Lost and Found: Hubble Finds Much of the Universe's Missing Hydrogen

1. How did Hubble detect this elusive hydrogen?

This hydrogen is so hot it escapes detection by normal observational techniques. So astronomers used Hubble's Space Telescope Imaging Spectrograph to hunt for oxygen, an element that mixed with and was heated by the hydrogen.

Astronomers detected this highly energized oxygen by using the light of a distant quasar to probe the invisible space between the galaxies, which is like shining a flashlight beam through a fog. The imaging spectrograph found the spectral "fingerprints" of intervening oxygen imprinted on the quasar's light. Slicing across billions of light-years of space, the quasar's brilliant beam penetrated at least four separate filaments of the invisible hydrogen laced with the telltale oxygen. The presence of oxygen between the galaxies implies there are huge quantities of hydrogen in the universe.

2. How did the hydrogen heat up?

Supercomputer models of the expanding, evolving universe have predicted an intricate web of gas filaments where hydrogen is concentrated along vast chain-like structures. Clusters of galaxies form where the filaments intersect. The models predict that vast hydrogen clouds flowing along the chains should collide and heat up. This would squelch the formation of more galaxies in the hottest regions, so star birth was more abundant in the early universe when the hydrogen was cool enough to coalesce. Observations with ground-based telescopes previously detected vast clouds of relatively cool hydrogen between galaxies in the early universe.

3. How was the oxygen created?

Exploding stars in galaxies probably spewed the oxygen into space where it mixed with the hydrogen.