I don't know enough General Relativity to give a solid explanation. But here is a rough over-simplification: Imagine you've spent the last 5 seconds blowing up a balloon, but there is an ant walking on that balloon, starting from a marked spot.
In the 1st second, the ant moved 10mm. But in the next four seconds you blew up the balloon, so that 10mm of rubber is now 70mm long. The total distance from the ant's starting mark to her end-point can now be well over than 70mm, even though she only walks at less than 10mm/second.
As the ant, so the photon.
Imagine that balloon is not inflated at all, but ant magically moves 70mm instead of 10mm. How this is possible???
I can accept, that our part of Laniakea is expanding, but not whole Universe.
If you think of the ant as a 2 dimensional creature living on a two dimensional surface of the balloon, the expansion is occurring in 3 dimensional space. The two-dimensional ant has no way to observe the space north or south (above and below, to us) its plane of existence.
Now, imagine that the balloon is in a vacuum. It had a burst of air (the big bang) for a moment to get it started but it's not the further addition of air that's driving the expansion, it's the desire to equalize pressure between the exterior vacuum and the interior matter.
With that in mind, push everything up a dimension. We're 3 dimensional creatures living in a 3 dimensional universe. That 3 dimensional universe exists as a plane in 4 dimensional space.
The 4 dimensional balloon (that our universe is surrounding) is continuing to expand into the "nothingness" on the ana plane of the 4th dimensional universe because of the "pressure" imbalance on the kata plane of the 4th dimensional universe. (In 4th dimensional spatial terms, ana and kata indicates directions on the w axis, similar to up/down on the x axis, left/right on the y axis, and north/south on the z axis)
At least, that's how I see it in my head, but I'm an odd ball who has never had a problem intuitively visualizing 4th spatial dimension objects and how they might interact with 3rd dimensional space.
As for the increased speed of the expansion... actually, I hadn't thought about that. Does it indicate that equilibrium will never be found? The balloon analogy might be a useful tool, but our universe isn't made of rubber so it might have the capability of being able to expand infinitely without breaking and without its "nature" constraining the expansion speed.
Anyway, if this is a reasonable analogy, the "nothingness" in the ana direction isn't as interesting to me as what might comprise the "pressure" that's in the kata direction that's causing the expansion. It's nothing we'd ever be able to observe or sample since it's outside of our plane of existence, but it's awfully fun to speculate about the composition and nature of it.
Thus the expansion isn't caused by the movement of the photon but from something else? Then what caused that movement in the first place? And what cause the expansion?
That is we see red-shifts when we observe distant objects. And comparing that to other distance estimates, we see that far away things are more red-shifted. As if they are moving away from us.
When we build cosmological models to explain this, we could choose reference coordinates for time and space in different ways. But if we choose to define time and distance in the usual way then we get a picture in which the distances between all galaxies is increasing with time.
However, at scales so large that gravitational attraction has attenuated to nothing, this repulsive component of gravity is still getting stronger. At the scale of galaxy clusters, it is pushing everything in the universe apart in all directions: faster the farther apart they are. At a certain distance, it becomes so strong that objects will appear to be moving away from you faster than the speed of light.
The distance at which that happens is called your Cosmological Horizon, and it has similarities to the event horizon of a black hole turned inside out. Unlike a black hole, however, there isn't a single unique event horizon. Every distinct gravitationally-bound object (galaxy cluster) in the universe has its own cosmological horizon; every other object is being pushed out towards it, and vice versa.
I think this just opens more questions than it resolves, and would really like to hear a better explanation.
(In fact space inside galaxies isn't expanding at all, only space between galaxies. But I digress.)
It's when you measure between one side of the observable universe to the oposite side that the expansion speeds get crazy.
Yes, if by "we" you means "large scale cosmological structures".
But galaxies stay roughly constant size because gravity holds them together. Smaller things are also held together by various forces.
Anyway, I see no shockwave of any kind in CMB. Where it is? Such massive explosion, with energy of 1E53 atomic bombs and no shockwave at all just miliseconds (in scale of Universe) after the Bing Bang. How this is possible?
Also the frequently depicted diagram of expanding from a point doesn't really intend to depict the shape of it. The universe might still be infinitely large and the big bang occurred "everywhere" within it. Where is the shockwave in something that has no edge?
