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| Hot dark matter | |
|---|---|
| Name | Hot dark matter |
| Type | Hypothetical dark matter component |
| Composition | Light, relativistic particles (e.g., neutrinos) |
| Significance | Impacts large-scale structure, cosmic microwave background |
| Introduced | 1970s–1980s |
Hot dark matter is a hypothetical component of cosmic dark matter composed of particles that were relativistic at the time of matter–radiation equality. It contrasts with cold dark matter candidates by suppressing the formation of small-scale structure and shaping the large-scale distribution of matter in the universe. Studies linking particle physics and observational cosmology have driven constraints on its contribution to the total matter density through combined analyses of cosmic microwave background anisotropies, large-scale structure surveys, and neutrino experiments.
In cosmology the classification of dark matter into hot, warm, and cold depends on the free-streaming velocity of particles and their decoupling history relative to the Big Bang and recombination epoch. Early proposals for relativistic dark matter drew on the properties of the neutrino as described in the Standard Model and in extensions studied at institutions such as CERN, Fermilab, and SLAC National Accelerator Laboratory. The impact of a relativistic population on growth of perturbations was examined in the context of theoretical frameworks like the Lambda-CDM model, alternatives proposed by researchers associated with the Institute for Advanced Study and the Kavli Institute for Cosmological Physics, and the numerical simulations produced by collaborations at the Max Planck Institute for Astrophysics.
Hot dark matter candidates are characterized by low mass and relativistic velocities during key cosmological epochs. The archetypal candidate is the three active species of electron neutrino, muon neutrino, and tau neutrino originally postulated by Wolfgang Pauli and observed via experiments at facilities including Super-Kamiokande, Sudbury Neutrino Observatory, and IceCube. Sterile neutrinos invoked in proposals by groups at Brookhaven National Laboratory and papers by Bruno Pontecorvo and Vladimir Gribov are sometimes discussed, but if sufficiently light they act as warm or hot components depending on production mechanisms considered by teams at DESY and Los Alamos National Laboratory. Relativistic axions produced in the early Peccei–Quinn theory frameworks have been considered in models associated with Harvard University and Princeton University, yet most axion scenarios behave as cold components when produced nonthermally. Mass limits from oscillation experiments by collaborations like Daya Bay Reactor Neutrino Experiment and KamLAND set physical constraints on whether neutrinos could supply the majority of the cosmic matter density.
A hot component dramatically alters linear growth: free-streaming washes out density perturbations below the free-streaming length, favoring top-down formation where superclusters collapse before galaxies. Theoretical analyses by researchers from University of Cambridge, California Institute of Technology, and University of Chicago compared predictions against observations from surveys such as the Sloan Digital Sky Survey, 2dF Galaxy Redshift Survey, and the Dark Energy Survey. Numerical work by groups at the European Southern Observatory and Lawrence Berkeley National Laboratory incorporated hot components into N-body simulations to test effects on the matter power spectrum and galaxy bias. Constraints on hot fractions were refined using cosmic microwave background measurements from COBE, WMAP, and Planck Collaboration, which are sensitive to relativistic degrees of freedom parameterized alongside inputs from Baryon Acoustic Oscillations analyses performed at Max Planck Institute for Extraterrestrial Physics.
Observations limit the contribution of relativistic particle species to the total matter density and to the effective number of neutrino species, Neff. Measurements by the Planck Collaboration, combined with baryon acoustic oscillation results from Baryon Oscillation Spectroscopic Survey, underpin tight upper bounds on the sum of neutrino masses derived by teams at Carnegie Mellon University and University of Copenhagen. Laboratory bounds from KATRIN and cosmological bounds from analyses led by researchers at University of Oxford and Harvard-Smithsonian Center for Astrophysics converge to exclude a cosmologically dominant hot dark matter component. Lyman-alpha forest observations from instruments at Keck Observatory and Very Large Telescope further constrain light relics by probing small-scale structure sensitive to free-streaming suppression.
Direct laboratory searches for light relativistic particles focus on neutrino mass experiments and oscillation studies. Projects such as KATRIN, PTOLEMY (planned), and tritium beta decay experiments conducted at Institut Laue-Langevin aim to measure absolute masses that determine cosmological impact. Indirect constraints derive from cosmic probes: analyses of anisotropies by Planck Collaboration and lensing measurements by Atacama Cosmology Telescope and South Pole Telescope constrain relativistic relic density. Terrestrial neutrino detectors including Super-Kamiokande, JUNO, and DUNE refine oscillation parameters; accelerator neutrino programs at CERN and Fermilab investigate sterile neutrino scenarios. Proposed surveys by Euclid (spacecraft) and Nancy Grace Roman Space Telescope will tighten limits by mapping large-scale structure and weak gravitational lensing.
Interest in relativistic dark matter arose in the 1970s and 1980s with early work by theorists at University of California, Berkeley and Princeton University who explored neutrino-dominated cosmologies. Debates during the 1990s between proponents of hot, warm, and cold models occurred in conferences hosted by International Astronomical Union and workshops at Kavli Institute for Theoretical Physics. The cold dark matter paradigm gained dominance after comparisons to galaxy surveys and cosmic microwave background results from COBE and WMAP favored nonrelativistic candidates, a consensus crystallized in reviews published by groups linked to Cambridge University Press and Annual Reviews. Ongoing interplay between particle physics experiments at CERN and cosmological observations from Planck Collaboration continues to refine the possible role of relativistic relics in the composition of the cosmos.
Category:Cosmology Category:Particle astrophysics