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iamthewalrus(:3=
06-13-2005, 02:23 PM
So, according to Wikipedia's article on The Universe (http://en.wikipedia.org/wiki/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.

Ike Witt
06-13-2005, 02:36 PM
IANACosmologist, but IIRC photons were not created until some point well after the Big Bang.

Podkayne
06-13-2005, 02:38 PM
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.

Exapno Mapcase
06-13-2005, 02:38 PM
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.

Colibri
06-13-2005, 02:56 PM
With everything starting from one point at the big bang,

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.

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.

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.

Ethilrist
06-13-2005, 02:58 PM
Maybe the monobloc was 68 billion light years in diameter?

Chronos
06-13-2005, 03:05 PM
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).

Mathochist
06-13-2005, 03:06 PM
Well, the Universe expands because spacetime is expanding.

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

Noone Special
06-13-2005, 03:19 PM
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.

Dani

Captain Amazing
06-13-2005, 03:41 PM
Help me out here, because I've never gotten an answer to this...if the universe is expanding, what's it expanding into?

Colibri
06-13-2005, 03:48 PM
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.

Mathochist
06-13-2005, 03:49 PM
Help me out here, because I've never gotten an answer to this...if the universe is expanding, what's it expanding into?

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.

ultrafilter
06-13-2005, 03:53 PM
Help me out here, because I've never gotten an answer to this...if the universe is expanding, what's it expanding into?

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

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

Mathochist
06-13-2005, 03:59 PM
As far as we can ever know, it's filled with gumbo.

Ah gay-rawn-tee!

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

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

Ike Witt
06-13-2005, 04:00 PM
Help me out here, because I've never gotten an answer to this...if the universe is expanding, what's it expanding into?
I asked this here once before and the best answer that I got was "A curious ether".

Fromage A Trois
06-13-2005, 04:05 PM
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:

sturmhauke
06-13-2005, 04:18 PM
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?

iamthewalrus(:3=
06-13-2005, 04:21 PM
Thanks for all the replies. I see now where my misconception was.

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

Exapno Mapcase
06-13-2005, 04:25 PM
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?
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.

Stranger On A Train
06-13-2005, 04:44 PM
So why is it that things are further away? :confused: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.

Stranger

CurtC
06-13-2005, 05:26 PM
There's an excellent article at the Scientific American site about this: Misconceptions about the Big Bang (http://www.sciam.com/article.cfm?articleID=0009F0CA-C523-1213-852383414B7F0147&chanID=sa009)

My favorite part:This ubiquity of the big bang holds no matter how big the universe is or even whether it is finite or infinite in size. Cosmologists sometimes state that the universe used to be the size of a grapefruit, but what they mean is that the part of the universe we can now see--our observable universe--used to be the size of a grapefruit.

Observers living in the Andromeda galaxy and beyond have their own observable universes that are different from but overlap with ours. Andromedans can see galaxies we cannot, simply by virtue of being slightly closer to them, and vice versa. Their observable universe also used to be the size of a grapefruit. Thus, we can conceive of the early universe as a pile of overlapping grapefruits that stretches infinitely in all directions. Correspondingly, the idea that the big bang was "small" is misleading. The totality of space could be infinite. Shrink an infinite space by an arbitrary amount, and it is still infinite.

Mathochist
06-13-2005, 09:51 PM
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.

Hold on there, chief. There are observable (well, observed) objects with redshifts corresponding to superluminal recessions. The picture explanation is in the print version of the SciAm article CurtC just linked to, but the basic idea is that the light ray is stretched as well, though the galaxy isn't. Basically, the galaxy was close enough when the light was emitted, but the stretching of the universe puts to too far away by now.

Exapno Mapcase
06-13-2005, 10:26 PM
That's not exactly how I read the article, but let's go to the source:
The idea of seeing faster-than-light galaxies may sound mystical, but it is made possible by changes in the expansion rate. Imagine a light beam that is farther than the Hubble distance of 14 billion light-years and trying to travel in our direction. It is moving toward us at the speed of light with respect to its local space, but its local space is receding from us faster than the speed of light. Although the light beam is traveling toward us at the maximum speed possible, it cannot keep up with the stretching of space. It is a bit like a child trying to run the wrong way on a moving sidewalk. Photons at the Hubble distance are like the Red Queen and Alice, running as fast as they can just to stay in the same place.

Still, there is a limit to how far away something can be and emit light that can still be seen:
An accelerating universe, then, resembles a black hole in that it has an event horizon, an edge beyond which we cannot see. The current distance to our cosmic event horizon is 16 billion light-years, well within our observable range. Light emitted from galaxies that are now beyond the event horizon will never be able to reach us; the distance that currently corresponds to 16 billion light-years will expand too quickly. We will still be able to see events that took place in those galaxies before they crossed the horizon, but subsequent events will be forever beyond our view.

Chronos
06-13-2005, 10:38 PM
The units for speed/distance would be 1/s, right? Correct, and the reciprocal of that is the Hubble time, which is a first approximation for the age of the Universe (for a better approximation, you need to know what sort of stuff the Universe is made of). It's more commonly expressed in kilometers/second/megaparsec, though. Currently, the value is around 60 km/s/mpc, which is called the Hubble constant (a misnomer, since in most cosmological models, it's not constant).

Marley23
06-13-2005, 10:47 PM
Look at it this way: the universe is expanding in multiple 'directions' (if I can accurately use that word), so its size would be increasing at a rate greater than the speed of light.

Mathochist
06-14-2005, 01:55 PM
Look at it this way: the universe is expanding in multiple 'directions' (if I can accurately use that word), so its size would be increasing at a rate greater than the speed of light.

Read the OP. He took care of that thought....

Mathochist
06-14-2005, 01:58 PM
Still, there is a limit to how far away something can be and emit light that can still be seen:

Oh, yes. I didn't mean to say that we can see everything, but within the range of what we can see are objects with redshifts corresponding to superluminal recessions.