Astronomy

With all the stars in the universe, why isn't the sky saturated with light?

With all the stars in the universe, why isn't the sky saturated with light?

Space is dark. But if we direct telescopes to seemingly dark portions of sky, we see that it is filled with galaxies. If the universe is theoretically infinite, wouldn't an infinite number of photons be reaching the earth and illuminating the night sky brighter than it appears?


Logic1: If the universe is infinite AND static, the sky will be saturated with light.

Note "AND."

Logic2: If the sky is not saturated with light, the universe is not infinite or not static.

Note "OR."

Logic1 = Logic2

Observation: the sky is not saturated with light. Therefore: the universe is not infinite or not static.

We know that the universe is dynamic, at least.


Stars Are Suns, Suns Are Bright—So, Why Is the Night Sky Dark?

Olbers' Paradox tells us that the night sky shouldn't be dark&mdashit should be as bright as the surface of the sun in all directions! Why isn't it?

  • In a steady unchanging universe, the night sky should not be dark! This seeming contradiction is known as Olbers' paradox.
  • The solution to the paradox lies in the reality that the universe expands and is not infinitely old.
  • The seemingly steady, calm night sky offers evidence that the universe around us is constantly changing.

Why is the night sky dark? This seemingly simple question has some significant implications.

We take for granted that the night sky is dark. At the day&rsquos end, the closest and brightest star in our sky, the sun, sinks below the horizon. It leaves behind the vast blackness of space, which is dotted with the light from distant stars, sometimes the Moon, and maybe, if you&rsquore lucky or in the southern hemisphere, a look at the rest of our Milky Way galaxy and our extragalactic neighbors, the Small and Large Magellanic Clouds. So why should we expect anything different?


My God, It’s Full of Stars! How do we know how far they are?

Launched in 2013, the Gaia satellite has been staring at the sky and multitasking like no telescope has ever done. One of its jobs is to measure the motions of stars, which astronomers can then use to geometrically determine how far away they are (the trick is getting those motions mapped with enough precision to enable the necessary mathematics, which Gaia’s keen eye can do). This data release contains the precise movements of 7 million stars in 3-D, and the 2-D motions on the sky for nearly 1.4 billion others. The team plans to release its final set of data in 2020, which will also include information about the distinct chemical signatures of those stars.


The Women Who Mapped the Universe And Still Couldn’t Get Any Respect

In 1881, Edward Charles Pickering, director of the Harvard Observatory, had a problem: the volume of data coming into his observatory was exceeding his staff’s ability to analyze it. He also had doubts about his staff’s competence–especially that of his assistant, who Pickering dubbed inefficient at cataloging. So he did what any scientist of the latter 19th century would have done: he fired his male assistant and replaced him with his maid, Williamina Fleming. Fleming proved so adept at computing and copying that she would work at Harvard for 34 years–eventually managing a large staff of assistants.

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So began an era in Harvard Observatory history where women—more than 80 during Pickering’s tenure, from 1877 to his death in 1919— worked for the director, computing and cataloging data. Some of these women would produce significant work on their own some would even earn a certain level of fame among followers of female scientists. But the majority are remembered not individually but collectively, by the moniker Pickering’s Harem.

The less-than-enlightened nickname reflects the status of women at a time when they were–with rare exception–expected to devote their energies to breeding and homemaking or to bettering their odds of attracting a husband. Education for its own sake was uncommon and work outside the home almost unheard of. Contemporary science actually warned against women and education, in the belief that women were too frail to handle the stress. As doctor and Harvard professor Edward Clarke wrote in his 1873 book Sex in Education, “A woman’s body could only handle a limited number of developmental tasks at one time—that girls who spent to much energy developing their minds during puberty would end up with undeveloped or diseased reproductive systems.”

Traditional expectations of women slowly changed six of the “Seven Sisters” colleges began admitting students between 1865 and 1889 (Mount Holyoke opened its doors in 1837). Upper-class families encouraged their daughters to participate in the sciences, but even though women’s colleges invested more in scientific instruction, they still lagged far behind men’s colleges in access to equipment and funding for research. In a feeble attempt to remedy this inequality, progressive male educators sometimes partnered with women’s institutions.

Edward Pickering was one such progressive thinker–at least when it came to opening up educational opportunities. A native New Englander, he graduated from Harvard in 1865 and taught physics at the Massachusetts Institute of Technology, where he revolutionized the method of scientific pedagogy by encouraging students to participate in experiments. He also invited Sarah Frances Whiting, an aspiring young female scientist, to attend his lectures and to observe his experiments. Whiting used these experiences as the basis for her own teaching at Wellesley College, just 13 miles from Pickering’s classroom at MIT.

