Theorists predicted the big bang to expand faster than the speed of light

[AstrophysicsX]

I've read that theorists predicted the big bang to expand faster than the speed of light. Is this true? If so, how can we say that nothing can travel faster than light today?

 

[pallidin]

Could you give some reference to that?

 

[BruceW]

Space itself was expanding faster than the speed of light. This doesn't disobey relativity. For example, if you had two objects in the early universe with no relative motion, then the expansion of the space between them would mean they end up very far away from each other.

 

[rock.freak667]

I've read that theorists predicted the big bang to expand faster than the speed of light. Is this true? If so, how can we say that nothing can travel faster than light today?

I believe they can say this since the space-time fabric has no mass and thus can expand faster than c.

Once you have any mass, you can't travel at or more than c.

 

[AstrophysicsX]

So, is space still expanding at this speed? Or has it slowed down a bit?

 

[Drakkith]

So, is space still expanding at this speed? Or has it slowed down a bit?

The distance between two points in the universe can be increasing greater than c, yes. However the distance required for this is enormous. I can't remember the exact distance, but I think it was bigger than the observable universe.

Compared to the inflationary period at the beginning of the universe, space is expanding MUCH MUCH MUCH slower than it was. The effect isn't even noticeable until you get into the multi-million lightyear distances I believe.

 

[bapowell]

Even in non-inflationary spacetimes, points separated by a distance equal to the Hubble radius will have relative velocities surpassing the speed of light. Consider Hubble's Law:

$$v = Hr$$

When the separation is $r = c/H$, the relative velocity satisfies $v=c$. This is true regardless of the time dependence of $H$ -- it holds in both inflationary and non-inflationary spacetimes.

During inflation, the scale factor (the function that governs the expansion of the universe) grows in time as $a \sim e^{Ht}$, giving an accelerated expansion ($\ddot{a}>0$). During non-inflationary expansion, the universe grows only at a rate $a \sim t^{1/2}$ for radiation-dominated expansion, and $a \sim t^{2/3}$ for matter-dominated expansion -- both of which are non-accelerating.

 

[gregorygregg1]

The distance between two points in the universe can be increasing greater than c, yes. However the distance required for this is enormous. I can't remember the exact distance, but I think it was bigger than the observable universe.

Compared to the inflationary period at the beginning of the universe, space is expanding MUCH MUCH MUCH slower than it was. The effect isn't even noticeable until you get into the multi-million lightyear distances I believe.

Could the universe appear to be expanding because dimension is stretching in two directions at once? In other words, if dimension were rushing toward two singularities, might an observer between the two percieve an expanding universe?

 

[Chronos]

No. Two singularities would establish a plane. This does not agree with observation.

 

[gregorygregg1]

If you envision the singularities as "holes" in the structure of space, Wouldn't the substanceless dimension of space rushing toward those holes appear to be moving toward an observer positioned near one singularity, own galaxy, while at the same time, at a distance, moving away toward the distant singularity. I am not implying a plane, but space formed of three dimensions. Like water going down two drains at the same time. If you are between them, It appears to be expanding.

 

[Drakkith]

If you envision the singularities as "holes" in the structure of space, Wouldn't the substanceless dimension of space rushing toward those holes appear to be moving toward an observer positioned near one singularity, own galaxy, while at the same time, at a distance, moving away toward the distant singularity. I am not implying a plane, but space formed of three dimensions. Like water going down two drains at the same time. If you are between them, It appears to be expanding.

IF space is "something" and IF it was being "sucked" into these holes, maybe. But we also are on the other side of many black holes and space that is further away. These "flows" would alternate in directions, from toward us to against us and so forth. The cumulative effect of all these holes would be to make the universe appear to NOT be expanding on a universal distance. Current evidence supports the inflation and expansion of the universe however.

 

[Chronos]

Simple geometry at work here. 2 points establish a line, 3 points can only establsh a plane, and a 4th point, assuming it is not in the same plane as the other three points, will establish a 4 sided pyramid.

 

[bcrowell]

FAQ: What does general relativity say about the relative velocities of objects that are far away from one another?

Nothing. General relativity doesn't provide a uniquely defined way of measuring the velocity of objects that are far away from one another. For example, there is no well defined value for the velocity of one galaxy relative to another at cosmological distances. You can say it's some big number, but it's equally valid to say that they're both at rest, and the space between them is expanding. Neither verbal description is preferred over the other in GR. Only local velocities are uniquely defined in GR, not global ones.

Confusion on this point is at the root of many other problems in understanding GR:

Question: How can distant galaxies be moving away from us at more than the speed of light?

Answer: They don't have any well-defined velocity relative to us. The relativistic speed limit of c is a local one, not a global one, precisely because velocity isn't globally well defined.

Question: Does the edge of the observable universe occur at the place where the Hubble velocity relative to us equals c, so that the redshift approaches infinity?

Answer: No, because that velocity isn't uniquely defined. For one fairly popular definition of the velocity (based on distances measured by rulers at rest with respect to the Hubble flow), we can actually observe galaxies that are moving away from us at >c, and that always have been moving away from us at >c.[Davis 2004]

Question: A distant galaxy is moving away from us at 99% of the speed of light. That means it has a huge amount of kinetic energy, which is equivalent to a huge amount of mass. Does that mean that its gravitational attraction to our own galaxy is greatly enhanced?

Answer: No, because we could equally well describe it as being at rest relative to us. In addition, general relativity doesn't describe gravity as a force, it describes it as curvature of spacetime.

Question: How do I apply a Lorentz transformation in general relativity?

Answer: General relativity doesn't have global Lorentz transformations, and one way to see that it can't have them is that such a transformation would involve the relative velocities of distant objects. Such velocities are not uniquely defined.

Question: How much of a cosmological redshift is kinematic, and how much is gravitational?

Answer: The amount of kinematic redshift depends on the distant galaxy's velocity relative to us. That velocity isn't uniquely well defined, so you can say that the redshift is 100% kinematic, 100% gravitational, or anything in between.

Let's take a closer look at the final point, about kinematic versus gravitational redshifts. Suppose that a photon is observed after having traveled to Earth from a distant galaxy G, and is found to be red-shifted. Alice, who likes expansion, will explain this by saying that while the photon was in flight, the space it occupied expanded, lengthening its wavelength. Betty, who dislikes expansion, wants to interpret it as a kinematic red shift, arising from the motion of galaxy G relative to the Milky Way Malaxy, M. If Alice and Betty's disagreement is to be decided as a matter of absolute truth, then we need some objective method for resolving an observed redshift into two terms, one kinematic and one gravitational. But this is only possible for a stationary spacetime, and cosmological spacetimes are not stationary. As an extreme example, suppose that Betty, in galaxy M, receives a photon without realizing that she lives in a closed universe, and the photon has made a circuit of the cosmos, having been emitted from her own galaxy in the distant past. If she insists on interpreting this as a kinematic red shift, the she must conclude that her galaxy M is moving at some extremely high velocity relative to itself. This is in fact not an impossible interpretation, if we say that M's high velocity is relative to itself *in the past.* An observer who sets up a frame of reference with its origin fixed at galaxy G will happily confirm that M has been accelerating over the eons. What this demonstrates is that we can split up a cosmological red shift into kinematic and gravitational parts in any way we like, depending on our choice of coordinate system.

Davis and Lineweaver, Publications of the Astronomical Society of Australia, 21 (2004) 97, msowww.anu.edu.au/~charley/papers/DavisLineweaver04.pdf