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The universe is bathed in a steady stream of ultraviolet light – but where this light comes from has long been a mystery.

One theory is that light comes from quasars, which are distant objects powered by black holes a billion times as massive as our sun.

But some astronomers believe that much dimmer galaxies, which are larger in number, are responsible for bathing the cosmos in light.

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The universe is bathed in a steady supply of ultraviolet light – but where this light comes from has long been a mystery. A computer model shows one scenario for how light is spread through the early universe on vast scales  of more than 50 million light years across

Now, new research by London-based scientists could finally solve the mystery of the UV light and understand how galaxies formed by analysing its distribution.

'Which produces more light? A country's biggest cities or its many tiny towns?' asked Dr Andrew Pontzen, lead author of the study at University College London (UCL).

'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 very bright quasars?'

 

Scientist explains how light is spread through the early universe

One theory is that light comes from quasars, which are objects powered by black holes a billion times as massive as our sun (artists's impression pictured). Their intensity means quasars are relatively easy to spot

Answering this question will give scientists a valuable insight into the way that the universe built its galaxies – the places where stars and planets are born, live and die.

It will also help researchers properly calibrate their measurements of dark energy, the mysterious force that is accelerating the universe's expansion and will determine its far future.

To help find the light source, astronomers use quasars as beacons.

Because of their intensity, quasars are relatively easy to spot in astronomers' observations, even at distances more than 95 per cent of the way across the observable universe.

'Because they can be seen at such great distances, quasars are a useful probe for finding out the properties of the universe,' Professor Pontzen says.

'Distant quasars can be used as a backlight, and the properties of the gas between them and us are imprinted on the light.'

As the light travels through the universe towards Earth, two things happen: first, the expansion of the universe causes the lights colours to become redder and wavelengths become longer.

Clouds of gas absorb some very specific colours of light, progressively eating chunks out of the full spectrum.

Analysing the light when it arrives at Earth allows scientists to reverse-engineer the process and so understand the intervening clouds.

Doing this for a million quasars should help researcher learn a great deal about the distribution of matter, including both the gas and the galaxies and quasars that bathe the gas in radiation.

If galaxies caused the light, scientists would expect the remaining dark gas to be relatively evenly-spread across the universe.

If quasars did it, they would find a much less uniform distribution.

The team plan to use the Dark Energy Spectroscopic Instrument survey which will include detailed measurements of about a million distant quasars.

Although these measurements are designed to reveal how the expansion of the universe is accelerating due to dark energy, the new research shows that results from DESI will also determine whether the intervening gas is uniformly illuminated.

In turn, the measurement of patchiness will reveal whether light in our universe is generated by 'a few cities' - quasars or by 'many small towns' – or galaxies.

'It's amazing how little is known about the objects that bathed the universe in ultraviolet radiation while galaxies assembled into their present form,' says co-author Hiranya Peiris.

'This technique gives us a novel handle on the intergalactic environment during this critical time in the universe's history.'

Some astronomers believe that much dimmer galaxies, which are larger in numbers, are responsible for bathing the cosmos in light. Pictured is the Triangulum Galaxy, nearly three million light years from Earth