Pickering’s approach toward astronomic techniques was also progressive instead of relying solely on notes from observations made by telescope, he emphasized examining photographs–a type of observation known today as astrophotography, which uses a camera attached to a telescope to take photos. The human eye, he reasoned, tires with prolonged observation through a telescope, and a photograph can provide a clearer view of the night sky. Moreover, photographs last much longer than bare-eye observations and notes.

Early astrophotography used the technology of the daguerreotype to transfer images from a telescope to a photographic plate. The process was involved and required long exposure time for celestial objects to appear, which frustrated astronomers. Looking for a more efficient method, Richard Maddox revolutionized photography by creating a dry plate method, which unlike the wet plates of earlier techniques, did not have to be used immediately–saving astronomers time by allowing them to use dry plates that had been prepared before the night of observing. Dry plates also allowed for longer exposure times than wet plates (which ran the risk of drying out), providing for greater light accumulation in the photographs. Though the dry plates made the prep work more efficient, their sensitivity to light still lagged behind what astronomers desired. Then, in 1878, Charles Bennett discovered a way to increase the sensitivity to light, by developing them at 32 degrees Celsius. Bennet’s discovery revolutionized astrophotography, making the photographs taken by the telescopes nearly as clear and useful as observations seen with the naked eye.

When Pickering became director of the Harvard Observatory in 1877, he lobbied for the expansion of the observatory’s astrophotography technology, but it wasn’t until the 1880s, when the technology greatly improved, that these changes were truly implemented. The prevalence of photography at the observatory rose markedly, creating a new problem: there was more data than anyone had time to interpret. The work was tedious, duties thought to lend themselves to a cheaper and less-educated workforce thought to be capable of classifying stars rather than observing them: women. By employing his female staff to engage in this work, Pickering certainly made waves in the historically patriarchal realm of academia.

But it’s hard to tout Pickering as a wholly progressive man: by limiting the assistants’ work to largely clerical duties, he reinforced the era’s common assumption that women were cut out for little more than secretarial tasks. These women, referred to as “computers,” were the only way that Pickering could achieve his goal of photographing and cataloging the entire night sky.

All told, more than 80 women worked for Pickering during his tenure at the Harvard Observatory (which extended to 1918), putting in six-day weeks poring over photographs, and earning 25 to 50 cents an hour (half what a man would have been paid). The daily work was largely clerical: some women would reduce the photographs, taking into account things like atmospheric refraction, in order to render the image as clear and unadulterated as possible. Others would classify the stars through comparing the photographs to known catalogs. Others cataloged the photographs themselves, making careful notes of each image’s date of exposure and the region of the sky. The notes were then meticulously copied into tables, which included the star’s location in the sky and its magnitude. It was a grind. As Fleming noted in her diary:

In the Astrophotographic building of the Observatory, 12 women, including myself, are engaged in the care of the photographs…. From day to day my duties at the Observatory are so nearly alike that there will be little to describe outside ordinary routine work of measurement, examination of photographs, and of work involved in the reduction of these observations.

Pickering’s assistants examine photographs for astronomical data. Photo from the Harvard College Observatory.

But regardless of the unequal pay and distribution of duties, this work was incredibly important the data provided the empirical foundations for larger astronomical theory. Pickering allowed some women to make telescopic observations, but this was the exception rather than the rule. Mostly, women were barred from producing real theoretical work and were instead relegated to analyzing and reducing the photographs. These reductions, however, served as the statistical basis for the theoretical work done by others. Chances for great advancement were extremely limited. Often the most a woman could hope for within the Harvard Observatory would be a chance to oversee less-experienced computers. That’s what Williamina Fleming was doing when, after almost 20 years at the observatory, she was appointed Curator of Astronomical Photos.

One of Pickering’s computers, however, would stand out for her contribution to astronomy: Annie Jump Cannon, who devised a system for classifying stars that is still used today. But as an article written in The Woman Citizen‘s June 1924 issue reported: “The traffic policeman on Harvard Square does not recognize her name. The brass and parades are missing. She steps into no polished limousine at the end of the day’s session to be driven by a liveried chauffeur to a marble mansion.”

Annie Jump Cannon at her desk at the Harvard Observatory. Photo from the Smithsonian Institution Archives.

