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Register a new userEmergent Universe and Genesis from the DHOST Cosmology
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In this article, we present an emergent universe scenario that can be derived from DHOST cosmology. The universe starts asymptotically Minkowski in the far past just like the regular Galileon Genesis, but evolves to a radiation dominated period at the late stage, and therefore, the universe has a graceful exit which is absent in the regular Galileon Genesis. We analyze the behavior of cosmological perturbations and show that both the scalar and tensor modes are free from the gradient instability problem. We further analyze the primordial scalar spectrum generated in various situations and discuss whether a scale invariance can be achieved.
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We improve the DHOST Genesis proposed in cite{Ilyas:2020zcb}, such that the near scale invariant scalar power spectrum can be generated from the model itself, without involking extra mechanism like a string gas. Besides, the superluminality problem of scalar perturbation plagued in cite{Ilyas:2020zcb} can be rescued by choosing proper DHOST action.
We present a new class of nonsingular bounce cosmology free from instabilities, using a single scalar field coupled to gravity within the framework of the Degenerate Higher-Order Scalar-Tensor (DHOST) theories. In this type of scenarios, the gradient instability that widely exists in nonsingular bounce cosmologies in the framework of scalar-tensor and Horndeski/Galileon theories is removed by the effects of new operators introduced by the DHOST, due to the modification that they later bring about to the dispersion relation of perturbations. Hence, our results demonstrate that there is indeed a loophole for this type of bounce scenarios to be free from pathologies when primordial perturbations evolve through the bounce phase, and thus the theoretical {it no-go} theorem for nonsingular bounce cosmology of Horndeski/Galileon theories can be delicately evaded in DHOST extensions.
In quadratic-order degenerate higher-order scalar-tensor (DHOST) theories compatible with gravitational-wave constraints, we derive the most general Lagrangian allowing for tracker solutions characterized by $dot{phi}/H^p={rm constant}$, where $dot{phi}$ is the time derivative of a scalar field $phi$, $H$ is the Hubble expansion rate, and $p$ is a constant. While the tracker is present up to the cubic-order Horndeski Lagrangian $L=c_2X-c_3X^{(p-1)/(2p)} square phi$, where $c_2, c_3$ are constants and $X$ is the kinetic energy of $phi$, the DHOST interaction breaks this structure for $p eq 1$. Even in the latter case, however, there exists an approximate tracker solution in the early cosmological epoch with the nearly constant field equation of state $w_{phi}=-1-2pdot{H}/(3H^2)$. The scaling solution, which corresponds to $p=1$, is the unique case in which all the terms in the field density $rho_{phi}$ and the pressure $P_{phi}$ obey the scaling relation $rho_{phi} propto P_{phi} propto H^2$. Extending the analysis to the coupled DHOST theories with the field-dependent coupling $Q(phi)$ between the scalar field and matter, we show that the scaling solution exists for $Q(phi)=1/(mu_1 phi+mu_2)$, where $mu_1$ and $mu_2$ are constants. For the constant $Q$, i.e., $mu_1=0$, we derive fixed points of the dynamical system by using the general Lagrangian with scaling solutions. This result can be applied to the model construction of late-time cosmic acceleration preceded by the scaling $phi$-matter-dominated epoch.
A specific theoretical framework is important for designing and conducting an experiment, and for interpretation of its results. The field of gravitational physics is expanding, and more clarity is needed. It appears that some popular notions, such as `inflation and `gravity is geometry, have become more like liabilities than assets. A critical analysis is presented and the ways out of the difficulties are proposed.
We derive the primordial power spectra and spectral indexes of the density fluctuations and gravitational waves in the framework of loop quantum cosmology (LQC) with holonomy and inverse-volume corrections, by using the uniform asymptotic approximation method to its third-order, at which the upper error bounds are $lesssim 0.15%$, and accurate enough for the current and forthcoming cosmological observations. Then, using the Planck, BAO and SN data we obtain the tightest constraints on quantum gravitational effects from LQC corrections, and find that such effects could be well within the detection of the current and forthcoming cosmological observations.
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