Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Cosmological Filaments and Minivoids: The Origin of Intergalactic
  Absorption   Soon after the first QSO was identified, Gunn & Peterson searched for the
expected characteristic absorption trough on the blueward side of Lya in the
spectrum of the QSO due to an Intergalactic Medium (IGM). They failed to find
it, placing a constraint on the density of neutral hydrogen in the IGM that was
less than 1 part in 10^5 of that residing in galaxies, and concluded that the
IGM must be highly ionized. Soon afterwards absorption by Lya was detected in
the IGM; not by a diffuse component, but by a clumpy component of intergalactic
gas clouds, the Lya forest. The goal then became to search for `excess'
absorption beyond that expected from the forest. No clear absorption by a
diffuse HI component, however, was ever detected. The results of recent
numerical hydrodynamics computations of the formation of the Lya forest appear
to indicate that the division between a clumpy component and a diffuse one may
be inappropriate. We find that in a CDM-dominated cosmology, no substantial
diffuse medium should be expected. Instead, the medium condenses into a network
of complex structures that reveal themselves as discrete absorption systems in
the spectra of QSOs. The lowest column density lines arise from the fine
structure in minivoids -- small regions with densities below the cosmic mean.
These results suggest that the long sought for diffuse HI Gunn-Peterson effect
may not exist.

32
The categories of this paper are: atom-ph
37
The categories of this paper are: supr-con
35
The categories of this paper are: mtrl-th
29
The categories of this paper are: funct-an
9
The categories of this paper are: nlin
7
The categories of this paper are: hep-th
11
The categories of this paper are: hep-lat
14
The categories of this paper are: math-ph
31
The categories of this paper are: ao-sci
34
The categories of this paper are: chem-ph
27
The categories of this paper are: comp-gas
30
The categories of this paper are: q-alg
24
The categories of this paper are: dg-ga
23
The categories of this paper are: solv-int
33
The categories of this paper are: plasm-ph
12
The categories of this paper are: nucl-th