Cannon was born in Dover, Delaware, on December 11, 1863. Her father, a shipbuilder, had some knowledge of the stars, but it was her mother who passed on her own childhood interest in astronomy. Both parents nourished her love of learning, and in 1880, when she enrolled at Wellesley College, she became one of the first young women from Delaware to go away to college. At Wellesley, she took classes under Whiting, and while doing graduate work there she helped Whiting conduct experiments on x-rays. But when the Harvard Observatory began to gain fame for its photographic research, Cannon transferred to Radcliffe College in order to work with Pickering, beginning in 1896. Pickering and Fleming had been working on a system for classifying stars based on their temperatures Cannon, adding to work done by fellow computer Antonia Maury, greatly simplified that system, and in 1922, the International Astronomical Union adopted it as the official classification system for stars.

In 1938, two years before Cannon retired and three years before she died, Harvard finally acknowledged her by appointing her the William C. Bond Astronomer. During Pickering’s 42-year tenure at the Harvard Observatory, which ended only a year before he died, in 1919, he received many awards, including the Bruce Medal, the Astronomical Society of the Pacific’s highest honor. Craters on the moon and on Mars are named after him.

And Annie Jump Cannon’s enduring achievement was dubbed the Harvard—not the Cannon—system of spectral classification.

Sources: “Annals of the Astronomical Observatory of Harvard College, Volume XXIV,” on Take Note, An Exploration of Note-Taking in Harvard University Collections, 2012. Accessed September 3, 2013 “Annie Cannon (1863-1914)” on She Is An Astronomer, 2013. Accessed September 9, 2013 “Annie Jump Cannon” on Notable Name Database, 2013. Accessed September 9, 2013 “Brief History of Astrophotography” on McCormick Museum, 2009. Accessed September 18, 213 “The ‘Harvard Computers’” on WAMC, 2013. Accessed September 3, 2013 “The History of Women and Education” on the National Women’s History Museum, 207. Accessed August 19, 2013 Kate M. Tucker. “Friend to the Stars” in The Woman Citizen, June 14, 1924 Keith Lafortune. “Women at the Harvard College Observatory, 1877-1919: ‘Women’s Work,’ The ‘New’ Sociality of Astronomy, and Scientific Labor,” University of Notre Dame, December 2001. Accessed August 19, 2013 Margaret Walton Mayhall. “The Candelabrum” in The Sky. January, 1941 Moira Davison Reynolds. American Women Scientists: 23 Inspiring Biographies, 1900-2000. Jefferson, NC: McFarland & Company, 1999 “Williamina Paton Stevens Fleming (1857�)” on the Harvard University Library Open Collections Program, 2013. Accessed September 3, 2013.


3) Some stars are unbelievably huge.

The largest stars are called red hypergiants. One absurdly large one is called VY Canis Majoris. If you stacked 1,420 of our suns on top of each other, you’d have the diameter of VY Canis Majoris. Here’s what it looks like next to the sun:

Or, to bring back the ping pong ball-size sun, that would make VY Canis Majoris the height of a 16-story building. It would take an airplane about 1,100 years to fly around it, and if VY Canis Majoris were in the center of our Solar System where our sun is, it would swallow up everything out to the orbit of Saturn.

Another red hypergiant almost as large as VY Canis Majoris is Betelgeuse. You can see Betelgeuse on any starry night as Orion’s upper left shoulder—


Are the first stars in the Universe invisible?

Why “letting there be light” in the Universe isn’t enough.

“Dwell on the beauty of life. Watch the stars, and see yourself running with them.” -Marcus Aurelius

I want you to imagine the night sky as you know it. Far away from the cities, on a moonless night, out in the darkest areas you’ve ever experienced. Maybe you lay back on the grass, gazing up at the heavens above. You look up, the air is cool, and the sky is clear: no clouds to be seen at all.

What is it that you’re likely to see?

Yes, there are planets, stars bright and dim, and even the Milky Way overhead. But perhaps the most striking thing about the night sky isn’t the presence of these few, dispersed lights, but rather the fact that — at almost every location you can point — the sky itself is dark.

If you think about it for a minute, it doesn’t make a whole lot of sense that this should be the case.

If the Universe were really, truly full of stars — of points of light in all directions — then you’d fully expect that wherever you looked, in any direction, eventually your line-of-sight would run smack into a star.

And once that happened, you wouldn’t see “dark” anywhere you looked. Every point, eventually, would be filled with light, no matter how distant that star, galaxy, or other point of light happened to be.

This was one of the great paradoxes of the 19th century: Olbers’ paradox, which showed that the idea of an infinite Universe filled with an infinite number of stars spread out over that space was incompatible with the dark night sky we could all see.

