What brightens up the universe?

Does ultraviolet light, that bathes the cosmos, come from numerous but faint galaxies or from a smaller number of quasars? The answer may not be too far in the future.

A new method developed by researchers at the University College London (UCL) shows we will soon uncover the origin of ultraviolet light (UV), helping scientists understand how galaxies were built.

The new method builds on a technique already used by astronomers in which quasars act as beacons to understand space.

Quasars are the brightest objects in the universe and their intense light is generated by a gas.

The intense light from quasars makes them easy to spot even at extreme distances, up to 95 percent of the way across the observable universe.

"By studying how this light interacts with hydrogen gas on its journey to earth will reveal the main sources of illumination in the universe, even if those sources are not themselves quasars," explained cosmologist Dr Andrew Pontzen from UCL.

Two types of hydrogen gas are found in the universe -- a plain, neutral form and a second charged form which results from bombardment by UV light.

These two forms can be distinguished by studying a particular wavelength of light called 'Lyman-alpha' which is only absorbed by the neutral type of hydrogen.

Scientists can see where in the universe this 'Lyman-alpha' light has been absorbed to map the neutral hydrogen.

Since the quasars being studied are billions of light years away, they act as time capsules -- looking at the light shows us what the universe looked like in the distant past.

Current samples of quasars aren't quite big enough for a robust analysis of the differences between the two scenarios -- however, a number of surveys currently being planned should help scientists find the answer.

Chief among these is the DESI (Dark Energy Spectroscopic Instrument) survey which will include detailed measurements of about a million distant quasars.

"The resulting map will reveal where neutral hydrogen was located billions of years ago as the universe was vigorously building its galaxies," researchers noted.

The study was published in The Astrophysical Journal Letters.