Is the universe expanding faster than the speed of light?

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Discussion Overview

The discussion revolves around the concept of the universe's expansion, particularly whether it can exceed the speed of light, especially in the context of the Big Bang. Participants explore the implications of general relativity and special relativity, and how these theories relate to the expansion of space versus the movement of mass.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants note that while mass cannot exceed the speed of light in space, the expansion of space itself can occur at speeds greater than c.
  • One participant highlights that general relativity allows for local application of special relativity, but globally, in a dynamic spacetime like the universe, the rules differ.
  • Another participant questions the distinction between "space itself" and "through space," seeking clarification on how space can expand faster than light.
  • A metaphor involving sound is used to illustrate the concept of light and the perception of speed in relation to expanding space.
  • Some participants suggest that if the spatial component of the speed of light is not constant over time, it could explain the observed faster-than-light expansion of the universe.
  • There is a mention of the Alcubierre Warp Drive as a theoretical framework related to the expansion of space, with a note that its original interpretation may be flawed according to some critiques.
  • One participant expresses difficulty in understanding the mathematical aspects of the Alcubierre drive and related concepts.

Areas of Agreement / Disagreement

Participants express various viewpoints on the relationship between the expansion of the universe and the speed of light, with no consensus reached. The discussion includes competing interpretations and clarifications regarding the implications of relativity.

Contextual Notes

There are unresolved questions about the nature of space and its expansion, as well as the assumptions regarding the constancy of the speed of light over time. The discussion also touches on complex mathematical theories that some participants find challenging to understand.

DrWarezz
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Hi all,
It's said that approximately 3 seconds after/into the Big Bang, the 'universe' expanded at a velocity > c.
However, this breaks the laws of SR, right? "No mass can equal/breach c".

So, could someone enlighten me on this subject?
Why are BOTH "No mass can equal/reach c" and "the universe expanded at > c" (for those that believe in the BB) accepted? They contradict each other, right?

Please enlighten me :)

Thanks in advance,
[r.D]
 
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It is a postulate of general relativity that in any spacetime the laws of special relativity apply locally. This means that the speed of light in vacuum cannot be exceeded by any object in a local experiment. Globally, the laws of special relativity apply only to a flat and static spacetime (Minkowski spacetime). The universe is assumed to be a Robertson-Walker spacetime (dynamic with spatial expansion) and thus the laws of special relativity apply locally but not globally. The fact that galaxies far away from us may appear to recede at speeds greater than c (which is also true today and not only during the first three minutes) is a consequence of Hubbles law (v = H d) which can be derived asuming a Robertson-Walker spacetime. This speed is merely due to expansion of space (galaxies are comoving with the expansion), although this galaxies may zero proper velocity "on" space. The proper velocity of galaxies "on" space is indeed limited by the laws of special relativity.
 
As hellfire said, a mass cannot move through space at a speed greater than c. But that law does not apply to space itself.
 
I do not understand "space itself" vs "through space". I posted a question related to this in the "General Astronomy" section. What is meant by "space itself [can expand faster than c]"?
 
Picture this. If a car passes you faster than the speed of sound, will you hear it? Of course you will. The sound may not appear to be where you thought it was, but, you will still hear it go by you. Why would light appear to behave any different? The light has no idea how fast the source was traveling when it departed from the emitter. Do you think the sound will 'wink' out of existence once it passes you?
 
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Gonzolo said:
I do not understand "space itself" vs "through space". I posted a question related to this in the "General Astronomy" section. What is meant by "space itself [can expand faster than c]"?

In Relativity, space is a physical part of the universe...not a void. Space is a thing that can be stretched, bent, compressed, etc.

Picture the universe as matter & energy on a backdrop of space & time. "Spacetime" is sometimes called the fabric of the universe (although that can be a misleading name) - - it's what the universe is made of.
 
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Phobos said:
In Relativity, space is a physical part of the universe...not a void. Space is a thing that can be stretched, bent, compressed, etc.

Picture the universe as matter & energy on a backdrop of space & time. "Spacetime" is sometimes called the fabric of the universe (although that can be a misleading name) - - it's what the universe is made of.

I follow, but could you check out this thread I started last week or so :
https://www.physicsforums.com/showthread.php?t=45605
 
DrWarezz said:
It's said that approximately 3 seconds after/into the Big Bang, the 'universe' expanded at a velocity > c.
However, this breaks the laws of SR, right? "No mass can equal/breach c".

So, could someone enlighten me on this subject?
Why are BOTH "No mass can equal/reach c" and "the universe expanded at > c" (for those that believe in the BB) accepted? They contradict each other, right?
If it were assumed that the spatial component of c is not constant over time, then this would explain why the uiverse expanded at a rate greater than c (as it is here, now, in our current part of the universe), and it could explain why the fringes of the universe are expanding at a rate greater than c (as it is here, now, in our current part of the universe).

If it is assumed that the spatial component of c has never changed, as is common and as you have seen on this forum, it is also possible to make things work by complex mathematical games and complex theories.
 
Phobos said:
As hellfire said, a mass cannot move through space at a speed greater than c. But that law does not apply to space itself.

Is that like how the Alcubierre (sp?) Warp Drive functions (in theory)?
 
  • #10
Alcubiere and Space Expansion

singleton said:
Is that like how the Alcubierre (sp?) Warp Drive functions (in theory)?

Closely related. But David Waite has pointed out that the naive view of the Alcubierre drive (which was Alcubierre's own original view) as expanding space behind the ship and shrinking it ahead, is wrong, and he has a counterexample in which the FTL effect is produced by a rotation on local spacetime.

See this chapter of his online textbook on GR.
 
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  • #11
selfAdjoint said:
Closely related. But David Waite has pointed out that the naive view of the Alcubierre drive (which was Alcubierre's own original view) as expanding space behind the ship and shrinking it ahead, is wrong, and he has a counterexample in which the FTL effect is produced by a rotation on local spacetime.

See this chapter of his online textbook on GR.

Yikes, scrolling a few pages I have trouble understanding this math, some of the symbols and constants :cry:
 

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