The Warm & Hot Universe
Scientific Rationale
Surveys of the sky at lower energies have unveiled a `cosmic
concordance' Universe, whose composition is largely unknown to us. Even
within the small fraction of what we know - the baryons - about half
of the total mass density is missing at low redshifts. X-ray observations offer the
only means of finding the other half. Moreover, X-ray observations can also shed
vital new light on the remaining cosmic content by providing important
data on large scale structures. For instance, the smoothness of the hot
X-ray gas in clusters and groups makes it an ideal tracer of the potential
well to discriminate dark matter from modified gravity; the formation and
evolution of structures as revealed by their X-ray spectra and morphology
represent a direct test of the existence of dark energy, or cosmic
repulsion, at larger distance scales. The standard LCDM model, based on an abundance of high precision
microwave and optical data, and the associated evolution of the visible matter, may meet their most stringent
tests in the X-ray band.
On Mpc scales and beyond, topics include the 50% missing baryons of the near
Universe, the thermodynamic history of clusters, and the relationship between
these two issues and with the soft X-ray excess of clusters. The evolution
of cluster number density is extremely sensitive to cosmological models
(because the concordance LCDM model expects clusters to have been virialized
only at or close to the present epoch) as are the scaling laws of cluster
parameters. Another extremely powerful tool for discriminating between LCDM
and other models is a Sunyaev-Zel'dovich survey. To achieve the goals for SZ surveys,
however, one must complement any SZ decrement data with knowledge of the total
mass of each cluster, and the latter can most effectively be
measured by X-ray observations.
On the scale of galaxy groups, the challenges are equally major. The
large uncertainty in the contribution to the total matter budget from groups,
and the mass profile of groups which includes the `missing sub-halos'
problem, are two key areas which the next generation X-ray missions can, and
must, address.
On galactic scales, the unique capability of X-ray surveys for detecting
distant AGNs enables us to chart the evolution of these very bright
sources back to perhaps even the epoch of First Light, and to tackle the
chronology of black hole versus galaxy formation.
The purpose of this workshop is to review each of these
(and related) issues in the spirit of finding the best way of conducting
future observations, keeping in mind both the need to obtain direct and
unambiguous verdicts and to cover as many areas as possible under the
roof of only one or two mission concept(s). For this reason, we explicitly solicit
contributions from instrumentalists, and to encourage them to attend
and participate in an active dialogue with the rest of the community.
Only in this way can one arrive at realistic designs that serve the
necessary scientific objectives.
