A new study from a team of international astronomers, led by astrophysicists from the University of Nottingham with support from the Royal Astronomical Society (RAS), has produced some astounding results: The universe contains at least two trillion galaxies, 10 times more than the highest previous estimates. What's more, the new study suggests that 90 percent of all galaxies are hidden from us, and only the remaining 10 percent can be seen at all, even with our most powerful telescopes. The paper detailing the study was published today in the Astrophysical Journal.
"We are missing the vast majority of galaxies because they are very faint and far away," said Nottingham Astrophysics Professor Christopher Conselice in an RAS press release. "The number of galaxies in the universe is a fundamental question in astronomy, and it boggles the mind that over 90 percent of the galaxies in the cosmos have yet to be studied. Who knows what interesting properties we will find when we study these galaxies with the next generation of telescopes?"
For two decades, astronomers have used the Hubble Space Telescope (HST) Deep Field images to try to estimate the number of galaxies in the observable universe. The previous estimate was 100 to 200 billion, and now we believe that huge number was too small by a factor of 10 or 20, depending on where your original estimate falls.
It is no easy task to count the galaxies in the entire universe. For one thing, as previously mentioned, we cannot see the vast majority of galaxies with our telescopes because they are too far or too faint or both. For another, the farther away we peer with the HST, the smaller the area of the sky we are observing is—Hubble Deep Field images cover about one millionth of the total area of the sky. This animation shows just how small an area a Deep Field image covers.
The results in the new study are the culmination of 15 years of work. An initial grant from the RAS allowed undergraduate student Aaron Wilkinson, now a phD student at Nottingham University, to perform initial galaxy-counting analysis work that laid the foundation for the larger study.
Professor Conselice, in partnerships with researchers at the University of Edinburgh and Leiden University in the Netherlands, used Wilkinson's work and data from telescopes around the world, particularly Hubble, to create 3D maps of different parts of the universe. Mathematical analysis of the models using the calculated density of the galaxies and the volume for each mapped region of space allowed the researchers to deduce how many galaxies we are missing in our observations, and in turn, how many there are in total spread across the universe.
In addition to pinning down a total number, the study analyzed the number of galaxies that were present in the distant past compared to the number of galaxies that exist now. By peering 13 billion light-years into the past, shortly after the Big Bang, the researchers found that there were 10 times more galaxies in the ancient universe than there are now (most of which were small, about the size of the satellite galaxies that orbit the Milky Way).
"This is very surprising as we know that, over the 13.7 billion years of cosmic evolution since the Big Bang, galaxies have been growing through star formation and mergers with other galaxies. Finding more galaxies in the past implies that significant evolution must have occurred to reduce their number through extensive merging of systems."
That "significant evolution" is the continuous merging of smaller galaxies into the larger ones we see today, and the new model could help researchers piece together the formation story of the modern universe with greater accuracy than ever before.
The sheer difference in the number of galaxies has far-reaching implications as well. Probabilistic equations that estimate the number of hypothetical alien civilizations, such as the Drake Equation, will need to be modified to account for the dramatic increase in the number of estimated galaxies out there—which makes it even more astronomically unlikely that we are alone among intelligent species.
In the face of such an expansive universe, it is easy to feel both awe and a sense of insignificance here on Earth. It is reminiscent of Carl Sagan's thoughts on the Pale Blue Dot image, a photo taken of Earth by Voyager 1 from a distance of 6 billion kilometers, almost as far as Pluto.
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