A cosmic web filament revealed in Lyman-alpha emission around a luminous
  high-redshift quasar
  Simulations of structure formation in the Universe predict that galaxies are
embedded in a "cosmic web", where the majority of baryons reside as rarefied
and highly ionized gas. This material has been studied for decades in
absorption against background sources, but the sparseness of these inherently
one-dimensional probes preclude direct constraints on the three-dimensional
morphology of the underlying web. Here we report observations of a cosmic web
filament in Lyman-alpha emission, discovered during a survey for cosmic gas
fluorescently "illuminated" by bright quasars at z=2.3. With a projected size
of approximately 460 physical kpc, the Lyman-alpha emission surrounding the
radio-quiet quasar UM287 extends well beyond the virial radius of any plausible
associated dark matter halo. The estimated cold gas mass of the nebula from the
observed emission is at least ten times larger than what is typically found by
cosmological simulations, suggesting that a population of intergalactic gas
clumps with sub-kpc sizes may be missing within current numerical models.
