In an observable universe, how many galaxies are there? According to NASA's New Horizon's probe, it's not as much as we expected. New studies calculated the universe's darkness and found that "only" a few hundred billion galaxies are possibly shining too softly to be seen by us.
You have to travel out of the town, away from light pollution, to get a significantly better view of the stars in the night sky. The internal solar system is full of sailing specks of dust that capture photons, so the probes that make it to the edges have a better view of the faintest light in the universe, and the same applies to spacecraft.
New Horizons is ideal for the job as one of the most remote human-made objects out there. The spacecraft reached more than 4 billion miles (6.4 billion km) from Earth since traveling to Pluto in 2015 and the Kuiper Belt object Arrokoth in 2019. The sky at that distance is 10 times darker compared to Hubble's result.
In a statement, Marc Postman of the Space Telescope Science Institute underscored the importance of knowing how many galaxies are there.
How Dark is Dark?
So Postman and his team investigated the celestial optical history. This dim visible light sparkles so faintly that it can't really be seen behind local light pollution by most instruments on or in orbit around Earth.
Postman said the cosmic optical background teaches us something about the total sum of all the stars that have evolved since then. He said the latest discovery places a restriction on the total number of universes created and where they could be in the future."
According to Postman's new study forthcoming in the Astrophysical Journal, there are potentially a few hundred billion unidentified galaxies concealed in the darkness. That's a lot, but it's much less than the two trillion or so previous estimates that have been abstracted from Hubble data.
Take all the galaxies that Hubble could detect twice the normal amount, and that's what we can see," says Tod Lauer, lead author of the study, but nothing more.
In short, space is so dark. There can't be as many galaxies out there that add a dim light to the background as astronomers have previously predicted.
How Did They Calculate The Darkness?
The team studied images from New Horizons collections to draw this conclusion, accounting for aspects like light from stars of the Milky Way reflecting from interstellar dust. That left the background glow incredibly faint but still detectable.
And where does this residual light come from? Astronomers believe that it may be extremely similar to very distant dwarf galaxies, or even more small galaxies at additional distances. The culprit may also be rogue stars unattached to galaxies, or maybe galaxy halos are brighter than we hold them responsible for.
Further studies, such as the James Webb Telescope's October launch, may help address these questions.
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