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J. A. S. Lima

(IAG/USP)

The Accelerating Universe: Dark Energy and Alternative Models

26th Texas Symposium

São Paulo, Brasil, 16/12/2012

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SUMMARY

1. SNe Ia, Acceleration and Dark Energy

2. Cosmic Concordance Model

3. Λ-Problems/Dark Energy Candidates (GR)

4. Alternative Approaches

5. Perspectives/Conclusion

How do we know there is

dark energy?

We infer its existence only indirectly…

via its influence on the expansion

history of the universe!

Hypothesis: Dark Energy (DE) is a new component

responsible by the present accelerating stage of the

Universe (as indicated by SNe).

Basic Question:

1. Supernovas and the Accelerating Universe

SNe Ia: Best Cosmic Distance Indicator

Measure: (via ) and Ld 2 / 4Ld L F z

Output: constraints on the matter content (m, Λ )

SNe observations provide two basic informations:

1. The current Universe is accelerating (z < zt )

2. The past Universe was decelerating in past (z > zt )

),,(1 zFHd MoL

Model:

There is a transition Redshift: Zt ~ 1

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85 local

SNe 13 distant

SNe 1 very

distant SN

1998 Results: Acceleration! (HZSS) (Riess et al. AJ 116, 1009, 1998)

Allen et al. 2008

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SNe data Transition redshift (zt)

Turner & Huterer (2007)

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Important: Both informations can also be obtained without

reference to a specific dynamic model (Cosmography/Kinematic)

Turner & Riess ApJ 2002; Elgaroy & Multamaki, JCAP 2006

Cunha PRD 2009; Guimarães & Lima CQG 2011

Cunha & Lima, MNRAS 2008

Ω Ω Ω

Ω Ω Ω

Transition Redshift (Flat ΛCDM)

1)(

)()1()(

zH

zHzzq

2)(

a

aatq

ΛCDM - ACCELERATING MODEL

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3/1

m

tz

Transition Redshift:

Lima et al. 2012, arXiv: 1205.4648 [astro-ph.CO]

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CMB (WMAP) SNe Ia

SNe Ia

LSS

BAO

Observational Basis (Flat Universe, k=0)

AGE CMB

LSS + BAO X-RAY/CLUSTERS

SNe Ia

(Cosmic Paradigm – ΛCDM – since 2008!)

Cosmic

Concordance

(ΛCDM)

X-Ray CLUSTERS

Statistical Errors Statistical + Systematic

Ωk = Ωm = Ωm = Ωk =

Amanullah et al. ApJ 2010 - Union2 (557 SNe Ia)

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Riess et al. ApJ 2007 – Gold Sample (182 SNe)

Hicken et al. 2009 (Constitution) (397 SNe Ia)

Kowalski et al. ApJ 2008 - Union1 (307 SNe)

Astier et al. A&A 2006 – Legacy Sample (115 SNe)

Supernovae + Complementary Probes

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“We can be reasonably sure that if a better cosmology is called for

it will describe a Universe that looks like much ΛCDM, because that

is what is observed” (P. J. E. Peebles, arXiv:0910.5142v1)

Paris Conference: The Invisible Universe (2009)

Cosmic Concordance Model

But...there are problems and some answered questions!

Is ΛCDM only an effective model?

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Cosmic Concordance Model (ΛCDM)

0.7

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3. Λ - PROBLEMS:

obs

matter

obs

G e VM cu to ff

1 91 0411010 eVtheory

2. Why Now? (Coincidence Problem - CP)

1. Why so small? (CCP) 41210 eVobs

12010

obs

theory

4. Alternative Models (General Relativity)

)3(3

4

)(3

82

2

XXM

XM

pG

a

a

G

a

k

a

a

0 :onaccelerati ) (3

1 , 0, ap XMXXM

15

21

10 ) 1(

3

1

X

MXXp

FLRW Metric: a(t), k = 0, ± 1

SUPERNOVA OBSERVATIONS

Late-time accelerating expansion of the Universe

What is causing the acceleration of the Universe?

Λ - term, XCDM, Quintessence Chaplygin Gas, Interacting Models, etc.

Extra-dimensions, Branes, Kaluza-Klein, F(R), Inhomogeneities, etc.

