Research box title
How dark is the space? If you stay away from the city lights and look up, the sky between the stars does look very dark. Above the earth’s atmosphere, the outer space darkens even further, gradually becoming jet black.However, even there, the space is not absolute black. The countless distant stars and galaxies in the universe emit faint glimmers.
New measurements of this faint background light show that the invisible galaxies are not as abundant as some theoretical studies suggest, and their number is only hundreds of billions, instead of the two trillion galaxies previously reported.
“Know that this is an important number-how many galaxies are there?”
Earlier estimates were based on very deep sky observations made by the Hubble Space Telescope of the National Aeronautics and Space Administration (NASA). It relies on mathematical models to estimate how many galaxies are too small and drowsy for Hubble to see. The team concluded that 90% of the galaxies in the universe exceed Hubble’s ability to detect in visible light. These new discoveries rely on measurement results from NASA’s distant New Horizons mission, indicating that this number is much smaller.
Todd Laue of NSF NOIRLab said: “Double the number of all the galaxies that Hubble can see. This is what we see, but nothing more.”
These results will be announced at the American Astronomical Society meeting on Wednesday, January 13, which is open to registered participants.
The cosmic optical background the research team is trying to measure is equivalent to visible light, equivalent to the more widely known cosmic microwave background-the faint afterglow of the Big Bang itself before the existence of stars.
“Although the cosmic microwave background tells us the first 450,000 years after the Big Bang, the cosmic optical background tells us the total number of all stars that have formed since then,” Postman explained. “This imposes a limit on the total number of galaxies that have been created and their location in time.”
As powerful as Hubble, the team cannot use it to make these observations. Despite being in space, Hubble orbits the earth and still suffers from light pollution. The inner solar system is full of tiny dust particles from decomposing asteroids and comets. Sunlight reflects off these particles, creating a glow called zodiacal light, which can be observed even by sky observers on the ground.
To escape the zodiacal light, the research team had to use an observatory that escaped the inner solar system. Fortunately, the “New Horizons” spacecraft has completed the closest imaging of Pluto and the Kuiper belt object Arrokoth to date. At a distance (more than 4 billion miles at the time of these observations), the sky surrounding New Horizons is 10 times darker than the darkest sky the Hubble can enter.
These measurements are very difficult. For a long time, many people have tried to do this. “” New Horizons provides us with a better position than anyone to measure the optical background of the universe. “
The team analyzed the existing images in the New Horizons archive. In order to understand the faint background light, they must correct many other factors. For example, they subtracted light that was too weak to be recognized from the galaxy expected to exist. The most challenging corrective action is to remove the light reflected from the interstellar dust from the stars of the Milky Way and entering the camera.
Although the remaining signal is very weak, it can still be measured. The postman compares it to living in a remote area away from city lights, lying in a bedroom with open curtains at night. If a neighbor along the road opens the refrigerator for midnight snacks, and the light from the refrigerator reflects on the bedroom wall, its brightness will be as bright as the background detected by New Horizons.
So, what might be the source of this residual glow? There may be a large number of dwarf galaxies in the relatively close universe, which is beyond detectability. Otherwise, the diffuse halo of the stars surrounding the galaxy may be brighter than expected. There may be a large number of rogue interstellar stars scattered throughout the universe. Perhaps the most fascinating thing is that there may be distant galaxies that are even more obscure than the theory suggests. This means that the smooth distribution of galaxy sizes measured so far rises sharply, beyond the weakest system we can see, just like there are more pebbles on the beach than rocks.
NASA’s upcoming James Webb Space Telescope may be able to help solve this mystery. If faint galaxies are the cause of individual galaxies, then Weber ultra-deep field observations should be able to spot them.
This research was accepted for publication in the Astrophysical Journal.