New measurements of the blackness of the sky reflect only galaxies in the hundreds of millions.
How deep is the sky, and what does that tell us about the number of galaxies in the visible universe? Astronomers estimated the total galaxies by counting everything that appeared in the deep region of Hubble and then multiplying them by the total area of the sky. But other galaxies are also very faded and distant that can be detected directly. Yet until we can count them, their light damages the space with a weak glow.
To measure that brightness, astronomical satellites have to avoid the inner solar system and its light pollution, which reflects dust from sunlight. A team of scientists have used the observations NASATo the New Horizons Mission Pluto And the Cooper belt for determining the brightness of this cosmic optical background. Their result sets an upper limit for the abundance of faint, unresolved galaxies, indicating that they number only billions in number, not about 2 trillion galaxies as previously assumed.
How dark does space get? If you step away from the city lights and look up, the sky between the stars appears very dark indeed. The outer space above the earth’s atmosphere becomes even deeper, which is of a harder pitch-black color. And still there is no space Absolutely Black. The universe has a strong flicker from innumerable distant stars and galaxies.
New measurements of that weak background brightness suggest that undiscovered galaxies are less plentiful than some theoretical studies, instead of the previously reported two trillion galaxies in just hundreds of billions.
“This is an important number to know – how many galaxies are there?” Said Mark Postman of the Space Telescope Science Institute in Baltimore, Maryland, a lead author on the study. “We just don’t see light from two trillion galaxies.”
Earlier estimates were contrasted with very deep sky observations by NASA Hubble Space Telescope. It relied on mathematical models to estimate how many galaxies were too small and blurred to see Hubble. That team concluded that 90% of the galaxies in the universe were beyond Hubble’s ability to detect in visible light. The new findings, which relied on measurements from NASA’s Far New Horizons mission, suggest much smaller numbers.
“Take all the galaxies, Hubble can double that number, and what we see – but nothing more,” said NSF’s NOIRLab Todd Lauer, a lead author on the study.
These results will be presented at a meeting of the American Astronomical Society on Wednesday, January 13, which is open to registered participants.
The cosmic optical background that the team has tried to measure is equivalent to the visible light of the more well-known cosmic microwave background – the weak shock of the big bang, before the stars came into existence.
Postman explained, “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 those stars,”. “This places a constraint on the total number of galaxies formed, and where they may occur in time.”
As powerful as Hubble is, teams cannot use them to make these comments. Although located in space, Hebbal orbits the Earth and still suffers from light pollution. The inner solar system is filled with small dust particles from fragmented asteroids and comets. Sunlight reflects those particles, creating a glow called zonal light that can also be seen by skywatchers on the ground.
To avoid zonal light, the team had to use an observatory that survived the internal solar system. Fortunately the New Horizons spacecraft, which delivered the closest images of Pluto and the Kuiper Belt object Aarkoth, is sufficient to make these measurements. At their distance (more than 4 billion miles away when these observations were made), New Horizons experience a celestial sky 10 times deeper than the deepest sky accessible to Hubble.
“These types of measurements are very difficult. A lot of people have tried to do this for a long time, ”said Lauer. “New Horizons has provided us with a vantage point to measure the cosmic optical background that anyone is capable of.”
The team analyzed existing images from the New Horizons archives. In order to tease out the weak background brightness, he had to correct for several other factors. For example, they subtracted light from galaxies, which were too faint to be detectable. The most challenging improvement was removing the light Galaxy Stars shown in interstellar dust and camera.
The remaining signal, although extremely faint, was still mediocre. The postman compared this to living in a remote area away from city lights, with open curtains in your bedroom at night. If a neighbor a mile down the road opened their refrigerator in search of a midnight snack, and the light from their refrigerator was reflected from the bedroom walls, it would be as bright as the New Horizons found.
So, what could be the source of this leftover glow? It is possible that an abundance of dwarf galaxies in a relatively nearby universe is beyond traceability. Or the spread of stars that surround the galaxies may be more spectacular than expected. There may be a population of crooks, inter-star stars scattered throughout the universe. Perhaps there are far more faint, distant galaxies than the most intriguing, theories suggest. This would mean that the apparent distribution of the size of the measured galaxy to date has been growing much faster than the inane systems we can see – like there are many more pebbles on the beach than on the rocks.
Upcoming NASA James Webb Space Telescope May be able to help solve the mystery. If faint, discrete galaxies are the reason for this, the web should be able to detect ultra-deep field observations.
This study is accepted for publication in the Astrophysical Journal.