29 research outputs found
The Cosmic Coincidence as a Temporal Selection Effect Produced by the Age Distribution of Terrestrial Planets in the Universe
The energy densities of matter and the vacuum are currently observed to be of
the same order of magnitude: . The cosmological window of time during which this occurs is
relatively narrow. Thus, we are presented with the cosmological coincidence
problem: Why, just now, do these energy densities happen to be of the same
order? Here we show that this apparent coincidence can be explained as a
temporal selection effect produced by the age distribution of terrestrial
planets in the Universe. We find a large () probability that
observations made from terrestrial planets will result in finding at
least as close to as we observe today. Hence, we, and any
observers in the Universe who have evolved on terrestrial planets, should not
be surprised to find . This result is
relatively robust if the time it takes an observer to evolve on a terrestrial
planet is less than Gyr.Comment: Submitted to Ap
Dark-Energy Dynamics Required to Solve the Cosmic Coincidence
Dynamic dark energy (DDE) models are often designed to solve the cosmic
coincidence (why, just now, is the dark energy density , the same
order of magnitude as the matter density ?) by guaranteeing for significant fractions of the age of the universe. This
typically entails ad-hoc tracking or oscillatory behaviour in the model.
However, such behaviour is neither sufficient nor necessary to solve the
coincidence problem. What must be shown is that a significant fraction of
observers see . Precisely when, and for how long, must a
DDE model have in order to solve the coincidence? We
explore the coincidence problem in dynamic dark energy models using the
temporal distribution of terrestrial-planet-bound observers. We find that any
dark energy model fitting current observational constraints on and
the equation of state parameters and , does have for a large fraction of observers in the universe. This demotivates DDE
models specifically designed to solve the coincidence using long or repeated
periods of .Comment: 16 pages, 8 figures, Submitted to Phys. Rev.
A comprehensive comparison of the Sun to other stars: searching for self-selection effects
If the origin of life and the evolution of observers on a planet is favoured
by atypical properties of a planet's host star, we would expect our Sun to be
atypical with respect to such properties. The Sun has been described by
previous studies as both typical and atypical. In an effort to reduce this
ambiguity and quantify how typical the Sun is, we identify eleven
maximally-independent properties that have plausible correlations with
habitability, and that have been observed by, or can be derived from,
sufficiently large, currently available and representative stellar surveys. By
comparing solar values for the eleven properties, to the resultant stellar
distributions, we make the most comprehensive comparison of the Sun to other
stars. The two most atypical properties of the Sun are its mass and orbit. The
Sun is more massive than 95 -/+ 2% of nearby stars and its orbit around the
Galaxy is less eccentric than 93 +/- 1% of FGK stars within 40 parsecs. Despite
these apparently atypical properties, a chi^2 -analysis of the Sun's values for
eleven properties, taken together, yields a solar chi^2 = 8.39 +/- 0.96. If a
star is chosen at random, the probability that it will have a lower value (be
more typical) than the Sun, with respect to the eleven properties analysed
here, is only 29 +/- 11%. These values quantify, and are consistent with, the
idea that the Sun is a typical star. If we have sampled all reasonable
properties associated with habitability, our result suggests that there are no
special requirements for a star to host a planet with life.Comment: Published in the Astrophysical Journal, 684:691-706, 2008 September
1. This version corrects two small errors the press could not correct before
publication - the errors are addressed in an erratum ApJ will release on Dec
1, 200