Used to be there were between 100 and 200 billion galaxies in the universe. Plenty to go around. That was the estimate up until this fall, when an international team, led by Christopher Conselice from the University of Nottingham (UK) concluded there are at least 10 times that number or 1 to 2 trillion!
Astronomers using data from the Hubble Space Telescope and other telescopes have been working on an accurate census of galaxies. The same way we do moose counts in Minnesota to evaluate the number and health of the herd, astronomers want to know how many galaxies are out there to get a better understanding of the contents of the universe. The Hubble Deep Field images, captured in the mid 1990s, gave the first real insight into this. The first deep field image, taken in December 1995, contained at least 3,000 galaxies in a thimbles-worth of sky not even a tenth the diameter of the full moon.
Conselice and his team converted the Hubble images and others into 3D in order to make accurate measurements of the number of galaxies at different times in the universe’s history. Recall that the farther away the galaxy, the deeper back in time we look. The light from nearby galaxies, traveling at 186,000 miles per second, takes millions of years to reach our eyeballs. Every time we point our binoculars at the Andromeda galaxy on a fall night, we see the galaxy as it was 2.5 million years ago. The most distant galaxies shine from more than 13 billion light years away; seeing them takes us back to nearly the beginning of the universe in the Big Bang 13.7 billion years ago.
The team combined their 3D census with new mathematical models which allowed them to infer the existence of galaxies we know are out there but can’t view until bigger telescopes come online. This led to the surprising realization that some 90% of the galaxies in the observable universe are actually too faint and too far away to be seen — yet.
“It boggles the mind that over 90% of the galaxies in the Universe have yet to be studied. Who knows what interesting properties we will find when we observe these galaxies with the next generation of telescopes,” said Christopher Conselice.
Their survey dug far back in time, more than 13 billion years ago, and revealed that galaxies aren’t evenly distributed throughout the universe’s history. It appears there are 10 more galaxies per unit volume when the universe was only a few billion years old compared with today. Most of these galaxies were relatively small and faint and resemble the satellite galaxies orbiting our own Milky Way. The two biggest and brightest Milky Way satellites are known as the Large and Small Magellanic Clouds, visible from the southern hemisphere.
These results are strong evidence that the universe is evolving. Just like small businesses merging to form larger corporations, early galaxies merged together to make bigger ones, dramatically reducing their total number over time.
The decreasing number of galaxies as time progresses also helps to solve Olbers’ paradox or why the sky is dark at night. It’s named for 19th century amateur astronomer Heinrich Olbers, who pondered the question of the dark night sky back before the Big Bang and expanding space were discovered. In an infinitely old and infinitely large universe, every single spot in the night sky would be filled with overlapping stars from near to infinitely far, so many that the night sky would shine as brightly as the surface of the sun. In a modern update, every point in the sky would contain part of a galaxy, each one made up of billions of stars. Either way, that’s not what we see — the night sky is dark.
Due to the rapid expansion of space on cosmic scales between galaxies and groups of galaxies (the solar system and planets are NOT expanding) since the Big Bang, the light from distant galaxies has been stretched or redshifted, so that many galaxies are no longer visible in ordinary light. The universe is also young — only 13.7 billion years old, far from infinite — so the light from some of those distant galaxies hasn’t had enough time to make its way to Earth for us to see them.
Only by invoking a young, expanding universe can we keep our night sky dark. Sometimes a simple fact touches at levels so deep we have to wait centuries for an adequate explanation.