43rd Recontres de Moriond , March 15-22 (2008)

VECTOR models

for DARK ENERGY

Antonio L. MAROTO and Jose BELTRÁN JIMÉNEZUniversidad Complutense de Madrid

arXiv:0801.1486 [astro-ph]

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The Dark Energy Coincidence ProblemThe Dark Energy Coincidence ProblemThe Dark Energy Coincidence ProblemThe Dark Energy Coincidence Problem

Dark Energy models (ΛΛΛΛ, scalar, f(R),…)require unnatural dimensional parameters

in their Lagrangians or initial conditions

Our aim is to find a model:

• without dimensional scales (apart from G),

• with the same number of parameters as ΛΛΛΛCDM, • with natural initial conditions,

• with good fits to SNIa data

Vector models can do the job !

Why ρDE ~ ρM today ?

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The modelThe modelThe modelThe model

• G only dimensional scale in the model

• No free parameters

• No potential terms (Kiselev, Armendariz, ’04, Boehmer, Harko ’07, Mota, Koivisto `07)

Vector-tensor action

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The modelThe modelThe modelThe model

Equations of motion

Flat Robertson-Walker metric

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The modelThe modelThe modelThe model

Explicit solutions: radiation and matter eras

Matter

Radiation

Scaling during

radiation era

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Energy densitiesEnergy densitiesEnergy densitiesEnergy densities

Matter

Radiation

Vector dark

energy

Cosmological

constant

FinalFinalFinalFinal

singularitysingularitysingularitysingularity

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Equation of stateEquation of stateEquation of stateEquation of state

Radiation

Vector field evolution

(MP units)

W=1/3W= -0.457

phantom line

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Fitting SNe type Ia dataFitting SNe type Ia dataFitting SNe type Ia dataFitting SNe type Ia data

Distance modulus

Luminosity distance

Data sets

SNLS (Astier et al. ’06) z<1Gold (Riess et al ’04) z<1.7

157 (previous)+ 16 (HST z>1) = 173 SNe 44 (previous)+ 71 (new z<1) = 115 SNe

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Fitting SNe type Ia data: VCDM vs. Fitting SNe type Ia data: VCDM vs. Fitting SNe type Ia data: VCDM vs. Fitting SNe type Ia data: VCDM vs. ΛΛΛΛCDMCDMCDMCDM

Gold set

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Other parametrizations with the Gold setOther parametrizations with the Gold setOther parametrizations with the Gold setOther parametrizations with the Gold set

Lazkoz, Nesseris,

Perivolaropoulos, ‘05

VCDM:

Vector dark energy:

best fit to Gold

dataset to date

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Initial conditions and the end of the universeInitial conditions and the end of the universeInitial conditions and the end of the universeInitial conditions and the end of the universe

Natural initial conditions:

Nojiri, Odintsov, Tsujikawa, ‘05

Bouhmadi, González, Martin, ’07

Type III Type III Type III Type III ““““BigBigBigBig----freezefreezefreezefreeze””””

singularitysingularitysingularitysingularity

“Imminent” final singularity:

tend –t0 = 690

million years

(h=0.7)

~ earliest fossils

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Problems at small scales?Problems at small scales?Problems at small scales?Problems at small scales?

PPN parameters

Static parameters

agree with GR

Preferred frame effects

Present bounds

Quantum instabilities: Ghosts

Negative energies for perturbations at sub-Hubble scales,

but can be gauged away in this model (preliminary)

Will, ’81,

Nordtvedt

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ConclusionsConclusionsConclusionsConclusions

Vector model for dark energy with:

• No potential terms

• No dimensional scales (apart from G)

• No free parameters

• Standard kinetic terms

Excellent fit to Gold dataset and compatible with SNLS:

definite predictions for cosmological parameters

Scaling behaviour during radiation and natural initial

conditions (No coincidence problem)

Phantom dark energy today: BAO, CMB?