English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Higher-derivative gravity theories, such as Lovelock theories, generalizeEinstein's general relativity (GR). Modifications to GR are expected whencurvatures are near Planckian and appear in string theory or supergravity. Butcan such theories describe gravity on length scales much larger than the Planckcutoff length scale? Here we find causality constraints on Lovelock theoriesthat arise from the requirement that the equations of motion (EOM) ofperturbations be hyperbolic. We find a general expression for the "effectivemetric" in field space when Lovelock theories are perturbed around somesymmetric background solution. In particular, we calculate explicitly theeffective metric for a general Lovelock theory perturbed around cosmologicalFriedman-Robertson-Walker backgrounds and for some specific cases whenperturbed around Schwarzschild-like solutions. For the EOM to be hyperbolic,the effective metric needs to be Lorentzian. We find that, unlike for GR, theeffective metric is generically not Lorentzian when the Lovelock modificationsare significant. So, we conclude that Lovelock theories can only be consideredas perturbative extensions of GR and not as truly modified theories of gravity.We compare our results to those in the literature and find that they agree withand reproduce the results of previous studies.

Causality Violations in Lovelock Theories