This artist’s illustration shows NASA’s spacecraft New Horizons in the outer solar system. In the background lies the sun and a glowing band representing zodiac sign caused by sunlight reflecting off dust. By traveling beyond the inner solar system and the associated light pollution, New Horizons was able to answer the question: How dark is space? At the bottom right are background stars of the Milky Way. Credit: Joe Olmsted (STScI)
New measurements of the sky’s blackness show that galaxies amount to only hundreds of billions.
How dark is the sky, and what does this tell us about the number of galaxies in the visible universe? Astronomers can estimate the total number of galaxies by counting everything visible in a Hubble field and then multiplying by the total area of the sky. But other galaxies are too faint and far to detect directly. Although we can not count it, they keep light space with a faint glow.
To measure the glow, astronomical satellites must escape from the inner solar system and its light pollution, caused by sunlight reflected by dust. A team of scientists has observations of NASANew Horizons mission to Pluto and the Kuiper belt to determine the brightness of this cosmic optical background. Their result sets an upper limit for the abundance of faint, unresolved galaxies, showing that they number only hundreds of billions, not 2 trillion galaxies as previously believed.
This photo shows a phenomenon known as zodiacal light. Bottom left extends a glowing spot at the top right in the direction of Jupiter, the bright object to the left of the center. Zodiac light is caused by sunlight reflecting small dust particles into the inner solar system – the decomposed remnants of comets and asteroids. Attempts to measure how dark space telescopes like Hubble use have been thwarted by this ambient glow. Credit: Z. Levay
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, there is no space 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 by deep air by NASA 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 asteroid light that can be observed even by aerial viewers 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 made), New Horizons experiences ambient air that is 10 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 light Milky Way stars reflected from interstellar matter and 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 snack at midnight, and the light from the fridge reflects off the bedroom walls, it will be just as bright as the background New Horizons detects.
So, what could be the source of this excess glow? It is possible that an abundance of dwarf galaxies in the relatively nearby universe are just beyond perceptibility. Or the diffuse ray circles 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 just beyond the faintest systems we can see – just as there are many more pebbles on the beach than rocks.
NASA’s upcoming James Webb Space Telescope may be able to 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.