Primordial Gravitational Waves to Probe Non-Gaussianity and Parity

A new study explores the capability of third-generation gravitational-wave detectors, such as the Einstein Telescope and Cosmic Explorer, to detect scalar-induced gravitational waves (SIGWs). These waves, generated in the primordial universe, could offer a unique pathway to investigate primordial non-Gaussianity and parity violation. Researchers have developed a framework that considers contributions to the energy density spectrum of stochastic gravitational waves (SGWB) arising from scalar non-Gaussianity, quantified by the primordial bispectrum and trispectrum. The particularity of this approach lies in the fact that the parity-odd component of the primordial scalar trispectrum induces circular polarization in the stochastic gravitational-wave background. This circular polarization would act as a direct signature of parity violation in the early universe. The analysis, based on simulated data from future interferometers, suggests that these instruments will be able to place competitive constraints on both the bispectrum and the scalar trispectrum (both its parity-even and parity-odd components). Furthermore, the study has taken into account the contribution from astrophysical sources of gravitational waves, which could act as a "foreground" for the primordial signals. Despite the presence of this astrophysical foreground, the results indicate that future detectors will be able to effectively constrain cosmological parameters related to SIGWs, as well as astrophysical parameters. This highlights the potential of primordial gravitational waves as a cosmological tool to unravel fundamental properties of the early universe, such as the distribution of density fluctuations and parity symmetry.

$f(R,T)$ Gravity Models Consistent with Standard Cosmological Model

A new analysis of modified $f(R,T)$ gravity models, which consider an arbitrary function of the Ricci curvature ($R$) and the trace of the energy-momentum tensor ($T$), has found these models to be consistent with the standard cosmological model ($\Lambda$CDM). Researchers examined the specific form $f(R,T) = R + \lambda T^\epsilon$, fitting its parameters to a variety of observational cosmological data. This approach seeks to explore alternatives to dark energy for explaining the accelerated expansion of the universe, without the need to introduce a cosmological constant or additional fields. The study utilized a combination of key cosmological datasets: the cosmic microwave background (CMB), baryon acoustic oscillations (BAO), cosmic chronometers, and Type Ia supernovae. Unlike previous work, this research explicitly incorporated correlations between the different datasets and radiation effects, allowing for a more robust characterization of the model parameters. The inclusion of these factors is crucial for obtaining precise and reliable fits in the context of current precision cosmology. The results of the analysis show that the best fit for the parameter $\epsilon$ is $0.010^{+0.013}_{-0.021}$. This value is remarkably close to $\epsilon = 0$, which corresponds to the standard cosmological model. The compatibility of this range with $\epsilon = 0$ suggests that, while $f(R,T)$ models offer a broader theoretical framework, current observations do not require a significant deviation from Einstein's gravity in the matter sector. This implies that dark energy remains the simplest and most consistent explanation with current data, although the door to subtle gravitational modifications remains open for future, higher-precision observations.