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Let's say I was at the very center of the enormous Boötes void, way out in deep, deep space. What could I see with the naked eye? I assume I could see no individual stars, but could I resolve any galaxies? If I gazed in the direction of a super-cluster of galaxies would it seem brighter than other directions? How dark would it be compared to, say, the far side of the moon when it is a full moon on earth?

I am told there are, in fact, a few galaxies in the void. So let's say I pick a spot in the void that is as far from any of those galaxies as possible.

costrom
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Paul Young
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  • The Bootes void has some galaxies in it, and I don't know if it has much of a centre. IMO this needs to be more specific about the conditions. – JMac Apr 17 '19 at 14:29
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    Let's pick the spot that is furthest from any galaxy. – Paul Young Apr 17 '19 at 14:32
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    @PaulYoung so you're saying "if there's a bright center to the Universe, you're at the [point] it's farthest from"? – User1000547 Apr 17 '19 at 21:47
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    @MikeTheLiar seems more like "let's get as far away from as many galaxies as possible" – user253751 Apr 18 '19 at 00:38
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    As a starter: What would we see from Earth if we removed all those celestial objects that belong to our Galaxy? – Hagen von Eitzen Apr 19 '19 at 10:05
  • https://en.wikipedia.org/wiki/KBC_Void Apparently we are in a void?? – Ryan Thorngren Apr 19 '19 at 12:10
  • Given that only 3-4 galaxies in the local group can be seen with naked eye, the answer to this question would seem totally obvious. – ProfRob Apr 20 '19 at 15:05
  • @RobJeffries - We can perceive the milky way in the sky even though we cannot perceive the individual stars. So, though we could not perceive, say, the individual galaxies of the Hercules super-cluster, could we perceive a kind of glow from it so that we were not in total darkness? That is not obvious to me – Paul Young Apr 20 '19 at 15:51
  • But the answer you seem happy with is the obvious one. – ProfRob Apr 20 '19 at 17:49
  • I added a comment ... so maybe we will get a bit more. – Paul Young Apr 20 '19 at 18:12
  • The Milky Way is already in a void that's about 1 billion light years across (see KBC Void/Local Void). And yet we can see galaxies like Andromeda and Triangulum. – Alastor Dec 31 '23 at 16:28

5 Answers5

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Individual sources

The number density of galaxies in a void is typically an order of magnitude lower than the average in the Universe (e.g. Patiri et al. 2006). In this astronomy.SE post, I estimate the number density of galaxies of magnitude $M=-17$ or brighter in the Boötes Void to be $n \sim 0.004\,\mathrm{Mpc}^{-3}$, or $10^{-4}\,\mathrm{Mlyr}^{-3}$ (i.e. "per cubic mega-light-year"). Hence, the typical distance to a galaxy from a random point in the Boötes Void is $$ d = \left( \frac{3}{4\pi n} \right)^{1/3} \simeq 13\,\mathrm{Mlyr}. $$

Although some galaxies will be brighter than $M=-17$, the number density declines fast with brightness; for instance, galaxies that are 10 times brighter are roughly 100 times rarer, meaning that they're on average 5 times more distant and hence appear 25 times fainter. On the other hand, the number density of galaxies fainter than $M=-17$ doesn't increase that fast (in astronomish: $-17$ is close to $M^*$; "M-star").

So for the sake of this calculation, let's assume that the closest galaxy is an $M=-17$ galaxy at a distance of $13\,\mathrm{Mlyr}$. That distance corresponds to a distance modulus of $\mu \simeq 28$, so the apparent magnitude of the galaxy would be $$ m = M + \mu \simeq 11. $$

Typically, humans cannot see objects darker than $m \simeq 6.5$ (the magnitude scale is backwards, so darker means "larger values than 6.5"), although some have claimed to be able to see $m\simeq8$ — still an order of magnitude brighter than the $m=11$ estimated above. Moreover, this threshold assumes point sources, whereas a galaxy has its brightness smeared out over a quite large area, lowering its surface brightness significantly!$^\dagger$. Note also that, as in the rest of the Universe, galaxies in voids are not completely randomly scattered throughout space, but tend to cluster in clumps and filaments, and that the number density is smaller in the center of the void, meaning that here the typical distance to the next galaxy is larger.

