Measuring primordial gravitational waves from CMB B-modes in cosmologies with generalized expansion histories

TL;DR

This paper assesses how generalized dark energy models impact the measurement of primordial gravitational waves via CMB B-modes, highlighting the importance of combining multiple observations to improve constraints.

Contribution

It introduces a framework for evaluating the effects of dynamic dark energy on CMB B-mode measurements and demonstrates how combined data can mitigate degeneracies.

Findings

Dark energy variations can alter lensing-to-primordial GW power ratio by up to 30%.

Upcoming satellite experiments' constraints on GWs weaken with extra dark energy parameters.

Combining CMB with SNe, BAO, and Hubble data improves constraints on tensor-to-scalar ratio r.

Abstract

We evaluate our capability to constrain the abundance of primordial tensor perturbations in cosmologies with generalized expansion histories in the epoch of cosmic acceleration. Forthcoming satellite and sub-orbital experiments probing polarization in the CMB are expected to measure the B-mode power in CMB polarization, coming from PGWs on the degree scale, as well as gravitational lensing on arcmin scales; the latter is the main competitor for the measurement of PGWs, and is directly affected by the underlying expansion history, determined by the presence of a DE component. In particular, we consider early DE possible scenarios, in which the expansion history is substantially modified at the epoch in which the CMB lensing is most relevant. We show that the introduction of a parametrized DE may induce a variation as large as 30% in the ratio of the power of lensing and PGWs on the degree scale. We find that adopting the nominal specifications of upcoming satellite measurements the constraining power on PGWs is weakened by the inclusion of the extra degrees of freedom, resulting in a reduction of about 10% of the upper limits on r in fiducial models with no GWs, as well as a comparable increase in the error bars in models with non-zero r. Moreover, we find that the inclusion of sub-orbital CMB experiments, capable of mapping the B-mode power up to the angular scales affected by lensing, can restore the forecasted performances with a cosmological constant. Finally, we show how the combination of CMB data with Type Ia SNe, BAO and Hubble constant allows to constrain simultaneously r and the DE quantities in the parametrization we consider, consisting of present abundance and first redshift derivative of the energy density. We compare this study with results obtained using the forecasted lensing potential measurement precision from CMB satellite observations, finding consistent results.