Seminář se koná v úterý ve 13:10 v posluchárně ÚTF MFF UK
v 10. patře katedrové budovy v Tróji, V Holešovičkách 2, Praha 8
By using a new perturbative approach we derive a fully covariant and gauge invariant theory of adiabatic, radial perturbations of perfect fluid stars within the theory of general relativity, which is equivalent to (but more powerful than) the Chandrasekhar pulsation equation. We discuss how the choice of frame in which perturbations are described can significantly simplify the mathematical analysis of the problem, and we single out two different relevant frames: comoving and static. The properties of the perturbation equations in both frames are discussed, and we show that on the one hand, it is possible to find analytical solutions for the perturbation equations if the background is sufficiently regular, and on the other hand the system is also suitable for numerical implementation. Exploiting the new formalism, we derive an upper bound for the maximum compactness of stable, perfect fluid stars, which is equation-of-state-agnostic and significantly smaller than the Buchdahl bound. Finally, the covariance of the theory allows us to draw some insights into the thermodynamics of perturbative non-equilibrium systems.
4-Dimensional Einstein-Gauss-Bonnet (4DEGB) gravity has garnered significant attention in the last few years as a phenomenological competitor to general relativity. We consider the theoretical and observational implications of this theory in both the early and late universe, (re-)deriving background and perturbation equations and constraining its characteristic parameters with data from cosmological probes. Our investigation surpasses the scope of previous studies by incorporating non-flat spatial sections. We explore consequences of 4DEGB on the sound and particle horizons in the very early universe, and demonstrate that 4DEGB can provide an independent solution to the horizon problem for some values of its characteristic parameter alpha. Finally, we constrain an unexplored regime of this theory in the limit of small coupling alpha (empirically supported in the post-Big Bang Nucleosynthesis era by prior constraints). This version of 4DEGB includes a geometric term that resembles dark radiation at the background level, but whose influence on the perturbed equations is qualitatively distinct from that of standard forms of dark radiation. In this limit, only one beyond-LambdaCDM degree of freedom persists, which we denote as alphaC. Our analysis yields the estimate alphaC = (-9 ± 6) × 10-6 thereby providing a new constraint of a previously untested sector of 4DEGB.
Jiří Bičák Oldřich Semerák