One of the most iconic photos ever taken by the Hubble Space Telescope is the Hubble Deep Field image taken in 1995. The image, which covers a tiny fraction of the sky roughly the width of a dime viewed from a distance of 75 feet, revealed at least 1,500 galaxies, and gave us an inkling of how mind-bogglingly huge the universe is.
The image has long been considered representative of the typical distribution of galaxies in the cosmos. Based on this assumption, astronomers calculated that the observable universe — a ginormous sphere over 90 billion light-years in diameter — contained 200 billion galaxies.
It turns out that this estimate, already a staggeringly huge one, may be 10 times too low — at least according to an international team of researchers. These scientists used deep-space images and other Hubble data, as well as a new mathematical model to infer the existence of galaxies that the current generation of telescopes are incapable of seeing, to create a 3D map of the known universe.
Their estimate of how many galaxies the observable universe has? 2 trillion.
This means that every point in the sky you look at contains part of a galaxy. The only reason the night sky appears dark instead of a blinding shade of white is because many of these galaxies are so redshifted due to the expansion of space that their radiation no longer falls in the visible spectrum.
“We are missing the vast majority of galaxies because they are very faint and far away. 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,” Christopher Conselice, a professor of astrophysics at the University of Nottingham, and lead author of a study detailing the findings, said in a statement. “Who knows what interesting properties we will find when we study these galaxies with the next generation of telescopes?”
In addition to providing an updated estimate of the actual number of galaxies, the results also reveal that galaxies were not evenly distributed throughout the universe’s history. According to the researchers, the universe, when it was only a few billion years old, contained 10 times as many galaxies in a given volume of space compared with today.
Most of these galaxies have since merged to form larger ones, reducing the galactic population density of the cosmos.
“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,” Conselice said. “Finding more galaxies in the past implies that significant evolution must have occurred to reduce their number through extensive merging of systems.”