Dark Energy Cosmology

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Dark Energy Cosmology
Robert Caldwell Dartmouth College

• INPE Winter School
• September 12-16, 2005

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Cosmic Evolution
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Cosmic Acceleration
observations of type 1a supernovae indicate our
universe is accelerating
RC, Physics World, May 2005 data Riess et al,
ApJ 607 (2004) 665
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Cosmic Acceleration
observations of type 1a supernovae indicate our
universe is accelerating
universe contains m, k, ?
k0 (CMB)
basic picture
spatially-flat RW spacetime
acceleration due to ?
Riess et al, ApJ 607 (2004) 665
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Cosmological Constant
The Cosmological Constant and the Theory of
Elementary Particles
Zeldovich, Sov. Phys. Uspekhi 11 (1968) 381
The genie (?) has been let out of the bottle.
A new field of activity arises, namely the
determination of ?
The quantum vacuum energy of scalar particles
resembles a cosmological constant
Problem IR phenomenon has UV divergence
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Casimir Effect
sketch of basic calculation
oscillator energy
energy density
with boundaries
change due to plates
pressure difference
uniform stress-energy between the plates
Milton, 2001
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Casimir Effect Measurements
Lamoreaux, PRL 78 (1997) 5
Mohideen Roy, PRL 81 (1998) 4549
Bressi et al, PRL 88 (2002) 041804
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Casimir Effect
Vacuum Energy is not Immutable
Casimir Effect in a Weak Gravitational Field
Energy density carried in vacuum fields distorts
in response to a weak gravitational field
Caldwell, astro-ph/0209312
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Cosmological Constant
Are unattached vacuum loops real? Do they
gravitate?
Casimir Effect a real manifestation of
fluctuations of the QED vacuum
Quantum vacuum contributes to inertia
Jaekel, Lambricht, Reynaud, New Astron. Rev. 46
(2002) 727
Casimir effect without the vacuum
Jaffe, PRD 72 (2005) 021301
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Constant Problems
Numerous sources of an effective ? in physics
beyond SM
Everywhere you look in QFT (even standard
model) Symmetry-breaking vacuum energy
(Higgs-like fields) Supergravity 3-form (may be
quantized)
Ways to get rid of it
Cancellation (unstable to dynamics) Supersymmetry
(broken) Euclidean QG (probabilistic)
Weinberg, Rev Mod Phys 61 (1989) 1
Any physics content? There must be fluctuations!
Ghost Condensation Arkan-Hamed et al, JHEP 0405
(2004) 074
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Action Principle
Einstein 1919 Anderson Finkelstein, Am J. Phys.
39 (1971) 901 van der Bij and van Dam, Physica
116A (1982) 307
Unimodular gravity
a theory of (linear quantum) gravitation
subject to symmetries satisfying -det(g)1
trace
nearly equivalent formulation of Einsteins GR
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Action Principle Multiplication
Linde, PLB 200 (1988) 272

• Two spacetimes, x y
• Identical matter content
• Antipodal symmetry
• Result zero vacuum and trace-free GR

quantum effects? observable consequences?
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Action Principle Subtraction
Kaplan Sundrum, hep-th/0505265

• One spacetime
• Identical matter content
• Energy parity symmetry
• Result zero vacuum but there are ghosts

ghost sector must be underpopulated
classically stable, but QG instability
Slow instability with QG cutoff ? 10-3 eV?
observable consequence
30? breakdown of gravitational potential
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Cosmological Constant a bookmark
Regard ? as a placeholder until observation,
experiment, and theory can determine the
underlying physical laws behind the dark energy
phenomenon.
Astrophysical phenomena not sensitive to the dark
energy
use ? as a simple, economical model
Cosmological phenomena sensitive to the dark
energy
Question observations / experiments Test
theoretical foundation Search for conflicts with
baseline ? What are the alternatives?
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Dark Energy
What is it?
The cosmological constant?
A new particle / field?
A new aspect of gravitation?
A misinterpretation of observations?
Astrophysical confusion?
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Lives of the Cosmological Constant
1917 Einstein, deSitter
1968 Zeldovich
History Kragh, Cosmology Controversy
1980s
? dark matter or baryonic models
Numerous authors investigated the cosmological
consequences of an additional smooth,
time-varying component of the cosmic fluid.
Kinematic Tests of Exotic, Flat Cosmological
Models
Charlton Turner, ApJ 313 (1987) 495
Cosmology with a Time-Variable Cosmological
Constant
Peebles Ratra, ApJ 325 (1988) L17
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Dark Energy
close the system of equations
evaluate classical tests of cosmology to
constrain w
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Dark Energy
w equation-of-state controls the slope of the
dark energy curve
past in the simplest model, the dark energy does
not play a significant role in cosmic evolution
before z2
present the time-evolution of w determines the
time-evolution of the dark energy
future unknown without a reliable model of the
dark energy
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Dark Energy
The dark energy phenomena could be due to a new,
very-light scalar field.
(-)
Why a scalar field? This is arguably the
simplest method of introducing new physics within
the well-tested framework of quantum physics.
Due to the similarity of the dark energy
phenomena with inflation, it seems plausible that
a similar mechanism is responsible.
Scalar fields, while not necessarily fundamental,
are ubiquitous in theories beyond the standard
model of particle physics.
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Dark Energy
The scalar field is a pioneer, sent out to
explore new worlds of physics!

• Waves, Optics
• Electrodynamics
• Quantum Mechanics
• Scalar QED
• Field Theory
• Symmetry Breaking
• Dilatons, Moduli

• Nordstroms Scalar Gravity
• Kaluza-Klein Unification
• Dirac and Jordans Cosmology
• Scalar-Tensor Gravity
• Inflaton
• Quintessence

?
Gravity and the Tenacious Scalar Field Carl
Brans, gr-qc/9705069