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Cornering Extended Starobinsky Inflation with CMB and SKA
by Tanmoy Modak, Lennart Röver, Björn Malte Schäfer, Benedikt Schosser, Tilman Plehn
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Submission summary
Authors (as registered SciPost users):  Tilman Plehn · Benedikt Schosser 
Submission information  

Preprint Link:  scipost_202210_00082v1 (pdf) 
Date submitted:  20221025 11:36 
Submitted by:  Schosser, Benedikt 
Submitted to:  SciPost Physics 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approaches:  Theoretical, Phenomenological 
Abstract
Starobinsky inflation is an attractive, fundamental model to explain the Planck measurements, and its higherorder extension may allow us to probe quantum gravity effects. We show that future CMB data combined with the 21cm intensity map from SKA will meaningfully probe such an extended Starobinsky model. A combined analysis will provide a precise measurement and intriguing insight into inflationary dynamics, even accounting for correlations with astrophysical parameters.
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Submission & Refereeing History
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Reports on this Submission
Report 1 by Lucien Heurtier on 2023224 (Invited Report)
 Cite as: Lucien Heurtier, Report on arXiv:scipost_202210_00082v1, delivered 20230224, doi: 10.21468/SciPost.Report.6790
Strengths
1 Important new direction of cosmological searches
2 Clarity of the paper
Weaknesses
1 The results are quite model dependent, but that is unavoidable
Report
Dear editor,
The authors propose to constrain Starobinsky inflation models, supplemented by $R^3$ terms using future CMB observatories such as LiteBird and CMBS4, and 21 cm intensity mapping experiments such as the square kilometre array SKA. Indeed, by focusing on redshifts and scales over which structure formation is known to be under control, and where the expected background is reduced, the authors hope to constrain inflation parameters using 21 cm line measurements.
After reviewing basic results about inflation and its treatment in perturbation theory, the authors describe how they obtain likelihoods for LiteBird and CMBS4 and derive bestfit values for the set of cosmological and inflation parameters considered. Then the authors explain how they derive the 21 cm line power spectrum in the redshift range they consider, and obtain the corresponding SKA likelihood. Using the combined sensitivities of the three detectors, the authors demonstrate that the three detectors can help measure better the value of the extra term they add to the action.
The manuscript is clear and wellwritten. The purpose of the paper is modest but wellmotivated. I believe it deserves to be published in SciPost Physics, as it opens a new window on searches for signatures of inflation (and modified gravity), not only using CMB observations but also other classes of cosmological measurements such as 21cm line measurements.
However, before the paper is accepted for publication, I would need the authors to address the following questions/comments:
1) Although it is understandable in the text by deduction that it corresponds to fiducial values, the parameter $\hat H$ and $\hat D$ used in Eq.(33) are not defined.
2) In Eq.(37), the choice of $\sigma_{\rm NL} = 1$Mpc as a fiducial value should be justified. How would a different choice affect the results?
3) After Eq.(44), it is mentioned that the authors "choose $\Delta z = 1$, which is slightly lower than the whole redshift range probed by the experiment zmax − zmin = 2." The authors should comment on the motivation and validity for such a choice.
4) The quoted bestfit value from Planck for the cparameter (times 1e5) is 4.135 in Eq.(27) and before Eq.(28). However, in Table 1, the bestfit value obtained from Planck is 4.315. The authors should make sure of which value is the correct one, and possibly crosscheck that the numbers in the Tables do not contain any other typos if there is one.
5) An open question: The authors notice in their analysis a correlation between the value of (M,c) and the value of the number of efolds of inflation N_star. Such a correlation is expected, as constraints on inflation are inherently entangled with the assumed postinflationary cosmic history. In this work the authors set the prior for N_star to be a gaussian centred on 55 with width 5, to cover a range of 5060 efolds. First I am wondering whether such choice for the prior affects significantly the final results. Second, while pure radiation domination from the end of inflation to the later BigBang cosmology would lead to a large value of N_star, a long period of early matter domination could lower this number even below 50. While this early cosmic history may not frankly affect CMB measurements beyond the single value of N_star, I am wondering whether it could have an impact on the 21cm intensity mapping as the matter power spectrum may evolve differently in the presence of a long period of early matter domination. Also, I am wondering whether CLASS assumes a specific cosmic history (maybe pure radiation domination right after inflation?), in which sense it may not be clear what it means to actually vary the number of efolds of inflation without modifying cosmic history.
Best regards
The referee
Author: Benedikt Schosser on 20230324 [id 3504]
(in reply to Report 1 by Lucien Heurtier on 20230224)Dear referee,
Thank you for your questions and remarks. Our answers are compactly attached in a file. Additionally, you can find a new version of the manuscript with the changes in red attached.
Best regards,
The editor
Attachment:
answers_report.pdf
Anonymous on 20230406 [id 3559]
(in reply to Benedikt Schosser on 20230324 [id 3504])Dear Editor,
I am happy with the answers provided by the authors and the amendment made to the draft.
I am thus now considering the manuscript acceptable for publication in SciPost.
Best regards
The referee