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Starts With A Bang

No, This Is Not A Hole In The Universe

The supposed ‘hole in the Universe’ that is touted to be a billion light-years across and contain no matter and emit no radiation. Reality is far more interesting than the lies included in this image’s text. (ESO, WITH TEXT BY IFLS)

There aren’t any holes in the Universe at all. What actually exists is far more interesting.

Somewhere, far away, if you believe what you read, there’s a hole in the Universe. There’s a region of space so large and empty, a billion light-years across, that there’s nothing in it at all. There’s no matter of any type, normal or dark, and no stars, galaxies, plasma, gas, dust, black holes, or anything else. There’s no radiation in there at all, either. It’s an example of truly empty space, and its existence has been visually captured by our greatest telescopes.

At least, that’s what some people are saying, in a photographic meme that’s been spreading around the internet for years and refuses to die. Scientifically, though, there’s nothing true about these assertions at all. There is no hole in the Universe; the closest we have are the underdense regions known as cosmic voids, which still contain matter. Moreover, this image isn’t a void or hole at all, but a cloud of gas. Let’s do the detective work to show you what’s really going on.

The dark nebula Barnard 68, now known to be a molecular cloud called a Bok globule, has a temperature of less than 20 K. It’s still quite warm when compared with the temperatures of the cosmic microwave background, however, and is definitely not a hole in the Universe. (ESO)

The first thing you should notice, when you take a look at this image, is that the points of light you see here are numerous, of varying brightnesses, and come in a variety of colors. The brighter ones have diffraction spikes, indicating that they’re point-like (rather than extended) sources. And the black cloud that appears is clearly in the foreground of all of them, blocking all of the background light in the center but only a portion of the light at the outskirts, allowing some of the light to stream through.

These light sources cannot be objects billions of light-years away; they are stars within our own Milky Way galaxy, which itself is only around 100,000 light-years across. Therefore, this light-blocking object has to be closer than those stars are, and has to be relatively small if it’s so nearby. It cannot be a great void in the Universe.

The dusty regions that visible-light telescopes cannot penetrate are revealed by the infrared views of telescopes like the VLT with SPHERE, or, as shown here, with ESO’s HAWK-I instrument. The infrared is spectacular at showcasing the sites of new and future star formation, where the visible light-blocking dust is densest. What appears to be a hole or void in visible light can be seen to be for what it actually is: foreground matter that is simply opaque to certain wavelengths.(ESO/H. DRASS ET AL.)

In fact, this is a cloud of gas and dust that’s a mere 500 light-years away: a dark nebula known as Barnard 68. Over 100 years ago, the astronomer E. E. Barnard surveyed the night sky, looking for regions of space where there was a dearth of light silhouetted against the steady background of the Milky Way’s stars. These “dark nebulae,” as they were originally called, are now known to be molecular clouds of neutral gas, and are sometimes also known as Bok globules.

The one we’re considering here, Barnard 68, is relatively small and nearby:

  • it’s located only 500 light-years away,
  • it’s extremely low in mass, at just twice the mass of our Sun,
  • and it’s quite small in extent, with a diameter of approximately half a light-year.
Visible (left) and infrared (right) views of the dust-rich Bok globule, Barnard 68. The infrared light is not blocked nearly as much, as the smaller-sized dust grains are too little to interact with the long-wavelength light. At longer wavelengths, more of the Universe beyond the light-blocking dust can be revealed. (ESO)

Above, you can see an image of Barnard 68, the same nebula, in the infrared portion of the spectrum. The particles that make up these dark nebulae are of a finite size, and that size is extremely good at absorbing visible light. But longer wavelengths of light, like infrared light, can pass right through them. In the infrared composite image, above, you can clearly see that this isn’t a void or a hole in the Universe at all, but just a cloud of gas that light can easily pass through. (If you’re willing to look at it properly.)

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Bok globules are abundant throughout all gas-rich and dust-rich galaxies, and can be found in many different locations in our own Milky Way, from the dark clouds in the plane of the galaxy to the light-blocking clumps of matter found amidst star-forming and future-star-forming regions.

The Eagle Nebula, famed for its ongoing star formation, contains a large number of Bok globules, or dark nebulae, which have not yet evaporated and are working to collapse and form new stars before they disappear entirely. While the external environment of these globules may be extremely hot, the interiors can be shielded from radiation and reach very low temperatures indeed. (ESA / HUBBLE & NASA)

So if that’s what this image is actually showing, what about the idea behind the caption: that somewhere out there is an enormous void in the Universe, more than a billion light-years across, that contains no matter of any type and that emits no radiation of any type at all?

Well, there are indeed voids out there in the Universe, but they’re probably not the same as what you might think. If you were to take the Universe as it was when it began — as a nearly perfectly uniform sea of normal matter, dark matter and radiation — you’d be compelled to ask how it evolved into the Universe we see today. The answer, of course, involves gravitational attraction, the expansion of the Universe, radiation and gravitational collapse, star formation, feedback, and time.

While the web of dark matter (purple) might seem to determine cosmic structure formation on its own, the feedback from normal matter (red) can severely impact galactic scales. Both dark matter and normal matter, in the right ratios, are required to explain the Universe as we observe it. Neutrinos are ubiquitous, but standard, light neutrinos cannot account for most (or even a significant fraction) of the dark matter. (ILLUSTRIS COLLABORATION / ILLUSTRIS SIMULATION)

These ingredients, when subject to the laws of physics over the past 13.8 billion years of our cosmic history, lead to the formation of a vast and intricate cosmic web. Gravitational attraction is a runaway process, where overdense regions not only grow, but grow more rapidly as they accumulate more and more matter. The lower-density regions around them, even from quite a distance away, don’t stand a chance.

