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Comment by goohle | Hacker News Reader

It looks like Big Shrink (BS) theory is easier to understand: we are shrinking, our rulers are shrinking, so cosmic distances are looking bigger in every direction. It explains why our Universe is flat and why we cannot find a Universe-big source of energy for expanding of the Universe.

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This doesn't make sense either - if I were shrinking and you were shrinking, then the distance between us would appear to be growing larger, but it's not. Gravitationally bound systems don't expand, only the galaxies themselves seem to be moving away from one another. Combined with no mechanism to explain the shrinking, nor any reason why the various other laws of physics don't seem to be affected and it doesn't seem any easier to understand at all.

Just curious, since gravity works over infinite distances (as I understand things):

When do two masses stop being gravitationally bound? Is that when each mass's relative speed exceeds the escape velocity of the other mass?

No, that can't be right: they could still end up in orbit - obviously gravitationally bound.

One complication is that since the universe is homogenous and infinite, there’s a boundary that is expanding away from us faster than the speed of light and this gravity as well.

The observable universe is of finite mass and size, while the total universe is of infinite or finite mass and size. On a macro scale, the total universe probably looks similar. It's definitely not micro homogeneous or entropy would be infinite and heat death would have already occurred. Heat death won't be the biggest problem because of the likelihood of the Big Rip.

In this case, bound means that the force of gravity is stronger than the expansion of space time. All of space is expanding, but things that are close (ie within a few hundred thousand light years) pull on eachother enough to resist being swept away by the "current" and only distant galaxies recede. But it's not like there is something special about our position, if you could magically teleport to another galaxy a few billion lightyears away the picture would be about the same: your own galaxy and those nearby don't go flying apart, but all the distant galaxies do.

But what energy is making us shrink?

From the article:

> “The memory is nothing but the change in the gravitational potential,” said Thorne, “but it’s a relativistic gravitational potential.” The energy of a passing gravitational wave creates a change in the gravitational potential; that change in potential distorts space-time, even after the wave has passed.

Gravitational waves are that energy.

Conservation of energy gets a bit screwy with these kinds of cosmic models. Partially because there isn't a single direction of 'time', but also because all kinds of energy gets stored in the fabric of space-time which can cancel out all kinds of things.

Not to mention that there's no reason to assume conservation of energy still holds if the laws of physics simply change over time (which would be the case in the simplest possible theory were all interaction distances simply shrink over time).

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