Geometry Matter/Energy

Dark Geometry

↑ Dark Matter / Dark Energy

↑

Gμν ＝ 8πGTμν

Modified

Gravity?

Accelerating Models (Inventory)

1. Cosmological Constant (Λ-Problem, Coincidence Problem)

2. Dark energy: p=wϱ, w=const. 3. Variable Dark Energy: w=w(z) 4. Quintessence – [ϕ, V(ϕ) ] - Scalars 5. Coupled Dark Matter-Dark Energy 6. K-essence 7. Chaplygin gas 8. Quartessence 9. Phantoms: p=wϱ, w < -1

1. Backreaction 2. Averaging Models 3. Production of CDM 4. F(R), F(T)... 5. Scalar-tensor models 6. Gauss-Bonnet 7. Brane dark energy 8. Cardassian 9. Degravitation

Dark Energy Models (GR ) : Models Without Dark Energy:

Gen. Rel.

Modified

Gravity

What is the correct solution? Add your solution here! 17

Unifying Dark Sector Models in GR (No Dark Energy!)

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1. XCDM (p=ωϱ , ω constant)

%9515.001.1

%9504.027.0

m

Amanullah et al. ApJ 2010 - Union2 (557 SNe Ia)

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2. Searching for Dark Energy: DETF* Strategy

1. Is the accelerating expansion consistent with Λ: Is w 1 ?

2. Probe the underlying dynamics. Measure the time evolution of DE: w(z) ?

3. Search for possible failures of general relativity (Modified Gravity?).

Goal: Compare the effects of DE on the expansion and in the growth of structures

(galaxies or galaxy clusters) – Difficult do quantify.

* Dark Energy Task Force (2006) – Comissioned by Nasa and NSF

Variable Dark Energy? Λ or not Λ?

pX = (z)X | (z)= o+ az/(1+z)

constant ΛCDM: o = -1 , a = 0

4 Basic Probes: SNe Ia, Galaxy Clusters, BAO and WL

H(z) ! Farooq, Mania & Ratra, et al. 2012, arXiv 1211.4253

DETF Cosmological Approach…

Parameterize dark-energy equation of state parameter :

• Today (z 0) w(1) w0

• In the far past (z>>1 ) w(0) w0 wa w(a) w0 + wa z / 1+z

Komatsu et al. WMAP7 (2011)

OBSERVATIONAL PERSPECTIVE (this decade!)

Measurements of H0 (2 -1%) – now is 3.2% (Riess at al. 2011)

A value of Ho (2% -1% ) would provide insights into fundamental physics!

Modified Gravity?

Accurate distances opportunity to learn about Gravitational Physics of a

locally inhomogeneous Universe

1. The nature of dark energy and its evolution

2. Curvature of the Universe (test for inflation)

3. Total number of relativistic particles

4. Limits on the sum of neutrinos mass (CMB data)

H0 is correlated with many other cosmological quantities!

Freedman & Madore ARAA 2010, Suyu et al. 2012

96% Dark Matter

Possibilities in GR:

REDUCING THE DARK SECTOR?

Modified Gravity: F(R)

Sotiriou & Faraoni, Rev. M. Phys. 2010

Remember: The theoretical status of DM is

greater than Dark Energy!

?

1) Averaging Models / Backreaction – inhomogeneities

Buchert GRG 2001, 2002; Elllis Nat 2008

Kolb et al. PRD 2008; Kolb CQG 2011

Basilakos & Lima PRD 2010; Lima, Basilakos & Costa PRD 2012

2) Grav. Production of CDM Particles - Lima et al. JCAP 2010

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The right framework is still unclear! One important task

is to explore all physically reasonable possibilities.

CONCLUSIONS

• Other DE candidates are not well motivated from FP

• Is the ΛCDM model only an effective cosmology?

More generally: Is dark energy an illusion. New Ether? MG?

Basic Cosmological Problem: What is the nature of DE?

SNe Ia, CMB, GC & BAO are consistent with ΛCDM (68% cl). But…

Any answer will have a fundamental

impact on astroparticle physics/cosmology!

• Vacuum density is plagued with the Λ-Problems

Theory provides either no guidance or far too much, depending on your point of view! Is DE a solution or a new Problem?

Λ or not Λ ?

Play your game and good lucky!

24 THANK YOU!