  The Hot Universe Baryon Surveyor (HUBS) mission is proposed to study
"missing" baryons in the universe. Unlike dark matter, baryonic matter is made
of elements in the periodic table, and can be directly observed through the
electromagnetic signals that it produces. Stars contain only a tiny fraction of
the baryonic matter known to be present in the universe. Additional baryons are
found to be in diffuse (gaseous) form, in or between galaxies, but a
significant fraction has not yet been seen. The latter (missing baryons) are
thought to be hiding in low-density warm-hot ionized medium (WHIM), based on
results from theoretical studies and recent observations, and be distributed in
the vicinity of galaxies (i.e., circum-galactic medium) and between galaxies
(i.e., intergalactic medium). Such gas would radiate mainly in the soft X-ray
band and the emission would be very weak, due to its very low density. HUBS is
optimized to detect the X-ray emission from the hot baryons in the
circum-galactic medium, and thus fill a void in observational astronomy. The
goal is not only to detect the missing baryons, but to characterize their
physical and chemical properties, as well as to measure their spatial
distribution. The results would establish the boundary conditions for
understanding galaxy evolution. Though highly challenging, detecting missing
baryons in the intergalactic medium could be attempted, perhaps in the
outskirts of galaxy clusters, and could shed significant light on the
large-scale structures of the universe. The current design of HUBS will be
presented, along with the status of technology development.