Inflation wasn’t something that happened from a single point - it happened as a result of vacuum collapse, spontaneously, all over the place. Some of these inflationary zones went much faster than others, and where they intersected, regions of increased density, and therefore matter, occurred.
The filamentary nature of the very macroscopic universe reflects these shock intersection zones - they’re full of galaxies. We think that the voids are likely centred around points where inflation occurred.
So yeah. You’re the shockwave. Sorry man.
Here’s a decent vid from PBS Space Time where they explain some of this nice and clearly - I recommend their channel generally if these topics interest you. https://youtu.be/72cM_E6bsOs
The thing is we cannot see such a star in visible light spectrum anymore. The light of the star has shifted to infrared or microwave. Why? Because while the light was traveling in spacetime, the spacetime itself expanded and stretched the light wavelength. When you take a redshift into account, you can correct the distance and figure out that the star must be, say, 40B light years away.
By the way, note that this is all about the observable universe. There might be stars beyond our observation horizon that we might never see due to rapid expansion of the universe.
The space between objects can also expand faster than light, the object itself can still not move faster than light. If the space between two objects is expanding faster than light then for all intents and purposes, the other objects no longer exists to another. There is no way to contact or observe that object other than the light that was send your way before space went FTL.
If you have a laser pointer and flick your wrist fast enough, the spot on the ground that your cat chases can move faster than light without violating any laws of physics.
That's perhaps easier to see, if you think of a giant laserpointer aimed at the moon.
Similarly, a shadow on the wall can move faster than the speed of light.
An outside observer that just sees the spot on the moon (but has no clue that it's produced by you flicking your wrists), just sees a spot that moves insanely fast.
No information, energy nor matter travels faster than light here, of course.
Quantum mechanics limit the spread of information to the speed of light, most wave functions including the fundamental forces propagate at the speed of light and acceleration requires increasing amounts of energy the close your come to the speed of light.
I guess one way to look at where this comes from is to look at Conway's Game of Life. A cell in this game spawns if enough neighbors are present and with some trickery you can make things that move. But due to the rules of the game, nothing moves faster than 1 square per round.
There is no explicit rule that the speed of light in that game is 1 square per round, it's just that the way the rules work, the fastest thing could only possibly be that fast. Everything is limited to this speed that doesn't exist in the game.
If you changed the rules to allow cells to die or spawn depending on cells up to 1 square inbetween in distance, the speed of light would be 2 squares and none of the rules will explicitly state this limit.
The expansion of space itself doesn't violate that - you couldn't use it to communicate with someone in an FTL way.
My friend and I both live at different points on the surface of a balloon with distance A. I throw a ball at light speed while the balloon is being inflated, and the distance between us is B when the ball reaches him after T time.
To my friend it seems that the ball was thrown at B / T speed, which is faster than the speed of light.
There is no meaningful spatial boundary at a point that is currently at a distance of 13.8 billion light years from us. We can detect (severely red-shifted) photons from beyond that point perfectly fine! The actual boundary is 46.5 billion light years away in both directions, hence the "observable" diameter of 93 billion.
But of course the meaning of "observed" is kinda strange here, since we're not directly measuring the distance but estimating it based on from other observations.
(Or is it flat like a torus, so doesn't need any curvature to go loop on itself?)
Basically, ignoring wormholes and black holes and assuming that spacetime is locally flat everywhere and it's mathematically a manifold, my question is: what's the shape of the (global) universe?
Global as opposed to observable. So we might have a hard time answering that question. How would you be able to distinguish between the (n-dimensional equivalent of) a torus vs a flat infinite space, if you can't see the repetition?
You'd even have a hard time distinguishing a hypersphere from a flat infinite space, if the hypesphere was big enough so that we can't tell it's curvature apart from no curvature.
Or the universe might be weirdly shaped, and we just happen to live in the flat part.
So I guess the question comes down to:
* assuming no edges * assume Copernicus at least for space (we might have a special position in time) * What's the simplest theory about the shape of the global universe that satisfies our observations?
I suspect general relativity toys around with such questions, because I know that they sometimes look at cosmological (toy) models for the whole universe, and not just what's in the light cone of one particular observer.
Just to fit a bunch of observations that wouldn't otherwise make sense for the current model.
I remember the story of a chap from Pisa having a hard time trying to budge a bunch of clerics to consider his observations proof that their model was broken. ;)