Cambridge Cosmology: Galaxies

The Dark Matter of the Universe

There is strong evidence that the universe consists primarily of dark (non-luminous) matter, also that thismatter is of an exotic non-baryonic form. Estimates of the density parameter Omega vary on different lengthscales:

Luminous matter:

A simple addition over all the luminous material---stars and hot gas---within the optical radius of galaxies yields a non-dynamical estimate for the mass density, Omega ~ 0.005. This is a lower limit for the baryonic density which can be extended by integrating over the total background luminosity of the universe, but it still yields only Omega ~ 0.04. Further baryons could have condensed into Jupiters or low luminosity red and brown dwarves.

Ellipticals and spirals:

Dynamical estimates for the mass-to-light ratio of elliptical galaxies apply the virial theorem by measuring the velocity dispersion; typically, this yields M/L~ 7. For spirals, the measurement of disk rotation curves out to the optical radius (ignoring the outer halo), yields M/L~ 4-10. Using these determinations, the mass in the universe is Omega ~ 0.016. However, the rotation curves of spirals can be measured further out into the halo by observing globular clusters beyond the disk. There are strong indications that there is up to ten times more dark matter associated with each galaxy than the previous estimate, yielding Omega ~ 0.15.

Clusters of galaxies:

The virial theorem can also be applied to determine the mass of clusters from the dynamics of its galaxies. However, there is some uncertainty as to whether clusters of galaxies have had sufficient time to properly virialize their internal motion. Nevertheless, such estimates indicate much larger mass-to-light ratios M/L~300, giving an overall mass density Omega ~ 0.2 (± 0.1).

Deep redshift galaxy surveys:

Comprehensive surveys of infra-red and other galaxies have gone out to distances in excess of 200Mpc. From large-scale velocities, it is possible using linear theory to estimate the homogeneous mass density. These gives rather uncertain determinations, Omega ~ 0.8 (± 0.5).

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