  Astronomical observations have shown that the expansion of the universe is at
present accelerating, consistently with a constant negative pressure or
tension. This is a major puzzle because we do not understand why this tension
is so small compared to the Planck density; why, being so small, it is not
exactly zero; and why it has precisely the required value to make the expansion
start accelerating just at the epoch when we are observing the universe. The
recently proposed conjecture by Afshordi that black holes create a
gravitational aether owing to quantum gravity effects, which may be identified
with this invisible tension, can solve this coincidence problem. The fact that
the expansion of the universe is starting to accelerate at the epoch when we
observe it is a necessity that is implied by our origin in a planet orbiting a
star that formed when the age of the universe was of the same order as the
lifetime of the star. This argument is unrelated to any anthropic reasoning.
