Warm dark matter (WDM) is a hypothesized form of
dark matter
Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not a ...
that has properties intermediate between those of
hot dark matter and
cold dark matter, causing structure formation to occur bottom-up from above their free-streaming scale, and top-down below their free streaming scale. The most common WDM candidates are
sterile neutrino
Sterile neutrinos (or inert neutrinos) are hypothetical particles (neutral leptons – neutrinos) that are believed to interact only via gravity and not via any of the other fundamental interactions of the Standard Model. The term ''sterile neutri ...
s and
gravitinos. The WIMPs (
weakly interacting massive particles
Weakly interacting massive particles (WIMPs) are hypothetical particles that are one of the proposed candidates for dark matter.
There exists no formal definition of a WIMP, but broadly, a WIMP is a new elementary particle which interacts via gra ...
), when produced non-thermally, could be candidates for warm dark matter. In general, however, the thermally produced WIMPs are
cold dark matter candidates.
keVins and GeVins
One possible WDM candidate particle with a mass of a few
keV Kev can refer to:
Given name
* Kev Adams, French comedian, actor, screenwriter and film producer born Kevin Smadja in 1991
* Kevin Kev Carmody (born 1946), Indigenous Australian singer-songwriter
* Kev Coghlan (born 1988), Scottish Grand Prix moto ...
comes from introducing two new, zero charge, zero
lepton number
In particle physics, lepton number (historically also called lepton charge)
is a conserved quantum number representing the difference between the number of leptons and the number of antileptons in an elementary particle reaction.
Lepton number ...
fermions to the
Standard Model of Particle Physics: "keV-mass inert fermions" (keVins) and "GeV-mass inert fermions" (GeVins). keVins are overproduced if they reach thermal equilibrium in the early universe, but in some scenarios the
entropy production from the decays of unstable heavier particles can suppresses their abundance to the correct value. These particles are considered "inert" because they only have suppressed interactions with the
Z boson.
Sterile neutrinos
Sterile neutrinos (or inert neutrinos) are hypothetical particles (neutral leptons – neutrinos) that are believed to interact only via gravity and not via any of the other fundamental interactions of the Standard Model. The term ''sterile neutri ...
with masses of a few keV are possible candidates for keVins.
At temperatures below the
electroweak scale their only interactions with standard model particles are
weak interactions due to their
mixing with ordinary
neutrinos. Due to the smallness of the mixing angle they are not overproduced because they freeze out before reaching thermal equilibrium. Their properties are consistent with astrophysical bounds coming from structure formation and the
Pauli principle
In quantum mechanics, the Pauli exclusion principle states that two or more identical particles with half-integer spins (i.e. fermions) cannot occupy the same quantum state within a quantum system simultaneously. This principle was formula ...
if their mass is larger than 1-8 keV.
In February 2014, different analyses
have extracted from the spectrum of X-ray emissions observed by
XMM-Newton
''XMM-Newton'', also known as the High Throughput X-ray Spectroscopy Mission and the X-ray Multi-Mirror Mission, is an X-ray space observatory launched by the European Space Agency in December 1999 on an Ariane 5 rocket. It is the second cornerst ...
, a monochromatic signal around 3.5 keV. This signal is coming from different
galaxy cluster
A galaxy cluster, or a cluster of galaxies, is a structure that consists of anywhere from hundreds to thousands of galaxies that are bound together by gravity, with typical masses ranging from 1014 to 1015 solar masses. They are the second-lar ...
s (like
Perseus and
Centaurus
Centaurus is a bright constellation in the southern sky. One of the largest constellations, Centaurus was included among the 48 constellations listed by the 2nd-century astronomer Ptolemy, and it remains one of the 88 modern constellations. ...
) and several scenarios of warm dark matter can justify such a line. We can cite, for example, a 3.5 keV candidate annihilating into 2 photons,
or a 7 keV dark matter particle decaying into a photon and a neutrino.
In November 2019, analysis of the interaction of various galactic halo matter on densities and distribution of stellar streams, coming off the satellites of the Milky Way, they were able to constrain minimums of mass for density perturbations by warm dark matter keVins in the GD-1 and Pal 5 streams. This lower limit on the mass of warm dark matter thermal relics mWDM > 4.6 keV; or adding dwarf satellite counts mWDM > 6.3 keV
See also
*
**
**
*
*
References
*
*
*
*
* Millis, John. Warm Dark Matter. About.com. Retrieved 23 Jan., 2013. http://space.about.com/od/astronomydictionary/g/Warm-Dark-Matter.htm.
Further reading
*
Dark matter
{{particle-stub