Does the expanding Universe have a knowable center?

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Discussion Overview

The discussion revolves around the concept of whether the expanding universe has a knowable center. Participants explore theoretical implications, observational perspectives, and the nature of cosmic expansion, with references to general relativity and the behavior of light in an expanding universe.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants propose that since the speed of light is constant, observers from any location would perceive the universe as expanding uniformly, suggesting that a center is unknowable.
  • Others argue that the universe likely does not have a center, as not everything necessarily possesses a central point.
  • It is suggested that defining a center may be impossible because the concept of a "center" may not apply to the universe.
  • A participant points out that while the speed of light is constant, the expansion of the universe can have different properties, indicating that the current understanding is not the only possible solution to the Einstein Field Equations.
  • Another participant notes that the concept of a "center of the universe" is meaningless rather than unknowable.
  • A different perspective is introduced, referencing Sydney Coleman's idea of "egocentric coordinate systems," where each observer could consider themselves the center of the universe.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the existence and definition of a center in the universe. There is no consensus on whether a center is knowable or meaningful.

Contextual Notes

Some claims depend on the definitions of "center" and "observer," and the discussion acknowledges the complexity of cosmic expansion and the nature of observational perspectives.

gymjim
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Since the speed of light is a constant, no matter what location from which a search of the universe is made, matter would appear to be accelerating in the expansion in the same way to the observer. It is understood that about 14 billion years ago the expansion started. From any vantage point, the universe would appear to have started the 14 billion years ago, and the size of the universe would appear to be the same. If the universe is appears expanding in the same way from any point in the universe, the center of the universe than is unknowable.
 
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It is unknowable and most likely doesn't exist. Not everything has a center and the Universe probably doesn't.
 
probably you cannot define a center, because there was never such a thing as a "center"...
 
gymjim said:
Since the speed of light is a constant, no matter what location from which a search of the universe is made, matter would appear to be accelerating in the expansion in the same way to the observer.

The latter part is true, but it's not because the speed of light is constant; it's because the universe happens to be expanding in that particular way. It's perfectly possible to come up with other valid solutions to the Einstein Field Equations with different expansion properties, and the speed of light will still be constant in all those solutions.

gymjim said:
It is understood that about 14 billion years ago the expansion started. From any vantage point, the universe would appear to have started the 14 billion years ago, and the size of the universe would appear to be the same.

True. (More precisely, it's true for "comoving" observers, observers who see the universe as homogeneous and isotropic. We here on Earth are not exactly "comoving" observers, but we're close enough to it for most purposes. But, for example, someone flying by Earth at nearly the speed of light would see the universe as highly non-isotropic, and would *not* assign the same age or size to the universe as we do.)

gymjim said:
If the universe is appears expanding in the same way from any point in the universe, the center of the universe than is unknowable.

No; the "center of the universe" is a meaningless concept, not unknowable.
 
We have a FAQ on this: https://www.physicsforums.com/showthread.php?t=506991
 
Last edited by a moderator:
There is a center of the universe per Sydney Coleman's "egocentric coordinate systems". Each sufficiently egocentric researcher defines themselves as the center of the universe.
 

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