The Lyman alpha Forest is the sum of absorption lines seen in spectra of distant galaxies and quasars, beginning from the Lyman alpha line at 121.6 nm to shorter wavelength (higher photon energies).
The forest is created by the fact that photons that come to us from distant light sources show Hubble redshift that is roughly proportional to the distance between us and the source of light.
This means the intergalactic neutral hydrogen clouds at different positions between Earth and the light source see the photons at different wavelength (see also redshift). Therefore each individual cloud leaves its fingerprint as an absorption line at a different position in the spectrum we see from Earth.
The reason for the Hubble redshift that causes the forest isn't universally accepted. The majority of astronomers attribute it to the hypothetical expansion of the universe. Some physicists attribute it to the conservation of energy since if energy is conserved it simulates an accelerating expansion of the universe through the Einsteinian general time dilation effect.
In astronomical spectroscopy, the Lymanalphaforest is the sum of absorption lines seen in spectra of distant galaxies and quasars, beginning from the Lymanalpha line at 121.6 nm to shorter wavelengths (higher photon energies).
The forest is created by the fact that photons that come to us from distant light sources show Hubble redshift that is roughly proportional to the distance between us and the source of light.
Since neutral hydrogen clouds at different positions between Earth and the distant light source see the photons at different wavelengths (due to the redshift), each individual cloud leaves its fingerprint as an absorption line at a different position in the spectrum as observed on Earth.
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