The resolution to this paradox, of course, is that when we look at the distant Universe, we’re actually looking back in time, and since the Universe existed in a hot, dense, more uniform early state, there was a time before which the Universe didn’t contain any stars, since it took time for gravitation to begin collapsing that primordial gas into stars for the very first time. Look out beyond a certain distance, and you won’t ever see even a single star.

After the Big Bang, the Universe was hot, dense and uniform, but also expanding and cooling. By time the Universe is around 380,000 years old, it’s cooled enough to form neutral atoms for the first time. But there are two barriers to seeing anything:

  1. There’s nothing to see until we start creating something that emits light.
  2. Even once you do that, the Universe needs to become transparent.

Although these two problems — the formation of the first stars and the Universe becoming transparent — are often conflated together as “the dark ages,” they’re two separate problems that the Universe needs to solve.

First, you simply don’t have anything to see until you form stars for the first time. While the Universe started off almost perfectly uniform, there are tiny imperfections, including some regions that start off with slightly more matter than average. Over time, gravitation works to pull more and more matter into these overdense regions, growing them into clumps of matter.

It takes tens of millions of years, but after enough time passes, these clumps grow large enough that gravity begins to collapse them under their own gravity. And when the cores of these clumps of atoms and molecules get dense enough, the process of nuclear fusion — burning hydrogen fuel into helium — can finally occur!

These sites of nuclear fusion become the cores of the very first stars in the Universe, burning hot and bright, and emitting the first visible light the Universe has seen since the early stages of the hot Big Bang.

This happens after as little as 50 million years into the Universe’s history, an incredibly short time to the first stars.

But there’s a problem: none of these stars are actually visible to us!

Sure, the stars are emitting light, but so are the stars behind the “dark nebula” above, Barnard 68. This nebula appears so dark because the light from the stars is blocked! Why’s this? Because the atoms and molecules that exist in there are of the right physical size to absorb — and hence appear opaque — to visible light.

While single atoms themselves only have specific atomic transitions they can absorb light at, when they’re bound together in all sorts of intricate configurations, they can actually block the full spectrum of visible light. And this type of opacity is exactly what’s going to happen when the first stars form: the Universe might be creating light, but there’s no way for it to travel to our eyes.

So how do we get out of this?

You have to ionize those atoms! Or, more specifically, you have to reionize them, since they were ionized once before: back before they became neutral in the first place.

But this doesn’t happen quickly: this is a process that requires billions upon billions of stars to form, emit ultraviolet, ionizing radiation, and strike more than 99% of the neutral atoms in the Universe. It’s a gradual process, one that takes around 550 million years to complete!

Up until recently, we thought that reionization — this last phase of the Universe becoming transparent to visible light — occurred 450 million years after the Big Bang, but this extra factor of 100 million years was determined by recent observations of the Planck satellite.

But this does not mean, as you might have read recently, that the Universe’s oldest stars formed 100 million years later than we’d previously thought.

It means that the first stars formed much, much earlier that we’re able to see them, and that we didn’t form enough of these stars — and they didn’t burn hot enough for long enough — to reionize the Universe and make it transparent to light until 100 million years after we had previously thought.

It’s not enough, in the Universe, to simply “let there be light” in order to see the first stars: you need for that light to be able to freely travel through space!

In visible light, there’s no way to see them: no matter how good the Hubble Space Telescope ever is, no matter how long it stares at these patches of sky, it will never see back to the first stars, because the Universe is still opaque to visible light.

But there is hope, and the James Webb Space Telescope has the potential to bring that hope to reality.

By looking in longer wavelengths of light, those dusty configurations of atoms and molecules might actually be transparent to those wavelengths. Even though Hubble might not be able to see those stars, James Webb, which will view infrared (and quite long infrared) wavelengths, will be able to see all the way out to epochs where the Universe was opaque to visible light.

In other words, in just a few years, we may truly be able to probe the first stars in the Universe, not merely hundreds of millions of years after the fact, when the Universe becomes transparent to visible light. The first stars in the Universe may be invisible for a time, but that’s a fault of our eyes, not a fault of the light!


Planetarium

We thank J.O from Washington, DC for submitting this query. We hope the answer won't be overly taxing. ha ha ha

German astronomer F.W. Olbers (1758-1840) posed a similar question, one that confounded astronomers and philosopher's alike. "If the infinite Universe is static and contains an infinite number of stars, why isn't the sky uniformly bright?" By Olbers' reasoning, every single line of sight should intersect a star somewhere. A Universe that is infinite in age, extent and stellar population should never produce a night sky.

