Luca amendola inaf osservatorio astronomico di roma
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Luca Amendola INAF/Osservatorio Astronomico di Roma. a viable candidate for dark energy?. Is higher-order gravity. Collaboration: D. Polarski, S. Tsujikawa, R. Gannouji, C. Charmousis, S. Davis, E. Magliaro. Firenze GGI 2006. Observations are converging…. …to an un expected universe.

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Luca amendola inaf osservatorio astronomico di roma

Luca Amendola

INAF/Osservatorio Astronomico di Roma

a viable candidate for dark energy?

Is higher-order gravity

  • Collaboration: D. Polarski, S. Tsujikawa, R. Gannouji, C. Charmousis, S. Davis, E. Magliaro

  • Firenze GGI 2006


Observations are converging

Observations are converging…

…to an unexpected universe


What do we know about cosmic expansion

What do we know about cosmicexpansion ?

Nucleosynthesis (z~109)

CMB (z~1000)

Standard candles (z~1)

Perturbations (z~0-1000)


Four hypotheses on dark energy

Four hypotheses on dark energy

A) Lambda & friends

B) scalar field

C) modified gravity

D) non-linear effect


Two classes of models

Two classes of models

A) Lambda & friends

B) scalar field

C) modified gravity

D) non-linear effect

1) Models which are almost impossible

to rule out

2) Models which are easy to rule out


Lambda friends

Lambda & friends

A) Lambda & friends

B) scalar field

C) modified gravity

D) non-linear effect


Scalar field

Scalar field

A) Lambda & friends

B) scalar field

C) modified gravity

D) non-linear effect


An ultra light scalar field

An ultra-light scalar field

  • It is more general

  • Scalars are predicted by fundamental theories

Observational requirements:

Slow evolution

Light mass

V()

Compton wavelength = Hubble length


The coupling

The coupling

  • But beside the potential there can be also a coupling…


Dark energy as scalar gravity

Dark energy as scalar gravity

T(m)= CT(m)

T= -CT(m)

coupled conservation laws :

First basic

property:

C2/G = scalar-to-tensor ratio


Dark energy a s scalar gravity

Dark energyasscalar gravity

Jordan frame

Einstein frame


An extra gravity

An extra gravity

Newtonian limit: the scalar interaction generates an attractive extra-gravity

in real space

Yukawa term


How about local gravity constraints

How about local gravity constraints ?

α

λ


The fourfold way out

The fourfold way out

NO DE

uninteresting

Chameleon models

Coupled DE....

Make mφvery large

Make βvery small

Make them density dependent

Make them species dependent


A viable candidate for dark energy

A species-dependent interaction

T(cdm)= CT(cdm)

T= -CT(cdm)

T(bar)= 0

T(rad)= 0


Two qualitatively different cases weak coupling strong coupling

Two qualitatively different cases:weak coupling strong coupling 


Weak coupling density trends

Weak coupling: density trends

today

No coupling

mat

rad

= 0

a ~ tp

p = 2/3

MDE:

field

coupling

rad

mat

= /9

a ~ tp

p = 6/(42+9)

MDE:

field

MDE

kinetic phase, indep. of potential!


Deceleration and acceleration

Deceleration and acceleration

Assume

V(f) = f-a

today

mat

rad

Dominated by

potential energy

α

Dominated by

kinetic energy

β

field

The equation of state w=p/r

depends on bduring fMDE and on aduring tracking:

we = 4b2/9: past value (decelerated)

wf = -2/(a+2): present value (accelerated)


Wmap and the coupling

cl)

WMAP and thecoupling 

Planck: 

Scalar force 100 times weaker than gravity


Modified 3d gravity

Modified 3D gravity

A) Lambda & friends

B) scalar field

C) modified gravity

D) non-linear effect

Higher order gravity !

Simplest case:

Capozziello,Turner,

Carroll, Odintsov…

L.A., S. Capozziello, F. Occhionero, 1992


Modified n dim gravity

Modified N-dim gravity

Simplest case:


Faces of the same physics

Faces of the same physics

Extra-dim.

degrees of freedom

Higher order

gravity

Coupled scalar field

Scalar-tensor gravity


The simplest higher order gravity

The simplest higher-order gravity

is in fact a scalar-tensor model in the Jordan frame

and a coupled dark energy model

in the Einstein Frame


General higher order gravity

General higher order gravity

In Einstein Frame

Coupled dark energy with a strong coupling !


Is this already ruled out by local gravity

Is this already ruled out by local gravity?

is a scalar-tensor theory with Brans-Dicke

parameter ω=0 or β=1/2

Therefore if

and f(R) is not yet ruled out

see eg. Nojiri & Odintsov 2003; Brookfield et al. 2006


The simplest case

The simplest case

In Einstein Frame


R 1 r model the mde

R+1/R model:the φMDE

today

mat

rad

field

rad

mat

field

MDE

Jordan F:

Caution:

Plots in the

Einstein frame!


R 1 r model the mde1

R+1/R model:the φMDE

More exactly

today

mat

rad

During this phase,

field

rad

mat

and therefore

field

MDE

Caution:

Plots in the

Einstein frame!


R r n model

R+Rn model

L.A., S. Tsujikawa, D. Polarski 2006


Distance to last scattering in r r n model

Distance to last scattering in R+Rn model


A view from the jordan frame

A view from the Jordan Frame

Plots by R. Gannouji, U. of Montpellier


How far can we extend this

How far can we extend this?

An autonomous dynamical system


Analytical results critical points

Analytical results: critical points

For all f(R) theories:

The wrong matter era

(the t1/2 behavior) exists always

The good matter era

(the t2/3 behavior)exists

only if m(-1)=0

This immediately rules out many cases:


A viable candidate for dark energy

Gallery of Failed Cosmologies

Plots by R. Gannouji, U. of Montpellier


Numerical results

Numerical results

a) Dark energy domination

is always preceded by

the wrong matter era (if any), except...

b) …when the acceleration is driven

by a Lambda term as in


General f r ricci riemann

General f(R, Ricci, Riemann)

we find that an exact matter era exists only

for very special combination of parameters

so probably most of these models are ruled out.


Back to second order gravity

Back to second order gravity

Is this the most general second-order scalar-tensor theory ?

No!

This is a Coupled Gauss Bonnet model. Can we put

constraints on it without specifying the potential/couplings?

Yes, if the scalar field is a DE field


Coupled gauss bonnet

Coupled Gauss-Bonnet

Everything depends on

Observable deviations

from linearized

Newtonian gravity


Coupled gauss bonnet1

Coupled Gauss-Bonnet

Observable deviations

from linearized

Newtonian gravity

in the slow-rolling and

small-coupling limit


Observing a gauss bonnet term

Observing a Gauss-Bonnet term

Direct observables

Growth of matter fluctuations

ISW effect

L.A., C. Charmousis,S. Davis,

astro-ph/0506306


Searching for a perfect dark energy model

Searching for a perfect dark energy model

Quanto a figure perfette o nobili,

credo che per murare le quadre

sieno più perfette che le sferiche,

ma per ruzzolare o condurre i carri

stimo più perfette le tonde che le

triangolari. (Il Saggiatore, 1623)

As to the perfection or nobility

of geometric figures, I think that

for works in masonry square figures

are more perfect than spherical ones,

but to roll or drive a carriage I consider

the circular figures more perfect

than the triangular ones.

(Il Saggiatore, 1623)


An ultra light scalar field1

An ultra-light scalar field

Hubble size

Galactic size

Adopting a PGB

potential

Abundance

Mass

L.A. & R. Barbieri 2005


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