matter content of the universe.

The sections below survey the physical basis for hot dark matter, how it affects the growth of cosmic structure, the key observational constraints, and how the idea sits in contemporary cosmology.

Physical basis and particle candidates

Hot dark matter is characterized by relativistic or near-relativistic velocities when particles decouple from the primordial plasma. The rate at which these particles move, and the scale over which they suppress density fluctuations, is governed by free-streaming: fast particles travel long distances, smearing out inhomogeneities on small scales. The canonical HDM candidate is the active neutrino species of the Standard Model, whose masses are small enough that their thermal motion remains substantial for a considerable epoch after decoupling. The potential presence of additional light species, such as sterile neutrinos, could also contribute to the hot component under certain conditions.

HDM is often discussed in relation to the broader category of dark matter. The small but nonzero velocity dispersion of HDM contrasts with the typically nonrelativistic nature assumed for CDM, and with the intermediate behavior sometimes described for WDM. Researchers model the influence of HDM on the matter power spectrum, the growth of density perturbations, and the timing of structure formation, comparing predictions to observations from multiple probes of the cosmos.

Implications for structure formation

The hallmark of hot dark matter is its tendency to erase density fluctuations below its free-streaming scale. In a universe with a substantial HDM component, early fluctuations on small scales would be suppressed, and large-scale structures would form first, subsequently fragmenting into smaller systems. This “top-down” picture is at odds with a wide range of observations indicating that galaxies and clusters appear at earlier times than a pure HDM scenario would predict.

  • The cosmic web and galaxy formation history are used to test HDM scenarios against data from the distribution of galaxies and the growth of large-scale structure. Observations of the Cosmic microwave background anisotropies, the distribution of galaxies, and the absorption features in the Lyman-alpha forest provide stringent constraints on how much HDM can be present.
  • The interplay with CDM is central: a subdominant hot component can, in principle, coexist with cold dark matter without spoiling the success of CDM-based structure formation, but the allowed fraction is tightly limited by data.
  • Related topics: Structure formation, Large-scale structure.

Observational constraints and current status

Cosmological measurements place robust limits on the amount of hot dark matter in the universe. The masses of the known active neutrinos, together with cosmological data, constrain the sum of neutrino masses to be small, which in turn limits the possible HDM fraction. Observations of the cosmic microwave background, especially when combined with large-scale structure data, imply that hot components cannot dominate the matter budget and must constitute only a subdominant portion of the total dark matter. This has led to a cosmological model in which the bulk of dark matter is cold, with a permissible but small hot admixture.

  • Key data sources: Cosmic microwave background, Planck (spacecraft), Lyman-alpha forest observations, galaxy surveys.
  • The role of neutrino mass measurements: laboratory experiments on neutrino oscillations and cosmological probes constrain the allowable neutrino mass scale, thereby limiting HDM contributions.
  • Sterile neutrino scenarios can be discussed in the context of HDM or as separate model variants, depending on their mass and production history.

The historical debate over HDM shaped how cosmologists approached the problem of structure formation. While a purely HDM universe proved incompatible with the observed abundance of small galaxies and the detailed shape of the matter power spectrum, acknowledging a nonzero hot component has helped refine mixed dark matter models. In contemporary cosmology, hot dark matter remains a real but constrained component, providing insight into the properties of the light particle zoo that interacts with the early universe.

See also