The Cosmological Horizon
People, of the superstitious sort, used to think that the world was flattish and that, at the horizon, the world ended, and there was a great waterfall and if you sailed over it, you'd die.
Well, eventually. For a little while, at least, you'd have a really good view of some really big oliphants before crashing into the bitter cold carapace of a gigantic turtle.
Obviously, we know different, because some fellow called Magellan decided to sail towards the horizon and prove them wrong, which he did.
But that's just one little planet. The universe itself is much bigger and has more surprises in store.
For example, we really don't know how big it actually is. We only know how far in time we can see using our best technology. But you've got a starship that can take you to any point --- and that means you can truly get away from it all!
And to boot, you can prove Magellan, ultimately & fundamentally, wrong.
Eventually, your starship is going to approach not just the edge of the Geocentric Observable Universe, but you're going to approach the edge of the Universe proper. There will be, beyond this line, no stars, no planets, no comets, no cupids, no galaxies, no black holes, white holes or blue holes. Only radiation & the information it carries with it. This is the Cosmological Girdle, a sort of hazy & dim region of sparse particles of energy.
But beyond even this region, your starship will approach the very edge of Absolutely Everything Whatsoever, which is the Cosmological Horizon. The horizon, on Earth, we know, is the demarcation line where the light of the Sun first peeks up over the arcking edge of the Earth.
The Cosmological Horizon is the ever advancing demarcation line between that place where you can discern the allermost, uttermost and lastestmost squimon of information originating from within the Universe itself on the one hand, and that other place where you can no longer discern anything at all from within the Universe. Could be a light particle or some bit of matter or radiation, doesn't matter. Once your starship reaches that ultima thule of cosmology, you can pull up alongside the speeding squimon and stand on the theoretical border between The Universe, towards the aft of your ship, and Not The Universe, towards the bow.
Detectors arrayed along the aft quarters of your ship might detect 1 or 2 particles of energy in a petrasqintijillion cubic parsecs of space, the dim glow of the Universe's bow wave. Detectors arrayed along the bow of the ship can detect nothing approaching from ahead, and will also be unable to detect anything approaching from abaft. You've just passed beyond the last anything of the universe.
So, just bop along for a few more lightyears (ha-ha! -- how can there even bé a lightyear where there is no light?) and come to a full stop.
At this point, distance ahead is meaningless, because there's nothing around you to compare with. You are now in THE VOID. No rinkydink intrauniversal void this! This is the place everything that the Universe actually is expands into and illumines with its radiance.
Looking out your windows, you will be able to see only these two things:
- NOTHING --- Nothing to see here, folks! You are beyond the physical & astronomical extent of the Universe, so there are no stars or galaxies here. You are now beyond even the radiation emanating from the objects that are along the outermost edge of the Universe. In every direction you look with your eyes, point a telescope or aim any kind of radiation sensor, you will receive no inbound data.
- EVERYTHING --- Curiously enough, having left the final frontier outpost of the Universe, you will interestingly and conversely enough, be able to see everything! Every particle of matter and every particle of radiation that exists is now located within your starship, or radiating from it, whether it's your blinking hazard lights or the residual emanations from you hyperdrive exhaust ports. If you sent out a probe from your vessel, it would be surrounded & bathed in the very dim radiation of your own private little universe.
Of Note: You specified the "known universe", so here's something to chew on. Wherever you are, you are at the centre of the Observable Universe. If you move ten billion lightyears away from Earth, the centre of the O.U. moves ten billion lightyears away from where it was before, and naturally the Known Universe grows along with it.
There is also the Unobservable Universe out there, beyond what we can detect. This is where your starship is now. But, paradoxically enough, now that you have actually reached the edge of the U.U., and have gone beyond it by some distance, you have actually not only increased the size of the O.U. by orders of magnitude, but you have also taken the Known Universe with you, out beyond itself. Kudos and welcome to the Twilight Zone!
I've read that the U.U. may be 23 trillion lightyears in diameter. I would take that to mean everything up to the Cosmological Horizon. I've also read that the Universe is about 13.8 billion years old. Since we really don't know where we are within that 23 trillion lightyear diameter volume, and thus don't know which direction the original centre is (where the Singularity was), it will be a matter of pure guessing as to which direction to fly towards. If we happen to be only 14.2 billion lightyears from the Edge of the Universe Proper, and the shortest direction is "down" from the solar system, then you'll only be travelling something like 28.5 billion lightyears to get beyond the Cosmological Horizon.
If you choose wrong, well, you're going to be travelling a long, looooong way! 13.8 billion lightyears beyond the 23 trillion light years to travel through Absolutely Everything.