New measurements of the blackness of the sky show that galaxies number only hundreds of billions.
How dark is the sky, and what does it 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 deep field and then multiplying them by the total area of the sky. But other galaxies are too faint and distant to directly detect. Still, we can not count them, but their light is enough with a faint glow.
To measure this glow, astronomical satellites must escape the inner solar system and its light pollution caused by sunlight reflecting dust. A team of researchers has used observations from NASA‘s New 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 thought.
How dark is the room? If you get away from the city lights and look up, the sky between the stars actually looks very dark. Above the Earth’s atmosphere, outer space is dimmed even more and fades to an ink black. And yet there is no room, even there absolutely black. The universe has a flooded faint glimpse of countless distant stars and galaxies.
New measurements of the faint background glow show that the unseen galaxies are smaller than some theoretical studies suggested, and they number only in hundreds of billions rather than the previously reported two trillion galaxies.
“It’s an important number to know – how many galaxies are there?” said Marc Postman of the Space Telescope Science Institute in Baltimore, Maryland, a lead author of the study. “We simply do not see the light of two trillion galaxies.”
The previous estimate was extrapolated from very deep sky observations from NASAs Hubble Space Telescope. It relied on mathematical models to estimate how many galaxies were too small and weak for Hubble to see. That 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, suggest a far more modest number.
“Take all the galaxies Hubble can see, double the number, and that’s what we see – but nothing more,” said 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 tried 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 of all the stars that have ever formed since then,” Postman explained. “It puts a limit on the total number of galaxies that have been created and where they can be in time.”
As strong as Hubble is, the team could not use it to make these observations. Although Hubble is in space, it orbits the Earth and still suffers from light pollution. The inner solar system is filled with tiny dust particles from decomposed asteroids and comets. Sunlight reflects these particles and creates a glow called the light of the zodiac that can be observed even by skywatchers on the ground.
To escape the light of the zodiac, the team had to use an observatory that escaped the inner solar system. Fortunately, the New Horizons spacecraft, which has provided the closest images ever of Pluto and the Kuiper Belt object Arrokoth, is far enough to make these measurements. At its distance (more than 4 billion miles away when these observations were made), New Horizons experiences a surrounding sky 10 times darker than the darkest sky available to Hubble.
“These measurements are extremely difficult. Many people have been trying to do this for a long time, ”Lauer said. “New Horizons provided us with a vantage point to measure the cosmic optical background better than anyone has been able to do.”
The team analyzed existing images from New Horizon’s archives. To tease out the faint background glow, they had to correct a number of other factors. For example, they drew light from the expected galaxies that are too faint to be identified. The most challenging correction was to remove light from The Milky Way stars reflected from interstellar dust and into the camera.
The remaining signal, although extremely weak, was still measurable. Postman compared it to living in a remote area far from the city lights and lying in your bedroom at night with the curtains open. If a neighbor a mile further down the road opened their refrigerator looking for a midnight snack, and the light from their refrigerator reflected from the bedroom walls, it would be as bright as the background New Horizons discovered.
So what could be the source of this remaining glow? It is possible that an abundance of dwarf galaxies in the relatively nearby universe is just beyond detectability. Or the diffuse halos of stars surrounding galaxies may be brighter than expected. There may be a population of eerie, intergalactic stars scattered 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 sharply just beyond the weakest systems we can see – just as there are many more pebbles on a beach than rocks.
NASA is coming James Webb Space Telescope can possibly help solve the mystery. If faint, individual galaxies are the cause, Webb ultra-deep field observations must be able to detect them.
This study is accepted for publication in the Astrophysical Journal.