Basic confusion about the Big Bang and light cones

[Joseph Flatt]

TL;DR

Was everything in the same light cone at some point, and if so, how did that cease to be the case?

I’m getting confused somewhere, and I’d be obliged if someone could pinpoint my error.

1. At or near the Big Bang, everything was so close as to be within each other’s light cones.
2. All parts of the cosmos are now outside of some other parts’ light cones.
Therefore,
3. Something traveled faster than light.

What have I missed?

 

[PeroK]

Summary:: Was everything in the same light cone at some point, and if so, how did that cease to be the case?

That makes no sense. A light cone is a set of events (points in spacetime). With an emphasis on "time" in spacetime.

 

[Joseph Flatt]

Sorry, I’ll try again.
I suppose another way of putting it is that in the distant past everything was condensed into such a small area that every event was causally relevant to every other event (at least that’s what I assume, perhaps erroneously). This is no longer the case because every event has an absolute elsewhere. How did the latter state of affairs come from the former if nothing travels faster than light?

I hope that makes a bit more sense.

 

[PeroK]

I suppose another way of putting it is that in the distant past everything was condensed into such a small area that every event was causally relevant to every other event (at least that’s what I assume, perhaps erroneously).

This is not correct. The universe was much denser and hotter, but not fundamentally different in terms of causality.

This is no longer the case because every event has an absolute elsewhere.

As every event always has.

How did the latter state of affairs come from the former if nothing travels faster than light?

The universe expanded.

 

[Bandersnatch]

in the distant past everything was condensed into such a small area that every event was causally relevant to every other event (at least that’s what I assume, perhaps erroneously).

Yes, this is a misconception. As you roll back the time, all casual patches shrink to zero. I.e. all light cones grow with time. At the limit of the singularity every point is causally disconnected.
Since you're familiar with light cones, take a look at the graphs below:

All three represent the same, best-fit model of the universe (LCDM), only drawn in different coordinates. The coordinates of the one at the bottom recover the basic shape of the light cone, so that what was said above can be readily seen. Here, each point on the x-axis can be seen as a galaxy moving with the expansion, but locally stationary, and the vertical lines are their world lines. Whichever event you choose to draw a light cone from, the earlier one on the same world line will have a smaller extent.

 

[Bandersnatch]

but not fundamentally different in terms of causality.

I don't know why you'd say that. The causal patches obviously vary with time.

 

[Joseph Flatt]

Thank you both for your replies. I think I now have a clearer idea of where my thinking was flawed.

 

[PeroK]

I don't know why you'd say that. The causal patches obviously vary with time.

In what way were light cones fundamentally different in the past?

 

[Bandersnatch]

In what way were light cones fundamentally different in the past?

In the bit you responded to the OP suggested the light cones change their extent with passing time in a particular way. Which does happen, only the change is in the opposite direction to what was imagined.
If by 'not fundamentally different in terms of causality' you meant to convey that light cones always do what light cones do, i.e. the extent of their base shrinks with time, then there's a full agreement. In which case the objection is purely pedagogical.
I.e. in the context of the question your response suggests that the causal patches somehow don't change their extent from today's.

 

[PeroK]

I.e. in the context of the question your response suggests that the causal patches somehow don't change their extent from today's.

Even though I said?

A light cone is a set of events (points in spacetime). With an emphasis on "time" in spacetime.

 

[Bandersnatch]

Lack of clarity is in the eye of the beholder. Let's leave it at that.

 

[PeterDonis]

1. At or near the Big Bang, everything was so close as to be within each other’s light cones.

This is wrong.

2. All parts of the cosmos are now outside of some other parts’ light cones.

This will be true for any pair of points in space whatever if you make your time frame short enough. If your time frame is 1 nanosecond, anything further than a foot away from you is outside your past light cone.

Therefore,
3. Something traveled faster than light.

Invalid because the separation of objects caused by the expansion of the universe has nothing to do with anything traveling faster than light. "Faster than light" would mean that some object with nonzero rest mass outruns a light ray going in the same direction. And that never happens.