{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,4,9]],"date-time":"2026-04-09T02:50:17Z","timestamp":1775703017274,"version":"3.50.1"},"reference-count":91,"publisher":"MDPI AG","issue":"4","license":[{"start":{"date-parts":[[2022,4,3]],"date-time":"2022-04-03T00:00:00Z","timestamp":1648944000000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"name":"Ministry of education and science of Kazakhstan","award":["Grant AP05135123"],"award-info":[{"award-number":["Grant AP05135123"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Symmetry"],"abstract":"<jats:p>In this work, we shall exhaustively study the effects of modified gravity on the energy spectrum of the primordial gravitational waves background. S. Weinberg has also produced significant works related to the primordial gravitational waves, with the most important one being the effects of neutrinos on primordial gravitational waves. With this short review, our main aim is to gather all the necessary information for studying the effects of modified gravity on primordial gravitational waves in a concrete and quantitative way and in a single paper. After reviewing all the necessary techniques for extracting the general relativistic energy spectrum, and how to obtain, in a WKB way, the modified gravity damping or amplifying factor, we concentrate on specific forms of modified gravity of interest. The most important parameter involved for the calculation of the effects of modified gravity on the energy spectrum is the parameter aM, which we calculate for the cases of f(R,\u03d5) gravity, Chern\u2013Simons-corrected f(R,\u03d5) gravity, Einstein\u2013Gauss\u2013Bonnet-corrected f(R,\u03d5) gravity, and higher derivative extended Einstein\u2013Gauss\u2013Bonnet-corrected f(R,\u03d5) gravity. The exact form of aM is presented explicitly for the first time in the literature. With regard to Einstein\u2013Gauss\u2013Bonnet-corrected f(R,\u03d5) gravity, and higher derivative extended Einstein\u2013Gauss\u2013Bonnet-corrected f(R,\u03d5) gravity theories, we focus on the case in which the gravitational wave propagating speed is equal to that of light in a vacuum. We provide expressions for aM expressed in terms of the cosmic time and in terms of the redshift, which can be used directly for the numerical calculation of the effect of modified gravity on the primordial gravitational wave energy spectrum.<\/jats:p>","DOI":"10.3390\/sym14040729","type":"journal-article","created":{"date-parts":[[2022,4,3]],"date-time":"2022-04-03T06:04:01Z","timestamp":1648965841000},"page":"729","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":65,"title":["Spectrum of Primordial Gravitational Waves in Modified Gravities: A Short Overview"],"prefix":"10.3390","volume":"14","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-3529-7030","authenticated-orcid":false,"given":"Sergei","family":"Odintsov","sequence":"first","affiliation":[{"name":"ICREA, Passeig Luis Companys, 23, 08010 Barcelona, Spain"},{"name":"Institute of Space Sciences (ICE, CSIC), C. Can Magrans s\/n, 08193 Barcelona, Spain"},{"name":"Institute of Space Sciences of Catalonia (IEEC), 08193 Barcelona, Spain"},{"name":"Ratbay Myrzakulov Eurasian International Centre for Theoretical Physics, Nur-Sultan 010009, Kazakhstan"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0125-4160","authenticated-orcid":false,"given":"Vasilis","family":"Oikonomou","sequence":"additional","affiliation":[{"name":"Department of Physics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece"}]},{"given":"Ratbay","family":"Myrzakulov","sequence":"additional","affiliation":[{"name":"Ratbay Myrzakulov Eurasian International Centre for Theoretical Physics, Nur-Sultan 010009, Kazakhstan"},{"name":"Faculty of Natural Sciences, Eurasian National University, Nur-Sultan 010008, Kazakhstan"},{"name":"Laboratory for Theoretical Cosmology, International Centre of Gravity and Cosmos, (TUSUR), Tomsk 634050, Russia"}]}],"member":"1968","published-online":{"date-parts":[[2022,4,3]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"1","DOI":"10.1007\/978-3-540-74353-8_1","article-title":"Inflationary Cosmology","volume":"Volume 738","author":"Linde","year":"2008","journal-title":"Lecture Notes in Physics"},{"key":"ref_2","doi-asserted-by":"crossref","unstructured":"Gorbunov, D.S., and Rubakov, V.A. 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