Hacker News Reader

Comment by mudil | Hacker News Reader

A question for a physicist from a layman. All objects in the universe are connected to each other through gravity, no matter how distant. So what happens to the energy or mass of an object (or total energy/mass) in the expanding universe? As space inflates, the distances are greater between objects, so where the excess of gravitational radiation went to? Can that account for the dark energy?

View on HN

Gravity is not radiation. The expansion of the universe does work against gravity but there are limits. Gravity decreases with distance. So if the universe is expanding fast enough (it is) it will overcome gravity. Imagine setting a bomb off between two asteroids. Use a small bomb and they move apart but eventually drift back. Gravity wins. But use a big enough bomb and they both hit escape velocity. Gravity looses. The objects are moving away from each other fast enough that the force of gravity falls away so fast that it will never turn them around let alone bring them back together.

Dark energy is the concept that while the universe is expanding, gravity wins / everything is at escape velocity, it shouldn't be expanding faster. Dark energy is the fact that the two asteroids separated by the bomb (the big bang) aren't just at escape velocity, something is causing them to accelerate.

> Gravity is not radiation.

'Gravitational radiation' is, apparently, a thing; gravity waves basically. It's not _electromagnetic_ radiation tho, which is what I'm guessing what you meant.

I don't think it's crazy to think that gravity is propagated at the speed of light but that we can't, and likely won't ever be able to, measure any gravity waves smaller than those generated by extremely large-gravity systems.

Non-flat spacetime doesn't necessarily conserve energy (Noether's theorem).

> All objects in the universe are connected to each other through gravity, no matter how distant.

I don't believe this is correct, as gravitational force/bending of spacetime moves at the speed of light.

Reply on news.ycombinator.com