Astronomical surveys will reveal whether the universe was lit by galaxies or quasarsReuters

We will soon be able to uncover the origin of the ultraviolet light in the cosmos, helping scientists understand how galaxies were built, according to new research.

Forthcoming astronomical surveys will reveal just what lit up the universe, explaining whether the light comes from the many galaxies or quasi-stellar radio sources - the most energetic members of a class of objects called active galactic nuclei.

"Which produces more light? A country's biggest cities or its many tiny towns?" asked University College London cosmologist Dr Pontzen, who led the study. "Cities are brighter, but towns are far more numerous. Understanding the balance would tell you something about the organisation of the country.

"We're posing a similar question about the universe: does ultraviolet light come from numerous but faint galaxies, or from a smaller number of quasars?"

Quasars are the brightest objects in the Universe. Their light is generated by gas as it falls towards a black hole and although galaxies can contain millions or even billions of stars, they are still dim by comparison.

Understanding whether the numerous small galaxies outshine the rare quasars will provide insight into the way the universe built up today's populations of stars and planets. It will also help scientists properly calibrate their measurements of dark energy, the agent thought to be accelerating the universe's expansion and determining its far future.

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

The intense light from quasars makes them easy to spot even at extreme distances. 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 they are not from quasars.

Two types of hydrogen gas are found in the universe – a plain, neutral form and a second charged by UV light. These two forms can be distinguished by studying a 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.

The quasars act as a time capsule as they are billions of light years away, and the resulting map will reveal where neutral hydrogen was located billions of years ago as the universe was building its galaxies.

An even distribution of neutral hydrogen gas would suggest numerous galaxies as the source of most light, whereas a more random pattern would indicate that quasars were the primary origin of light.

A number of surveys are being planned to gather more information about quasars. Chief among these is the Dark Energy Spectroscopic Instrument (DESI) which will determine whether light in the universe is generated by quasars or galaxies.

Co-author Dr Hiranya Peiris told

It's amazing how little is known about the objects that bathed the universe in ultraviolet radiation while galaxies assembled into their present form. This technique gives us a novel handle on the intergalactic environment during this critical time in the universe's history".

The study was published in the Astrophysical Journal Letters.