Cosmological models and constraints 247consequently have greater power to constrain parameters, efficient techniques of
parameter space exploration will become increasingly important.
To this point, the discussion has assumed that the microwave background
power spectrum is perfectly described by linear perturbation theory. Since the
temperature fluctuations are so small, parts in a hundred thousand, linear theory is
a very good approximation. However, on small scales, nonlinear effects become
important and can dominate over the linear contributions. The most important
nonlinear effects are the Ostriker–Vishniac effect coupling velocity and density
perturbations (Jaffe and Kamionkowski 1998, Hu 2000), gravitational lensing by
large-scale structure (Seljak 1996), the Sunyaev–Zeldovich effect which gives
spectral distortions when the microwave background radiation passes through
hot ionized regions (Birkinshaw 1999) and the kinetic Sunyaev–Zeldovich effect
which Doppler shifts radiation passing through plasma with bulk velocity (Gnedin
and Jaffe 2000). All three effects are measurable and give important additional
constraints on cosmology, but more detailed descriptions are outside the scope of
this chapter.
Finally, no discussion of parameter determination would be complete
without mention of galactic foreground sources of microwave emission. Dust
radiates significantly at microwave frequencies, as do free–free and synchrotron
emission; point source microwave emission is also a potential problem. Dust
emission generally has a spectrum which rises with frequency, while free–free and
synchrotron emission have falling frequency spectra. The emission is not uniform
on the sky, but rather concentrated in the galactic plane, with fainter but pervasive
diffuse emission in other parts of the sky. The dust and synchrotron/free–
free emission spectra cross each other at a frequency of around 90 GHz.
Fortunately for cosmologists, the amplitude of the foreground emission at this
frequency is low enough to create a frequency window in which the cosmological
temperature fluctuations dominate the foreground temperature fluctuations. At
other frequencies, the foreground contribution can be effectively separated from
the cosmological blackbody signal by measuring in several different frequencies
and projecting out the portion of the signal with a flat frequency spectrum.
The foreground situation for polarization is less clear, both in amplitude and
spectral index, and could potentially be a serious systematic limit to the quality
of cosmological polarization data. However,, it may be no greater problem for
polarization fluctuations than for temperature fluctuations. For an overview of
issues surrounding foreground emission, see Bouchet and Gispert 1999 or the
WOMBAT web site, http://astro.berkeley.edu/wombat.
7.5.5 Current constraints and upcoming experiments
As the Como School began, results from the high-resolution balloon-born
experiment MAXIMA (Hananyet al2000) were released, complementing the
week-old data from BOOMERanG (de Bernardiset al2000) and creating a
considerable buzz at coffee breaks. The derived power spectrum estimates are