SciPost Submission Page
Elements of cosmology beyond FLRW
by Pierre Fleury
This is not the latest submitted version.
Submission summary
| Authors (as registered SciPost users): | Pierre Fleury |
| Submission information | |
|---|---|
| Preprint Link: | scipost_202508_00049v1 (pdf) |
| Date submitted: | Aug. 21, 2025, 12:21 p.m. |
| Submitted by: | Pierre Fleury |
| Submitted to: | SciPost Physics Lecture Notes |
| for consideration in Collection: |
| Ontological classification | |
|---|---|
| Academic field: | Physics |
| Specialties: |
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| Approach: | Theoretical |
Abstract
Modern cosmology is based on the cosmological principle, which states that the Universe is statistically homogeneous and isotropic. When applied in its strict — rather than statistical — sense, the cosmological principle leads to the Friedmann—Lemaître—Robertson—Walker (FLRW) model, which serves as background spacetime. This background is used to predict: (1) the dynamics of cosmic expansion; and (2) the kinematics of light propagation through the Universe, which dictates the interpretation of cosmological observations. In this lecture, we shall discuss the performance of the FLRW model for those purposes, and present some results on the so-called backreaction and fitting problems.
Current status:
Reports on this Submission
Strengths
1) The notes provide a very clear and transparent outline of the cosmological principle, with a specific focus on its possible limitations and pitfalls.
2) The content is highly relevant in the current era of cosmology, where several datasets hint at a potential breakdown of the cosmological principle. This set of notes serves as an excellent starting point to understand why (if the data are indeed correct) it is important to study the theoretical, physical, and philosophical implications of inhomogeneities.
3) The development of ideas is extremely approachable: starting from the very basics and stopping just before entering specialized scenarios. Ample and appropriate references are provided to guide readers toward further reading.
4) The figures are crisp and well chosen, greatly helping to convey the main ideas of the notes.
Weaknesses
Report
While the FLRW framework remains, as outlined, highly robust against current data, the notes appropriately emphasize the need for caution in alternative cosmological models. In particular, they highlight the importance of re-examining backreaction and light propagation effects from first principles, especially in exotic theories and non-trivial beyond-FLRW scenarios.
Requested changes
Nothing major, except for a few minor typographical and stylistic suggestions:
1) Lines 30–31: Inserting "of" would make the sentence read more smoothly.
2) Consider replacing "cst" with "const" or "const.".
3) There are minor inconsistencies in punctuation before and after equations. Please recheck for uniformity.
4) Line 126 does not read well grammatically and may benefit from rephrasing.
5) Line 215: Consider using "Similar" instead of "Similarly".
6) Line 353: Consider using "these" instead of "those".
Recommendation
Publish (easily meets expectations and criteria for this Journal; among top 50%)
Strengths
1- The paper provides a clear overview about cosmological backreaction and the fitting problem, detailing what these topics really mean; 2- It helps fix the main ideas for an audience that has some experience with the topics being treated.
Report
Requested changes
1- To improve the quality of the manuscript, I would recommend extending the bibliography, to help non-experts identify further relevant literature. 2- In particular, in Section 3.2, in which light-cone coordinates are introduced, it would be worth mentioning that there are other definitions of light-cone coordinates (e.g. geodesic light-cone coordinates), as another possible approach to compute cosmological observables.
Recommendation
Ask for minor revision