Hence, you would — at a random position in the Böoted Void — be most likely to be floating in complete darkness.$^\ddagger$

Background radiation

The combined light from all astrophysical and cosmological sources comprises a cosmic background radiation (CBR), meaning that at any time your eye does indeed receive photons across the entire electromagnetic spectrum. Thus the term "complete darkness" may be debated. On average, this background is dominated by the cosmic microwave background (if you're close to a star or a galaxy, those sources will dominate, but then it isn't really a "background" any longer).

In this answer, I estimate the total background in the visible region (from sources outside the Milky Way) to be roughly $3.6\times10^{-8}\,\mathrm{W}\,\mathrm{m}^{-2}$. If I've done my maths right, this corresponds to a the light from a 25 W light bulb, smeared out over a 15 km diameter sphere with you in the center. The Böotes Void would have an even lower background than this. I'm not a physiologist, but I think this qualifies as "complete darkness" (to the human eye; not to a telescope).

$^\dagger$For instance, the Andromeda galaxy has an apparent magnitude of $m=3.44$ which, if its light were concentrated in a point, would make it easily visibly even under light-polluted conditions.

$^\ddagger$Your eye might be able to detect individual photons, at stated in Árpád Szendrei's answer, but that hardly counts as "seeing anything".

pela
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  • I agree with your conclusion, but I would not agree with your arguments. Yes, one will not be able to see individual galaxies. But that doesn't mean that there will be total darkness. One can't see individual stars forming the Milky Way, but still can see the bright area on the sky. – lesnik Apr 19 '19 at 13:26
  • @lesnik The stars that are too far away in the MW to see individually are at kpc distance, a factor 1e3-1e4 closer than the galaxy I consider, and hence the light you see from them are a factor 1e6-1e8 brighter. That's why you can still see the light from them. – pela Apr 19 '19 at 21:15
  • @pela - given the density of the galaxies in the super-clusters around the edge of the void, is there any chance we could perceive them as a misty glow? Or did you comment effectively just answer that as a "no"? Would not it be worth comparing star density to galaxy density, the absolute magnitudes of each, and then the distances? – Paul Young Apr 20 '19 at 15:57
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    @PaulYoung Yes, anywhere in space there is a "cosmic background radiation", which is the combined field of all photons of cosmological and astrophysical origin. On average, it is dominated by the cosmic microwave background (see this answer for an overview), but if you're close to a galaxy, a star, or a flashlight, those sources will dominate. On average, optical photons (mostly from stars, and hence galaxies), is several orders of magnitudes less than the CMB, and in a void, it will be correspondingly smaller. – pela Apr 20 '19 at 19:11
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    So: Yes, there is light, but none detectable by a human eye. – pela Apr 20 '19 at 19:12
  • @pela - thanks, I think this is an important part of the answer. So, from deep within the void, even if I stare at the Hercules Supercluster, it will not stand out from the Cosmic Optical Background enough to be noticed by the naked eye. And, of course, the COB itself is too dim to be perceived. But I don't quite see the math for this ... Do we kinda need to show that once you are far enough away from a supercluster for it to become "an object" that it is not meaningfully brighter than the COB? Of course, if I already can't see superclusters from the earth, that might be proof enough? – Paul Young Apr 21 '19 at 15:51
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    @PaulYoung Good point! I edited the answer to discussing the background. I also modified the claim of "complete darkness", since 1) it is only on average (a random position could place you close enough to a galaxy) and 2) the background might give the eye a different sensation than "complete darkness", but as an astronomer I cannot answer this; you might want to ask a physiologist. – pela Apr 22 '19 at 08:46
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    @PaulYoung Wrt. superclusters and clusters of galaxies, those are absolutely not visible to the naked eye from Earth. They are too distant, and too smeared-out. Only four galaxies (Andromeda, LMC, SMC, Triangulum) are generally visibly to the naked eye from Earth, though there has been claims from a few people that three more (Centaurus A, M81, and M83) are visible. – pela Apr 22 '19 at 08:50
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    @PaulYoung I estimated the background quantitatively and updated the answer. – pela Apr 23 '19 at 09:17
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    @pela - that quantitative estimate really closes the coffin on the case. A 25W bulb's light smeared out over a 15km radius sphere ... – Paul Young Apr 23 '19 at 14:42
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From the Wikipedia page on the Boötes void (the same that you linked to in your question):

