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Comment by sslayer | Hacker News Reader

I've always maintained that we are slowly falling toward the center of a black hole that is larger than our current known universe. The rate of expansion is just the speed the universe is "falling, yet due to the absolutely fantastic size of the black hole it will take billions of years to collapse in on ourselves.

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That would add a directionality component as some objects closer to said black hole would accelerate faster.

A more plausible but unprovable option is that we are close to the center of a big void (the density of the matter in our neighborhood is much lower than outside the observable universe). So the higher mass concentration attracts objects in our neighborhood giving the impression of an accelerating expansion. The problem with this theory are that we would have to be almost at the center of this bubble of almost nothing (so the notion that earth is not in a special position is lost) and that if spacetime is not expanding we cannot explain why very distant galaxies recede from us faster than light. FTL travel though spacetime should be impossible but spacetime expanding faster than light is.

If we are within the event horizon, then the direction towards the center of the black hole is not a spatial direction, but a temporal one. The cental singularity is in the future. As it turns out, the temporal direction is the only direction of spacetime which is not symetric. Objects in the future are accelerating faster, which is presisly the prediction you made.

If I understand correctly, if you add up the mass in the universe, the Schwarzchild radius of that mass is 13 billion light years, which is a really interesting number.

If we were inside a universe-sized black hole, would we be able to tell? Can we experimentally distinguish a black hole universe from a not-black-hole universe?

Good point! However, inside an event horizon we should have a preferred direction in which all the light cones that can catch up to us actually catch up to us (if Penrose diagrams hold true within an event horizon). That would mean that looking in that direction the universe would appear brighter and given how smooth is the CMB that doesn't seem to happen. It could still work if we assume that when this black hole was created either everything or nothing was causally connected to us.

Fascinating point!

I think a theory winning when it comes to “end of the universe - scary version” is what is known as vacuum decay : https://bigthink.com/hard-science/vacuum-decay-end-of-the-un...

Is it not a bit odd to consider things that will happen long after the sun is a cinder "scary." There are enough genuinely scary things that could flash-sterilize the planet, or smash a big enough hole in it to end all complex life forms. Nearer-term scary things are likely to come from how poorly humans can adapt to long term life in space or on planets with different gravity.

Time stretches beyond the ability of life to survive in both directions. Nobody will be around to be scared.

I agree my comment is way too antropomorphic centric. As you say, in terms of the timescales involved it might not be a time humans are around.

At the speed of light we’d all be gone. Even if the Mars colony was snuffed out first, we wouldn’t even know, since the information about the change wouldn’t be able to reach us before the decay did!

Here’s a website that explores that idea: http://www.flatuniversesociety.com/

Fun idea. However, acceleration would be different in different directions, were this the case. In our universe, the acceleration is the same in every direction.

Expansion wouldn't be isotropic in that model, so no?

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