ERE 2008 September 15-19, 2008 1Spanish Relativity Meeting 2008, Salamanca, September 15-19 (2008)

Avoiding the

DARK ENERGY coincidence problem with a

COSMIC VECTOR

Antonio L. MAROTO and Jose BELTRÁN JIMÉNEZ Universidad Complutense de Madrid

Phys. Rev. D 78, 063005 (2008)

ERE 2008 September 15-19, 2008 2

The accelerated expansion of the universe

dL(z) relation from SN Ia data

Supernova Cosmology Project: 42 SNe Ia, z<0.83

High-z Supernova Search Team: 14 SNe Ia, z < 0.62,

Hubble Space Telescope

(Gold Set): 157 SNe Ia, + 16 with z > 1.25.

Deceleration - acceleration transition at z0.46 0.13

SNLS 71 SNe Ia z < 1 Reduced systematics

(single instrument)

1998

2004

2005

AcceleratedDecelerated-acceleratedDecelerated

ERE 2008 September 15-19, 2008 3

The need for dark energy

Friedmann equation

Einstein’s equations in flat FRW backgrounds

Acceleration requires negative pressure:

DARK ENERGY

ERE 2008 September 15-19, 2008 4

Dark energy equation of state pDE = wDE DE

Gold Set

Flat models (M + DE = 1)

Favors wDE < -1

Gold set

SNLS setwDE= const.

The need for dark energy

ERE 2008 September 15-19, 2008 5

Crossing the phantom divide wDE < -1?

Gold Set

ODEP: CMB, BAO, growth factor

Nesseris,Perivolaropoulos, ‘07

wDE= wDE(z)

The need for dark energy

ERE 2008 September 15-19, 2008 6

Cosmological constant p= -

G = (1019 GeV)-2

G8G (Tg)

~ (10 - 3 eV) 4

Coincidenceproblem

The dark energy coincidence problem

ERE 2008 September 15-19, 2008 7

Quintessence: scaling regime

Scaling

M has to be fixed in order to get DE

The dark energy coincidence problem

ERE 2008 September 15-19, 2008 8

The dark energy coincidence problem

Dark Energy models 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 CDM, • with natural initial conditions,• with good fits to SNIa data

Vector models can do the job !

Why DE ~ Mtoday ?

ERE 2008 September 15-19, 2008 9

The 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

ERE 2008 September 15-19, 2008 10

The model

Equations of motion

Flat Robertson-Walker metric

ERE 2008 September 15-19, 2008 11

The model

Explicit solutions: radiation and matter eras

Matter

Radiation

Scaling duringradiation era

ERE 2008 September 15-19, 2008 12

Energy densities

Matter

Radiation

Vector dark energy

Cosmological constant

Final

singularity

Scaling

ERE 2008 September 15-19, 2008 13

Equation of state

Radiation

Vector field evolution (MP units)

W=1/3 W= -0.457

phantom line

ERE 2008 September 15-19, 2008 14

Fitting 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

ERE 2008 September 15-19, 2008 15

Fitting SNe type Ia data: VCDM vs. CDM

Gold set

ERE 2008 September 15-19, 2008 16

Other parametrizations with the Gold set

Lazkoz, Nesseris,Perivolaropoulos, ‘05

VCDM:

Vector dark energy: best fit to Gold dataset to date

ERE 2008 September 15-19, 2008 17

Initial conditions and the end of the universe

Natural initial conditions:

Nojiri, Odintsov, Tsujikawa, ‘05Bouhmadi, González, Martin,

’07

Type III “Big-freeze”

singularity

“Imminent” final singularity:

tend –t0 = 690 million years

(h=0.7)

~ earliest fossils

ERE 2008 September 15-19, 2008 18

Stability and local gravity tests

PPN parameters

Static parametersagree with GR

Preferred frame effects

Present bounds

Classical stability

Will, ’81,Nordtvedt

Real propagation speed for scalar, vector and tensor perturbations

Quantum stability: ghosts?

ERE 2008 September 15-19, 2008 19

Conclusions

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?

ERE 2008 September 15-19, 2008 20

Evolution of density perturbations

Sub-Hubble modes

Radiation era

Matter era

Matter or radiation eras

Density contrast(matter and radiation)

PRELIMINARY

ERE 2008 September 15-19, 2008 21

Evolution of density perturbations

Super-Hubble modes

Matter and radiation eras

Density contrast(matter and radiation)

PRELIMINARY