The shared causal pasts and futures of cosmological events
Abstract
We derive criteria for whether two cosmological events can have a shared causal past or a shared causal future, assuming a FriedmannLemaitreRobertsonWalker (FLRW) universe with bestfit cosmological parameters from the Planck satellite. We further derive criteria for whether either cosmic event could have been in past causal contact with our own worldline since the time of the hot “big bang,” which we take to be the end of earlyuniverse inflation. We find that pairs of objects such as quasars on opposite sides of the sky with redshifts z≥3.65 have no shared causal past with each other or with our past worldline. More complicated constraints apply if the objects are at different redshifts from each other or appear at some relative angle less than 180°, as seen from Earth. We present examples of observed quasar pairs that satisfy all, some, or none of the criteria for past causal independence. Given dark energy and the recent accelerated expansion, our observable Universe has a finite conformal lifetime, and hence a cosmic event horizon at current redshift z=1.87. We thus constrain whether pairs of cosmic events can signal each other’s worldlines before the end of time. Lastly, we generalize the criteria for shared past and future causal domains for FLRW universes with nonzero spatial curvature.
 Publication:

Physical Review D
 Pub Date:
 August 2013
 DOI:
 10.1103/PhysRevD.88.044038
 arXiv:
 arXiv:1305.3943
 Bibcode:
 2013PhRvD..88d4038F
 Keywords:

 04.20.Gz;
 98.80.k;
 Spacetime topology causal structure spinor structure;
 Cosmology;
 Astrophysics  Cosmology and Extragalactic Astrophysics;
 Astrophysics  Galaxy Astrophysics;
 Astrophysics  High Energy Astrophysical Phenomena;
 General Relativity and Quantum Cosmology
 EPrint:
 40 pages, 16 figures, modified to more closely reflect published version