Hacker News Reader

Comment by SkeuomorphicBee | Hacker News Reader

> The largest the universe could have been at the time is 2 billion light-years across if the universe expanded at the speed of light after the Big Bang.

That is the first mistake in your train of thought, the universe doesn't expand "at the speed of light", it is not a ball expanding outwards with an outer shell expanding at a measurable speed.

Instead it expands everywhere all at once, all distances are being stretched. And this streatching doesn't have a "speed" (m/s), there is nothing traveling, so it is not bound by the speed of light. Instead it has a rate of expansion (m/m/s) as in how much each meter will grow in a second.

Sorry, I'm not an expert so I don't know how to explain properly.

View on HN

In other words, the speed of light is irrelevant. It (or relativity) doesn't dictate at all a limit on the relative speeds of two objects. This much is obvious if you consider two objects moving in oppositive directions at 51% the speed of light. Certainly valid, even though combined their relative speed from each other is >c

You can extrapolate the same to the expansion of the universe, I think, but I may be simplifying too much.

> two objects moving in oppositive directions at 51% the speed of light

> combined their relative speed from each other is >c

We've got to be very precise with the language here. That's not accurate as stated, even though I know what you mean. The relative speed of one of these objects from the other one is not greater than c. You cannot directly add relativistic velocities without making adjustments for time dilation. Adding .51 + .51 to get 1.02 is not how the math works out.

If I am sitting in the middle, observing both objects, then I see the distance between them increasing at greater than the speed of light. But that concept should not be called "relative velocity" of one object with respect to the other. That's different.

No observer can witness an object receding at or above the speed of light. At that point the redshift would completely suppresses the information from arriving.

Thanks, that's exactly what I meant!

Not to say there is no such a boundary, it just might not be what intuition would tell us like a baloon, a sphere or similar. Geometry of universe might be a torus, but also not really. Shit gets weird when you start reading research, and it takes a lot of effort to even start reading with comprehension there.

I like this.

My 'bad' mental model is: In the beginning there was an infinite amount of space and somewhere within that space the Big Bang happened. The universe is all the matter and energy produced by the Big Bang and that universe is expanding at a maximum speed of C through the infinite amount of space.

But you are saying, I think, that before the Big Bang there was no space. The Big Bang produced all the matter, energy, and space itself. And space is expanding, NOT just the matter and energy expanding through the infinite pre-existing space that I imagined.

I still have a very hard time imagining 'no space' before the Big Bang and the concept of space being something besides 'nothing'. Because if space is expanding it must be something more than nothing.

it's not known whether the Big Bang was the beginning of space or time. there's one theory (eternal inflation) that our universe fell out of a continuous Big Bang when a false vacuum collapsed. all the pieces of that field then became particles, since they're high energy compared to the newly-dropped floor of our spacetime.

the kind of expansion happening between galaxies is a different kind of inflation.

it's even more mind-blowing though, because it would imply that the 10^-32 second inflation of our universe from a grain of sand to only 1000x smaller than it is now is just... the steady state, outside our microscopic bubble of observable reality.

Reply on news.ycombinator.com