Gravitational Waves: The Theorist’s Swiss Knife
Round 1
Reviewer 1 Report
A few small typos in line 165 (vector), line 204 (from), after line 453 (neutron), line 460 (with) and 469 (contrained). No further comments.
Author Response
Typos have now been corrected. Thanks.
Reviewer 2 Report
Review article
Gravitational Waves: the theorist's Swiss knife
Mairi Sakellariadou
The article is a review on the gravitational wave research in recent years and covers gravitational wave background formed by different sources as compact binaries coalescences, cosmic strings, first-order phase transitions in the early Universe, and primordial black holes. It covers also tests of the General Relativity theory considering the LIGO/Virgo upper bounds on the relative energy density of non-tensor polarisations. Tests of General Relativity and modified gravity models are also considered in the scope of the detection of GW transients from the LVK Collaboration. Constraints on axions parameters and possible information that could be obtained on quantum gravity theories are also discussed.
In general, the article is well written and useful for a general view on the current status of gravitational waves research, and I have just a few observations listed below. My main concern is the lack of a final section about prospects on the GW research both, with the future ground-based and space-based detectors (in this case, a short discussion about supermassive black holes could be interesting). Please consider including such a Section or extend the 'Conclusion' or 'Final Remarks' section.
Title of the subsections: it would be better to write 'Compact binaries coalescence' instead of 'CBC', 'Cosmic strings' instead of 'CS', and so on. This becomes more informative to the reader.
page 4 Sentence after the Eq. (14). A brief explanation of why \alpha = 2/3 is not valid for mergers of binaries arising from population III stars is instructive at this point. Or a phrase like "this will be explained in the next section".
page 8 References for the mechanism that originates the asymmetry between right and left-handed circularly-polarized isotropic GWs are missing.
In Section 2.5 what is the frequency range for the scalar induced GWB? Make this information clearer in that section.
page 10 After Eq. (23) the peak in the relative energy density of the GWB is quoted, but what is the peak of the integrated power spectral density?
page 14 The \Psi_{GR} is the phase of the waveform evaluated without the presence of axions. In the form it is written, ("... is the phase of the waveform within General Relativity") it sounds like one is working with a modified gravity theory, but this is not the case.
Typos
page 5 line 121: Where one reads "black holes ate", please correct the word "ate".
page 5 Something is wrong in the sentence of line 140. Consider rewriting it.
page 6 Correct "tangent vecto" at line 165.
page 9 The first paragraph of Section 2.5 is unclear, consider rewriting it.
pages 9 and 10 Starting in line 311: The first sentence ends by saying that "... the scalar induced GWB remains unobservable by current or planned experiments". But the second phrase says that GWB reaches the GW-observatories. It is not clear if the considered GWB can be detected or not. Please include something (maybe in the first sentence) to make the whole paragraph clearer.
page 10 Correct "scalar induces GWB'' in the first paragraph. Correct "the integrated power of the peak" after Eq. (23).
p. 11 There are two upper bounds to \Omega^{(S)} before line 367. Which one of them is for tensor polarization?
Author Response
The suggestions have been considered in the revised version.
Note the following:
-Discussion about improvements with 3g detectors and LISA are already in the contribution, of course referring to the aspects discussed here.
-In GR there is no scalar radiation, so indeed axion models have been studied in extended gravity models
-About the scalar induced GWB, as was said in the text, I refer to the LIGO/Virgo frequency band.
-Regarding detectability of the scalar induced GW spectrum, already in the text there is a distinction between large and small scales.
