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 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.
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.
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.)
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.
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.
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.
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.
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.
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.
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.
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.