Research box title
How dark is the space? If you get away from city lights and look up, the sky between the stars indeed looks very dark. Above the Earth’s atmosphere, outer space fades even further and fades to an ink pitch black. And yet, even there, space is not absolutely black. The universe has a muted, soft glow of innumerable stars and galaxies.
New measurements of the faint background glow show that the invisible galaxies are less abundant than some theoretical studies have suggested, and this counts only hundreds of billions instead of the two trillion galaxies previously reported.
“This is an important number to know – how many galaxies are there?” says Marc Postman of the Space Telescope Science Institute in Baltimore, Maryland, a lead author of the study. “We simply do not see the light from two trillion galaxies.”
The earlier estimate was extrapolated from observations in the deep sky 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. The team concluded that 90% of the galaxies in the universe were beyond Hubble’s ability to detect in visible light. The new findings, based on measurements from NASA’s distant New Horizons mission, indicate a very modest number.
“Taking all the galaxies that Hubble can see doubles the number, and that’s what we see – but nothing more,” says Tod Lauer of NSF’s NOIRLab, a lead author of the study.
These results will be presented on Wednesday, January 13, at a meeting of the American Astronomical Society, which is open to registered participants.
The cosmic optical background that the team wanted to measure is the visible light equivalent of the more well-known cosmic microwave background – the faint 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,” Postman explained. “This puts a limit on the total number of galaxies that have been created, and where they may be on time.”
As powerful as Hubble is, the team could not use it to make these observations. Although it is in space, it orbits the earth and still suffers from light pollution. The inner solar system is filled with small dust particles of disintegrated asteroids and comets. Sunlight reflects off the particles and creates a glow called the zodiac sign, which can even be observed by air guards on the ground.
To escape the zodiac sign, the team had to use an observatory that escaped the inner solar system. Fortunately, the New Horizons spacecraft, which produced the closest images of Pluto and the Kuiper belt object Arrokoth ever, is far enough away to make these measurements. At its distance (more than 4 billion kilometers away when these observations were taken), New Horizons experiences ambient air that is ten times darker than the darkest sky accessible to Hubble.
‘This kind of measurement is extremely difficult. “A lot of people have been trying to do that for a long time,” Lauer said. “New Horizons gave us a vantage point to better measure the cosmic optical background than anyone could do.”
The team analyzed existing images from the New Horizons archives. To tease the faint background glow, they had to correct for a number of other factors. For example, they diverted light from the galaxies that were expected to exist that are too faint to be identified. The most difficult correction was to remove the light from the Milky Way stars reflected by interstellar dust in the camera.
The remaining signal, although extremely faint, was still measurable. Postman compared it to living in a remote area far from city lights while lying in your bedroom at night with the curtains open. If a neighbor opens the fridge a mile away in search of a midnight snack, and the light from the fridge reflects off the bedroom walls, it will be just as bright as the background that New Horizons detects.
So, what could be the source of this excess glow? It is possible that an abundance of dwarf systems in the relatively near universe is just beyond perceptibility. Or the diffuse orbits of stars surrounding galaxies may be brighter than expected. There may be a population of rogue, intergalactic stars scattered in the cosmos. The most interesting, perhaps, are many more faint, distant galaxies than theories suggest. This would mean that the smooth distribution of galaxy measurements measured so far rises steeply beyond the faintest systems we can see – just as there are many more pebbles on a beach than rocks.
NASA’s upcoming James Webb Space Telescope may help solve the mystery. If faint, individual galaxies are the cause, Webb ultra-deep field observations should be able to detect them.
This study is accepted for publication in the Astrophysical Journal.