# The expansion of the Universe and time travel (By a non-scientist)

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• Sarg
In summary, the expansion of the universe does not have a speed and going faster than the speed of light would not make time go backward.
Sarg
Hello I have a question about space. I am no physicist nor student of any kind of science but I'm curious and I've heard some things about the speeding up of the expansion of the universe which got me thinking...

If the expansion is accelerating without end and believed to even surpass the speed of light at some point, will all the energy in the universe reach a speed that's sufficient for time travel? I heard about one problem with going as fast or faster then light because anything except light moving that fast would gain infinit mass.

So considering that, could the expansion of the universe lead to a point where and infinit amount of energy is traveling back in time all at once, creating the circumstances for a big bang over and over again in an infinite, self sustaining time loop paradox?

I have no idea if all this sounds idiotic or not and that's why I'm here

Sarg said:
If the expansion is accelerating without end and believed to even surpass the speed of light at some point, will all the energy in the universe reach a speed that's sufficient for time travel? I heard about one problem with going as fast or faster then light because anything except light moving that fast would gain infinit mass.
This is all wrong.

The expansion does not have a speed. Not a speed like 10 miles per hour or 50 kilometers per second anyway.

The rate can be thought of as a percentage increase per unit time. Like ten percent per billion years. That number is made up, but should be in the ballpark. One can also express the rate in units of kilometers per second per megaparsec. In those units, the expansion rate is currently about 70 km/s/Mpc.

Now the rest of your idea falls apart.

Since the expansion is a rate and not a speed, it cannot be compared to the speed of light. It cannot be less than the speed of light. It cannot be greater. The two concepts do not go together. In addition, the speed of light limit is a local limit. The expansion of the universe is a global phenomenon. So the speed of light limit does not constrain the expansion of the universe.

Going faster than the speed of light does not make time go backward. Superman movies notwithstanding. That said, if one accepts the possibility of faster than light travel (or communication) and retains the principle of relativity which Einsteins special relativity adheres to then there would be problems with causality. Most scientists take this as forbidding faster than light travel.

Finally, going faster than light speed would not result in infinite mass. If one blindly follows the equations where they lead then it would result in an imaginary mass instead.

One should be aware that "mass" in the sense that modern scientists use the term means "invariant mass". An object's "invariant mass" does not change with the object's speed. It does not become infinite at light speed. It does not become imaginary past light speed.

There is a concept called "relativistic mass" which does increase with increasing speed. However, given the formula ##E=mc^2##, relativistic mass is just another name for energy with a factor of ##c^2## tacked on. So most modern scientists have discarded the notion of relativistic mass in favor of just saying "energy".
Sarg said:
So considering that, could the expansion of the universe lead to a point where and infinit amount of energy is traveling back in time all at once, creating the circumstances for a big bang over and over again in an infinite, self sustaining time loop paradox?
This is the problem with pop science. You get little nuggets here and there. Each containing a bit of truth. Each lacking the context to tell you how widely applicable that truth is. You try reasoning from those nuggets to a logical whole and... you just can't. The result is nonsense.

ramuma5309 and malawi_glenn
Ok thanks for the fast answer ^^

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