Ghost Condensates and Pure Kinetic k-Essence Condensates in the Presence of Field–Fluid Non-Minimal Coupling in the Dark Sector

Round 1

Reviewer 1 Report

Title: Ghost condensates and pure kinetic k-essence condensates in presence of field-fluid non-minimal coupling in the dark sector

Authors: Saddam Hussain, Anirban Chatterjee, Kaushik Bhattacharya

 

I have gone through this manuscript precisely and come up with my decision, which is mentioned below.

In this article, the authors have shown the link between pure kinetic k-essence fields and ghost fields in cosmology. The linkage is not new. All ghost fields are pure kinetic k-essence fields, whereas all pure kinetic k-essence fields may not be ghost fields. This manuscript probes a particular feature about the fate of the universe in the presence of a ghost field and a barotropic fluid. It is seen that in isolation, a ghost field in cosmology will in general, always lead to a stable accelerating phase of the universe. Does it do so even in the presence of a barotropic fluid, even when a non-minimal coupling is present between the field and the fluid? The authors have discussed the mentioned problem precisely. Furthermore, they have found a novel phase of pure kinetic k-essence condensate which only forms in the presence of a barotropic fluid when the field and fluid sectors are interacting non-minimally.

Moreover, this manuscript is concluded by stating that most of the time, ghost fields and a barotropic fluid can produce a stable accelerating phase when the ghost condensate is formed but not always. There can be cases where pure kinetic k-essence fields, which are not ghost fields, can also produce a stable accelerating expansion phase of the universe where matter energy density remains subdominant. These results are interesting and would help the readers explore more on this topic.

 

Therefore, I recommend this manuscript for publication in the Universe journal.

Author Response

As the referee did not ask any question we have not replied to Reviewer one.

Reviewer 2 Report

In the manuscript under review, the authors consider the idea of a non-minimal coupling between a barotropic fluid and a ghost field in order to produce a stable accelerating expansion phase of the universe.

They found that under certain conditions this is possible depending on the formation of a ghost condensate. They show that, a pure kinetic k-essence field condensates when interacting (non-minimally) with the fluid, which in turn can give rise to a stable accelerating expansion phase.

The results are clearly presented, and the conclusions are supported by the results. Although the topic has been discussed in the community for almost 20 years, the special case studied by the authors by considering a non-minimal coupling with a barotropic fluid could be interesting in this area of research.

However, precisely because of the huge amount of information we have about the role played by ghost fields and k-essence in inflation, it is also possible to compute some observables which support the ideas discussed in this proposal.

Hence, I want to request the authors to add some information about how observation can discard or not this proposal if possible. For instance, are there some predictions that distinguish it from standard slow-roll inflation ? what about non-gaussianities, are they observable in this model? I would like to see this for each example in  section 5.  In case the authors consider that this is not possible,  I would also like to read the reasons behind it.

 

 

Author Response

The reply has been uploaded as a PDF file.

Author Response File: Author Response.pdf

Reviewer 3 Report

In this paper, authors consider pure kinetic k-essence and ghost field that interact with barotropic fluid.

These k-essence and ghost fields can condensate for realizing stable acceleration of the universe.

After reviewing general formalism, they show condition in which ghost condensate happens.

In particular some dynamical analyses are carried out for non-minimally coupled field-fluid system.

I think their results are interesting and useful. 

Thus the paper can be published after considering following comments.

 

1. Authors consider general relativistic fluid. Is there some realistic example of such fluid? 

If there is an example, it is good to mention.

 

2. In the analysis parameters A and B in Eq.(45) is taken to be same order.

Is there problem if these parameters are hierarchical? Or is there some criterion to choose A and B?

Author Response

The reply has been uploaded as a PDF file

Author Response File: Author Response.pdf