2 Questions

Symbreak
1. In general relativity, why is there a requirement for spacetime, in the context of the Universe, to expand/contract?
It is often said that the theory predicts the Universe to expand, but Einstein missed out on this by adding the 'cosmilogical constant'. But where in the theory is the condition that the Universe must expand/contract?
The only way I can see this condition maintained is if the Universe is bounded - and thus has enough matter to ensure a contraction, or energy pressue to expand, but not remain static.

2. Why doesn't a charge increase (like an electrons charge) with velocity, in the same way as mass? Does not a charge have a kind of inertia, or radiation resistance, so therefore must increase for speeds near c?

Spin_Network
Symbreak said:
1.

2. Why doesn't a charge increase (like an electrons charge) with velocity, in the same way as mass? Does not a charge have a kind of inertia, or radiation resistance, so therefore must increase for speeds near c?

If two highly charged energetic particles were traveling near c, then their trajectories would be 'Attracted' to 'Opposite' charged particles, thus Gravity would be Electrically Charged Induced? A highly charged Blackhole, say at the core of Galaxies, would cause all finite particles with any sort of charge signature to be attracted inwards, towards the Charge Source, and Particles would never be found outside Galactic Blackholes.

Homework Helper
Gold Member
1. In general relativity, why is there a requirement for spacetime, in the context of the Universe, to expand/contract?

Gravity is a purely attractive force. If you set up a situation in which nothing in the universe was moving, then immediately all objects would start to fall towards each other, causing the entire universe to "clump".

We don't observe that happening, so there are two possible solutions:
1. There is a "repulsive gravity" which we haven't taken into account, or
2. Galaxies must have started off moving away from each other.

Of course, maybe both of these are true.

It is often said that the theory predicts the Universe to expand, but Einstein missed out on this by adding the 'cosmilogical constant'.

Actually, Einstein added the constant to try to avoid the universe collapsing in his theory, since he knew that we don't see all the galaxies coming towards us. Einstein originally aimed for a "steady state" universe. Then he found out that the universe was actually expanding, and so called the cosmological constant his "biggest blunder".