When astronomers first trained the Hubble Space Telescope on the deepest, darkest patch of the night sky, they knew they’d find a lot of faint, never-before-seen galaxies.
And they did. Ten years of monthslong exposures, taken with different instruments and in different wavelengths of light, revealed more and more galaxies.
The final tally of the Hubble ultradeep field photo, above, contains some 10,000 visible galaxies. By extrapolating that tiny patch to the rest of the cosmos, scientists estimated some 100 billion galaxies exist in the observable universe.
But this count may be off, and big time – possibly by a factor of 10 or 20.
According to a new study in The Astrophysical Journal, we can see only about 10% of all the galaxies that should be within sight, which means some 1.8 trillion to 1.9 trillion are missing.
“This question is not only of passing interest as a curiosity, but is also connected to many other questions in cosmology and astronomy,” a team of four astronomers, led by Christopher Conselice at the University of Nottingham, wrote in the study.
They say their discovery may also put a centuries-old paradox to rest.
Counting ancient galaxies
The new study checked the 100 billion galaxies number by estimating the density of galaxies from how many seemed to be in a fixed volume of space.
The team of astronomers began by reprocessing photos of the deepest, darkest patches of space. It included the Hubble ultradeep field image, which reveals galaxies that existed when the universe was as young as 400 million to 700 million years old. (The universe, from our vantage point, is 13.8 billion years old.)
Then they started tallying up the galaxies, from bright ones nearby to dim ones toward the edge of the observable universe. And because the speed of light is finite – it can take billions of years to reach Earth if it’s far enough away – as they looked farther out, they also looked back in time toward the cosmos’ youngest eras.
They counted galaxies in multiple wavelengths, charted them in 3-D, and figured out how many there were at various distances and times:
They discovered that the density of galaxies increased the further back in time that they looked. This made sense, since galaxies regularly merge and grow over time and they were looking at earlier eras. (Our Milky Way galaxy, for example, is on a collision course with the nearby Andromeda galaxy.)
But the density of galaxies increased only to a certain point, then fell off.
“These observations do not reach the faintest galaxies,” the authors concluded, adding, “We know that there should be many more faint galaxies beyond our current observational limits.”
By extrapolating the rates they saw and assuming that something was blocking their view, they say previous estimates of the number of galaxies in the observable universe may be off by a factor of 10, 20, or more.
Put another way, there could be 2 trillion galaxies in the observable universe instead of 100 billion.
This doesn’t mean we’ve suddenly found out the universe is 10 to 20 times as heavy, since these galaxies existed billions of years ago and have likely shaped up to look the way our local (and less dense) cosmic neighborhood looks today.
Also, gravity’s influence can be “seen” through its effects on the visible structure of the universe – and more than 80% of it is dark matter anyway, not visible matter such as stars and galaxies.
Solution to a centuries-old mystery?
Each time we gaze up at the night sky, we take for granted that it’s a pitch-black void studded with twinkling stars.
This is called Olbers’ paradox, and though the idea was posited in the 1500s, it still remains somewhat controversial.
It asks the question: If the universe is essentially infinite and ageless, why don’t we see a uniform field of stars – and a blindingly bright glow of visible light, like the sun’s, instead of vast darkness?
We’ve since discovered cosmic background radiation, which does illuminate the sky uniformly and in a few different wavelengths of light. Trouble is, we can’t see that energy with our eyes – so we built sensitive instruments, such as the COBE satellite, to detect it for us.
But the question still stands: Why isn’t there a similar background glow for visible light?
Modern explanations suggest the universe is finite, has an age, and is expanding faster and faster, which, according to a discovery by Edwin Hubble in 1929, shifts the light of the most distant stars to colors that human eyes can’t see. Paradox solved – mostly.
Conselice and his colleagues have put forth one more explanation, though, after discovering that so many galaxies are unaccounted for.
They suggest that absorption of light by gas and dust that’s drifting through space – a long-discarded piece of Olbers’ paradox that was originally thought to make the bright-sky problem worse – plays a darkening role.
That is, the old rationale was that an infinite field of stars would infinitely heat up the gases and dust until they, too, were as bright as a star.
But we now know the observable universe is finite – it’s about 90.68 billion light-years wide – and the authors suggest that distant and red-shifted (though otherwise visible) galaxies could have their light absorbed by gas and dust in the void of space. That energy would be reemitted by cosmic debris – but in infrared and ultraviolet wavelengths, which are invisible to human eyes.
“It would thus appear that the solution to the strict interpretation of Olbers’ paradox, as an optical light detection problem, is a combination of nearly all possible solutions – redshifting effects, the finite age and size of the universe, and through absorption,” the researchers wrote.
In the next 10 years or so, as bigger and more sensitive telescopes on the ground and in space go online, the team hopes to take advantage of the deepest images of space ever made – and in wavelengths the human eye can’t see – to test if their hunch pans out.
“It boggles the mind that over 90 percent of the galaxies in the universe have yet to be studied,” Conselice said in a NASA press release.