What does expand: Space or Spacetime?

[Buzz Bloom]

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

 

[George Jones]

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

 

[Buzz Bloom]

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

 

[w4k4b4lool4]

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.

 

[N123]

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.)

 

[phinds]

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?"

 

[N123]

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?

 

[phinds]

3. Which characteristic?

Expansion in a physical dimension.