The Milky Way is just a speck in a universe filled with countless galaxies. But if we had to make an educated guess, how many galaxies are there in the universe?
This sounds like a simple question, but it is anything but. The first problem is that even with our most powerful telescopes, we can only see a small part of the universe.
“The observable universe is only that part of the universe from which light has had time to reach us,” the astrophysicist Kai Noeskenow a liaison officer at the European Space Agency, told Live Science.
The universe is 13.8 billion years old, but the observable universe extends more than 13.8 light years in any direction. This is because the universe is expanding and light started in the beginning when the universe was smaller.
“Now, the total size in each direction is about 46 billion light years“, said Noeske.
This is much smaller than even our smallest estimates for the entire universe. “We see at most 3% of the universe,” Pamela Gaya senior scientist at the Institute of Planetary Sciences, told Live Science.
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The second problem is that there are so many galaxies that we can make estimates of the total number based on what we can observe in small regions of the universe.
“You look at a small part of the sky and count everything in that small part and then multiply by the size of the sky,” Gay said.
But even this requires a break. “What do we define? [as] a galaxy?” Noeske said. “We have really giant galaxies that must be a factor of 10 more” the mass of our galaxy, “and we have many small galaxies, from smaller galaxies that are about 10 times more few masses … to dwarf galaxies”.
At some point, scientists must determine a minimum mass for a galaxy to make estimates possible.
“If we put 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 history of the universe than there are today, which is why galaxy estimates are an average over time.
“But these results come from Hubble [telescope] — The James Webb Space Telescope is starting to talk about these results — which are close to Earth, inside our solar system, and are limited to what they can see of all the things in our solar system that adds light to the sky,” Gay said. “We have a spacecraft with a camera that has gone over all the debris within our solar system, and this is New Horizons spacecraft.”
A 2021 study used the camera on New Horizons to measure the total amount of light in different parts of the sky and estimated how many galaxies would be needed to create that much light.
“And suddenly, since they’re outside of all the light sources in our solar system, they realize that 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 on the high end and 100 billion on the low end is the number of galaxies in our observable universe,” she said.
If you assume that it is 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 see less of the universe than we think, there will be a smaller total number of galaxies.
But considering that we don’t actually know the size of the universe, these estimates are fuzzy. “If it’s an infinite universe, you’ll have infinite galaxies,” Gay said.