How many galaxies in the universe?

  Estimated No of galaxies in the universe      

If we can see at most 3% of the universe, how can we estimate the total number of galaxies in it? Galaxies are vast collections of stars that populate our universe. But how many galaxies are there? Counting them seems like an impossible task. Sheer numbers is one problem, once the count gets into the billions, it takes a while to do the addition. Another problem is the limitation of our instruments. To get the best view, a telescope needs to have a large aperture (the diameter of the main mirror or lens) and be located above the atmosphere to avoid distortion from Earth's air. 

The Milky Way is just a speck in a universe filled with an untold number of galaxies. But if we had to take an educated guess, how many galaxies are in the universe? That sounds like a simple question, but it's anything but. The first problem is that even with our most powerful telescopes, we can see only a tiny fraction of the universe. Perhaps the most resonant example of this fact is the Hubble eXtreme Deep Field (XDF), an image made by combining 10 years of photographs from the Hubble Space Telescope. The telescope watched a small patch of sky in repeat visits for a total of 50 days, according to NASA. If you held your thumb at arm's length to cover the moon, the XDF area would be about the size of the head of a pin. By collecting faint light over many hours of observation, the XDF revealed thousands of galaxies, both nearby and very distant, making it the deepest image of the universe ever taken at that time. So if that single small spot contains thousands, imagine how many more galaxies could be found in other spots.

"The observable universe is only that part of the universe from which the light has had time to reach us," astrophysicist Kai Noeske, now outreach officer at the European Space Agency said. The universe is 13.8 billion years old, but the observable universe stretches more than 13.8 light-years in every direction. That's because the universe is expanding and light got a head start early on, when the universe was smaller. "Now, the total size in each direction is about 46 billion light-years," Noeske said. That's much smaller than even our smallest estimates of the entire universe. "We see at most 3% of the universe," Pamela Gay, a senior scientist at the Planetary Science Institute said.

While estimates among different experts vary, an acceptable range is between 100 billion and 200 billion galaxies, said Mario Livio, an astrophysicist at the Space Telescope Science Institute in Baltimore, Maryland. The James Webb Space Telescope is expected to reveal even more information about early galaxies in the universe. Another problem is that there are so many galaxies that we can only make estimates of the total number based on what we can observe in small regions of the universe. "You look at a small patch of the sky, and you count everything in that small patch and then multiply over the size of the sky," Gay said. But even that requires a cutoff. "What do we define [as] a galaxy?" Noeske said. "We have really giant galaxies that have to have a factor of 10 more" the mass of our galaxy, "and we have a lot of small galaxies, from lower-mass galaxies that have about 10 times less mass … down all the way to dwarf galaxies."

The Hubble Space Telescope has been successful for galaxy counting and estimation. The telescope, launched in 1990, initially had a distortion on its main mirror that was corrected during a shuttle visit in 1993. Hubble also went underwent several upgrades and service visits until the final shuttle mission there in May 2009. In 1995, astronomers pointed the telescope at what appeared to be an empty region of Ursa Major, and collected 10 days' worth of observations. The result was an estimated 3,000 faint galaxies in a single frame, going as dim as 30th magnitude, according to Weber State University. (For comparison, the North Star or Polaris is at about 2nd magnitude.) This image composite was called the Hubble Deep Field and was the farthest anyone had seen into the universe at the time. 

At some point, scientists need to define a minimum mass for a galaxy to make estimates possible. "If we set a mass cutoff and try to make this conservative, like a million solar masses, we end up with an average number of galaxies in the universe from the beginning to today of about 1 to 2 trillion," Noeske said. Scientists think there were more galaxies earlier in the universe's history than there are today, which is why galaxy estimates are an average over time. "But those results come from the Hubble [telescope], the James Webb Space Telescope is starting to speak to these results, which are near Earth, inside of our solar system, and are limited on what they can see by all the stuff in our solar system that adds light to the sky," Gay said. "We do have one spacecraft with a camera that has gotten beyond all the garbage within our solar system, and that's the New Horizons spacecraft."

As the Hubble telescope received upgrades to its instruments, astronomers repeated the experiment twice. In 2003 and 2004, scientists created the Hubble Ultra Deep Field, which in a million-second exposure revealed about 10,000 galaxies in a small spot in the constellation Fornax. In 2012, again using upgraded instruments, scientists used the telescope to look at a portion of the Ultra Deep Field. Even in this narrower field of view, astronomers were able to detect about 5,500 galaxies. Researchers dubbed this the eXtreme Deep Field. All in all, Hubble reveals an estimated 100 billion galaxies in the universe or so, but this number is likely to increase to about 200 billion as telescope technology in space improves.

A 2021 study used the camera aboard New Horizons to measure the total amount of light in various patches of sky and estimated how many galaxies would be needed to create that much light. "And suddenly, as they're outside of all the light sources in our solar system, they realize we don't need as many galaxies as we thought," Gay said. "And so their estimates put us at, like, 200 billion, maybe even 100 billion galaxies in the visible universe. "So somewhere between 2 trillion galaxies at the top edge and 100 billion at the lower edge is the number of galaxies in our observable universe," she said. If you assume that's 3%, at most, of our universe, you can multiply that range of galaxies to get the total number of galaxies in the universe. If we're seeing less of the universe than we think, there will be a smaller total number of galaxies. But considering we don't actually know the size of the universe, those estimates are murky. "If it's an infinite universe, you're going to have infinite galaxies," Gay said."

Whatever instrument is used, the method of estimating the number of galaxies is the same. You take the portion of sky imaged by the telescope (in this case, Hubble). Then, using the ratio of the sliver of sky to the entire universe, you can determine the number of galaxies in the universe. This is assuming that there is no large cosmic variance, that the universe is homogenous. We have good reasons to suspect that is the case. That is the cosmological principle. The principle dates back to Albert Einstein's theory of general relativity. Einstein said that gravity is a distortion of space and time. With that understanding in hand, several scientists (including Einstein) tried to understand how gravity affected the entire universe. "The simplest assumption to make is that if you viewed the contents of the universe with sufficiently poor vision, it would appear roughly the same everywhere and in every direction," NASA stated. "That is, the matter in the universe is homogeneous and isotropic when averaged over very large scales. This is called the cosmological principle."

Measurements of the universe's expansion, through watching galaxies race away from us, show that it is about 13.82 billion years old. As the universe gets older and bigger, however, galaxies will recede farther and farther from Earth. This will make them more difficult to see in telescopes. The universe is expanding faster than the speed of light (which does not violate Einstein's speed limit because the expansion is of the universe itself, rather than of objects traveling through the universe). Also, the universe is accelerating in its expansion. This is where the concept of the "observable universe",  the universe that we can see, comes into play. In 1 trillion to 2 trillion years, this means that there will be galaxies that are beyond what we can see from Earth.

Galaxies also change over time. The Milky Way is on a collision course with the nearby Andromeda Galaxy, and both will merge in about 4 billion years. Later on, other galaxies in our Local Group, the galaxies closest to us, will eventually combine. Residents of that future galaxy would have a much darker universe to observe. As the early universe inflated, there are some theories that say that different "pockets" broke away and formed different universes. These different places could be expanding at different rates, include other types of matter, and have different physical laws than our own universe. While it is interesting to count the number of galaxies in our universe, astronomers are more interested in how galaxies reveal how the universe was formed. According to NASA, galaxies are a representation of how matter in the universe was organized, at least, on the large scale. (Scientists are also interested in particle types and quantum mechanics, on the small side of the spectrum.) Because Webb can look back to the early days of the universe, its information will help scientists better understand the structures of the galaxies around us today.