[thx4allthestuff]

Super weird food for thought, but I used to think about the universe a lot as a kid, and back then we were under the assumption that the expansion of the universe was slowing. At some point that viewpoint changed and we now believe it is accelerating, hence the emergence of so called dark matter. Anyway, this led me to envision a fourth dimension, a sphere. Imagine that our universe began at any arbitrary point on the inside of this sphere, and then orient the sphere so that we are at the bottom (like a penny inside of an inflated balloon). Now imagine (don’t believe, just imagine) that our universe is expanding at a constant rate. As we approach the equator of this sphere, the area that we must cover grows larger, but once we pass the equator it begins to grow smaller. And so what might appear to us as slowing down and speeding up could just be the shape of space changing, and not the speed of expansion. Again, just food for thought.

[brian-armstrong]

Minor correction: The accelerating expansion of the universe implies the existence of dark energy, not dark matter.

[InclinedPlane]

This is not a minor correction simply because they both contain the word "dark". It would have been slightly more correct to confuse the terms dark matter and dark chocolate. Dark energy is an entirely different kettle of fish.

[thx4allthestuff]

Or maybe it's nothing at all ;)

[thx4allthestuff]

Correction noted and accepted. Thanks. In this particular thought experiment, dark energy (not dark matter as originally stated) has been replaced by the shape of space, and due to our inability to directly detect such extra-dimensional curvature we have defined this energy as a placeholder (which would not exist if this experiment proved to be true).

[isaachier]

I used to be a bit interested in astrophysics, so I can't vouch 100% for this being accurate, but it's my understanding:

The universe isn't the penny, it's the balloon. Physicists believe we are living on the surface of a hypersphere. One important consequence of this idea is that the big bang didn't occur at a specific point in our 3D space, but at the center of the sphere.

Furthermore, the concept of a balloon expanding vs. deflating is a bit of a misconception. The argument used to be that whether or not the balloon is expanding, depending on the rate of the expansion, gravity could eventually win out and cause the matter to collapse back together (big crunch scenario). The problem with that theory is that we now know that galaxies are speeding away from us at a growing speed that (not sure the exact details, probably based on red shift in light from nearby galaxies). So the idea of gravity winning out was not based on evidence, just one of a number of possibilities, but the evidence proved it wrong beyond a doubt.

[JProthero]

The analogy of an inflating balloon is a useful one because, as in the universe, an observer at any given position on the surface of the balloon sees all other points receding from them. This leads to the illusion that any observer's position is the 'center' of the expansion, but there is actually no center on the surface of the balloon, just as there is no unique origin point for the expansion of space in the universe.

The analogy isn't perfect though. I don't think it's quite right that cosmologists believe we are living on the surface of an expanding hypersphere; that would imply that the expansion of the universe had a real spatial center somewhere in a large extra dimension, just as the inflating surface of a balloon has a real spatial center in the balloon's three dimensional interior, inaccessible to observers that can only probe the surface.

That the universe has a real, albeit extra-dimensional spatial center isn't a mainstream idea, but there are theorists exploring the possibility that the universe exists on the surface of a brane in a higher dimensional 'bulk', and that the big bang resulted from a collision between branes in that higher dimensional space [1].

There might be some utility in thinking of the universe as an inflating hypersphere whose radius corresponds to time, rather than to an additional spatial dimension. In that analogy, the center of the hypersphere (or balloon) would represent a point in time, rather than a point in higher-dimensional space. The surface of the hypersphere (corresponding to the space of our universe) would appear to expand the further an observer was from the temporal 'center', which would be equivalent to the big bang. There is a consensus among cosmologists that the big bang appears to be a special point in time, if not in space.

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

[WhitneyLand]

Wouldn’t that be not the best fit you can imagine with the holographic principle?

[stephengillie]

Think of 4 dimensional spacetime as an oddly-shaped rectangle, which is much wider at one end than another. Maybe more like a pyramid with the top bit missing. All of matter and energy started as an explosion at the narrow end, and is traveling across towards the much broader end. The expansion of the universe is merely the expansion of the barrel, and the dark energy spreading apart the universe is really the slight trajectory differences caused by the shape of the charge that started the process.

To me, it's like 3d canon shot flying down a 4d barrel.

Or trace the worldlines of these particles. Any given point in time represents a 3d slice of the 4d pyramid-rectangle.

[ianai]

That’s a really awesome thought experiment! I got a related idea during undergrad. What if the apparent symmetry between the three special and one temporal dimensions was at one point complete? Ie time could at one point be swapped out with any of the other three dimensions. But somehow that symmetry collapsed- and maybe it’s collapse was the result or creation of mass-energy. I imagine a hypercube being “pulled down” into a confined region. But the confinement “pushed” the two directions of time into the one direction and mass-energy.

[monkeydreams]

Interesting ideas.

Wouldn't this result in relative acceleration which correlates to the angle of the body being observed relative to the viewer? Anything aligbed directly between the viewer and a pole would move at a constant speed while speeds would appear to increase as you approach perpendicular? I can't imagine how this would play out in a 3d projection of a 4d surface however.

[bordercases]

Observations of "cosmic inflation" are not uniformly distributed, so this could very well be true.

[pas]

Is that the current consensus? Do you happen to have sources for that?