Tonight someone asked, “If at one second after Big Bang, our universe expanded to a size slightly larger than it is now, doesn't that mean that our universe expanded faster than the speed
of light?”
If I understand the metric properly, the answer is yes.
My
question is, “If the “space” of the “universe” can expand
faster than the speed of light, but matter that exists within that
space cannot travel faster than the speed of light, is that saying
that “space” is not made up of matter?”
I've
always understood the universe as something where everything is
connected to something else, that – well -
What
exactly is being said here?
Two points that might interest you.
ReplyDeleteFirst, there is a mistake in the way the question if formulated. The universe did not expand to a size larger than it is now 1 sec after the Big Bang, because that would mean that the universe shrank since then. The evidence points to a universe that has been expanding since the Big Bang, and accelerating. What the evidence also suggests, which the question correctly states, is that the universe did expand faster than the speed of light for a short period of time during its infancy.
Second, how is that even possible if nothing goes faster than the speed of light? This is explained in terms of Relativity because saying that nothing can go faster than the speed of light is actually a bit of an over-simplification. What the law actually says is that objects cannot go faster than the speed of light relative to each other.
Two analogies can help understand the strangeness of this principle. For instance, take a spaceship going at the speed of light. The spaceship then turns on its headlight. At what speed is the beam of light travelling? One could think that because the spaceship is already going at the speed of light, and it is projecting light in front of it at the speed of light, then the beam should go at twice the speed of light! But of course that would violate the principle, so it is the wrong answer. What really happens? It's all relative. For the spaceship, the beam of light does appear to go at the speed of light, relative to the ship. But for an observer on the side, the question is more complicated. In this case, what will change is the time required for the light bouncing on the ship to arrive to the observer (the observer needs some light to see the ship...) versus the time required for the light from the beam of light to arrive. Because the beam of light goes faster than the ship, relative to the ship, the observer will have the impression that the beam of light took a longer time to arrive because it covered a longer distance, being in front of the ship. The time and distance are thus bigger, keeping the speed constant with the speed of light.
The second analogy helps explain how the universe can expand faster than the speed of light. We need to imagine someone running on a treadmill, but a special treadmill which is actually expanding while the runner is one it. The treadmill appears to be moving under the feet of the runner, and the runner moves relative to the treadmill. The speed at which is treadmill is moving relative to the runner, and the speed at which the runner move relative to the treadmill are bound by the speed of light. However, the rate of expansion of the treadmill is not bound by the speed of light because that expansion does not represent a movement of one object with respect to another, it's the track itself which is getting bigger. It's weird because this is obviously an imperfect analogy; for us, the treadmill is also a physical objects bound by the same rules, but for the universe, the treadmill is actually what contain all the physical objects so the expansion of that thing that contain the objects is not bound by the same rules as what's found inside it.
Hope this helps...
You might be right about the expansion. I don't remember where I read it, it was probably 20 years ago but I distinctly remember reading that the universe expanded to a greater size than it is now, shrank in kind of a rebound and is now expanding again. This was before the discovery of dark matter and its expansion properties.
ReplyDeleteRegardless, it's pretty cool.
Yep, fascinating! I could be wrong on some details too :)
ReplyDeleteNicely explained, Hugo. Of course, it really theoretical physics, which is all math. And Einstein's model breaks down at the singularity. Neil Turok is currently working on a model that links the breakdown of dark matter to producing a Big Bang, which could change our understanding completely. Stay tuned, so to speak...
ReplyDelete