Size of Known Universe / Age of Known Universe > c. What gives?

So, according to Wikipedia’s article on The Universe (aside: they have everything on there), the approximate age of the universe is 13.7 billion years. In the same article, they state that the approximate size of the known universe is 78 billion light years.

With everything starting from one point at the big bang, it seems like the maximum possible diameter for the universe would be 13.7 x 2 = 27.4 billion light years. Somehow, it seems that the outer reaches of the universe have been moving at an average speed of almost 3 times the speed of light, which is against the law. Something must be done.

Assuming that Wikipedia’s not totally on crack (a distinct possibility), physicists, please enlighten me.

IANACosmologist, but IIRC photons were not created until some point well after the Big Bang.

Well, the Universe expands because spacetime is expanding.

The universal speed limit of c only applies to motions through space.

So two galaxies on opposite sides of the observable Universe can travel apart at a speed faster than c because they are not flying through space, they’re being carried by space.

All cosmologists agree that space itself can expand faster than the speed of light. Most agree that during the period of what is called inflation, spacetime did exactly that. The speed-of-light restriction applies only to matter that exists inside of spacetime.

It is a popular misconception that the Big Bang happened in one place, and now everything is rushing away from that point. Since space itself did not exist before the Big Bang, there was no single point at which it occurred.

Although different objects in the universe are becoming farther apart, they are not moving. Instead, space itself is expanding. The objects within it are (essentially) stationary.

Maybe the monobloc was 68 billion light years in diameter?

It’s misleading to say that space expanded faster than light during inflation. Yes, it did that (presuming as most cosmologists do that inflation actually occured), but it’s also expanding faster than light right now. The expansion of space is not measured in terms of speed, but rather in terms of speed per distance. The more distant objects you look at, the faster they’re moving away from you. No matter when you look, a sufficiently distant object will be moving away at faster than c. The only difference during Inflation was that then, “a sufficient distance” was much, much shorter than it is now (less than the size of an atom, in fact).

No, families of spacelike slices of spacetime are expanding. Spacetime is (in classical GR) fixed.

iamthewalrus(:3= – Not a really good analogy (and IANA professional), but something that helps me wrap my own head around this particular fact.

Think of a balloon. Think of “matter” existing as some sort of manifestation within the rubber which the balloon is made of. Imagine that this “matter” is constained, by the nature of the fabric if the balloon, to move no faster than, say, 1 mm/sec against the fabric.

Now imagine the balloon is being blown up. The whole “balloon universe” (the rubber skin), along with the “balloon matter” within it, is expanding. points of “balloon matter” far apart from each other are receding from each other much faster than 1 mm/sec. But the “Ballon Matter” still cannot move relative to the (stretching) fabric of its “universe” any faster than the “Balloon Speed Limit” of 1 mm/sec.

Apologies to the real cosmologists here, but hopefully this lame analogy, offered from one layman to another, will help you understand the difference between motion in the universe and motion of the universe.


Help me out here, because I’ve never gotten an answer to this…if the universe is expanding, what’s it expanding into?

Nothing. Since there is no space “outside” the universe, it is not expanding into anything.

In classical GR that’s not part of the theory. The big problem with the balloon analogy is that when you think of the 2-dimensional surface of the balloon, you think of it being embedded into 3-dimensional space around it. If you think about the 3-dimensional region the balloon’s surface sweeps out as time passes, you think of it being embedded into 4-dimensional spacetime.

In GR, spacetime is just the surface. Everything takes place on the surface and never refers to anything “outside” the surface. Yes, the surface could be thought of as embedded in some higher-dimensional space, but it doesn’t affect the physics at all. It doesn’t matter, and may as well not be there. To answer your question with “nothing” even overstates it. The embedding space just isn’t part of the model at all.

As far as we can ever know, it’s filled with gumbo.

The units for speed/distance would be 1/s, right?

Ah gay-rawn-tee!

speed = distance/time, so speed/distance = 1/time. yes.

I asked this here once before and the best answer that I got was “A curious ether”.

Right… I really should know this, considering the amount of study of this sort of thing I’ve done.

If spacetime is expanding, at a rate relative to the distance from the point you’re observing, why don’t the objects in it expand too? Obviously the distance from my feet to my head is tiny in comparison to billions of light years, but I should be getting taller.

And if this is the case, then my metre rule is expanding too. Meaning that when I measure the distance to a galaxy a billion light years away, it’s always the same distance from me (although a metre rule may not be the best way to measure it).

So why is it that things are further away? :confused:

If distant objects are receding faster than c, how can we detect them? Wouldn’t the light be redshifted to 0 Hz before it reaches us?

Thanks for all the replies. I see now where my misconception was.

Is there any theoretical limit on the expansion/compression of space-time?

Exactly. We can’t make any observations of any material receding at more than the apparent speed of light. That’s why people talk of the “observable” universe, that part of the universe that is receding at less than C. It’s the observable universe that has a radius of approximately 13 billion light years.

Space is expanding, but the forces (electromagnetic on the molecular scale, strong and weak nuclear on the subatomic, and gravity at the macro scale) remain constant (AFAWK). Think of a pool with shallow slopes; as you add water, the surface area of the pool increases, but the lillies floating on top don’t get any bigger.

Space is not “stuff”, and “stuff” is not space.