Washington, Oct 14 There are at least 10 times more galaxies in the observable universe than previously thought, said astronomers.
One of the most fundamental questions in astronomy is that of just how many galaxies the universe contains, and astronomers earlier estimated that the observable universe contained about 200 billion galaxies.
The new research - to be published in The Astrophysical Journal -- shows that this estimate is at least 10 times too low.
The researchers led by Christopher Conselice of University of Nottingham in Britain reached this conclusion using deep-space images from NASA's Hubble space telescope and the already published data from other teams.
They converted the images into 3-D, in order to make accurate measurements of the number of galaxies at different epochs in the universe's history.
In addition, they used new mathematical models, which allowed them to infer the existence of galaxies that the current generation of telescopes cannot observe.
This led to the surprising conclusion that in order for the numbers of galaxies we now see and their masses to add up, there must be a further 90 per cent of galaxies in the observable universe that are too faint and too far away to be seen with present-day telescopes.
These myriad small faint galaxies from the early universe merged over time into the larger galaxies we can now observe.
"It boggles the mind that over 90 per cent of the galaxies in the universe have yet to be studied. Who knows what interesting properties we will find when we discover these galaxies with future generations of telescopes? In the near future, the James Webb Space Telescope will be able to study these ultra-faint galaxies," Conselice said.
"These results are powerful evidence that a significant galaxy evolution has taken place throughout the universe's history, which dramatically reduced the number of galaxies through mergers between them - thus reducing their total number," Conselice explained.
The decreasing number of galaxies as time progresses also contributes to the solution for Olbers' paradox (first formulated in the early 1800s by German astronomer Heinrich Wilhelm Olbers): Why is the sky dark at night if the universe contains an infinity of stars?
The team came to the conclusion that indeed there actually is such an abundance of galaxies that, in principle, every patch in the sky contains part of a galaxy.
However, starlight from the galaxies is invisible to the human eye and most modern telescopes due to other known factors that reduce visible and ultraviolet light in the universe.
Those factors are the reddening of light due to the expansion of space, the universe's dynamic nature, and the absorption of light by intergalactic dust and gas.
All combined, this keeps the night sky dark to our vision, the researchers said.