No abstract

Considering high-energy modifications of Einstein gravity during inflation is an interesting issue. We can constrain the strength of the new gravitational terms through observations of inflationary imprints in the actual universe. In this paper we analyze the effects on slow-roll models due to a Chern-Simons term coupled to the inflaton field through a generic coupling function f (φ). A well known result is the polarization of primordial gravitational waves (PGW) into left and right eigenstates, as a consequence of parity breaking. In such a scenario the modifications to the power spectrum of PGW are suppressed under the conditions that allow to avoid the production of ghost gravitons at a certain energy scale, the so-called Chern-Simons mass M CS . In general it has been recently pointed out that there is very little hope to efficiently constrain chirality of PGW on the basis solely of two-point statistics from future CMB data, even in the most optimistic cases. Thus we search if significant parity breaking signatures can arise at least in the bispectrum statistics. We find that the tensor-tensor-scalar bispectra γγζ for each polarization state are the only ones that are not suppressed. Their amplitude, setting the level of parity breaking during inflation, is proportional to the second derivative of the coupling function f (φ) and they turn out to be maximum in the squeezed limit. We comment on the squeezed-limit consistency relation arising in the case of chiral gravitational waves, and on possible observables to constrain these signatures.

We consider light dark matter candidates originated from the evaporation of Schwarzschild primordial black holes, with masses in the range $$10^{-5}$$ 10 - 5 –$$10^9$$ 10 9 g. These candidates are beyond standard model particles with negligible couplings to the other particles, so that they interact only gravitationally. Belonging to the category of warm dark matter, they nevertheless spoil structure formation, with a softer impact for increasing values of the candidate spin. Requiring such candidates to fully account for the observed dark matter, we find that the scenario of black hole domination is ruled out for all spin values up to 2. For the scenario of radiation domination, we derive upper limits on the parameter $$\beta $$ β (the primordial black hole energy density at formation over the radiation one), which are less stringent the higher the candidate spin is.

Chern-Simons gravity coupled to the scalar sector through a generic coupling function f (φ) can be tested at the very high energies of the inflationary period. In ref.[1], we computed the theoretical parity breaking signatures of the γγζ primordial bispectrum which mixes two gravitons and one scalar curvature perturbation. We defined a parameter Π which measures the level of parity breaking of the corresponding bispectrum. In this work we forecast the expected 1σ error on Π using the cosmic microwave background (CMB) angular bispectra. We find that, given the angular resolution of an experiment like Planck, Π ∼ 10 6 is detectable via the measurement of BBT or BBE angular bispectra if the tensor-to-scalar ratio r = 0.01. We also show that, from the theoretical point of view, Π can be greater than 10 6 . Thus, our conclusion is that BBT or BBE CMB angular bispectra can become an essential observable for testing Chern-Simons gravity in the primordial universe.

Parity violation is a powerful observable to distinguish a cosmological background of Gravitational Waves (GWs) from an astrophysical one. Planar single GW interferometers, both on ground and in space, are unable to measure the net circular polarization of an isotropic Stochastic Gravitational Wave Background (SGWB). In this paper, we explore the possibility of detecting circular polarization of an isotropic SGWB by cross-correlating two space-based detectors planned to be launched around 2034: LISA and Taiji. We compute the response of such a network to chirality and we perform a Fisher forecast analysis on the I and V Stokes parameters for the SGWB. We find that a clear measurement of chirality can be claimed for a maximally chiral flat signal with amplitude h 2 ΩGW ≃ 10−12 at the frequency scales of LISA and Taiji.

Gravitons are the quantum counterparts of gravitational waves in low-energy theories of gravity. Using Feynman rules one can compute scattering amplitudes describing the interaction between gravitons and other fields. Here, we consider the interaction between gravitons and photons. Using the quantum Boltzmann equation formalism, we derive fully general equations describing the radiation transfer of photon polarization, due to the forward scattering with gravitons. We show that the Q and U photon linear polarization modes couple with the V photon circular polarization mode, if gravitons have anisotropies in their power-spectrum statistics. As an example, we apply our results to the case of primordial gravitons, considering models of inflation where an anisotropic primordial graviton distribution is produced. Finally, we evaluate the effect on cosmic microwave background (CMB) polarization, showing that in general the expected effects on the observable CMB frequencies are very small. However, our result is promising, since it could provide a novel tool for detecting anisotropic backgrounds of gravitational waves, as well as for getting further insight on the physics of gravitational waves.

The Laser Interferometer Space Antenna (LISA) has the potential to reveal wonders about the fundamental theory of nature at play in the extreme gravity regime, where the gravitational interaction is both strong and dynamical. In this white paper, the Fundamental Physics Working Group of the LISA Consortium summarizes the current topics in fundamental physics where LISA observations of gravitational waves can be expected to provide key input. We provide the briefest of reviews to then delineate avenues for future research directions and to discuss connections between this working group, other working groups and the consortium work package teams. These connections must be developed for LISA to live up to its science potential in these areas.

In this work we analyse in detail the possibility of using small and intermediate-scale gravitational wave anisotropies to constrain the inflationary particle content. First, we develop a phenomenological approach focusing on anisotropies generated by primordial tensor-tensor-scalar and purely gravitational non-Gaussianities. We highlight the quantities that play a key role in determining the detectability of the signal. To amplify the power of anisotropies as a probe of early universe physics, we consider cross-correlations with CMB temperature anisotropies. We assess the size of the signal from inflationary interactions against so-called induced anisotropies. In order to arrive at realistic estimates, we obtain the projected constraints on the non-linear primordial parameter F NL for several upcoming gravitational wave probes in the presence of the astrophysical gravitational wave background. We further illustrate our findings by considering a concrete inflationary realisation and use it to underscore a few subtleties in the phenomenological analysis.