Dark matter is the name given to additional mass whose presence is inferred only from gravitational attraction but which does not interact with light. It accounts for roughly 25% of the energy density of the universe. Dark matter is presumed to be a massive (~100 GeV scale) particle which is basically non-interacting except through gravity. There are several ongoing efforts to directly or indirectly detect the presumed dark matter particle.
Reasons we believe in dark matter:
Supersymmetry suggests that there should be a "lightest supersymmetric particle" which would be an ideal dark matter particle candidate. Other proposals are the axion or any next lightest particle from some string theory model. Excluded models are MACHOs (MAssive Compact Halo Objects) and neutrino dark matter (neutrinos would be too hot to cluster on the appropriate scales). One umbrella name for non-specific models isWIMPs (Weakly Interacting Massive Particles).
Direct detection searches operate under the assumption that dark matter must have some very small interaction cross section which allows it to scatter off of material in a detector which they work very hard to isolate from all external interactions. Another approach is to hope that at very high energies, a collider experiment will generate particles of dark matter which would show up as missing momentum/energy in the collision tracking.
Indirect detections operate under the assumption that dark matter may have some small annihilation cross section and look for the signature of annihilation in regions of high dark matter density.
Alternative gravity theories try to explain the above effects by modifying General Relativity rather than introducing a new massive particle. MOND (Modified Newtonian Dynamics) is generally discredited for being a phenomenological model rather than a fundamental model and requires tuning to match different scales, besides not being able to reproduce lensing, CMB, or BAO data. TeVeS is another alternative which is a fundamental theory (I don't know enough about it to comment on its validity).
Dark matter is part of the standard model of cosmology. It is true that there are several candidates for what particle dark matter may be and we aren't sure which one is correct. However, the field is fairly confident that Dark Matter is a sign of new physics beyond the standard model of particle physics, and not of a modification to General Relativity. This is due to the high level of consistency between the above mentioned data, assuming WIMP dark matter. As we believe that there must be a model beyond the standard model, it is natural to accept that there are more massive particles which we don't yet understand.
Reasons we believe in dark matter:
- Galactic rotation curves: spiral galaxies's angular rotation as a function of radius allows one to measure the mass interior to that radius. This method gives a mass roughly 10 times the mass we can see from luminous matter.
- Galaxy cluster masses from galaxy dispersion velocities: if one assumes that galaxy clusters are "virialized", which is a form of kinetic equilibrium, then one can determine the mass from the velocities of the member galaxies. This gives a mass roughly 20 times the mass of the luminous material in galaxies.
- Galaxy cluster masses from hot X-ray emitting gas: massive galaxy clusters have hot, diffuse gas (roughly the same mass as in galaxies!) which glows in the X-ray band. This gas is also close to virialized, and measurements of the X-ray spectrum can give a mass agreeing with the above.
- Gravitational lensing: according to General Relativity, light is deflected from the path it would take in Newtonian gravity due to the presence of mass or energy. The amount of deflection or the statistics of the distortions of many images around an object can be used to measure the total gravitating mass of the lens, regardless of the type of gravitating mass. This allows us to map out where the mass is and compare this with where the light is. A famous example of this is the Bullet Cluster, which can be mapped using visible light to determine where the stars are; X-rays, to determine where the hot inter-galactic gas is; and lensing, to determine where most of the mass is. The X-ray gas is as massive or more than the stars, and is stripped from the galaxies due to a recent merger event. The lensing data tell the tale that most of the mass (which is more than that seen from the X-ray gas mass plus stars) has not collided and has just passed right through after this merger event.
- Cosmic Microwave Background: analysis of the CMB determines the scale of clustering of matter at the time of last scattering. There are several "acoustic peaks" in the multipole spectrum of the CMB, corresponding to compressions and rarefactions of plasma at scales corresponding to half-cycles of the universe's age at that time. Since normal matter has pressure support but dark matter in principle would not, these would affect the compressions in different ways. The fits to the data require dark matter in order to be consistent with the pattern of compressions and rarefactions.
- Baryon Acoustic Oscillation is similar to the above effect but on the length scales of the universe today, not at the time of last scattering.
Supersymmetry suggests that there should be a "lightest supersymmetric particle" which would be an ideal dark matter particle candidate. Other proposals are the axion or any next lightest particle from some string theory model. Excluded models are MACHOs (MAssive Compact Halo Objects) and neutrino dark matter (neutrinos would be too hot to cluster on the appropriate scales). One umbrella name for non-specific models isWIMPs (Weakly Interacting Massive Particles).
Direct detection searches operate under the assumption that dark matter must have some very small interaction cross section which allows it to scatter off of material in a detector which they work very hard to isolate from all external interactions. Another approach is to hope that at very high energies, a collider experiment will generate particles of dark matter which would show up as missing momentum/energy in the collision tracking.
Indirect detections operate under the assumption that dark matter may have some small annihilation cross section and look for the signature of annihilation in regions of high dark matter density.
Alternative gravity theories try to explain the above effects by modifying General Relativity rather than introducing a new massive particle. MOND (Modified Newtonian Dynamics) is generally discredited for being a phenomenological model rather than a fundamental model and requires tuning to match different scales, besides not being able to reproduce lensing, CMB, or BAO data. TeVeS is another alternative which is a fundamental theory (I don't know enough about it to comment on its validity).
Dark matter is part of the standard model of cosmology. It is true that there are several candidates for what particle dark matter may be and we aren't sure which one is correct. However, the field is fairly confident that Dark Matter is a sign of new physics beyond the standard model of particle physics, and not of a modification to General Relativity. This is due to the high level of consistency between the above mentioned data, assuming WIMP dark matter. As we believe that there must be a model beyond the standard model, it is natural to accept that there are more massive particles which we don't yet understand.
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