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
Quantum fields can notoriously violate the null energy condition (NEC). In a cosmological context, NEC violation can lead to, e.g., dark energy at late times with an equation-of-state parameter smaller than -1 and non-singular bounces at early times. However, it is expected that there should still be a limit in semiclassical gravity to how much “negative energy” can accumulate over time and in space as a result of quantum effects. In the course of formulating quantum-motivated energy conditions, the smeared null energy condition has emerged as a recent proposal. This condition conjectures the existence of a semi local bound on negative energy along null geodesics, which is expected to hold in semiclassical gravity. In this work, we show how the smeared null energy condition translates into theoretical constraints on NEC-violating cosmologies. Specifically, we derive the implied bounds on dark energy equation-of-state parameters and an inequality between the duration of a bouncing phase and the growth rate of the Hubble parameter at the bounce. In the case of dark energy, we identify the parameter space over which the smeared null energy condition is consistent with the recent constraints from the Dark Energy Spectroscopic Instrument.
General relativity describes the Universe with extraordinary success, yet black hole horizons, the Big Bang, and cosmic acceleration point toward regimes where its classical description may be incomplete. Can observations turn quantum gravity into an empirical science? I will discuss three possible windows. Quantum structure near black-hole horizons may produce delayed gravitational wave “echoes” and departures from the Kerr geometry. In the early Universe, the quantum running of couplings in quadratic gravity can generate inflation and make testable predictions for the cosmic microwave background. At late times, DESI’s apparent preference for evolving dark energy might instead reflect a “cosmic glitch in gravity”: a small mismatch between the strength of gravity measured locally and on cosmological scales. A recent CMB and DESI analysis finds cosmological gravity to be roughly one percent weaker than Newtonian gravity. Intriguingly, the inferred glitch is of the same order as the inverse de Sitter scrambling time in Hubble units. The same logarithmic dependence on horizon entropy controls the black-hole echo delay, suggesting that quantum horizons might leave related observable imprints across vastly different scales.
TBA
TBA
TBA
Hermiticity is usually treated in an axiomatic way in quantum mechanics, ensuring key properties of observables in a Hilbert space. In this talk I will discuss how Hermiticity may be something deeper; a symmetry law associated with the conservation of the inner product current in space-time. We will see that this symmetry breaks in the presence of gravitational fields, causal horizons, or space-time curvature for a restricted observer. I will motivate the background for such a symmetry showing what cosmology and near-horizon black hole thermodynamics tell us about it, namely that to preserve global flatness of the universe one needs effective Hermiticity to hold true, but local Hermiticity needs to break near black hole horizons to ensure the second law of thermodynamics. I then discuss how all of this leads to the notion that Probability itself, like energy in general relativity, becomes Quasilocal in curved spacetimes. We then conclude by discussing the implications of all this on quantum field theory in curved spacetimes and quantum gravity in general.
TBA
TBA
David Kubizňák Oldřich Semerák