Galaxies — those vast collections of stars that populate our universe — are all over the place. Perhaps the most resonant example of this fact is the Hubble eXtreme Deep Field, a collection of photographs from the Hubble Space Telescope revealing thousands of galaxies in a single composite picture.
While estimates among different experts vary, an acceptable range is between 100 billion and 200 billion galaxies, Mario Livio, an astrophysicist at the Space Telescope Science Institute in Baltimore, told Space.com. 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, Livio said, this means that there will be galaxies that are beyond what we can see from Earth.
"We can only see light from galaxies whose light had enough time to reach us," Livio said. "It doesn’t mean that that’s all there is in the universe. Hence, the definition of the observable universe."
"The numbers are not going to change much," Livio added, pointing out the first galaxies probably formed not too long before that. "So a number like 200 billion [galaxies] is probably it for our observable universe."
