Parity-violation in bouncing cosmology

The parity violating model based on teleparallel gravity is a competitive scheme for parity violating gravity, which has been preliminary studied in the literature. To further investigate the parity violating model in teleparallel gravity, in this paper, we construct all independent parity-odd terms that are quadratic in torsion tensor and coupled to a scalar field in a way without higher-order derivatives. Using these parity-odd terms, we formulate a general parity violating scalar-tensor model in teleparallel gravity and obtain its equations of motion. To explore potentially viable models within the general model, we investigate the cosmological application of a submodel of the general model in which terms above the second power of torsion are eliminated. We focus on analyzing cosmological perturbations and identify the conditions that preserve the parity violating signal of gravitational waves at linear order while avoiding the ghost instability.

“…Other recent studies on PV gravities can be found in refs. [23][24][25][26][27][28][29][30][31][32][33][34][35][36].…”

Section: Jhep08(2023)070mentioning

confidence: 99%

Parity violating scalar-tensor model in teleparallel gravity and its cosmological application

Rao,

Zhao

2023

“…Basically, the violation of NEC implies an increase in the Hubble parameter H (i.e., dH/dt > 0). Given that the power spectra of scalar and tensor perturbations are proportional to H 2 , an intermediate NEC violation during inflation leads to intriguing phenomena of observational interest at certain scales, including: 1) a significant enhancement in the power spectrum of primordial GWs [122,123], 2) a notable amplification of the parity-violation effect in primordial GWs [124] (see also [125]), 3) the generation of primordial black holes with masses and abundances of observational interest [126], along with the associated scalar-induced GWs. Remarkably, the scenario proposed by [122] naturally yields a broken power-law power spectrum, which may potentially be consistent with observations from both CMB and PTA, see also [55] for recent studies.…”

Section: Introductionmentioning

confidence: 99%

Null energy condition violation during inflation and pulsar timing array observations

Ye,

Zhu,

Cai

2024

Self Cite

Recently, evidence of stochastic gravitational wave background (SGWB) signals observed by pulsar timing array (PTA) collaborations, has prompted investigations into their origins. We explore the compatibility of a proposed inflationary scenario, incorporating an intermediate null energy condition (NEC)-violating phase, with the PTA observations. The NEC violation potentially amplifies the primordial tensor power spectrum, offering a promising explanation for PTA observations. Numerical analyses, primarily focused on NANOGrav’s 15-year results, reveal the model’s compatibility with PTA data. Notably, the model predicts a nearly scale-invariant GW spectrum in the mHz frequency range, which sets our scenario apart from other interpretations predicting a red primordial GW spectrum on smaller scales.

One-loop effective action of the IKKT model for cosmological backgrounds

Battista,

Steinacker

2024

We study cosmological solutions of the IKKT model with k = –1 FLWR geometry, taking into account one-loop corrections. A previously discussed covariant quantum spacetime is found to be stabilized through one-loop effects at early times, without adding a mass term to the model. At late times, this background is modified and goes through a period of acceleration, before settling down to a coasting FLRW geometry with scale parameter a(t) ~ t. This is qualitatively close to observation without any fine-tuning, irrespective of the detailed matter content of the universe.