Two Trillion Galaxies

Two trillion! That’s 2,000,000,000,000 galaxies. A trillion is a million times a million.

For a long time astronomers have said that our observable universe has about 100 billion galaxies. Two trillion is 2,000 billion, or 20 times larger.

It’s still smaller than the U.S. national debt at about $19 trillion, but those are just dollars, not galaxies. There are only 7 billion people on Earth, this number is 300 times larger.

Why so many?

Galaxies are not static. They evolve.

So what’s the twist, why were astronomers so wrong for so long? Well, it’s because now we can see galaxies much further away than before, thanks to the Hubble Space Telescope and other advanced telescopes. In fact we are now detecting many galaxies that were formed in the first billion years of the universe’s history.

And the universe is about 13.8 billion years old.

A team of astronomers from the University of Nottingham, the Leiden Observatory, and the University of Edinburgh, have built extremely detailed 3D maps of distant galaxies in order to estimate the density of these. They have used data from the Hubble Space Telescope and various ground-based telescopes.

When we look at distant galaxies we are also looking back into the past, since light travels at a finite speed. The researchers, lead by Prof. Christopher Consilice at the University of Nottingham, found the density of galaxies when the universe was a few billion years old to be about 10 times higher than at present (after correcting for the expansion of the universe). He noted that “we are missing the vast majority of galaxies because they are very faint and far away”.

These earlier galaxies were smaller, less massive, much less so. Large galaxies today like the Milky Way and the Andromeda galaxy have masses of around a trillion times the Sun. These galaxies were much more like the two dozen satellite galaxies found around the Milky Way, such as the Magellanic Clouds.

The Large Magellanic Cloud has a mass of about 1% of our Milky Way Galaxy. Image credit: NASA (C141 flight)

The main conclusion of the study? We know that galaxies undergo mergers. Apparently there have been many more mergers than previously assumed. Large galaxies such as our Milky Way have been formed by multiple successive mergers.


NGC 3921 is actually two galaxies in the process of merging. Note the strange and twisted orientation of the spiral arms, and the appearance of two disk like structures.Image credit: ESA/Hubble & NASA

So while there were originally trillions of galaxies in the early universe, in today’s universe the number has been reduced by mergers to order hundreds of billions.

Mergers will continue, in fact the Andromeda Galaxy is headed our way, and it’s twice as big as we are!

Reference – Royal Astronomical Society press release


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