Just as the overdense regions grow, the surrounding regions that are underdense, of average density, or even of above-average density (but less “above-average” than the most overdense nearby region) will lose their matter to the denser ones. What we wind up with is a network of galaxies, galaxy groups, galaxy clusters, and large-scale filaments of structure, with enormous cosmic voids between them.

The evolution of large-scale structure in the Universe, from an early, uniform state to the clustered Universe we know today. The type and abundance of dark matter would deliver a vastly different Universe if we altered what our Universe possesses. Note that in all cases, small-scale structure arises before structure on the largest scales comes about, and that even the most underdense regions of all still contain non-zero amounts of matter. (ANGULO ET AL. 2008, VIA DURHAM UNIVERSITY)

Does this mean, though, that these cosmic voids are completely empty of normal matter, dark matter, and emit no detectable radiation of any kind?

Not at all. Voids are large-scale underdense regions, but they aren’t completely devoid of matter at all. While large galaxies within them may be rare, they do exist. Even in the deepest, sparsest cosmic void we’ve ever found, there is still a large galaxy sitting at the center. Even with no other detectable galaxies around it, this galaxy — known as MCG+01–02–015 — displays enormous evidence of having merged with smaller galaxies over its cosmic history. Even though we cannot detect these smaller, surrounding galaxies directly, we have every reason to believe they are present.

The galaxy shown at the center of the image here, MCG+01–02–015, is a barred spiral galaxy located inside a great cosmic void. It is so isolated that if humanity were located in this galaxy instead of our own and developed astronomy at the same rate, we wouldn’t have detected the first galaxy beyond our own until the 1960s. (ESA/HUBBLE & NASA AND N. GORIN (STSCI); ACKNOWLEDGEMENT: JUDY SCHMIDT)

We see, in many of these cosmic voids, evidence for molecular clouds of gas that are less dense than the Bok globules we talked about earlier, but still that are dense enough to absorb distant starlight or quasar light. These absorption features tell us, quite definitively, that these voids do contain matter: typically in about 50% the abundance of the average cosmic density.

These are low-density regions, not regions completely devoid of all types of matter.

The light from ultra-distant quasars provide cosmic laboratories for measuring not only the gas clouds they encounter along the way, but for the intergalactic medium that contains warm-and-hot plasmas outside of clusters, galaxies, and filaments. Because the exact properties of the emission or absorption lines are dependent on the fine structure constant, this is one of the top methods for probing the Universe for time or spatial variations in the fine structure constant, as well as the properties of the intervening regions of space. (ED JANSSEN, ESO)

We see evidence for the presence of dark matter as well, as the background starlight shows the effects of both gravitational changes (via the integrated Sachs-Wolf effect) and of weak gravitational lensing. Even the cold spots that appear in the cosmic microwave background can be cross-correlated with these underdense regions.

The magnitude of how cold these cold spots get teach us something very important: these voids cannot have zero matter in them at all. They might have just a fraction of the density of a typical region, but as far as underdensities go, a density that’s ~0% the average density is inconsistent with the data.

The cold fluctuations (shown in blue) in the CMB are not inherently colder, but rather represent regions where there is a greater gravitational pull due to a greater density of matter, while the hot spots (in red) are only hotter because the radiation in that region lives in a shallower gravitational well. Over time, the overdense regions will be much more likely to grow into stars, galaxies and clusters, while the underdense regions will be less likely to do so. The gravitational density of the regions the light passes through as it travels can show up in the CMB as well, teaching us what these regions are truly like. (E.M. HUFF, THE SDSS-III TEAM AND THE SOUTH POLE TELESCOPE TEAM; GRAPHIC BY ZOSIA ROSTOMIAN)

You might, then, begin worrying why we cannot detect any radiation or light of any type from them. It should be true that these regions would emit light. The stars that formed within them must emit visible light; the hydrogen molecules that transition from a spin-aligned state to an anti-aligned state should emit 21-cm radiation; the contracting clouds of gas should emit infrared radiation.

Why don’t we detect it? Simple: our telescopes, at these great cosmic distances, aren’t sensitive enough to pick up photons of such low densities. This is why we have worked so hard, as astronomers, to develop other methods of directly and indirectly measuring what’s present in space. Catching emitted radiation is an extremely limiting proposition, and isn’t always the best way to make a detection.

In between the great clusters and filaments of the Universe are great cosmic voids, some of which can span hundreds of millions of light-years in diameter. While some voids are larger in extent than others, spanning a billion light-years or more, they all contain matter at some level. Even the void that houses MCG+01–02–015 likely contains small, low surface brightness galaxies that are below the detection limit.(ANDREW Z. COLVIN (CROPPED BY ZERYPHEX) / WIKIMEDIA COMMONS)

It is absolutely true that billions of light-years away, there are enormous cosmic voids in space. Typically, they can extend for hundreds of millions of light-years in diameter, and a few of them might extend for a billion light-years in size or even many billions of light-years. And one more thing is true: the most extreme ones don’t emit any detectable radiation.

But that is not because there is no matter in them; there is. It’s not because there aren’t stars, gas molecules, or dark matter; all are present. You just can’t measure their presence from emitted radiation; you need other methods and techniques, which show us that these voids still contain substantial quantities of matter. And you definitely shouldn’t confuse them with dark gas clouds and Bok globules, which are small, nearby clouds of light-blocking matter. The Universe is plenty fascinating exactly as it is; let’s resist the temptation to embellish reality with our own exaggerations.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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