Ning Bao, Jason Pollack, Grant N. Remmen

The recently proposed ER=EPR correspondence postulates the existence of wormholes (Einstein-Rosen bridges) between entangled states (such as EPR pairs). Entanglement is famously known to be unobservable in quantum mechanics, in that there exists no observable (or, equivalently, projector) that can accurately pick out whether a generic state is entangled. Many features of the geometry of spacetime, however, are observables, so one might worry that the presence or absence of a wormhole could identify an entangled state in ER=EPR, violating quantum mechanics, specifically, the property of state-independence of observables. In this note, we establish that this cannot occur: there is no measurement in general relativity that unambiguously detects the presence of a generic wormhole geometry. This statement is the ER=EPR dual of the undetectability of entanglement.

Netta Engelhardt, Gary T. Horowitz

Two quantum field theories whose Hilbert spaces do not overlap cannot transmit energy to one another. From this simple principle, we deduce some highly nontrivial consequences for holographic quantum gravity. These include: (1) certain cosmological bounces are forbidden, (2) generic singularities inside black holes cannot be resolved, and (3) traversable wormholes do not exist. At the classical level, this principle rules out certain types of naked singularities and suggests that new singularity theorems should exist.

Eleonora Di Valentino, Alessandro Melchiorri, Joseph Silk

The recent measurements of Cosmic Microwave Background temperature and polarization anisotropies made by the Planck satellite have provided impressive confirmation of the $\Lambda$CDM cosmological model. However interesting hints of slight deviations from $\Lambda$CDM have been found, including a $95 \%$ c.l. preference for a "modified gravity" structure formation scenario. In this paper we confirm the preference for a modified gravity scenario from Planck 2015 data, find that modified gravity solves the so-called $A_{lens}$ anomaly in the CMB angular spectrum, and constrains the amplitude of matter density fluctuations to $\sigma_8=0.815_{-0.048}^{+0.032}$, in better agreement with weak lensing constraints. Moreover, we find a lower value for the reionization optical depth of $\tau=0.059\pm0.020$ (to be compared with the value of $\tau= 0.079 \pm 0.017$ obtained in the standard scenario), more consistent with recent optical and UV data. We check the stability of this result by considering possible degeneracies with other parameters, including the neutrino effective number, the running of the spectral index and the amount of primordial helium. The indication for modified gravity is still present at about $95\%$ c.l., and could become more significant if lower values of $\tau$ were to be further confirmed by future cosmological and astrophysical data.

Philip Bull

Tests of general relativity (GR) are still in their infancy on cosmological scales, but forthcoming experiments promise to greatly improve their precision over a wide range of distance scales and redshifts. One such experiment, the Square Kilometre Array (SKA), will carry out several wide and deep surveys of resolved and unresolved neutral hydrogen (HI) 21cm line-emitting galaxies, mapping a significant fraction of the sky from $0 \le z \lesssim 6$. I present forecasts for the ability of a suite of possible SKA HI surveys to detect deviations from GR by reconstructing the cosmic expansion and growth history. SKA Phase 1 intensity mapping surveys can achieve sub-1% measurements of $f\sigma_8$ out to $z\approx 1$, with an SKA1-MID Band 2 survey out to $z \lesssim 0.6$ able to surpass contemporary spectroscopic galaxy surveys such as DESI and Euclid in terms of constraints on modified gravity parameters if challenges such as foreground contamination can be tackled effectively. A more futuristic Phase 2 HI survey of $\sim 10^9$ spectroscopic galaxy redshifts would be capable of detecting a $\sim 2\%$ modification of the Poisson equation out to $z\approx 2$.