This is much darker than the Hubble Space Telescope leads us to believe.
You may have gazed at the sky while standing in a rural area far from all the city lights. Countless twinkling stars shine in the night sky. The space between those stars is filled with complete darkness. But how dark is this really? And what does this say about the number of galaxies in the observable universe?
Not really black
Just as the sky may appear dark at times, the interstellar space is not completely black even there. The universe is lit by a faint glow of countless distant stars and galaxies. It’s hard to say how strong this glow is. Satellites and telescopes within our solar system – such as the Hubble Space Telescope – cannot properly measure the light emitted. In fact, space around Earth and the inner solar system is filled with dust particles that are illuminated by the sun, creating a diffuse glow across the sky.
However, there is a mission here for the New Horizons space probe; The unmanned probe, launched in 2006, visited the dwarf planet Pluto and later the asteroid Arocoth. This spacecraft is currently located far from all prominent sources of light pollution, billions of miles from Earth. Thus, the surrounding sky is about 10 times darker than the darkest sky visible for the Hubble telescope. This is interesting. Because this also allows New Horizons to make a more accurate estimate of the overall brightness of all galaxies in the universe.
However, the Hubble Space Telescope has already ventured with this before. Astronomers can estimate the total number of galaxies by counting everything visible to Hubble and then multiplying it by the total area of the sky. But other galaxies are too dim and too distant to be directly detected. 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 galaxies in the universe are invisible to the Hubble telescope.
But it now appears that the universe is actually much darker. Using New Horizons, researchers have once again measured the visible light emitted from galaxies. This now shows that invisible galaxies are fewer in number than previously thought. This would not be 2 trillion, but only hundreds of billions.
Cosmic visual background
The researchers are relying on measurements of what’s called a “cosmic optical background,” which is the visible light equivalent to the most common cosmic background radiation (the thermal radiation emitted shortly after the Big Bang). “As the 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,” researcher Mark Postman explains. “It puts a limit on the total number of galaxies and where they can be located in time.”
So while we cannot count all galaxies, their light penetrates into space in a faint, mysterious glow. The researchers analyzed current images of New Horizons and filtered all known sources of visible light, until all that remained was light from outside our galaxy. However, the remaining signal, although very weak, is still measurable. This means that the unexplained light is still too bright. This postman compares it to living in a remote area far from city lights. If you were lying in bed at night with the blinds open, and a neighbor turned on their fridge a mile away, this light could bounce off your bedroom walls. This will then be as bright as the light New Horizons captured.
Who or what exactly is responsible for this unwarranted light? It is possible that dwarf galaxies exist in the relatively near universe and cannot be detected for some reason. Or, the scattered halos of the stars surrounding galaxies may be brighter than expected. Or, there may be many more distant, fainter galaxies than theory suggests. At this point, we can only speculate. Next NASA James Webb Telescope May help solve the mystery. If individual galaxies are the cause, James Webb’s observations should be able to reveal this.
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