Two trillion galaxies in the universe

Astronomers have long sought to determine how many galaxies there are in the universe. This is a fundamental question that we have only been able to address with any certainty due to new scientific results.

During the past 20 years very deep Hubble Space Telescope images have found a myriad of faint galaxies, and it was approximated that the observable Universe contains about 100 billion galaxies in total. Now, an international team, led by Christopher Conselice, Professor of Astrophysics at The University of Nottingham, has shown that the actual number is much higher than this.

Professor Conselice and his team has shown that the number of galaxies in our universe is at least two trillion — 10 times more than previously thought — the often quoted value of around 100 Billion.

Current astronomical technology allows us to study a fraction of these galaxies — just 10 per cent. It means that over 90 per cent of the galaxies in our universe have yet to be discovered, and will only be seen once bigger and better telescopes are developed.

The research — ‘The Evolution of Galaxy number density at Z < 8 and its implications’ — is published today (October 13, 2016) in the Astrophysical Journal — the foremost research journal in the world dedicated to recent developments, discoveries and theories about astronomy and astrophysics. The results have clear implications for galaxy formation, and also help solve an ancient astronomical paradox — why is the sky dark at night?

Professor Conselice, said: “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 number we would like to know, and 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 study these galaxies with the next generation of telescopes. These galaxies will likely hold the clues to many outstanding astrophysical issues.”

Intergalactic archaeological dig

Professor Conselice’s research is the culmination of 15 year’s work. His team converted pencil beam images of deep space from telescopes around the world, and especially from the Hubble telescope into 3D maps to calculate the volume as well as the density of galaxies of one tiny bit of space after another. This painstaking research enabled him to establish how many galaxies we have missed - much like an intergalactic archaeological dig.

The results of this study are based on the measurements of the number of galaxies at different epochs — different instances in time — through the universe’s history.

When Professor Conselice and his team at Nottingham, in collaboration with scientists from the Leiden Observatory at Leiden University in the Netherlands and the Institute for Astronomy at the University of Edinburgh, examined how many galaxies there were in a given value they found that this increased significantly at earlier times.

In fact, it appears that there are a factor of 10 more galaxies in a given volume of space when the universe was a few billion years old compared with today. Most of these galaxies are low mass systems with masses similar to those of the satellite galaxies surrounding the Milky Way.

Professor Conselice, said: “This is very surprising as we know that over the 13.7 billion years of cosmic evolution galaxies are growing through star formation and merging with other galaxies. Thus, to find that there were in fact more galaxies in the past implies that that significant evolution in galaxies must have occurred to reduce the number of galaxies through extensive merging of systems. This also gives us a verification of the top-down formation of structure in the universe.”

Probing cosmic history answers astronomical questions

By probing deep into space Professor Conselice and his team have been able to go way back in time — more than 13 billion years in the past — to find out how our universe evolved and answer some vexing questions.

The implications of this research are many, for instance; galaxies are likely to be forming by merging together. This decreases the number of systems as time progresses which provides a possible solution to Oblers' paradox — why the sky is dark at night? Solutions to this in the past were based on the fact that the universe is finite in size as well as in time. However, if we consider all the undiscovered galaxies then in principle the critiera for Oblers’ paradox is met. However, most galaxies in the universe are very distant and their light is absorbed by gas in intergalactic space. Otherwise, we would see the night sky lit up everywhere.