Study says observable universe contains two trillion galaxies

 Observable universe is filled with at least two trillion galaxies, ten times the old estimate

Using data from deep-space surveys taken by NASA's Hubble Space Telescope and other observatories, astronomers have performed a census of the number of galaxies in the universe. The team came to the surprising conclusion that there are at least 10 times as many galaxies in the observable universe than previously thought. The results have clear implications for our understanding of galaxy formation and also helps shed light on an ancient astronomical paradox, why is the sky dark at night? Two trillion, and most of them out of sight, a team led by Christopher Conselice at the University of Nottingham put the number of galaxies in the observable universe at about two trillion, roughly ten times the figure astronomers had been working with. The galaxy count has climbed and fallen as instruments changed, from a few hundred billion in the 1990s to two trillion in 2016 and back toward the hundreds of billions in the New Horizons reading. Each revision says as much about the reach of the telescope as about the universe itself.

A team led by Christopher Conselice at the University of Nottingham put the number of galaxies in the observable universe at about two trillion. The twist sits inside the result: the great majority of those galaxies are too faint for any telescope now operating to detect. The paper states it plainly. There are “at least 2 × 10¹² (two trillion) galaxies in the currently visible universe, the vast majority of which cannot be observed with present day technology as they are too faint.” In other words, the count is less a tally of dots in our images than an estimate of how many dots the images are missing. The universe suddenly looks a lot more crowded, thanks to a deep-sky census assembled from surveys taken by NASA's Hubble Space Telescope and other observatories. The team did not add up galaxies one by one. Instead they used galaxy stellar mass functions, which describe how many galaxies of each mass exist in a given slice of space. They measured these at many points in cosmic history, reaching back to within roughly 650 million years of the Big Bang. Much of the deep data came from the Hubble Space Telescope, whose narrow, long-exposure images act like core samples drilled through the universe.

Fitting those measurements with a standard mathematical curve, the researchers worked out the density of galaxies at each epoch, then integrated across the whole observable volume and the whole age of the universe. The arithmetic ran down to galaxies as small as a million times the mass of the Sun, faint dwarfs which no survey can see at great distance. Most of these galaxies were relatively small and faint, with masses similar to those of the satellite galaxies surrounding the Milky Way. As they merged to form larger galaxies the population density of galaxies in space dwindled. This means that galaxies are not evenly distributed throughout the universe's history, the research team reports. "These results are powerful evidence that a significant galaxy evolution has taken place throughout the universe's history, which dramatically reduced the number of galaxies through mergers between them, thus reducing their total number. This gives us a verification of the so-called top-down formation of structure in the universe," explained Conselice.

Working with colleagues at Leiden Observatory in the Netherlands and the University of Edinburgh, the team turned narrow, deep images of the sky into three-dimensional maps. From those they measured the volume and the galaxy density of one small slice of space after another. Conselice likened the effort to an intergalactic archaeological dig, a way of estimating how many galaxies had been missed rather than counting the ones already on the photographic plates. The faintest galaxies the model implies would shine at around magnitude 29 on the astronomers’ brightness scale, far below the reach of the unaided eye and beyond all but the deepest exposures ever taken. Two independent versions of the calculation gave 1.2 trillion and 2.8 trillion. The University of Nottingham reported the averaged figure of two trillion as the headline result, the product of what Conselice described as roughly fifteen years of work. “We are missing the vast majority of galaxies because they are very faint and far away,” Conselice said when the study appeared. “It boggles the mind that over 90 percent of the galaxies in the universe have yet to be studied.”

One of the most fundamental questions in astronomy is that of just how many galaxies the universe contains. The landmark Hubble Deep Field, taken in the mid-1990s, gave the first real insight into the universe's galaxy population. Subsequent sensitive observations such as Hubble's Ultra Deep Field revealed a myriad of faint galaxies. This led to an estimate that the observable universe contained about 100 billion galaxies. The two-trillion figure is a model-dependent extrapolation, not a head count, and the authors are careful about its limits. The result hinges on two choices: the smallest galaxy mass they counted and the earliest epoch they reached. They note that pushing the mass limit lower would multiply the total by about seven, which is why they frame two trillion as a floor rather than a ceiling. The bigger caveat arrived later. In 2021 a team used NASA’s New Horizons spacecraft, far enough from the Sun to escape the dust glow which brightens the sky near Earth, to measure the cosmic optical background, the summed light of all galaxies. From out there the sky was about ten times darker than the darkest view Hubble can manage. The researchers argued in public that the faintness of that glow points to a total closer to the hundreds of billions than to trillions. “We simply don’t see the light from two trillion galaxies,” said researchers.

So the famous number is one side of an open argument, the high end of a range that newer data has pulled downward. What survives the dispute is the underlying lesson rather than the digit: most of the universe’s galaxies, on any of these estimates, are fainter than we can currently observe. Conselice and his team reached this conclusion using deep-space images from Hubble and the already published data from other teams. They painstakingly converted the images into 3-D, in order to make accurate measurements of the number of galaxies at different epochs in the universe's history. In addition, they used new mathematical models, which allowed them to infer the existence of galaxies that the current generation of telescopes cannot observe. This led to the surprising conclusion that in order for the numbers of galaxies we now see and their masses to add up, there must be a further 90% of galaxies in the observable universe that are too faint and too far away to be seen with present-day telescopes. These myriad small faint galaxies from the early universe merged over time into the larger galaxies we can now observe. The count is high partly because the young universe was busier than the present one. When the team tracked how galaxy numbers changed over time, the trend ran backward. A given volume of space held about ten times more galaxies when the universe was a few billion years old than it does today.

That sounds backward, since the universe has only grown older and larger. The explanation is that small galaxies have spent billions of years merging into bigger ones, so the population thins even as individual galaxies grow. Most of the missing multitude are low-mass systems, no larger than the dwarf galaxies, and they are exactly the ones too dim to register at great distance. Before this work, the count was usually put in the hundreds of billions. Even careful counts from the deepest direct images, such as the Hubble Ultra Deep Field, reached only a couple of hundred billion galaxies. The number of galaxies feeds into the total amount of starlight filling space, and into the centuries-old puzzle known as Olbers’ paradox, which asks why the night sky is dark if the universe is full of stars. A sky packed with two trillion galaxies might seem to make the paradox worse, not better. The team’s answer is that the light of those distant galaxies never reaches us at full strength. Most of it is absorbed by gas in intergalactic space, and the expansion of the universe stretches the rest toward redder, fainter wavelengths. So the universe can be far more crowded than telescopes show while the night sky stays dark, with most of its contents waiting in the gloom for sharper eyes. 

This may be the honest takeaway. The number of galaxies is not a settled fact carved into the sky but a running estimate, one that shifts every time a new instrument sees a little deeper into the dark. The decreasing number of galaxies as time progresses also contributes to the solution for Olbers' paradox (first formulated in the early 1800s by German astronomer Heinrich Wilhelm Olbers): Why is the sky dark at night if the universe contains an infinity of stars? The team came to the conclusion that indeed there actually is such an abundance of galaxies that, in principle, every patch in the sky contains part of a galaxy. However, starlight from the galaxies is invisible to the human eye and most modern telescopes due to other known factors that reduce visible and ultraviolet light in the universe. Those factors are the reddening of light due to the expansion of space, the universe's dynamic nature, and the absorption of light by intergalactic dust and gas. All combined, this keeps the night sky dark to our vision.