Alleviating H0H_0 Tension with New Gravitational Scalar Tensor Theories

We investigate the cosmological applications of new gravitational scalar-tensor theories and we analyze them in the light of $H_0$ tension. In these theories the Lagrangian contains the Ricci scalar and its first and second derivatives in a specific combination that makes them free of ghosts, thus corresponding to healthy bi-scalar extensions of general relativity. We examine two specific models, and for particular choices of the model parameters we find that the effect of the additional terms is negligible at high redshifts, obtaining a coincidence with $\Lambda$CDM cosmology, however as time passes the deviation increases and thus at low redshifts the Hubble parameter acquires increased values ($H_0\approx 74 km/s/Mpc$) in a controlled way. The mechanism behind this behavior is the fact that the effective dark-energy equation-of-state parameter exhibits phantom behavior, which implies faster expansion, which is one of the theoretical requirements that are capable of alleviating the $H_0$ tension. Lastly, we confront the models with Cosmic Chronometer (CC) data showing full agreement within 1$\sigma$ confidence level.Comment: 9 pages, 7 figures, version to appear in Phys.Rev.

Alleviating both H0H_0 and σ8\sigma_8 tensions in Tsallis cosmology

We present how Tsallis cosmology can alleviate both $H_0$ and $\sigma_8$ tensions simultaneously. Such a modified cosmological scenario is obtained by the application of the gravity-thermodynamics conjecture, but using the non-additive Tsallis entropy, instead of the standard Bekenstein-Hawking one. Hence, one obtains modified Friedmann equations, with extra terms that depend on the new Tsallis exponent $\delta$ that quantifies the departure from standard entropy. We show that for particular $\delta$ choices we can obtain a phantom effective dark energy, which is known to be one of the sufficient mechanisms that can alleviate $H_0$ tension. Additionally, for the same parameter choice we obtain an increased friction term and an effective Newton's constant smaller than the usual one, and thus the $\sigma_8$ tension is also solved. These features act as a significant advantage of Tsallis modified cosmology.Comment: 14 pages, 4 figure

Alleviating the <i>H</i>₀ Tension in Scalar–Tensor and Bi-Scalar–Tensor Theories

Herein, we investigate scalar–tensor and bi-scalar–tensor modified theories of gravity that can alleviate the H0 tension. In the first class of theories, we show that by choosing particular models with a shift-symmetric friction term we are able to alleviate the tension by obtaining a smaller effective Newton’s constant at intermediate times, a feature that cannot be easily obtained in modified gravity. In the second class of theories, which involve two extra propagating degrees of freedom, we show that the H0 tension can be alleviated, and the mechanism behind this is the phantom behavior of the effective dark-energy equation-of-state parameter. Hence, scalar–tensor and bi-scalar–tensor theories have the ability to alleviate the H0 tension with both known sufficient late-time mechanisms