Is there a non hypothetical particle [aka axion] that can be the make up of Dark Matter?
Partially true. There is a hypothetical particle known as axion which could make up the dark matter if it is proven to exist.
Some hold that normal neutrinos can partially explain dark matter, and these are not hypothetical.
However unless we have a big misunderstanding of neutrinos they cannot possibly explain the amount of dark matter which apparently exists.
I thought that the axion is an supersymmetric particle and that super-symmetry has not been proved
No, the axion does not a priori have anything to do with supersymmetry. It is introduced as a way of solving the strong CP problem and at the same time results in a dark matter candidate. You can do both, i.e., introduce the axion and supersymmetry, the superpartner of the axion would be the axino.
The "standard" SUSY dark matter candidate would be a neutralino, i.e., the superpartner of some linear combination of Z, photon, and neutral Higgses.
According to Wiki.
Theory suggests that axions were created abundantly during the Big Bang. Because of a unique coupling to the instanton field of the primordial universe (the "misalignment mechanism"), an effective dynamical friction is created during the acquisition of mass following cosmic inflation. This robs all such primordial axions of their kinetic energy.
If axions have low mass, thus preventing other decay modes, theories predict that the universe would be filled with a very cold Bose–Einstein condensate of primordial axions. Hence, axions could plausibly explain the dark matter problem of physical cosmology. Observational studies are underway, but they are not yet sufficiently sensitive to probe the mass regions if they are the solution to the dark matter problem. High mass axions of the kind searched for by Jain and Singh (2007) would not persist in the modern universe. Moreover, if axions exist, scatterings with other particles in the thermal bath of the early universe unavoidably produce a population of hot axions.
So Axions would be hot dark matter, not cold dark matter?
This would require them to come into thermal contact with the rest of the Universe. I cannot speak for the paper they cite, but as far as I understand from people working on axions, this is not generally accepted and the idea is that they are not thermally produced.
This paper shows that Axions can be hot or cold, more likely hot
We have examined a possibility that the pNG bosons, especially the QCD axions, account for both HDM and CDM in the Universe, the former of which has been suggested by the recent observations (cf. Eqs. (1) and (2)). We divide the production process of the axion HDM into thermal and non-thermal ones. In the thermal case, the QCD axion can explain HDM for the decay constant fa ≈ 3 × 106 − 107 GeV, which however is in tension with the SN1987A limit even for the hadronic axion models. On the other hand, the axion HDM can be naturally produced by the saxion decay. This is possible for the saxion mass ranging from O(103 ) GeV to O(1010) GeV and fa . 3 × 1010 GeV. Note that the non-thermally produced axions need to be “colder” than the ambient plasma, in order to explain the hierarchy between the effective HDM mass of O(0.1) eV and the physical axion mass ma = 0.006 eV(fa/109 GeV)−1 (cf. Eq. (11)). We have discussed two cases in which such axions are produced. In the case (i), there is a latetime entropy production which dilutes the axions produced by the saxion decay, assuming that the saxion dominates the Universe and decays dominantly into a pair of axions. In the case (ii), the saxion decays into a pair of axions when it is subdominant. Our analysis can be also applied to the case where the saxion coherent oscillations partially evaporates into plasma after being trapped at the origin by the thermal effects . We have pointed out that the axion HDM can be a natural outcome of the saxion trapped at the origin. The axion CDM can be produced by either the initial misalignment mechanism or domain wall annihilation
One possible WDM candidate particle with a mass of a few keV comes from introducing two new, zero charge, zero 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 with masses of a few keV are possible candidates for keVins. At temperatures below theelectroweak 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 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, a monochromatic signal around 3.5 keV. This signal is coming from different galaxy cluster (like Perseus or Centaurus ) 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 photon and neutrino.
Do you have further questions? Simply repasting from Wikipedia articles is not going to further any productive discussions.
According to this site warm dark matter is a better candidate, than cold dark matter, my question is why do cosmologist stick to cold dark matter in their theories
While we are at this, has the MACHO hypothesis been completely ruled out?
I do know it's no longer popular, but could this explain at least partly what the missing mass is?
A large amount of ordinary baryonic matter that never quite aggregated enough to become galaxies, maybe more failed brown dwarf stars than we thought?
rootone, this is from Wiki, it seems some thing around 1% of Dark Matter is Machos
A MACHO may be detected when it passes in front of or nearly in front of a star and the MACHO's gravity bends the light, causing the star to appear brighter in an example ofgravitational lensing known as gravitational microlensing. Several groups have searched for MACHOs by searching for the microlensing amplification of light. These groups have ruled out dark matter being explained by MACHOs with mass in the range 0.00000001 solar masses (0.3 lunar masses) to 100 solar masses. One group, the MACHO collaboration, claims to have found enough microlensing to predict the existence of many MACHOs with mass of about 0.5 solar masses, enough to make up perhaps 20% of the dark matter in the galaxy. This suggests that MACHOs could be white dwarfs or red dwarfs which have similar masses. However, red and white dwarfs are not completely dark; they do emit some light, and so can be searched for with the Hubble Telescope and with proper motion surveys. These searches have ruled out the possibility that these objects make up a significant fraction of dark matter in our galaxy. Another group, the EROS2 collaboration does not confirm the signal claims by the MACHO group. They did not find enough microlensing effect with a sensitivity higher by a factor 2. Observations using the Hubble Space Telescope's NICMOS instrument showed that less than one percent of the halo mass is composed of red dwarfs. This corresponds to a negligible fraction of the dark matter halo mass. Therefore, the missing mass problem is not solved by MACHOs.
The axion doesn't have to be necessarily supersymmetric. Of course it can be extended into supersymmetric and supergravity theories.
The axion was first proposed as a resolution to the Strong CP-problem (by Peccei-Quinn and Weinberg-Wilczek) and was later on seen as CDM candidate.
I think that CDM is needed to explain the structure formation and CMB density anisotropies, whereas HDM can't do that.
As for whether axions are HDM or CDM, I think it's more interesting to look at them as candidates for CDM (for the size of which we don't have a verified component). It all depends on when axions were first created (at what temperatures the PQ symmetry broke relative to the inflation).
Are Axions the only dark matter candidate?
Certainly not. The most commonly-thought of is a supersymmetric particle, where if supersymmetry is true then the lightest supersymmetric particle would become dark matter. The neutralino is the most common, but there are other possibilities as well.
Axions are usually considered a more exotic dark matter candidate, actually, because their production mechanism is a bit elaborate. But personal tastes always vary with this sort of thing.
In addition to the mainstream candidates , many more exotic candidates have been suggested - WIMPzillas, Q-balls, gravitinos, etc. are these still plausible
Sterile neutrinos still appear viable for at least some fraction of dark matter. The plus side is they fit the standard model.
I would say this is something purely subjective. They certainly are not part of the standard model and there are several possibilities of extending it which would be fine with having the SM as a limit. The introduction of right-handed neutrinos into the SM may be minimalistic to some extent, but that is another issue.
Dark matter is really a mysterious monster, if susy exists, then supersymmetric particle must be its candidate, and if not, I think we should jump out the old thinking way and envisage a new theory to solve all of thorny problems completely.
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