SciPost Submission Page
Gravitational waves from the early universe
by Rafael R. Lino dos Santos, Linda M. van Manen
Submission summary
| Authors (as registered SciPost users): | Rafael Robson Lino dos Santos · Linda van Manen |
| Submission information | |
|---|---|
| Preprint Link: | scipost_202501_00047v1 (pdf) |
| Date submitted: | 2025-01-24 11:22 |
| Submitted by: | Lino dos Santos, Rafael Robson |
| Submitted to: | SciPost Physics Lecture Notes |
| Ontological classification | |
|---|---|
| Academic field: | Physics |
| Specialties: |
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| Approaches: | Theoretical, Phenomenological |
Abstract
“Even though one could already constrain different models in cosmology and Beyond Standard Model physics using CMB data, these models remained unconstrained at shorter wavelength scales, and knowledge of new physics at higher energy scales relied on theoretical assumptions and extrapolations to these scales. Recently, however, we have experienced the advent of gravitational-wave and multi-messenger astronomy, including the outstanding detections by the LIGO-Virgo collaboration over the past decade and the latest searches for Hellings-Downs correlations in pulsar timing data. Ongoing and future gravitational wave collaborations explore different frequency ranges of the gravitational wave spectrum. In these lecture notes, we focus on how we can probe cosmology and Beyond Standard Model physics with primordial gravitational waves. For this purpose, we review the formalism of gravitational waves in General Relativity, introduce stochastic gravitational waves, and derive the Hellings-Downs correlation for pulsar timing array searches. We comment on detection efforts and present some of the most important cosmological sources that could produce a background. Ultimately, one could compare model-dependent gravitational wave density spectra using gravitational wave data from ground-based, space-borne, and pulsar timing array searches. These lecture notes were inspired by the course "Gravitational Waves from the Early Universe" given at the 27th W.E. Heraeus "Saalburg" Summer School 2021 by Valerie Domcke. ”
Author comments upon resubmission
List of changes
A copy of our reply for the 18 points raised by the referee on the v2 report can be found here:
https://docs.google.com/document/d/1VagEG3OWC5_HqgghCvPwSwUDPi1ZVsR_qBvTHZ1gIDI/edit?usp=sharing
Other modifications are:
1. We corrected some typos and also introduced some new references.
2. We decided not to include new references that appeared after the release of v2 of these notes.
3. For most cases, we changed “gravitational wave” to “GW”.
4. We change instances of the word “lecture” to “section” since each section does not necessarily need to correspond to a lecture.
5. We make the notation uniform (for instance, Eq. XX and Fig. YY instead of equation (XX)).
6. On page 7, we slightly modified the explanation about the gauge transformation of the tensor field. We moved the discussion of the TT projection operator to this section, to improve readability.
7. We have improved Sec. 3.4, where we define the GW spectrum.
8. We have improved Sec. 4 with more precise sentences and reviewed the previous text.
9. We have moved the subsections ‘searches with interferometers’ and ‘PTAs; to a new section – Sec. 5 - Searching for the background.
10. Improved discussion about antenna patterns at the end of current 5.1.3.
11. Included comments about millisecond pulsars.
12. Discussion of Sec. 6 revised and improved. Some sentences regarding the Hubble horizon in the main text were included.
13. A plot of the SM effective number of relativistic and entropy degrees of freedom has been added to the current Sec. 6.4.
14. The sections on phase transition and cosmic strings have been reviewed and improved.
15. New affiliation, contact, and funding information added.