Conservation of energy in cosmology

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Discussion Overview

The discussion revolves around the conservation of energy in cosmology, particularly in the context of models like Vilenkin's tunnelling from nothing. Participants explore the implications of energy conservation laws in relation to gravity and matter, questioning the necessity of balancing energies when energy conservation may not apply in cosmological settings.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • Some participants note that Vilenkin's model claims the universe originated from "nothing" and suggests that the negative energy of gravity can cancel the positive energy of matter, raising questions about the relevance of energy conservation in this context.
  • Others reference Sean Carroll's assertion that energy is not conserved in cosmology, which complicates the discussion about the necessity of balancing energies in Vilenkin's model.
  • It is mentioned that while Carroll states that the energy of matter alone is not conserved, Vilenkin's approach considers the total energy of matter and gravity together, which is claimed to be conserved.
  • Participants highlight that the energy of gravity is not uniquely defined in a covariant way, leading to debates about the validity of definitions that allow for total energy conservation.
  • Some express that only the energy of matter is well-defined and not conserved, according to certain purists in the field.

Areas of Agreement / Disagreement

Participants do not reach a consensus; there are multiple competing views regarding the conservation of energy in cosmology, particularly concerning the definitions and implications of energy in relation to gravity and matter.

Contextual Notes

The discussion reveals limitations in the definitions of energy in cosmology, particularly regarding the covariant nature of gravitational energy and the assumptions underlying different models. There are unresolved questions about the applicability of energy conservation laws in cosmological scenarios.

windy miller
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Models like Vilenkin's tunnelling from nothing model described here:
https://www.sciencedirect.com/science/article/abs/pii/0370269382908668
claim the universe came from "nothing". It is claimed this doesn't violate any conservation laws because the negative energy of gravity and the positive energy of matter can cancel each other out. However according to Sean Carroll energy is not conserved in cosmology anyway ,see here:
http://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/comment-page-2/
So my question is why do people like Vilenkin even worry about whether or not the positive energy of matter is balanced by the negative energy of gravity if energy is not conserved in cosmology?

Note: please let's not get into a phislophical debate as to the meaning of "nothing: here , my question is not about that.
 
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Energy of matter alone is not conserved, that's what Carroll is saying. Vilenkin however talks about total energy of matter and gravity together, which is supposed to be conserved. The problem is that the energy of gravity is, in general, not defined uniquely in a covariant way. It can be defined such that the total energy is conserved, but some purists do not take this definition seriously because it's not covariant. For those purists only the matter energy is well defined, which is not conserved.

See also https://www.physicsforums.com/insights/does-gravity-gravitate/ by @PeterDonis .
 
Last edited:
Demystifier said:
Energy of matter alone is not conserved, that's what Carroll is saying. Vilenkin however talks about total energy of matter and gravity together, which is supposed to be conserved. The problem is that the energy of gravity is, in general, not defined uniquely in a covariant way. It can be defined such that the total energy is conserved, but some purists do not take this definition seriously because it's not covariant. For those purists only the matter energy is well defined, which is not conserved.

See also https://www.physicsforums.com/insights/does-gravity-gravitate/ by @PeterDonis .
Ah ok thanks very much , appreciate the reply.
 

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