This is much darker than the Hubble Space Telescope leads us to believe.
You may have looked at the sky as you stand in a rural area away from all the lights in the city. Countless bright stars shine in the night sky. The space between these stars is filled with complete darkness. But to what extent is it dark? And what does this say about the number of galaxies in the observable universe?
It’s not really black
Just as the sky may seem dark at times, interstellar space is not completely black, even there. The universe is illuminated by a faint glow of innumerable distant stars and galaxies. It’s hard to say how strong this glow is. Satellites and telescopes in our solar system, such as the Hubble Space Telescope, cannot properly measure the light emitted. In fact, the space around the Earth and the inner solar system is full of dust particles illuminated by the sun, creating a diffuse glow in the sky.
However, here is a mission for the New Horizons spacecraft; The unmanned spacecraft, launched in 2006, visited the dwarf planet Pluto and later the asteroid Arocoth. This probe is currently far from all major sources of light pollution, billions of miles from Earth. Thus, the surrounding sky is approximately ten times darker than the darkest sky visible through the Hubble Telescope. This is interesting. Because this also allows New Horizons to make a more accurate estimate of the overall brightness of all the galaxies in the universe.
However, the Hubble Space Telescope has already ventured into this before. Astronomers can estimate the total number of galaxies by counting everything visible to Hubble and multiplying it by the total area of the sky. But other galaxies are too faint and too far away to detect them directly. The researchers used mathematical models to estimate the number of galaxies too small and too weak for Hubble to detect. The team concluded that 90% of the galaxies in the universe are invisible to the Hubble Telescope.
But now it seems that the universe is actually much darker. Using New Horizons, researchers have once again measured the visible light emitted from galaxies. Invisible galaxies are now shown to be fewer in number than previously thought. That would not be 2 trillion, but only hundreds of billions.
Cosmic visual background
Researchers rely on measurements of what is called the “cosmic optical background,” which is visible light equivalent to the most common cosmic background radiation (thermal radiation emitted shortly after the Big Bang). “As cosmic background radiation tells us more about the first 450,000 years after the Big Bang, the cosmic optical background tells us more about all the stars that have formed since then,” explains researcher Mark Postman. “Set a limit on the total number of galaxies and where they can be located in time.”
Thus, although we cannot count all the galaxies, their light penetrates space with a faint and mysterious glow. The researchers analyzed current images of New Horizons and filtered out all known sources of visible light, until only light was left outside our galaxy. However, the remaining signal, although very weak, can still be measured. This means that the inexplicable light is still too bright. This postman compares it to living in a remote area away from city lights. If you were lying in bed at night with the shutters open and a neighbor turned on the fridge a mile away, that light could bounce off the walls of your bedroom. Then it will be as bright as the light captured by New Horizons.
Who or what exactly is responsible for this unwarranted light? It is possible that dwarf galaxies exist in the relatively close universe and cannot be detected for some reason. Or, the scattered halos of the stars surrounding the galaxies may be brighter than expected. Or there may be many more distant and distant galaxies than the theory suggests. At this point, we can only speculate. NASA’s next James Webb telescope can help solve the mystery. If individual galaxies are the cause, James Webb’s observations should be able to reveal it.
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