If I point in the sky and draw a line from the tip of my finger to the end of the universe, how many galaxies would this line cross? I had a discussion with my dad and he said due to the universe being so empty the line wouldn't cross any solid matter. I said that the universe is so enormous in size that there would be many galaxies and stars crossing the line. Is this calculable? If yes which one is correct?
4 Answers
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Take a look at the "Deep field" image from Hubble: http://www.spacetelescope.org/science/deep_fields/
You can see in this tiny part of the sky, hundreds of galaxies, but between there is lots of dark sky. Most lines of sight don't end on a galaxy. I'd say dad was right. I would estimate, from the deep field, that you have about a 1/10 chance of pointing at a galaxy.
Even if your finger does point at a galaxy, the distances between stars are vast. Think of two footballs separated by the distance between Europe and China as a model for the distances between stars. A finger pointing at a galaxy would be unlikely to point exactly at one of the stars.
This is probably a Good Thing. Olber's paradox supposed that if each line of sight ended on a star, the night sky would be a uniform blaze of light.
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How thick is the line? An extended finger is about 30 arcminutes across, the size of the moon, but a line is a line, 0 arcminutes. The Hubble Deepfield, as mentioned by Kilfiger, is only 2.5 arcminutes across, 1/12th of a finger-width, yet contains hundreds of galaxies.
If you want to be a stickler for true lines, the image does contain dark space too. At a guess, there's not easily accessible software for counting such things, it looks like 70-80% blackness (ignoring obvious stars), so 1 chance in 5 your line is pointing at a galaxy.
answered Sep 6, 2015 at 16:37
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On average, you will point at one galaxy.
The argument goes as follows:
Looking at the spectrum of quasars, which are effectively point sources, and which lie at distances of the order of the size of the observable Universe, it turns out that typically, roughly one damped Lyman $\alpha$ system (DLA) is detected. DLAs are huge reservoirs of neutral gas, which are early stages of galaxy formation.
At least in a handful of cases, the galaxy counterpart of the DLA has been confirmed. These observation are very challenging, since they are a faint sources at very small projected distances from the very bright quasar. Here's the setup: 
So, accepting a DLA as a galaxy, I'd say that you will typically point at one galaxy.
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Actually we can't have an specific answer to that. Because we don't have enough knowledge about the size of the universe and the density of galaxies in other parts of the universe.
You both might be true.
What happens if we are in a low density of matters but far from us we have a high density of galaxies and stars but we can't see them because of being too much far or being hidden from us behind something like Dark Matter or anything else.
Human are just guessing and are not too sure about anything.
