Abstract
Primordial gravitational waves (GWs) are said to be a smoking gun in cosmic inflation, while, even if they are detected, the specification of their origins are still required for establishing a true inflationary model. Testing non-Gaussianity in the tensor-mode anisotropies of the cosmic microwave background (CMB) is one of the most powerful ways to identify sources of GW signals. In this paper, we review studies searching for tensor non-Gaussianities employing the CMB bispectrum and forecast future developments. No significant signal has so far been found from temperature and E-mode polarization data, while orders-of-magnitude improvements in detection limits can be achieved by adding the information of B-mode polarization. There is already an established methodology for bispectrum estimation, which encourages a follow-up investigation with next-decadal CMB B-mode surveys.
Highlights
Recent cosmic-variance-limited-level measurements of the scalar sector in primordial fluctuations using the temperature and E-mode polarization fields of the cosmic microwave background (CMB), indicate that the Universe experienced an inflationary expansion at very early stages (Aghanim et al, 2018; Akrami et al, 2018)
In the model where the axion is coupled to the U(1) gauge field, a characteristic bump appears in the gravitational waves (GWs) bispectrum, and, depending on its location, it is detectable by the CMB BBB bispectrum measurement (Namba et al, 2016; Shiraishi et al, 2016)
We start by reviewing how to compute the CMB bispectra generated from the primordial scalar, vector, and tensor NGs based on the general formalism developed in Shiraishi et al (2010, 2011b)
Summary
Recent cosmic-variance-limited-level measurements of the scalar sector in primordial fluctuations using the temperature and E-mode polarization fields of the cosmic microwave background (CMB), indicate that the Universe experienced an inflationary expansion at very early stages (Aghanim et al, 2018; Akrami et al, 2018). There is the possibility of a post-inflationary generation of NG GWs due to magnetic fields (Shiraishi et al, 2011c) Another possible way to produce large tensor NGs comes from non-trivial non-linear gravitational interactions predicted in some modified theories of gravity (e.g., Gao et al, 2011, 2013; Maldacena and Pimentel, 2011; Akita and Kobayashi, 2016; Bartolo and Orlando, 2017; Domenech et al, 2017; Naskar and Pal, 2018; Anninos et al, 2019; Ozsoy et al, 2019). The induced bispectra can have distinctive shapes compared with that from the usual Einstein term In this sense, the CMB tensor-mode bispectrum is a key indicator of the inflationary particle content and/or high energy gravity. The final section is devoted to the conclusions of this paper
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
