What does expand: Space or Spacetime?

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Discussion Overview

The discussion revolves around the nature of the universe's expansion, specifically whether it refers to the expansion of space, spacetime, or both. Participants explore implications related to inflation, dark energy, and the conceptual understanding of expansion in cosmology.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation
  • Mathematical reasoning

Main Points Raised

  • Some participants assert that the expansion refers solely to space, with distances between far-away objects increasing over time, while the time coordinate does not expand.
  • Others argue that describing expansion as merely spatial can be misleading, suggesting that it is more accurate to say that objects are getting farther apart rather than space itself expanding.
  • A participant mentions a German astrophysicist's view that both space and spacetime expanded during the Big Bang, leading to confusion about the nature of expansion as a temporal process.
  • There is a discussion about dark energy, with some suggesting it is responsible for inflation and the universe's current accelerated expansion, while others clarify that inflation pertains to the early universe's expansion and that dark energy relates to present-day acceleration.
  • Some participants propose that the density of dark energy remains constant, leading to more dark energy being present as space expands, while others challenge this notion, emphasizing the distinction between the amount of dark energy and its density.
  • There are differing views on whether the tendency to expand can be considered an intrinsic property of space, with some asserting it is not known for certain.
  • Participants discuss the implications of a cosmological constant as a simple explanation for expansion, while also noting that the nature of dark energy remains a topic of active research and uncertainty.

Areas of Agreement / Disagreement

Participants express multiple competing views regarding the nature of expansion, the role of dark energy, and the relationship between space and spacetime. The discussion remains unresolved, with no consensus reached on these complex topics.

Contextual Notes

Limitations include varying interpretations of expansion, the distinction between inflation and current acceleration, and the complexities surrounding dark energy and its properties. Participants acknowledge the uncertainty in measurements and the ongoing research in cosmology.

  • #31
Chalnoth said:
We do know that a massive particle on the order of tens to hundreds of times the mass of a proton that interacts with the weak force could fit the currently-available evidence.
Hi @Chalnoth:

Thanks for your post.

I would like to understand better these ideas about DM. Can you recommend a reference?

Regards,
Buzz
 
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  • #32
Buzz Bloom said:
I would like to understand better these ideas about DM. Can you recommend a reference?

Try 3.3.2 "Dark Matter Relics" from Baumann's excellent lecture notes

http://www.damtp.cam.ac.uk/user/db275/Cosmology/Lectures.pdf
 
  • #33
Hi George:

Thanks very much for the link. The lectures look quite interesting, and I expect they will entertain/educate me for quite a while.

Regards,
Buzz
 
  • #34
recall however that the distinction between space and time is coordinate-dependent.

that is, you can always pick a time coordinate that does not ``expand'' (with respect to your `proper time'), or you can use conformal coordinates where both space and time components of the metric ``expand'' by the same factor.
 
  • #35
I am confused by this whole thread. I have only a superficial knowledge, so probably nothing fundamental.
1. how do you know things are moving away from each other when there is no constant scale? (Any physical scale itself would expand, would it not?)
2. If it is red-shift, how do you know that photons don't gradually expand in wavelength (like a slow decay from blue to red)?
3. What makes time so great that space always has to change (expand) but time does not have to? Is this related to the fact that we can traverse space at will in any direction but time only in forward direction? (By we I mean matter with a positive rest mass.)
 
  • #36
N123 said:
I am confused by this whole thread. I have only a superficial knowledge, so probably nothing fundamental.
1. how do you know things are moving away from each other when there is no constant scale? (Any physical scale itself would expand, would it not?)
2. If it is red-shift, how do you know that photons don't gradually expand in wavelength (like a slow decay from blue to red)?
3. What makes time so great that space always has to change (expand) but time does not have to? Is this related to the fact that we can traverse space at will in any direction but time only in forward direction? (By we I mean matter with a positive rest mass.)
1. The scale is irrelevant. WHATEVER the scale you use, things are moving apart and things farther away are moving apart faster than things closer together.
2. The "tired light" theory was debunked ages ago.
3. You are trying to apply a characteristic to time that does not apply. It's like asking "why isn't the number 3 green?"
 
  • #37
phinds said:
2. The "tired light" theory was debunked ages ago.
3. You are trying to apply a characteristic to time that does not apply. It's like asking "why isn't the number 3 green?"

2. Thank you for pointing me to the "tired light" theories.
3. Which characteristic?
 
  • #38
N123 said:
3. Which characteristic?
Expansion in a physical dimension.
 

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