Dark Energy Models

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Dark Energy Models

• Robert Caldwell
• Dartmouth College

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Numerous observations and experiments support the
hypothesis that our universe is filled by some
sort of dark energy which is responsible for the
cosmic acceleration.
January 2005
large-scale structure
cosmic microwave background
supernova distances
simple explanation?
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Expansion, Geometry, Gravity
metric
distances
progress!
Kamionkowski RC astro-ph/0403003
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What is dark energy?
Our view of the cosmos requires some
interpretation
Interpret metric information energy curves
space
dark energy accelerates the cosmic expansion
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dark energy models

• Cosmological Constant
• Dynamical Dark Energy, Quintessence
• Gravitational Transition
• many other ideas

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dark energy model 1
Cosmological Constant
a perfect substance?
is it physical?
is it thermodynamic?
is it a sign of total theory breakdown?
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dark energy model 2
Dynamical Dark Energy, Quintessence
an ultra-light scalar

• Dark energy axion?
• Vacuumless field?

recast equation of motion

• consistent w/eqn?

• sensible mass?

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Dark Energy with w lt -1?
A phantom? Systematic effects or strange, new
physics!
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dark energy model 2
Quest for the Equation-of-State
A generalized scalar cannot cross
what goes wrong?

• Sound speed becomes imaginary!
• Pressure fluctuations diverge!
• Perturbative treatment breaks down

A fluid with a well-behaved sound speed can cross
A pair of scalars can compose a crossing dark
energy

• Small-scale fluctuations vanish at instant w-1
• No distinct trace on cmb, lss

Vikman 04 Hu 04 Doran RC 05
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dark energy model 2
bulk viscosity generates negative pressure

• Cosmological particle production can generate
• a large, negative bulk pressure (Zeldovich
  70)

• Bulk pressure in the dark sector (Zimdahl 93,
  98)

Some simple models
detailed analysis?
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dark energy model 4
A Gravitational Transition
(some) gravitational theories

• Late-time transition leads to acceleration
• Both (apparent) w gt -1 and w lt -1 are possible
• Tests of GR turn up interesting constraints

deSitter attractor
need for higher dimensions?
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gravity
Is the universe is like a strange gravitational
sea with Einstein-islands?
cluster-size?
consider implications for lensing, structure
evolution
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dark energy model 4
A Gravitational Transition
Evolve towards constant curvature
background like w(a) fluctuations novel
constraints from GR tests
Parker Raval, PRL 86, 749 (2001) Carroll et al,
PRD 70, 043528 (2004) Parker, Vanzella, RC 05
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gravitational lensing
influence of dark energy?
classic test of gravity
non-Einstein gravity anisotropic shear

ISW P(k)
Bekenstein Sanders, ApJ 429, 480 (1994)
Acquaviva et al, PRD 70, 023515 (2004)
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structure evolution
influence of dark energy?
Additional clustering a la quintessence,
k-essence, etc
Transition to constant curvature does not
automatically squash inhomogeneities
gravitational waves gain a mass
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dark energy models

• Cosmological Constant
• Dynamical Dark Energy, Quintessence
• Gravitational Transition
• many other ideas