CMB data are beginning to put limits on parameters which are directly
relevant for particle physics models. For example there is a limit on
the neutrino contribution
_{}
*h*^{2} < 0.0076 (95% confidence)
from a combination of *WMAP* and galaxy clustering data from the
2dFGRS project
[47].
This directly implies a limit on neutrino mass, assuming the usual
number density of fermions which decoupled when they were relativistic.

A combination of the *WMAP* data with other data-sets gives some hint
of a running spectral index, *i.e.*,
*dn* / *d* ln *k*
0
[42].
Although this is still far from resolved
[48],
things will certainly improve as new data come in.
A convincing measurement of a non-zero running of the index would be quite
constraining for inflationary models
[49].

One other hint of new physics lies in the fact that the quadrupole
and some of the other low
modes seem anomalously low compared with the best-fit
CDM model
[32].
This is what might be expected in a universe which has
a large scale cut-off to the power spectrum, or is
topologically non-trivial. However, because of cosmic variance, possible
foregrounds *etc*., the significance of this feature is still a
matter of debate
[50].

In addition it is also possible to put limits on other pieces of physics [51], for example the neutrino chemical potentials, time variation of the fine-structure constant, or physics beyond general relativity. Further particle physics constraints will follow as the anisotropy measurements increase in precision.

Careful measurement of the CMB power spectra and non-Gaussianity can in
principle put constraints on high energy physics, including ideas of
string theory, extra dimensions, colliding branes, *etc*. At the moment
any calculation of predictions appears to be far from definitive. However,
there is a great deal of activity on implications of string theory for
the early Universe, and hence a very real chance that there might be
observational implications for specific scenarios.