How dark is space? Universe may have fewer galaxies than scientists previously estimated, finds study
How dark is the sky and what does that tell us about the number of galaxies that are out there? The universe is less crowded than we thought, according to scientists, a finding that has implications for what is known about its makeup.
Researchers used data from NASA’s New Horizons mission to Pluto and the Kuiper Belt to determine how dark the cosmic optical background is. In the report, which has been accepted for publication in the Astrophysical Journal, experts took new measurements of the weak background glow from unseen galaxies.
Analysis reveals that the unseen galaxies are less plentiful than some theoretical studies suggested, numbering only in the hundreds of billions rather than the previously reported two trillion galaxies. “It’s an important number to know – how many galaxies are there? We simply don’t see the light from two trillion galaxies,” says Marc Postman, a lead author from the Space Telescope Science Institute in Baltimore, Maryland.
The cosmic optical background that the researchers sought to measure is the “visible-light equivalent of the more well-known cosmic microwave background,” the weak afterglow of the big bang itself before stars ever existed.
“While the cosmic microwave background tells us about the first 450,000 years after the big bang, the cosmic optical background tells us something about the sum total of all the stars that have ever formed since then. It puts a constraint on the total number of galaxies that have been created, and where they might be in time,” explains Postman.
The earlier estimate was extrapolated from very deep-sky observations by NASA’s Hubble Space Telescope. It relied on mathematical models to estimate how many galaxies were too small and faint for Hubble to see.
“Astronomers can estimate the total number of galaxies by counting everything visible in a Hubble deep field and then multiplying them by the total area of the sky. But other galaxies are too faint and distant to directly detect. While they cannot be counted, their light suffuses space with a feeble glow,” the team emphasizes.
The inner Solar System is filled with tiny dust particles from disintegrated asteroids and comets. Sunlight reflects off those particles, creating a glow called the “zodiacal light” that can be observed even by skywatchers on the ground. Thus, to measure the glow, astronomical satellites have to escape the inner Solar System and its light pollution.
While it is powerful, Hubble still suffers from light pollution. The investigators conclude that 90% of the galaxies in the universe were beyond Hubble’s ability to detect in visible light.
To escape the zodiacal light, the team had to use an observatory that has escaped the inner Solar System. They turned to the New Horizons spacecraft, which has delivered the closest-ever images of Pluto and the Kuiper Belt object Arrokoth, and is far enough to make these measurements. New Horizons experiences an ambient sky that is 10 times darker than the darkest sky accessible to Hubble.
“These kinds of measurements are exceedingly difficult. A lot of people have tried to do this for a long time. New Horizons provided us with a vantage point to measure the cosmic optical background better than anyone has been able to do it,” states Tod Lauer of NSF’s NOIRLab, a lead author on the study.
The experts analyzed existing images from the New Horizons archives. To tease out the feeble background glow, they had to correct for several other factors. For example, they subtracted the light from the galaxies expected to exist that are too faint to be identifiable. The most challenging correction was removing light from Milky Way stars that was reflected off interstellar dust and into the camera. The remaining signal, though extremely faint, was still measurable.
The current findings indicate a much more modest number. “Take all the galaxies Hubble can see, double that number, and that’s what we see – but nothing more,” writes Lauer.
So, what could be the source of the leftover glow? According to scientists, an abundance of dwarf galaxies in the relatively nearby universe may lie just beyond detectability, or the diffuse halos of stars that surround galaxies might be brighter than expected.
“There might be a population of rogue, intergalactic stars spread throughout the cosmos. Perhaps most intriguing, there may be many more faint, distant galaxies than theories suggest. This would mean that the smooth distribution of galaxy sizes measured to date rises steeply just beyond the faintest systems we can see – just as there are many more pebbles on a beach than rocks,” the findings state.
The James Webb Space Telescope, which is slated for launch in October 2021, may be able to answer the question. If faint, individual galaxies are the cause, then Webb ultra-deep field observations should be able to detect them, say experts.