The Sunyaev-Zeldovich effect (SZE) is an observed
anisotropy in the
cosmic microwave background (CMB) spectrum,
caused by Compton scattering of
microwave photons by ions in the gas that fills
clusters of galaxies. Although it is a less important perturbation of the microwave background in comparison to that caused by primordial density fluctuations, the SZE is important in astronomy and cosmology.
Theorized by Soviet
physicists Rashid Sunyaev and Yakov Zel'dovich in 1969, the
SZE can be used to calibrate the
Hubble constant.
Thermal Sunyaev-Zeldovich effect
The universe is filled with galaxies. By all accounts, there are likely
billions of them. Many of these galaxies lie within
clusters that formed from the densest regions of the
primordial universe. Today, we see these clusters as aggregations of
thousands of galaxies, all gravitationally bound and orbiting about the
center of the cluster. What is not evident to the eye is that these
clusters are also filled with very hot gas. The gas can have temperatures
of several million Kelvins, and can be very bright in X-rays.
However, this gas also has an effect on the
CMB.
As light from the microwave background
passes though these galaxy clusters, the microwave photons are
scattered by electrons in the intra-cluster gas,
giving low-energy photons much higher energy in the process. This
results in a fainter CMB spectrum at low photon energies, which
appears as a cooler microwave background in the direction of the
galaxy cluster.
This is called the thermal Sunyaev-Zeldovich
effect, because the CMB is being distorted by the
thermal motions of electrons within
the cluster gas.
By measuring how much the spectrum is changed, you can make a good guess as
how large the cluster is.
This measurement is important, because we have no simple way
to measure distances in the universe. Measuring the
linear size of
the cluster and the
angular size (as it appears to us
on the sky) lets us use
trigonometry to determine the
distance to
the cluster. If we then measure the
redshift of the cluster, we can
calibrate the Hubble constant, i.e.
What Hubble constant is required to
obtain the measured redshift for the cluster at known distance?
In reality, using the SZE to measure the Hubble constant is very difficult,
and requires several critical assumptions. In particular, it requires that
one knows the distribution of hot gas within the galaxy cluster, since the
density of the gas determines how the CMB photons will scatter. One must
also assume a specific cosmological model for the universe, particularly
for the deceleration parameter, q0. Neither of these
is particularly well-known, though the distribution of gas in the cluster can
be roughly measured with the X-ray radiation it emits.
Measuring the redshift of the cluster is also difficult, because the
individual galaxies within the cluster have their own large pecular
velocities in addition to the Hubble velocity of the cluster.
The Sunyaev-Zeldovich effect has been used many times to measure the Hubble
constant, and values derived from the SZE method seem to agree with those
measured from other means (e.g. extragalactic
Cepheid variables and
high-redshift supernovae). The current best-guess for
the Hubble constant is about 65 ± 10
km/s/megaparsec.
Kinetic Sunyaev-Zeldovich effect
There is a second, weaker effect observed in some galaxy clusters, called
the kinetic Sunyaev-Zeldovich effect. Like the thermal SZE, the
cluster gas has an effect on the CMB, but in this case, there is a further
modification to the spectrum caused by the bulk motion of the cluster
itself, relative to the Hubble expansion. In the thermal effect, we assumed
that the cluster was moving relative to us only because of the expansion of
the universe. In fact, this was an important assumption because we assume
that the redshift we measure is purely due to the Hubble effect. However,
the cluster itself may have a peculiar motion, particularly if it is
gravitationally interacting with something else. You then
have to take into account scattering by a gas moving at the velocity of
the cluster relative to the Hubble flow, which may be several
thousand kilometers per second. This actually produces a
tiny brightening of the spectrum, but the increase is about a factor
of ten smaller than the dimming caused by the thermal effect.
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