Lyman-alpha forest

The Lyman-alpha forest is a special pattern seen in the light from faraway stars and bright objects called quasars. When we study this light using a tool called a spectroscope, we see many thin lines where the light has been blocked. These lines are caused by clouds of gas, mostly hydrogen, that the light passes through on its journey to us.

Each cloud of gas moves at a slightly different speed, which changes how we see the light from the distant object. This creates many lines that look like a forest when drawn on a graph, hence the name “Lyman-alpha forest.”

These lines help scientists learn about the universe’s structure and history. By studying the Lyman-alpha forest, we can discover how matter is spread out across huge distances and how it has changed over time. This gives us clues about how galaxies formed and evolved.

History

The Lyman-alpha forest was first seen in 1970 by astronomer Roger Lynds when he studied a very faraway object called a quasar named 4C 05.34. He noticed many lines in the light pattern that did not usually appear, and he thought they were all caused by the same process involving hydrogen atoms. Later observations by John Bahcall and Samuel Goldsmith found the same lines in other faraway quasars. Roger Lynds was the first to call these lines the "Lyman-alpha forest." Jan Oort suggested that these lines came from gas clouds between galaxies, not from inside the quasars themselves.

Physical background

When an electron in a hydrogen atom moves between energy levels, it creates lines of light called spectral lines. The Lyman series happens when the electron moves between the lowest energy level and higher levels. The Lyman-alpha line is special because it occurs when the electron moves from the lowest level to the next one. This line has a specific wavelength of 1216 Å, which is in the ultraviolet part of the light spectrum.

We see these Lyman-alpha lines in the light from distant galaxies and quasars because the light passes through clouds of hydrogen gas on its way to us. These clouds are moving at different speeds, so their lines appear at different positions in the spectrum, creating what looks like a "forest" of lines.

Use as a tool in astrophysics

The Lyman-alpha forest helps scientists study the space between galaxies. By looking at the light from faraway stars and galaxies, we can learn about the clouds of gas that the light passes through. These clouds contain hydrogen, and by studying how the light changes, scientists can figure out how much gas is there and how hot it is.

We can also learn about the early universe using the Lyman-alpha forest. When we look at very distant light, we see more lines, showing us how the universe has changed over time. This helps us understand big ideas about space, time, and what makes up the universe.