4 ms·

The thing I don't get with that analogy is the ballon IS expanding into something - eg if you put the balloon in a cup, it won't expand (unless you have REALLY

The thing I don't get with that analogy is the ballon IS expanding into something - eg if you put the balloon in a cup, it won't expand (unless you have REALLY strong lungs :-) ). Also, curious where the extra energy comes from to keep the 'vacuum energy' (zero point energy?) from going down - IIRC, its remained constant even though the universe is expanding

consilient 4y ago
> Also, curious where the extra energy comes from to keep the 'vacuum energy' (zero point energy?) from going down - IIRC, its remained constant even though the universe is expanding Energy is ill-defined in a global sense: it's the conserved quantity associated with time-translation symmetry, but there's no single choice of "time translation" that can be extended to all of spacetime.
- mech422 4y ago
  Hmm...This goes beyond what PBS space-time taught me about it :-) My understanding was we have more space (eg space is expanding) but the 'vacuum energy' is staying constant.. If I understand what you're saying, that statement depends on the time frame you measure against ? edit: s/your/you're/
  - consilient 4y ago
    No, it's just not meaningful. The vacuum energy density is (likely) constant. The total energy of the universe is simply not a thing.
    - mech422 4y ago
      Hmm..ok - thank you. I'm gonna have to work on that more, atm I'm having a hard time comprehending how volume can increase without density decreasing. To google! Thanks!
      - consilient 4y ago
        Google won't help you here. Popsci stuff is useless at best, and more often actively misleading. You need a GR textbook. An Introduction to General Relativity, Spacetime, and Geometry (https://fma.if.usp.br/~mlima/teaching/PGF5292_2021/Carroll_SG.pdf https://fma.if.usp.br/~mlima/teaching/PGF5292_2021/Carroll_S...) is relatively accessible.
        mech422 4y ago
        oh! Thanks!
tzs 4y ago
This article might help: "Energy Is Not Conserved" by Sean Carroll [1]. Here are a couple of paragraphs to give you the gist of it. > But many people have just this reaction. It’s clear that cosmologists have not done a very good job of spreading the word about something that’s been well-understood since at least the 1920’s: energy is not conserved in general relativity. (With caveats to be explained below.) > The point is pretty simple: back when you thought energy was conserved, there was a reason why you thought that, namely time-translation invariance. A fancy way of saying “the background on which particles and forces evolve, as well as the dynamical rules governing their motions, are fixed, not changing with time.” But in general relativity that’s simply no longer true. Einstein tells us that space and time are dynamical, and in particular that they can evolve with time. When the space through which particles move is changing, the total energy of those particles is not conserved. [1] https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/ https://www.preposterousuniverse.com/blog/2010/02/22/energy-...
- mech422 4y ago
  oh! weirder and weirder! Thanks for the link! edit: that was a good read! Thanks again

consilient 4y ago

> Also, curious where the extra energy comes from to keep the 'vacuum energy' (zero point energy?) from going down - IIRC, its remained constant even though the universe is expanding Energy is ill-defined in a global sense: it's the conserved quantity associated with time-translation symmetry, but there's no single choice of "time translation" that can be extended to all of spacetime.

mech422 4y ago
Hmm...This goes beyond what PBS space-time taught me about it :-) My understanding was we have more space (eg space is expanding) but the 'vacuum energy' is staying constant.. If I understand what you're saying, that statement depends on the time frame you measure against ? edit: s/your/you're/
- consilient 4y ago
  No, it's just not meaningful. The vacuum energy density is (likely) constant. The total energy of the universe is simply not a thing.
  - mech422 4y ago
    Hmm..ok - thank you. I'm gonna have to work on that more, atm I'm having a hard time comprehending how volume can increase without density decreasing. To google! Thanks!
    - consilient 4y ago
      Google won't help you here. Popsci stuff is useless at best, and more often actively misleading. You need a GR textbook. An Introduction to General Relativity, Spacetime, and Geometry (https://fma.if.usp.br/~mlima/teaching/PGF5292_2021/Carroll_SG.pdf https://fma.if.usp.br/~mlima/teaching/PGF5292_2021/Carroll_S...) is relatively accessible.
      - mech422 4y ago
        oh! Thanks!

tzs 4y ago

This article might help: "Energy Is Not Conserved" by Sean Carroll [1]. Here are a couple of paragraphs to give you the gist of it. > But many people have just this reaction. It’s clear that cosmologists have not done a very good job of spreading the word about something that’s been well-understood since at least the 1920’s: energy is not conserved in general relativity. (With caveats to be explained below.) > The point is pretty simple: back when you thought energy was conserved, there was a reason why you thought that, namely time-translation invariance. A fancy way of saying “the background on which particles and forces evolve, as well as the dynamical rules governing their motions, are fixed, not changing with time.” But in general relativity that’s simply no longer true. Einstein tells us that space and time are dynamical, and in particular that they can evolve with time. When the space through which particles move is changing, the total energy of those particles is not conserved. [1] https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/ https://www.preposterousuniverse.com/blog/2010/02/22/energy-...

mech422 4y ago
oh! weirder and weirder! Thanks for the link! edit: that was a good read! Thanks again