While estimates among different experts of how many galaxies there are in the universe vary, an acceptable range is between 100 billion and 200 billion, according to 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 the universe’s earliest galaxies, according to The Astrophysical Journal.
According to Livio, the Hubble Space Telescope has been very successful in counting and estimating galaxies. Launched in 1990, it initially had a distortion in its main mirror that was corrected during a shuttle visit in 1993. Hubble also went through several upgrades and service visits until the last shuttle mission in May 2009.
The result was an estimated 3,000 faint galaxies in a single frame. This composite of images was called the Hubble Deep Field and was the furthest anyone had seen in the universe at the time.
In 1995, astronomers pointed this one at what appeared to be an empty region of the Big Dipper and collected observations for ten days. The result was an estimated 3,000 faint galaxies in a single frame. This composite of images was called the Hubble Deep Field and was the furthest anyone had seen in the universe at the time.
When Hubble received improvements to its instruments, the experts repeated the experiment twice. In 2003 and 2004, scientists created the Hubble Ultra Deep Field, which in a million-second exposure revealed some 10,000 galaxies in a tiny dot in the constellation Fornax.
In 2012, again using improved instruments, they used the telescope to look at a part of the ultra-deep field. Even in this narrower one, they were able to detect some 5,500 galaxies. The researchers called this the eXtreme Deep Field.
In total, Hubble has revealed approximately 100 billion galaxies in the universe, but this number is likely to increase to around 200 billion.
In all, Hubble has revealed roughly 100 billion galaxies in the universe, but this number is likely to increase to around 200 billion as space-based telescope technology improves, Livio told Space.com.
COUNTING STARS
Whichever instrument is used, the method of estimating the number of galaxies is the same. The part of the sky photographed by the telescope (in this case, the Hubble) is taken. Then, using the relationship between the strip of sky and the entire universe, the number of galaxies in it can be determined.
“This assumes that there is no great cosmic variation, that the universe is homogeneous,” Livio pointed out. “We have good reason to suspect that this is the case. That is the cosmological principle.”
The principle dates back to Albert Einstein’s theory of general relativity, who said that gravity is a distortion of space and time. With that understanding in hand, various scientists (including Einstein) tried to understand how gravity affected the entire universe.
“The simplest assumption you can make is that if you look at the contents of the universe with poor enough vision, it would look pretty much the same everywhere and in all directions,” NASA said. “That is, matter in the universe is homogeneous and isotropic when averaged over very large scales. This is called the cosmological principle.”
An example of the cosmological principle at work is the cosmic microwave background (CMB), radiation that is a remnant of the early stages of the universe after the Big Bang. Using instruments like NASA’s Wilkinson Microwave Anisotropy Probe, astronomers have discovered that the CMB is virtually identical wherever you look.
WOULD THE NUMBER OF GALAXIES CHANGE?
Measurements of the expansion of the universe, through observation of galaxies receding from us, show that it is about 13.82 billion years old. However, as the universe ages and grows, the galaxies will move further and further away from Earth. This will make them more difficult to see in telescopes.
Galaxies also change over time. The Milky Way is on a collision course with the nearby Andromeda Galaxy, and the two will merge in about 4 billion years.