A Forest Made of Missing Light
Report — Field Note
A Forest Made of Missing Light
Hundreds of dark lines in the light of one quasar. A name borrowed from a walk in the woods, and the places it stops being true.
A quasar is the core of a young galaxy, lit by matter falling into a massive black hole and bright enough to be seen across most of the observable universe. Its light travels to us along a single line. On the way it crosses long stretches of hydrogen — thin gas drawn out along the filaments of large-scale structure, with no edges worth the word.
Pass that light through an instrument that spreads it by wavelength, the way a prism spreads sunlight, and you get a band running from blue to red. Uninterrupted, it would be smooth.
Hydrogen absorbs at one particular ultraviolet wavelength — 1216 ångströms, called Lyman-alpha after Theodore Lyman, who measured that family of lines early in the last century. A patch of hydrogen along the path removes a thin slice of light there and leaves a dark line behind. One patch. One line.
The universe expands, so light stretches as it travels. A patch nearer to us leaves its line at one position; a patch farther away leaves its line at another. Hundreds of patches leave hundreds of lines. In 1970 Roger Lynds saw this in the light of a quasar catalogued as 4C 05.34 and called it a forest. The name has never been replaced. Note what the picture is made of: not light, but light that was taken out.
It is right in one respect. Stand at the edge of woods, look straight ahead, and trunks at many depths each block part of your view along a single line of sight — which is the arrangement exactly. Then it slips. Trees are solid things with edges; these are diffuse. A wood can be walked into and sampled in any direction; this is one needle-thin core, and a second line needs a second quasar. And a forest stands in the present, while reading this band from one end to the other is reading across billions of years.
Those lines record how clumpy matter is at fine scales. The cosmic microwave background — the leftover glow of the early universe — records how clumpy it is at very large ones. The two measurements do not entirely agree, and the gap has sat there for years. It is now the place where a theory with no measurements of its own can finally be handed a bill.
note: the line positions here are schematic. Real absorbers all sit blueward of the quasar's own emission line, which the drawing respects, but the left-to-right spacing is drawn for legibility rather than from a redshift calculation
Sources: KCL · Lyman-alpha forest (Wikipedia) · Keel, U. Alabama — the Lyman-alpha forest · NASA APOD, 1998 · ScienceDaily · Quanta, Mar 23