We use the 2015 Planck likelihood in combination with the Bicep2/Keck likelihood (BKP and BK14) to constrain the chirality, chi, of primordial gravitational waves in a scale invariant scenario. In this framework, the parameter chi enters theory always coupled to the tensor -to -scalar ratio, r, e.g. in combination of the form chi . r. Thus, the capability to detect chi critically depends on the value of r. We find that with present data sets chi is de facto unconstrained. We also provide forecasts for chi from future CMB experiments, including COrE+, exploring several fiducial values of r. We find that the current limit on r is tight enough to disfavor a neat detection of chi. For example, in the unlikely case in which r similar to 0.1(0.05), the maximal chirality case, i.e. chi = +/- 1, could be detected with a significance of similar to 2.5(1.5)sigma at best. We conclude that the two-point statistics at the basis of CMB likelihood functions is currently unable to constrain chirality and may only provide weak limits on chi in the most optimistic scenarios. Hence, it is crucial to investigate the use of other observables, e.g. provided by higher order statistics, to constrain these kinds of parity violating theories with the CMB.
Testing chirality of primordial gravitational waves with Planck and future CMB data: no hope from angular power spectra
GRUPPUSO, Alessandro;NATOLI, Paolo;
2016
Abstract
We use the 2015 Planck likelihood in combination with the Bicep2/Keck likelihood (BKP and BK14) to constrain the chirality, chi, of primordial gravitational waves in a scale invariant scenario. In this framework, the parameter chi enters theory always coupled to the tensor -to -scalar ratio, r, e.g. in combination of the form chi . r. Thus, the capability to detect chi critically depends on the value of r. We find that with present data sets chi is de facto unconstrained. We also provide forecasts for chi from future CMB experiments, including COrE+, exploring several fiducial values of r. We find that the current limit on r is tight enough to disfavor a neat detection of chi. For example, in the unlikely case in which r similar to 0.1(0.05), the maximal chirality case, i.e. chi = +/- 1, could be detected with a significance of similar to 2.5(1.5)sigma at best. We conclude that the two-point statistics at the basis of CMB likelihood functions is currently unable to constrain chirality and may only provide weak limits on chi in the most optimistic scenarios. Hence, it is crucial to investigate the use of other observables, e.g. provided by higher order statistics, to constrain these kinds of parity violating theories with the CMB.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
