Measurement of the small-scale structure of the intergalactic medium
  using close quasar pairs
  The distribution of diffuse gas in the intergalactic medium (IGM) imprints a
series of hydrogen absorption lines on the spectra of distant background
quasars known as the Lyman-$\alpha$ forest. Cosmological hydrodynamical
simulations predict that IGM density fluctuations are suppressed below a
characteristic scale where thermal pressure balances gravity. We measured this
pressure-smoothing scale by quantifying absorption correlations in a sample of
close quasar pairs. We compared our measurements to hydrodynamical simulations,
where pressure smoothing is determined by the integrated thermal history of the
IGM. Our findings are consistent with standard models for photoionization
heating by the ultraviolet radiation backgrounds that reionized the universe.
