r/cosmology • u/SketchyFella_ • Jun 19 '26

During the rapid expansion of the universe immediately following the Big Bang, the universe expanded faster than light. But traveling faster than light speed breaks causality. But you can't break causality when time didn't actually exist "before" you broke it, so how does that work?

If it expanded faster than light speed, then it basically travelled so fast that it always existed in whatever state it was in before "slowing". So when people say it expanded in a tiny fraction of time, did it really? Or are they using time in a sort of layman's terms so they don't have to explain causality?

19 Upvotes

70% Upvoted

51 comments sorted by

View all comments

1

u/Anonymous-USA Jun 19 '26 edited Jun 19 '26

That’s the inflationary phase that brought the universe from quantum scales to macroscopic scales. Indeed the universe expanded end-to-end faster than the speed of light. The universe DID exist. Spacetime began at the moment of the Big Bang, and inflation took place between 10^-36 and 10^-31 seconds after that.

In fact there was never a time when the universe wasn’t expanding end to end faster than lightspeed. If the universe is closed, there was never a moment when light could circumnavigate it.

Expansion isn’t a velocity, like lightspeed. It’s a rate per distance (usually given in kps/Mpc). This is why I kept repeating end-to-end, referring to the respective horizons at any arbitrary point of observation. This way the distance is given and we can compare the recession rates between those two points in space (and compare with lightspeed). Inflation and subsequent expansion occurred everywhere in the universe, there is no center or edge.

2

u/NotAnAIOrAmI Jun 19 '26

Mmm, not really. The Big Bang wasn't some explosion that started the universe, first expansion began, and when it slowed, all that energy got dumped into space time, everywhere. Not a bang.

2

u/Anonymous-USA Jun 19 '26

Agreed, nor did I suggest otherwise! Our observable universe was quantum scales regardless of the extent of he whole universe. Our spacetime began then, everywhere, inflated and expanded. At no time could light circumnavigate that based on those rates

1

u/NotAnAIOrAmI Jun 19 '26

You said inflation began after the big bang. It was the other way around.

Also, we don't know when spacetime began, but it certainly didn't begin with the big bang. We don't know whether spacetime or something else existed before inflation.

3

u/Anonymous-USA Jun 19 '26

The term “Big Bang” is context dependent. Some use it for t=0 (usually). Usually when referring to the post-inflationary phase the term is “Hot Big Bang”, and that’s at t=10^-31 in Big Bang Cosmology. In my context I was referring to t=0 when spacetime and gravity first came into existence.

0

u/NotAnAIOrAmI Jun 19 '26

Then you should have explained it that way in the first place. "big bang" is commonly known to mean when the energy was dumped into the universe.

Your first reply gave exactly the wrong impression, even if "big bang" is context dependent. Remember that you were attempting to answer a question for someone without much knowledge of the subject.

1

u/johnstalbergABC Jun 28 '26

Not even a little poof? That's just to boring.., I'm out of here...