According to astronomer Greg Aldering, the scale of the void is such that "If the Milky Way had been in the center of the Boötes void, we wouldn't have known there were other galaxies until the 1960s."

(The original article can be found thanks to the Wayback Machine.)

This means that if we were in the center of the Boötes void, we wouldn't be able to see any galaxies with the naked eye, and not even with some telescopes. Again according to Wikipedia, telescopes were invented at the beginning of the 17th century, radio telescopes in the 1930s and infrared telescopes in the 1960s.

Fabio says Reinstate Monica
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In fact you would see only individual stars. Like on Earth, you can only see individual stars, and no galaxies with the naked eye. Why? Because galaxies are too dim as a whole.

Now, if you would just float in empty space in the void you would be better off then on Earth. It is because here on Earth there is some light pollution.

But in the void, there is basically no light pollution, so you could see even single photons arriving from far away giant stars.

And yes, the human eye is able to detect even single photons.

By the way, all the stars that you can see with the naked eye from Earth are from the Milky Way. But again, that is too because of the light pollution and because our night sky even if there would be no light pollution here on Earth would be filled with brighter start from the Milky way. Our eyes would get used to that, and the start from other galaxies would just appear too dim to see.

Now on the dark side of the moon it would be a little different, but only that there is no light pollution like on Earth. Still, from the dark side of the Moon you could still only see stars from the Milky way.

Now in the void in your case, I think it is different. It is so dark and void, that even a single photon coming from a far away large star would be visible to our eyes that get used to the darkness.

Please see here:

Do all the individual stars that we can see in the night sky belong to Milky Way?

Árpád Szendrei
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    You can see Andromeda with the naked eye, under optimal conditions. It's mag +3.44. Sagittarius Dwarf Spheroidal Galaxy and M33 should also be visible – CSM Apr 17 '19 at 20:04
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    the human eye is able to detect even single photons Sort of. A human rod cell can be activated by a single photon. But the retinal circuitry extinguishes the noise of such isolated "pixels" as part of its pre-processing. However, the frog retina doesn't contain that neural circuitry, so frog brains can detect single photon events. – PM 2Ring Apr 18 '19 at 01:06
  • It depends on the number of photons per second (obviously). You can observe light sources which result in as few as one or two photons per second hitting the same rod, so I think that counts as single photons. You're right that a single photon hitting the retina randomly will not be visible. – jcupitt Apr 18 '19 at 09:28
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    The first sentence is completely wrong. As mentioned by CSM, you can see Andromeda Galaxy (one trillion stars), the Magellanic clouds (30 + 3 billion stars) and many clusters (e.g. Great Hercules Cluster with 300000 stars and Omega Centauri with 10 million stars) perfectly well with the naked eye. It's hard to write a credible answer after such an intro. – Eric Duminil Apr 18 '19 at 13:41
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    Could you please edit your answer? The question was featured on hot network questions. – Eric Duminil Apr 19 '19 at 21:30
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It seems that even supernovas and quasars would not be bright enough to be seen from 100 million light-years away (Boötes Void's diameter is 330 million light-years).

If you see something that looks like a star from the Boötes Void, it could be a Gamma-ray burst.

This one was visible from Earth with the naked eye (mag. 5.8) even though it was 7.5 billion light-years away.

Eric Duminil
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I don't think it's likely you would see anything, considering the fact it's very dark and the human eye can normally see in the dark. I do think it's a good question though.

Roo
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