The observable universe has more galaxies than initially thought. This finding follows the deep-sky census that assessed the data from NASA's Hubble Space Telescope and other observatories.
The real population of galaxies might be 10 times more than the observable universe, according to astronomers who did the data-crunching.
Leading the study was Christopher Conselice of the University of Nottingham in the UK. The team's research paper will soon appear in The Astrophysical Journal.
It makes a significant contribution in offering clues to galaxy formation and in solving the astronomical puzzle as to why the sky looks dark at night.
"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. This gives us a verification of the so-called top-down formation of structure in the universe," explained Conselice.
According to the study, most galaxies in the past were small and their masses are comparable to satellite galaxies at the Milky Way.
Conselice and his team reached important conclusions using deep-space images from Hubble and published data from other sources.
The study explained that starlight from the galaxies is invisible to the human eye and modern telescopes because extraneous factors reduce visible and ultraviolet light in the universe. The expansion of space causes reddening of light due to the dynamic nature and absorption of light by intergalactic dust and gas.
Top-Down Structure In The Universe
Conselice and his team projected a theory of smaller galaxies merging to form larger galaxies and thus leading to a dwindling of galaxy population in space. This supports the argument that galaxies are not evenly distributed across the universe.
Many studies in the past on the topic included the landmark Hubble Deep Field image taken in the 1990s, which revealed the presence of a vast array of faint galaxies. The image led scientists to estimate that there are 200 billion galaxies inhabiting the observable universe.
However, the new research shows that such an estimate is woefully short of the actual numbers and the real numbers might be 10 times more than that.
Using 3D images, the team made accurate measurements of galaxies at different epochs in the universe. They also used mathematical models for inferring galaxies that the current telescopes are unable to observe.
Their studies led to the conclusion that there must be an extra 90 percent of galaxies in the observable universe that are too faint to be seen with present-day telescopes. The observable larger galaxies are indeed merged clusters of small faint galaxies from the early universe.
"It boggles the mind that over 90 percent 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," said Conselice.
The diminishing number of galaxies with the passage of time answers the paradox proposed by German astronomer Heinrich Wilhelm Olbers on why the sky is dark at night if it contains an infinity of stars.
The Hubble Space Telescope project signals international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the Hubble telescope.