Implications of the NANOGrav results for inflation
Abstract
The NANOGrav pulsar timing array experiment reported evidence for a stochastic common-spectrum process affecting pulsar timing residuals in its 12.5-yr data set, which might be interpreted as the first detection of a stochastic gravitational wave background (SGWB). I examine whether the NANOGrav signal might be explained by an inflationary SGWB, focusing on the implications for the tensor spectral index nT and the tensor-to-scalar ratio r. Explaining NANOGrav while complying with upper limits on r from BICEP2/Keck Array and Planck requires $r \gtrsim {\cal O}(10^{-6})$ in conjunction with an extremely blue tensor spectrum, 0.7 ≲ nT ≲ 1.3. After discussing models, which can realize such a blue spectrum, I show that this region of parameter space can be brought in agreement with big bang nucleosynthesis constraints for a sufficiently low reheating scale, $T_{\rm rh} \lesssim 100\, {\rm GeV} \!-\! 1\, {\rm TeV}$ . With the important caveat of having assumed a power-law parametrization for the primordial tensor spectrum, an inflationary interpretation of the NANOGrav signal is therefore not excluded.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- March 2021
- DOI:
- 10.1093/mnrasl/slaa203
- arXiv:
- arXiv:2009.13432
- Bibcode:
- 2021MNRAS.502L..11V
- Keywords:
-
- gravitational waves;
- cosmological parameters;
- inflation;
- Astrophysics - Cosmology and Nongalactic Astrophysics;
- General Relativity and Quantum Cosmology;
- High Energy Physics - Phenomenology;
- High Energy Physics - Theory
- E-Print:
- 5 pages, 2 figures. v2: revised conclusions, NANOGrav can in principle be due to an inflationary SGWB provided the reheating temperature is sufficiently (=very) low. References added, version accepted for publication in MNRAS Letters. Comments are welcome