Abstract:We study the continuum limit of the Benincasa-Dowker-Glaser causal set action on a causally convex compact region. In particular, we compute the action of a causal set randomly sprinkled on a small causal diamond in the presence of arbitrary curvature in various spacetime dimensions. In the continuum limit, we show that the action admits a finite limit. More importantly, the limit is composed by an Einstein-Hilbert bulk term as predicted by the Benincasa-Dowker-Glaser action, and a boundary term exactly propor… Show more
“…It states the continuum limit of the BD action, evaluated on a causal set generated from sprinkling on a compact region with null boundaries, is proportional to the Einstein-Hilbert action plus a joint term proportional to its co-dimension 2 volume. This conjecture is recently verified for causal diamonds in perturbative regimes [18,19]. Here we consider the region X with more complex joints than the simple lightcone-light-cone intersection.…”
We discuss how to define a kinematical horizon entropy on a causal set. We extend a recent definition of horizon molecules to a setting with a null hypersurface crossing the horizon. We argue that, as opposed to the spacelike case, this extension fails to yield an entropy local to the hypersurface-horizon intersection in the continuum limit when the causal set approximates a curved spacetime. We then investigate the entropy defined via the spacetime mutual information between two regions of a causal diamond truncated by a causal horizon, and find it does limit to the area of the intersection.
“…It states the continuum limit of the BD action, evaluated on a causal set generated from sprinkling on a compact region with null boundaries, is proportional to the Einstein-Hilbert action plus a joint term proportional to its co-dimension 2 volume. This conjecture is recently verified for causal diamonds in perturbative regimes [18,19]. Here we consider the region X with more complex joints than the simple lightcone-light-cone intersection.…”
We discuss how to define a kinematical horizon entropy on a causal set. We extend a recent definition of horizon molecules to a setting with a null hypersurface crossing the horizon. We argue that, as opposed to the spacelike case, this extension fails to yield an entropy local to the hypersurface-horizon intersection in the continuum limit when the causal set approximates a curved spacetime. We then investigate the entropy defined via the spacetime mutual information between two regions of a causal diamond truncated by a causal horizon, and find it does limit to the area of the intersection.
“…It states the continuum limit of the BD action, evaluated on a causal set generated from sprinkling on a compact region with null boundaries, is proportional to the Einstein-Hilbert action plus a joint term proportional to its co-dimension 2 volume. This conjecture is recently verified for causal diamonds in perturbative regimes [18,19]. Here we consider the region X with more complex joints than the simple light-cone-light-cone intersection.…”
We discuss how to define a kinematical horizon entropy on a causal set. We extend a recent definition of horizon molecules to a setting with a null hypersurface crossing the horizon. We argue that, as opposed to the spacelike case, this extension fails to yield an entropy local to the hypersurface-horizon intersection in the continuum limit when the causal set approximates a curved spacetime. We then investigate the entropy defined via the Spacetime Mutual Information between two regions of a causal diamond truncated by a causal horizon, and find it does limit to the area of the intersection.
“…I thank Ludovico Machet and Jinzhao Wang for sharing with me their calculations of the mean of the discrete random action of Riemann normal neighbourhoods which stimulated this work. The results on causal intervals are a special case of their general result on Riemann normal neighbourhoods in all dimensions [24]. We have used different methods both of which can be useful in future work and have agreed to publish both sets of calculations.…”
Evidence is provided for a conjecture that, in the continuum limit, the mean of the causal set action of a causal set sprinkled into a globally hyperbolic Lorentzian spacetime, M, of finite volume equals the Einstein Hilbert action of M plus the volume of the co-dimension 2 intersection of the future boundary with the past boundary. We give the heuristic argument for this conjecture and analyse some examples in 2 dimensions and one example in 4 dimensions.
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