How the Cosmic Web Moves on Large SCales
It is now generally believed that most of the ordinary matter in the
universe resides in the form of a connected network of galaxies and gas, the so-called cosmic web.
Galaxies are embedded in a gaseous matrix (the intergalactic medium or IGM) consisting mostly of a hydrogen-helium plasma; they grow partly by accreting gas from the IGM and partly by merging with other galaxies.
The traditional picture of galaxies as "Island Universes", separated by vast
oceans of nothingness, has been superseded by a scenario where galaxies
are more like raisins in a cake, with most of the matter in the universe
being in the "dough", i.e., the intergalactic medium, as opposed to having fallen into the galaxies themselves.
Until recently, very
little was known about how the cosmic web at high redshift actually moves.
Does it strictly follow the general expansion of the universe, or can we
see the pull of gravity directing streams of gas into future galaxies and
galaxy clusters ?
My collaborators and I have been attempting to measure the kinematics of the gaseous cosmic web at redshifts (2-3.5) to answer these questions.
We observed hundreds of absorption systems that simultaneously appeared in the lines of sight to a number
of close pairs of QSOs and measured the velocity differences between the
lines of sight for each absorber, as a function of perpendicular distance across the sky.
The spatial separation between the points where the absorption was measured are large enough that these absorbing "clouds" generally must
be large scale filaments or sheets of gas, rather than galaxies.
The velocity differences (stricly speaking we only observe the one-dimensional velocities
projected along the lines of sight to the QSOs) allow us to estimate the gradients in
the velocity field between two points in space. On large enough scales
most of these gradients
are expected to be due to the stretching of the cosmic web, following the general expansion of the universe, but this cannot
be the whole story: without
local departures from a uniform Hubble flow (i.e., regions in the universe breaking away from
the expansion) there could never be any galaxies, for example.
Our velocity measurements showed that most of the IGM does indeed follow
the Hubble expansion quite closely, but there are important departures:
On scales of a few tens to a couple hundred kiloparsec the average expansion velocity of the IGM falls short of the general expansion by up to about 20 percent
(at the lowest redshifts (z~2) that we observed). This average is dominated
by a few absorbers that appear to be contracting (perhaps into galaxies).
Our data are not yet good enough to draw further conclusions, but a look at
a computer simulation of the IGM reveals that the picture is likely to
be more complex yet: the average expansion velocity in the simulation is indeed
somewhat smaller than the Hubble flow, but
the majority of the clouds appear to be expanding even faster (by 5 - 20 percent)
than the general universe. This is probably because the absorption systems we observed
most often are likely to arise in gaseous filaments that are stretching under the pull
of gravity. We may be seeing the gas streaming into galaxies or galactic clusters at either end of the absorbing filaments.
Publications:
Rauch, Michael; Becker, George D.; Viel, Matteo; Sargent, Wallace L. W.; Smette, Alain; Simcoe, Robert A.; Barlow, Thomas A.; Haehnelt, Martin G: Expansion and Collapse in the Cosmic Web, Astrophysical Journal, 632:58, 2005