The Net Advance of Physics: The Nature of Dark Matter, by Kim Griest -- Section 2C.

Large Scale Flows

It would be best to measure the amount of dark matter on the

largest possible scales so that the sample is representative

of the entire Universe. Within the past several years a host of

large-scale flow methods have been tried and are giving impressive

results [10]. These methods have the advantage stated above

but the disadvantage that they depend upon assumptions about

galaxy formation--that is, they depend upon gravitational

instability theory, the assumption of linear biasing, etc. Also,

the errors in these measurements are still large and the

calculations are complicated, but they do have great promise,

and tend to give values of near unity.

A simple example comes from the observation that the local group

of galaxies moves at km/sec with respect to the cosmic

microwave background (CMB) (measured from the amplitude of

the CMB dipole). If this motion comes from gravity, then the

direction of the motion should line up with the direction where

there is an excess of mass, and the velocity should be determined by

the size of this excess. Thus, taking into account the expansion of

the Universe, one has

where the linear bias factor b has been introduced to relate the

observed excess in galaxy number counts to the excess in mass

density . Using galaxy counts from the IRAS satellite survey,

Yahil et al. [11] find that the direction of the excess agrees

with the direction of the velocity vector to within about 20 degrees,

and that

Thus with the very conservative limit b>0.5, one has , and

with the reasonable limit b>1, one finds . For this method to

be reliable, must be measured on very large scales to ensure

that convergence has been reached, and it is not sure that this is

the case.

The above technique is only one of many related methods used to

determine on large scales. Another example is the detailed

comparison of the peculiar velocities of many galaxies with the

detailed maps of . This should not only determine , but serve

as a stringent test for the theory that large-scale structure is formed

by gravitational instability. A recent review by Dekel [10] surveys

many such methods and concludes that reasonable evidence exists

for . Although these techniques holds much promise, it

should be noted that different analyses of the same data sometimes

lead to different conclusions. So for the time being, these estimates

of should not yet be viewed as robust [12].

In conclusion, the observational evidence for large amounts of dark

matter on galactic halo scales is overwhelming. On larger scales, the

observational evidence for in the 0.1 to 0.2 range is strong. On the

largest scales, substantial observational evidence exists for ,

and some evidence for near unity exists, although this may be in

conflict with observations on cluster scales.

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