The Milky Way is merely a tiny fragment within a vast cosmos brimming with countless galaxies. However, if we were to venture an informed estimate, approximately how many galaxies might populate our universe?
That appears to be an easy query, yet it’s far from straightforward. The primary issue is that despite having our strongest telescopes, we can observe just a minuscule part of the cosmos.
As per an astrophysicist, the observable universe encompasses just those regions from where light has sufficiently traveled to be seen by us. Kai Noeske , who currently serves as an outreach officer at the European Space Agency, told Live Science.
The universe is 13.8 billion years old , yet the observable universe extends over 13.8 billion light-years in all directions. This is due to the expansion of the universe; initially, when the cosmos were more compact, light had an earlier beginning.
The overall dimension in every direction is approximately 46 billion. light-years ," Noeske said.
That's significantly tinier compared to what we had anticipated for the whole cosmos. As we understand it, 'we can observe merely 3% of the universe,' Pamela Gay , a senior scientist at the Planetary Science Institute, told Live Science.
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The second issue is that due to the vast number of galaxies, we can only estimate the overall count by examining limited areas within the observable universe.
"As you examine a tiny section of the heavens, you tally all objects within that segment and then extrapolate for the entire sky," explained Gay.
However, this still necessitates a boundary. “How do we classify something as a galaxy?” asked Noeske. “We encounter extremely large galaxies that must be at least ten times more massive than ours,” he continued, “while also having numerous smaller galaxies; these range from low-mass ones which possess roughly one-tenth the mass…all the way down to dwarf galaxies.”
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 illuminates the sky," Gay commented. "We do possess one spaceship equipped with a camera that has managed to travel past all the debris within our solar system, and that’s the New Horizons spacecraft ."
A 2021 study utilized the onboard camera of New Horizons to gauge the overall quantity of light across different sections of space and then calculated the number of galaxies required to produce such luminosity.
“And then, as they find themselves beyond all the light sources within our solar system, they conclude that we require fewer galaxies than previously believed,” Gay stated. “Consequently, their calculations suggest there might be around 200 billion, perhaps even down to 100 billion, galaxies observable in the universe.”
"As stated, the count of galaxies within our observable universe falls somewhere between approximately 2 trillion at the higher end and around 100 billion at the lower end," she explained.
If you estimate that figure to be around 3%, or possibly even lower, of our observable universe, then multiplying this ratio with the known galaxy count would yield an estimated total for all galaxies within the cosmos. Should our visible portion represent only a fraction of what actually exists beyond our view, the cumulative tally of galaxies across the entire universe could potentially be reduced accordingly.
However, since we lack knowledge about the actual size of the universe, these estimations remain unclear. "In the case of an infinite universe, you would expect to find infinite galaxies," explained Gay.
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