Dark energy with negative and positive kinetic energy and the Big Rip

[xX_minecraftMLGPRO2019_Xx]

I was reading some articles about dark energy and its possible equation of state. I stumbled upon this article https://arxiv.org/abs/1804.02987 which stated that "a dark energy field with
w < -1 is a viable option", "phantom dark energy does not categorically have negative kinetic energy" and "kinetic energy is positive for w≳−1.22, which includes virtually all values of constant w allowed by cosmological data constraints". I'm a bit confused with negative and positive kinetic energy of dark energy, but all sources I've read have stated that Big Rip can happen due to a phantom dark energy field with a negative kinetic energy. Does it not happen if the field has w > -1 but positive kinetic energy? Apologies if I'm misunderstanding something.

[Mordred]

Might help to know that in order for the cosmological constant to stay constant the. W=-1 too far from that value and it will vary over time. The options are still viable for an evolving cosmological constant but so far research is showing non evolving. If Lambda does evolve then you may have an eventual collapse as opposed to a big rip.

Still going through the article however they seems to be using as negative pressure however still unclear on that myself till  have a chance to better study the math.

Yeah looks looks like the second terms in equation one is describing a vacuum scalar field. The positive and negative kinetic energy sign flips directly apply to that same equation.

The stress tensor components in the article equation 10. The -T^00 component is the energy density term T^0_i is the mixed covariant/contravariant momentum Flux in the I direction with T^0_j being in the j direction.

 Just to help you better understand some of the equations in the article. Insofar as he is describing the stress momentum terms of H_{ij}.

The majority of the other equations are fairly standard from thevFLRW metric including the related equations of state.

Hope that helps.

Edit; are you familiar with how the equations of state are derived ? That might help if the answer is no.

I should also note that the article is in the Newton limit under GR and does not include quantum field theory itself (QFT equations are second order).

Here is an article on first order perturbation theory as applied to QM. https://courses.physics.illinois.edu/phys485/fa2015/web/perturb.pdf

 

Edited December 14, 2022 by Mordred

[xX_minecraftMLGPRO2019_Xx]

On 12/13/2022 at 10:14 PM, Mordred said:

 Just to help you better understand some of the equations in the article. Insofar as he is describing the stress momentum terms of H_{ij}.

...

Edit; are you familiar with how the equations of state are derived ? That might help if the answer is no.

I'm afraid you are massively overestimating my knowledge of physics. I remember that equations of state are the relation of density and pressure of gases and fluids, but I don't really understand much in the article itself, I just wanted to know if a hypothetical Big Rip could be caused by a phantom field with positive kinetic energy.

[Mordred]

No the big rip requires the negative kinetic energy term to occur. However current evidence support that the big rip as being unlikely to occur as the cosmological constant shows strong supportive evidence of being constant.

 Provided it remains constant the heat death scenario is more likely

[xX_minecraftMLGPRO2019_Xx]

On 12/15/2022 at 9:15 PM, Mordred said:

No the big rip requires the negative kinetic energy term to occur. However current evidence support that the big rip as being unlikely to occur as the cosmological constant shows strong supportive evidence of being constant.

 Provided it remains constant the heat death scenario is more likely

Thanks a lot!