Dark Energy: Hopes and Expectations. Mario Livio Space Telescope Science Institute. What Causes Cosmic Acceleration?. Dark Energy? Standard General Relativity? Alternative Theories of Gravity?. Standard GR?. Premise: Universe not homogeneous today (fractal bubble model).
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Premise: Universe nothomogeneous today (fractal bubble model).
Result: Non-linear gravitational effects, averages, local time => distance-redshift relation as observed.
Strengths: 1. No need for unseen stuff.
2. No need to change GR.
3. Could perhaps be resolved soon.
Problems: 1. Anti-Copernican (Milky Way in special place)? (Not necessarily, but different angular scales in CMB.)
2. Not clear if solution can match all observations (e.g. H0, peaks in CMB).
Strengths: 1. Motivated by ideas from string theory.2. Testable in principle.
Weaknesses: 1. All but ruled out by lab and lunar ranging experiments
Inverse square law holds down to d = 56 m!
Measurements of lunar perihelion precession with an accuracy of δΦ = 1.4x10-12
2. Not clear if self-consistent model exists.
Yellow region excluded at 95% confidence level.
Strengths: 1. Clearly exists!2. If dark energy behaves like an ideal fluid:
This is unstable for any negative value of w, except w = -1, vacuum energy!
Problems: 1. Why so small and positive?
(But SUSY testable by LHC)
2. Why now?
Cosmological constant (t) very slowly varying function, most time spent at small and positive value.
String theory suggests that ~101000 vacua may exist.
Inflation may produce an ensemble of pocket universes.
If V can take a broad range of values in the multiverse, then complexity can arise only in some subuniverse. In generalV/3 is constrained.
For a flat probability distribution:
Principle of mediocrity: Is our universe typical?
By contrast, in cyclic model complexity can arise in almost every patch of space.
Dynamical and anthropic reasoning are not mutually exclusive!
What should we do if multiverse is predicted?