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Title: On the dark radiation problem in the axiverse

Abstract

String scenarios generically predict that we live in a so called axiverse: the Universe with about a hundred of light axion species which are decoupled from the Standard Model particles. However, the axions can couple to the inflaton which leads to their production after inflation. Then, these axions remain in the expanding Universe contributing to the dark radiation component, which is severely bounded from present cosmological data. We place a general constraint on the axion production rate and apply it to several variants of reasonable inflaton-to-axion couplings. The limit merely constrains the number of ultralight axions and the relative strength of inflaton-to-axion coupling. It is valid in both large and small field inflationary models irrespectively of the axion energy scales and masses. Thus, the limit is complementary to those associated with the Universe overclosure and axion isocurvature fluctuations. In particular, a hundred of axions is forbidden if inflaton universally couples to all the fields at reheating. In the case of gravitational sector being responsible for the reheating of the Universe (which is a natural option in all inflationary models with modified gravity), the axion production can be efficient. We find that in the Starobinsky R {sup 2}-inflation even a singlemore » axion (e.g. the standard QCD-axion) is in tension with the Planck data, making the model inconsistent with the axiverse. The general conclusion is that an inflation with inefficient reheating mechanism and low reheating temperature may be in tension with the presence of light scalars.« less

Authors:
;  [1]
  1. Institute for Nuclear Research of Russian Academy of Sciences, 117312 Moscow (Russian Federation)
Publication Date:
OSTI Identifier:
22676171
Resource Type:
Journal Article
Journal Name:
Journal of Cosmology and Astroparticle Physics
Additional Journal Information:
Journal Volume: 2017; Journal Issue: 06; Other Information: Country of input: International Atomic Energy Agency (IAEA); Journal ID: ISSN 1475-7516
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; AXIONS; COUPLING; FLUCTUATIONS; GRAVITATION; INFLATONS; MASS; QUANTUM CHROMODYNAMICS; STANDARD MODEL; UNIVERSE; VISIBLE RADIATION

Citation Formats

Gorbunov, Dmitry, and Tokareva, Anna. On the dark radiation problem in the axiverse. United States: N. p., 2017. Web. doi:10.1088/1475-7516/2017/06/016.
Gorbunov, Dmitry, & Tokareva, Anna. On the dark radiation problem in the axiverse. United States. https://doi.org/10.1088/1475-7516/2017/06/016
Gorbunov, Dmitry, and Tokareva, Anna. 2017. "On the dark radiation problem in the axiverse". United States. https://doi.org/10.1088/1475-7516/2017/06/016.
@article{osti_22676171,
title = {On the dark radiation problem in the axiverse},
author = {Gorbunov, Dmitry and Tokareva, Anna},
abstractNote = {String scenarios generically predict that we live in a so called axiverse: the Universe with about a hundred of light axion species which are decoupled from the Standard Model particles. However, the axions can couple to the inflaton which leads to their production after inflation. Then, these axions remain in the expanding Universe contributing to the dark radiation component, which is severely bounded from present cosmological data. We place a general constraint on the axion production rate and apply it to several variants of reasonable inflaton-to-axion couplings. The limit merely constrains the number of ultralight axions and the relative strength of inflaton-to-axion coupling. It is valid in both large and small field inflationary models irrespectively of the axion energy scales and masses. Thus, the limit is complementary to those associated with the Universe overclosure and axion isocurvature fluctuations. In particular, a hundred of axions is forbidden if inflaton universally couples to all the fields at reheating. In the case of gravitational sector being responsible for the reheating of the Universe (which is a natural option in all inflationary models with modified gravity), the axion production can be efficient. We find that in the Starobinsky R {sup 2}-inflation even a single axion (e.g. the standard QCD-axion) is in tension with the Planck data, making the model inconsistent with the axiverse. The general conclusion is that an inflation with inefficient reheating mechanism and low reheating temperature may be in tension with the presence of light scalars.},
doi = {10.1088/1475-7516/2017/06/016},
url = {https://www.osti.gov/biblio/22676171}, journal = {Journal of Cosmology and Astroparticle Physics},
issn = {1475-7516},
number = 06,
volume = 2017,
place = {United States},
year = {Thu Jun 01 00:00:00 EDT 2017},
month = {Thu Jun 01 00:00:00 EDT 2017}
}