Olbers' 'paradox' and JO's query sound similar, but are not exactly alike, JO's question differs because its very phrasing contains the answer. It also resolves Olbers' Paradox.

The resolution pertains to the Universe's age. The Universe is not infinitely old, but, instead, as JO indicated, is about 13 billion years old. The Universe hasn't been generating stars forever, only for billions of years. Consequently, the cosmos hasn't had sufficient time to fill the sky with stars. Lord Kelvin (1824-1907) proposed this age solution, which was corroborated years later by Big Bang theorists George Lemaître and Edwin Hubble. The dark night sky is direct observational evidence that we live in a Universe that was born at a specific time.

Or, in other words, the Universe doesn't have enough stars to fully illuminate our skies. While the Universe contains trillions of stars distributed through billions of galaxies and also dispersed through intergalactic space, the distances between us and most of those stars is unfathomable. The light intensity diminishes with the square of the distance. So, not every line of sight will connect with a star that we can actually see.

Of course, if you ever have a chance to observe the night sky through binoculars, you will see far more stars than are visible with the unaided eye. The sky is truly alight with stars, although some of them are hidden from our view.


The Olbers’ Paradox: Why is the Night Sky Dark?

Even if have never heard of Olbers’ paradox, you might still be familiar with its basic premise. It basically asks the question, ‘why isn’t the entire night sky as bright as the sun?’ You might be a bit taken back as to why this question is even be considered by astronomers, but it isn’t unfounded. Since the universe could extend infinitely in all directions, there might be an infinite number of stars. This means that, no matter where we looked, every point in the sky should shine with light.

Ultimately, this is a serious question that may help answer more of the universe’s mysteries however, this is by no means a modern question. As far back as 1610, individuals were considering this question. Kepler was one of the first to bring this question to light (get the pun), though it wasn’t until the 19 th century that Heinrich Wilhelm Olbers popularized it as a paradox. There have been several proposed attempts to solve this mystery, and I’ll run through some of the major ones.

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First off, if stars are uniformly distributed there will be 100 times as many in a certain section of sky at 10 times a unit of distance, and collectively they will be as bright as one star in that section. So each section contributes to a part of the sky adding this up means the whole sky should shine. Since the whole sky does not shine, it indicates that the locations of these stars and galaxies might not be equally spaced around the cosmos, and so then we see expanses of darkness because stars could hide behind one another or just concentrate into certain areas of the cosmos (we call this a non-uniform distribution).

A second reason that the sky is not full of light could be because something is blocking the light from reaching us. Interstellar dust clouds, perhaps? However, if a dust cloud were to be situated between us and another star, the effect of being bombarded with millions or billions of years of photons would heat it up so that (eventually) this energy would just be re-radiated back into space towards us. This is why nebulae glow brightly when they have no energy source of their own.

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Next, it has been proposed that the universe may be infinite in size, but it has a finite amount of mass so this would mean that stars are limited in number. The matter would also very gradually be used up by stars, leaving less and less for future generations, and if the universe is infinitely old, much of this original matter could already have been used up. This is also a possible explanation for why the night sky isn’t illuminated, but we still have a problem! The sheer number of stars in just the observable Universe (which we have estimated to be 10^22 or 10^24 via extrapolation) is so large that we’d still expect the glowing sky.

The next two are a lot more credible because they are based more upon the Big Bang theory and universal expansion (these explanations are how most scientists answer this question). Essentially, the universe is only 13.7 billion years old, so we can’t see any objects farther than 13.7 billion light-years away because the universe is not old enough for light farther away to have reached us yet. Our ‘observable universe’ is as far as we can see. As a result we live in a bubble, confining our vision.

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And finally, Edwin Hubble discovered that our universe is expanding and that we can measure how fast things are moving away or towards us by looking at how the wavelength of the radiation that we receive has shifted. This phenomenon is called redshift. We now know that the further away galaxies are from us the faster they are travelling, and so this could mean that very distant stars and galaxies may well have been redshifted too far down the spectrum to show up at night as visible light.

The final 2 of these are sure to have a considerable effect, whereas the first few are up for more debate. Be sure to watch the minutephysics video for a great breakdown of this question.


Why is the night sky black?

Space: it’s full of stars … isn’t it? Beth Scupham/flickr, CC BY-SA

It sounds obvious. That's what night is. The sun has set and when you look up at the sky, it's black. Except where there's a star, of course. The stars are bright and shiny.

