Why do domains expand in a way that makes each point equivalent?

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Discussion Overview

The discussion revolves around the concept of domain expansion in cosmology, particularly in relation to inflationary theory and the nature of space. Participants explore the implications of this expansion on the equivalence of points within domains and the structure of the universe.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • Some participants reference a visual representation of cosmic scales, noting inaccuracies regarding the size of the observable universe and the implications of a finite universe.
  • There is a discussion about the concept of domain walls in inflationary theory, with some arguing that these walls do not have a center.
  • One participant suggests that the structure of domains can be likened to bubbles in foam, raising questions about how to define the center of such irregular shapes.
  • Another participant claims that in domains, time flows differently, with time near the walls being close to zero and increasing towards the center, which they argue corresponds to time infinity.
  • There is a debate on whether the expansion of domains implies that each point within them is equivalent, with some asserting that there is no preferred point or distance to the domain wall.
  • One participant challenges the idea that expansion leads to equivalence, using the analogy of light cones to illustrate that while they expand, they still have a center relative to inertial frames.
  • Another participant posits that the domain wall represents the Big Bang, suggesting that the initial state of the universe had a finite size.
  • Participants express uncertainty about the definitions and implications of their claims, particularly regarding the nature of time and space in different cosmological models.

Areas of Agreement / Disagreement

The discussion contains multiple competing views regarding the nature of domain walls, the implications of expansion, and the concept of time within domains. There is no consensus on these topics.

Contextual Notes

Participants acknowledge limitations in their understanding and the complexity of defining concepts such as "center" and "equivalence" in the context of expanding domains. Some statements rely on specific interpretations of cosmological models that may not be universally accepted.

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FawkesCa said:
I didn't make this, but I think it's one of the coolest things ever.

http://www.newgrounds.com/portal/view/525347

enjoy
Cool, a nice variation on the old Powers of Ten movie (itself an adaptation of the neat 1957 book Cosmic View which can be viewed here). I did notice one inaccuracy, though--due to the expansion of space, the size of the observable universe (i.e. the maximum distance some object, at rest relative to the microwave background, could be from us today and for it still be possible to us to today be receiving light from that object when it was younger) is much larger than 14 billion light years--see wikipedia's Observable universe article.

edit: Also, no idea where they got the claim that the size of the whole universe is 93 billion light years! If space is flat it could be infinite. And saying "we're probably not at the center of the universe" in that final image would seem to be meaningless since even a finite universe isn't expected to have any outer boundaries to define a center, unless they're talking about something like the idea of "domain walls" in inflationary theory (beyond which space would continue, but certain basic fields would be in different ground states and thus the high-level laws of physics would appear different)
 
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unless they're talking about something like the idea of "domain walls" in inflationary theory
Domain walls also do not have a center.
 
JesseM said:
And saying "we're probably not at the center of the universe" in that final image would seem to be meaningless since even a finite universe isn't expected to have any outer boundaries to define a center...)

i don't believ that's what they meant. they were pointimg toward the center of the screen and SOME dumbass might believe they were saying "WE ARE THE CENTER OF THE UNIVERSE! WORSHIP ME!" :rolleyes: just as some still believe the Earth is only 6500 years old... (i got into a fist fight on my construction site over this.)
 
haael said:
Domain walls also do not have a center.
Diagrams like the one on http://www.damtp.cam.ac.uk/research/gr/public/cs_top.html seem to suggest the structure of the different domains is something like bubbles in foam, and each bubble has a center:

cs_dw_kibble.gif


Why do you say they wouldn't have centers? Relative to whatever cosmological coordinate system we find most natural, can't we find the distance between any point inside the domain and any point on the domain wall? There might be different ways to define the "center" of an irregular shape, but we could pick some simple definition, like the point inside the domain that has the largest distance to the closest point on the domain wall...
 
Diagrams like the one on this page seem to suggest the structure of the different domains is something like bubbles in foam, and each bubble has a center:
It's not that easy...

There might be different ways to define the "center" of an irregular shape, but we could pick some simple definition, like the point inside the domain that has the largest distance to the closest point on the domain wall...
Your picture looks like there was some universal time, common for all domains. This is not correct. In domains time flows "inside", that means near the wall the time is near 0 and in the center it is a big number. Actually, the point in the center of a domain corresponds to time infinity.

Domain wall is not something static, it rather moves at the speed of light for any observer.

Also, each domain does expand, so each point of it is equivalent.

can't we find the distance between any point inside the domain and any point on the domain wall?
Maybe it is possible, but I think the line connecting two points from different domains would have to go through singular points, so it would be hard to define its length.

Lol, I read it so long ago, so I don't remember where I saw all of this.
 
haael said:
Your picture looks like there was some universal time, common for all domains. This is not correct. In domains time flows "inside", that means near the wall the time is near 0 and in the center it is a big number. Actually, the point in the center of a domain corresponds to time infinity.
Surely the rate that time "flows" can only be defined relative to some coordinate system? If so, what coordinate system are you using? Can you provide a link or other source that backs up what you're saying here?
haael said:
Domain wall is not something static, it rather moves at the speed of light for any observer.

Also, each domain does expand, so each point of it is equivalent.
Why does expansion imply "each point of it is equivalent"? As an analogy, successive spacelike cross-sections of a future light cone give a light sphere which is also expanding at the speed of light, but relative to any given inertial frame the light sphere does have a center (although there is no frame-independent definition of the center)
 
Why does expansion imply "each point of it is equivalent"?
Domains expand in such a way, so each point is equivalent. There's no preferred point. There's no "closer" and "further" from any particular point to a domain wall. In fact, from the inside of domain point of view, the domain wall is in your past. You would have to go back in time to touch it. Digging deeper - the domain wall is the Big Bang itself, except in this theory the initial state of Universe has finite nonzero size.

As an analogy, successive spacelike cross-sections of a future light cone give a light sphere which is also expanding at the speed of light, but relative to any given inertial frame the light sphere does have a center (although there is no frame-independent definition of the center)
You assume that you are in special relativity flat space. Domains expand in different way, more like cosmological inflation.
Our Universe was once compressed in one point, but we don't have any center today, have we?

Can you provide a link or other source that backs up what you're saying here?
Sorry, I can't. It was too long ago. Perhaps it's time for me to shut up.
 

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