The Shared Causal Pasts and Futures of Cosmological Events

TL;DR

This paper establishes criteria for shared causal pasts and futures of cosmological events within FLRW universes, analyzing observational data and the effects of dark energy on causal contact and horizons.

Contribution

It introduces new criteria for causal connectivity of cosmic events in FLRW universes, including effects of dark energy and spatial curvature, with observational applications.

Findings

Quasar pairs with z >= 3.65 have no shared causal past with each other or our worldline.

The observable universe has a finite conformal lifetime due to accelerated expansion.

Criteria for causal independence are extended to universes with nonzero spatial curvature.

Abstract

We derive criteria for whether two cosmological events can have a shared causal past or a shared causal future, assuming a Friedmann-Lemaitre-Robertson-Walker universe with best-fit \Lambda CDM 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 early-universe 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 degrees, 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.