Yes, it's called Alcubierre Drive, where spacetime is compressed in front and expanded behind a ship to drive it through the spacetime like a gravitational wave.

https://en.wikipedia.org/wiki/Alcubierre_drive

Or the Krasnikov tube, where time is tilted such that travel through it (in one direction) takes you back in time (from a stationary perspective).

That's very similar to a wormhole, but without the topological flaw.

Just because the universe is expanding at c doesn't mean everything inside it is getting bigger at c. Due to the way mass curves spacetime, mass stays more or less the same size (in comparison to the rest of the empty universe, anyway) - it is mostly only the EMPTY parts of space that get stretched out at c. This means that distances between things are getting bigger all the time... BUT things are still able to travel through space, so they get further away even faster.

Imagine two people (which are galaxies) walking in the same direction on a travelator (which is spacetime). Ordinarily one person would reach the end, and then in a predictable amount of time so would the other. But if the travelator is expanding evenly in both directions, then the distance between the two people will increase, even though they're walking at the same speed.

One of the interesting things is that if everyone of us did manage to travel almost at the speed of light then time stops and you can be in universe wherever you want in matter of your seconds. In other words, whole universe just becomes giant wormhole and you teleport from wherever to wherever in matter of seconds at whim. If you reach to the edge of time dimension, the space dimension basically seize the exist.

I'm not sure that's right since things are light-years away it means you also need to travel just as long (or is this just as far as outside obeservers are concerned?). But even so, the universe around you would still get older while you travel like that so in a few trips you will reach the heat death of the universe.

That is just for outside observers. With time and space dilation, things do become instantaneous at light speed. It is actually kind of fun to think about that in the frame of reference of a photon which ‘is’ constantly coming in and out of existence as it interacts with charged mass.

> what's the difference between 'universe expansion' and 'travel'?

Travel happens inside the universe and expansion doesn't? If a rock is sliding along a stretchy sheet, stretching the sheet will make other things on the sheet move farther away from the rock, potentially faster than the fastest that the rock can slide, even though the rock doesn't move any faster.

The first one means the space time metric changes

Is it possible that the universe stays the same size and everything within it shrinks?

Don't think there is a way to tell the difference without stepping outside the universe to look back on the system relative to another frame.

But AFAIU, there is no edge to the universe with an unknown something beyond the barrier. Space is not an expanding balloon inside a bigger balloon. Ie, there is no space beyond space.

Not that i'm truly able to wrap my head around it, but at the moment of the big bang, time and space themselves exploded into existence.

The expanding balloon analogy is often used with the wrong audience and badly misleading there: the surface of the balloon is a 2d analogy for the universe and it's expansion, rather than the interior of the balloon being a 3d analogy.

But that would be a wrong analogy. Cool thing about a balloon surface is that it expands distances between fixed points on it (ants if you want). But that doesn’t happen exactly the same way to a space inside of a balloon, it just grows at its border without dragging any reference frames.

The “nothing” is the one thing I find fascinating ~ maybe it’s our human brains that can’t get it but it just doesn’t work for me!

There has to be something doesn’t there?

A way of thinking about it is that space isn't "something". Space is just the gaps between "something".

It seems like a lot of people think of the universe as a sort of container/volume that things exist inside, and then what is "outside the box" is a natural question, because there is a boundary to pass.

If you instead think of it as simply a description of the extent of a set of objects, it's easier. Picture a simulated universe with no fixed boundary, just a set of objects with coordinates.

"The universe" is just the set of objects, and it's volume is just the volume encompassing all of those objects at any time. The simulation doesn't contain any "space". Space in the simulation is just an absence of objects, and there's nothing special about "outside" or "inside" in that situation. If the objects move further away, the universe gets bigger. If you "travel" to the edge of the universe, and travel further, the universe gets bigger. If you try to traverse the current "edge of the universe", the edge moves with you.

There are theories where there might be something "outside" our universe in one or more dimensions, but there's no reason why something absolutely needs to have an "outside".

Does that include photons? Photons that have travelled the furthest are, by the above definition, at the boundary of the universe - an ever expanding boundary - which they will never exceed. If this is so and you were 'outside' the universe, would it look like a black hole? Does a universe have an event horizon?

Don't you find the fact there is something to be equally weird? I mean, first there was nothing and everything was as it should be. Then: something! And time!

This is why I think the notion of free will is wrong. The initial nothingness wasn't nothing. It was mass, but without time. The way the particles were arranged in this nothingness determines a future that has already been carved in mass.

> But AFAIU, there is no edge to the universe with an unknown something beyond the barrier. Space is not an expanding balloon inside a bigger balloon. Ie, there is no space beyond space.

Eternal inflation, if correct, states that there is indeed something which our universe is expanding into. (But even if that's the case, that still has nothing to do with the expansion we've been talking about.)

> Eternal inflation, if correct, states that there is indeed something which our universe is expanding into.

I don't think that's right. The fact that space gets bigger doesn't mean that space is inside something, it just means space gets bigger.

Disclaimer: not an astrophysicist

Space does get bigger, but that's separate. What eternal inflation is claiming is that some fourteen billion years ago there was a false vacuum collapse, and we're living inside the bubble that resulted, but said bubble is itself still expanding.

It won't ever get anywhere, since space outside the bubble is inflating much, much faster than inside. It means there's a border to our universe, which is expending at lightspeed, but it's far outside the Hubble horizon and thus entirely unreachable.

I think those are just two equivalent ways of thinking of the same thing.

The standard way is probably much easier to reason about.