But wait. Imagine you are deep in a forest. All around you there are trees. Wherever you look, you are looking at a tree. Maybe a big tree close up or a bunch of small trees further away. Surely it should be the same with stars. We're deep in the universe and whatever direction we look in, there ought to be stars there – billions and billions and billions of them. You would have thought that they'd fill the whole night sky, with the more distant ones fainter but more numerous.

This is called "Olbers' Paradox" after a 19th-century astronomer, although the conundrum was around for a couple of centuries before him. And the answer – at least, now – is fairly clear.

The reason the night sky isn't just a blaze of light is because the universe isn't infinite and static. If it were, if the stars went on forever, and if they had been there forever in time, we would see a bright night sky. The fact that we don't tells us something very fundamental about the universe we live in.

A limit to the universe may seem a natural explanation – if you were in a forest and you could see a gap in the trees, for example, you might surmise that you were near the edge. But it's dark on all sides of us, which would mean not just that the universe is bounded, but that we're in the middle of it, which is pretty implausible.

Alternatively, the universe could be limited in time, meaning that light from far-away stars hasn't had time to reach us yet.

Blame the Doppler effect

But actually the explanation is neither of these. Light from the far-away stars gets fainter because the universe is expanding.

Edwin Hubble discovered in 1929 that distant galaxies and stars are travelling away from us. He also found that the furthest galaxies are travelling away from us at the fastest rate – which does make sense: over the lifespan of the universe, faster galaxies will have travelled further.

And this affects how we see them. Light from these distant, fast-moving galaxies and stars is shifted to longer wavelengths by the Doppler effect. In the case of these stars, the effect shifts visible light into invisible (to the human eye) infra-red and radio waves, essentially making them disappear. Indeed, the blackness of the night sky is direct evidence of an expanding universe.

So if you want evidence of the Big Bang, you don't need the Hubble Telescope or the Large Hadron Collider. You just need your own eyes and a clear, dark night.

This story is published courtesy of The Conversation (under Creative Commons-Attribution/No derivatives).


Rentevrees bij beleggers blijkt schijnbeweging: techaandelen zetten jacht op records in en zo hard kan dat gaan

There could be another piece of the puzzle, according to a new study in The Astrophysical Journal, and astronomers may finally be able to fully put the centuries-old question to rest.

Astronomers previously estimated that the observable universe contains about 100 billion galaxies.

The new study checked that figure by estimating the density of galaxies from close by all the way to the farthest edges of the universe that we can see. Because the speed of light is finite – and can take billions of years to reach Earth – as they looked farther out, they also looked back in time toward the cosmos’ youngest eras.

The team of four astronomers, led by Christopher Conselice at the Leiden Observatory in the Netherlands, began by reprocessing photos of the deepest, darkest patches of space.

That data included an ultra-deep photo taken by NASA’s Hubble Space Telescope, which reveals galaxies that existed when the universe was as young as 400 to 700 million years old. (The universe from our vantage point is 13.8 billion years old.)

They counted galaxies in multiple wavelengths, charted them in three dimensions, and figured out how many there were at various distances and epochs of time:

They discovered the density of galaxies increased the farther back in time that they looked. This made sense, since galaxies regularly merge and grow larger 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 went up only up to a certain point – then fell off.

“[T]hese 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 think 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 are 2 trillion galaxies in the universe instead of 100 billion.

“This question is not only of passing interest as a curiosity, but is also connected to many other questions in cosmology and astronomy,” the team wrote in their study.

What is hiding 90% of galaxies brings us back to Olbers’ Paradox, and why the night sky is dark.

The researchers say most solutions to the paradox fall into two buckets: one, they explain how stars and galaxies vanished or two, they explain why a lot of stars and galaxies are out there but can’t be seen from our earthly vantage.

The most popular idea is a bit of both. It suggests that an expanding universe has red-shifted galaxies out of view, combined with the facts that the universe has a finite age and an observable size.

But Conselice and his colleagues went a step further and added another answer to the riddle of why there isn’t a similar background glow for visible light, especially with all of these newly discovered galaxies.

They suggest that absorption of light by gas and dust that’s drifting through space – a long-discarded piece of Olbers’ paradox, which was originally thought to make the bright-sky problem worse – is playing a darkening role.

The old rationale was that an infinite field of stars would infinitely heat up the gas and dust until it, too, was as bright as a star.

But 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, then re-emitted in infrared and ultraviolet wavelengths that 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.