The observable Universe contains about two trillion galaxies—more than ten times as many as previously estimated, according to the first significant revision of the count in two decades.
Since the mid-1990s, the working estimate for the number of galaxies in the Universe has been around 120 billion. That number was based largely on a 1996 study called Hubble Deep Field. Researchers pointed the Hubble Space Telescope at a small region of space for a total of ten days so that the long exposures would reveal extremely faint objects.
This view encompassed galaxies up to 12 billion light years away, which we see as they existed less than two billion years after the Big Bang. Astrophysicists then counted the galaxies within that narrow field of view and extrapolated the number to the full sky—under the assumption that it would look similar in all directions—to get to the 120 billion figure.
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However, there weren't enough galaxies in the Hubble Deep Field image to account for the density of matter distributed throughout the Universe. The missing matter had to be in the form of galaxies too faint to see, as gas and dark matter. “We always knew there were going to be more galaxies than that,” says astrophysicist Christopher Conselice of the University of Nottingham, UK. “But we didn't know how many existed because we couldn't image them.”
More recent deep-field studies conducted using Hubble—after NASA astronauts upgraded the observatory in 2009—and other telescopes enabled Conselice and collaborators to count visible galaxies out to distances of 13 billion light years. They were able to plot the number of galaxies of a given mass that corresponded to various distances away from Earth. The researchers then extrapolated their estimates to encompass galaxies too small and faint for telescopes to pick up. Based on this, they calculated that the observable Universe should contain 2 trillion galaxies. The paper1 will be published in the Astrophysical Journal.
Moments in time
The team’s count was not too surprising, says astronomer Steven Finkelstein at the University of Texas at Austin, but it’s still helpful to put a number on it. “I don't know of anyone who has done this before,” he says. Conselice says that theorists had expected the number to be even higher; he and his collaborators now plan to look into this discrepancy.
At present, researchers can only directly observe about 10% of the 2 trillion galaxies. But that will change in two years once Hubble's successor, the James Webb Space Telescope, is deployed, Conselice says. That telescope should also be able to peer much further back in time, to see how galaxies started to form, he adds.
The study might lead to an improved understanding galaxies by refining galaxy formation simulations and enabling more detailed assessments of how they grow.
But for now, his results are consistent with the current general theory of how galaxies form, in which most start very small, and then undergo a furious period of mergers and acquisitions, says Debra Elmegreen, an astronomer at Vassar College in Poughkeepsie, New York.
Because the Universe as seen today is a snapshot in time, many of the galaxies included in the new estimate no longer exist. They have merged into larger galaxies in the billions of years it took their light to reach Earth. So the current number of galaxies is therefore expected to be much lower than 2 trillion.
This article is reproduced with permission and was first published on October 14, 2016.