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Comment by aszantu | Hacker News Reader

I've been wondering for a while if light could be the thing that pushes away the boundaries of the universe and that's the reason for the expansion, if u can push a solar sail... anyways, I don't understand enough about this.

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There are no known boundaries of the universe. The universe expands everywhere continuously. I think of it as though a little bit of extra volume is injected in between everything on a regular basis. Also when we look out as far back in time as we can, everything looks very similar, and there's no "boundary". Each year we can see a little bit farther out into the universe because those photons from billions of years ago finally got to us and so far things keep looking the same the farther out we can see. So as far as we know the universe is far larger than what we can see (what has had enough time to send us photons). I also don't understand enough about this.

https://en.wikipedia.org/wiki/Expansion_of_the_universe https://en.wikipedia.org/wiki/Observable_universe

But at the same time we're also racing away. There is light from objects that will never get to us because we're moving away from it, due to expansion, faster than it'd ever get to us.

Eventually we'll have moved so far away from everything that we'll start seeing less and less (granted, "we" likely won't be around, our planet having long since been incinerated when Sol dies).

I think its likely we will be around in some form, though perhaps very different to how we see ourselves today.

We can calculate the force exerted on an object by a photon of light of a given frequency. I'm guessing it is not sufficient to produce the effect you envisage, although I have not done the calculation. Interestingly this photon pressure does put an upper limit on the mass of a star; the Eddington Limit [1]. If a star is too large (and therefore luminous) its photon pressure pushes away other material and stops is from growing larger!

[1] https://en.wikipedia.org/wiki/Eddington_luminosity

Would recommend reading Chapter 3 "From Eternity to Here" by Sean Carroll (or maybe read the entire book) for a "light weight" introduction. If you don't have the time then TL;DR : universe is expanding means literally that, i.e it is almost as if new space is getting created rather than "cosmological scale objects" hurtling away from each other because they "gained momentum (if one can even call it that)" because of that big bang.

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