Differing Manifestations of Spatial Curvature in Cosmological FRW Models
Round 1
Reviewer 1 Report
Comments and Suggestions for AuthorsThe work find statistical evidence for a mismatch between the (global) spatial curvature parameter K in the geodesic equation for incoming photons, and the corresponding parameter in the Friedmann equation that determines the time evolution of the background spacetime and its perturbations.
The main results could possibly be interpreted as an inherent inconsistency between the (idealized) maximally symmetric nature of the FRW metric, and the dynamical evolution of the GR-based homogeneous and isotropic LCDM model. This result is very interesting and valuable.
However, there are some problems in the structure and expressions, so I have the following suggestions for the authors.
- The Introduction part is too long, so the contexts which are not so related to the main subject should be removed. For example, the fourth paragraph “Inflation addresses the flatness problem by positing…” should be deleted.
- Table 2 should be deleted, since the related content has been included in the text.
- The caption of Table 4 has grammatical error.
- In the third paragraph of the Introduction part, the full name of “GUT” should be provided.
Due to the above flaws, I cannot recommend the publication in the current form of this manuscript.
Author Response
Please see the attached.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Comments and Suggestions for AuthorsDear Editor,
I have reviewed for Universe the paper #3582061 titled
"Differing Manifestations of Spatial Curvature in Cosmological FRW Models"
by Meir Shimon and Yoel Rephaeli.
The authors present an analysis of cosmological datasets
for a modified Universe model featuring a curvature slip, i.e.
an anomaly of the curvature terms appearing in the
Friedmann equation and in the transverse comoving distance.
The model is phenomenological in nature and not very well
justified from first principles, but I feel the proposal is interesting and warrants to
be considered in the light of the analysis on cosmological datasets performed by the authors.
However, before recommending the paper for publication
I have a few suggestions for improvements:
* There is a kind of mismatch from what the authors discuss at length
in Sections 1-2-4-5 as to the dark energy issue
and what is really performed in the analysis of Section 3.
I clarify better my meaning: at a first look to the paper the
reader is left with the impression that the main
aim of the authors is to solve the dark energy issue, because this is
repeatedly mentioned throughout the text as one of the main problem of the
standard scenario (there are full paragraphs on this point).
But in the end the issue is not addressed at all, since the model by the authors still includes
dark energy (in the form of a cosmological constant, if I understood correctly).
From the cornerplots in Figure 1, the Omega_DE parameter for their models is inferred to be
even larger than in the standard LCDM scenario.
Thus at the end the reader feels a bit cheated by these broken promises about
the dark energy issue...this may hinder the reader from fully appreciating the paper.
My strong suggestion is to clearly state since the beginning what is the true aim of the
authors' model, and to shorten the text considerably avoiding to mumble
too much on issues that actually are not addressed at all.
* Generally speaking, models with a curvature slip tend to imply that the standard
redshift vs. scale factor relationship a=1/(1+z) may be altered, i.e.,
a non-trivial redshift drift may arise. Is this the case for the models discussed here?
Or is it implicitly assumed that the redshift drift is negligible?
This point should be clearly stated and discussed in the text.
* Recent DESI data analyzed in the context of a w0waLCDM model (also referred to as CPL
parameterization) seem to tentatively point toward an evolving dark energy equation of state
becoming phantom, i.e. violating the null energy condition of general relativity,
in the past. How does you model comply with these recent BAO data (Abdul-Karim et al. 2025, arXiv:2503.14738)?
May the curvature slip help to reconcile these data with a more standard
dark energy equation of state? It is worth at least mentioning this point in the text.
* I do not think that a curvature slip may necessarily indicate a violation of general relativity, as the authors
state in the manuscript. In fact, it could be present in models where the spatial
curvature parameter evolves on different constant-time slices,
e.g. when Omega_k = k(t)/a^2 H^2 applies, where k(t) depends on time.
This may occurr in scenarios with slight deviation from perfect homogeneity or isotropy,
of which the curvature slip as considered by the authors could be an effective description.
* Lines 161-181 report well-known fact and are not instrumental to the paper. I suggest to remove them.
* Around lines 248 the authors may consider citing
the recent paper by Lapi et al. 2025, JCAP, 4, 15
as to a specific model where the curvature component is modified and actually acts like a dark energy
term driving acceleration at late times. Additional references on modified curvature models which is also worth adding are
Koksbang 2020, MNRAS, 498, L135 and Larena et al. 2009, PRD, 79, 311(https://ui.adsabs.harvard.edu/abs/2009PhRvD..79h3011L/abstract).
Author Response
Please see the attached.
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
Comments and Suggestions for AuthorsThe major points mentioned in the previous report were all successfully addressed. In my opinion the paper is now ready to be accepted for publication.
