COSMIC ACCELERATION
Dark energy and modified theories
M. SAMI
Centre for Theoretical Physics
Jamia Millia University New Delhi
Kobayashi-Maskawa Institute for the
Origin of Particles and the Universe,
Nagoya University, Nagoya .
4/13/2015
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REFERENCES


MS, “ A primer on problems and prospects of dark
energy “ Curr Sci, V 97, 848 (2009) [arXiv:0904.3445].
M. Turner, D. Huterer, “Cosmic Acceleration, Dark
Energy and Fundamental Physics
“J.Phys.Soc.Jap.76:111015,2007.


E. Copeland, MS, S. Tsujikawa, “ Dynamic of Dark
Energy”, hep-th/0603057.
Zeldovic and Novikov, Structure and evolution of
universe, V 2
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BRIEF OVER VIEW
Late time inconsistency of hot big bang: Resolution of
age crisis—cosmological constant.
Theoretical issues related cosmological constant (Sakharov)
Coincidence ; Fine tuning.
Quintessence and its problems.
Modified theories of gravity:
Local physics---Screening mechanisms.
Chameleon, Symmetron , Vainshtein mechanism.
Scope of chameleon/symmetron : Self acceleration.
Non-linear massive gravity
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Inconsistencies of standard model
EARLY TIMES:
Flatness problem
Primordial inhomogeneities
INFLATION
Horizon problem
LATE TIMES: Age crisis
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AGE CRISIS IN HOT BIG
BANG
Matter dominated universe:
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AGE CRISIS IN HOT BIG BANG
Let us ignore gravity for a moment :
Present epoch:
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AGE CRISIS :Repulsive effect
Cosmological constant or Hook’s
law
Static universe
Neumann (1895) and Seeliger(1896)
Einstein 1917
If no quasi-static universe then
away with cosmological constant!
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MS, arXiv: 0904.3445
AGE CRISIS AND ITS RESOLUTION WITH
Open universe (K<0)
Repulsive effect
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AGE CRISIS AND ITS
RESOLUTION WITH
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ENERGY OF VACUUM
SAKHAROV 1968
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The effective Cosmological constant
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COSMOLOGICAL CONSTANT
Wrong formulation
Old cosmological
constant problem
New cosmological constant problem
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The correct level of fine tuning
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String Landscape
We are
there!
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FINE TUNING PROBLEM : TWO
DISTANT SCALES TOGETHER
THE BIG AND THE SMALL
PUTTING THEM TOGETHER
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CINCIDENCE PROBLEM: SHAME
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WHERE IS THE COINSIDENCE ?
z
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SCALAR FIELD AS DARK ENERGY
Quintessence
for steep pot.
Predictive power of scalar fields
For a priori given cosmic history, it is always possible to construct a field potential
such that it gives rise to the desired result. Thus the scalar field models should be judged
by their generic features.
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Scalar Field Dynamics in presence of background
matter : Tracker or Freezing Models
Scaling Solution:
MS, Curr Sci, V 97, 848 (2009) [arXiv:0904.3445]
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Absence of trackers: Thawing
Models
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Trackers and non-trackers
• Trackers (Freezing models): Potentials which mimic steep exponential
at early epochs and become shallow at late times.
Thawing models:
Most of the systems belong to this category
, Scaling Pot.
Phantom field:
A. Sen, HEP0204:048,2002
N Mazumdar, S. Panda, A. Perez-Lorenzana,
Phys.. B614,101(2001)
E.Copeland, M. Garousi, MS, S. Tsujikawa
PRD71:043003,2005, S. Tsijikawa, MS,
PRD71:043003,2005 .
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Quintessence problem
C. Kolda and D Lyth,
hep-th.9811375
Unstable under radiative corrections when coupled to
other fields unless severely fine tuned.
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MODIFIED THEORIES OF GRAVITY
LARGE SCALE MODIFICATION
Chameleon theories.
• Galileon modified theories
• Massive gravity
•
Scalar degree(s) of freedom
Requirements
• Local physics be intact
• Cosmic acceleration
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SCALARON: Requirements
DE:
Solar Physics:
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Chameleon Field
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MASS SCREENING OR FIFTH FORCE SUPPRESSION
(Basic Idea)
Localized source:
Chameleon or Symmetron
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Kinetic suppression
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Chameleon versus Galileon(Vainshtein )
Chameleon
Galileon
Small distances:
Large distances:
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Chameleon at work
R, M
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Vainstein Effect- dynamical suppression of fifth force
Short distances:
Large distances:
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Vainshtein at work
R, M
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Symmetron: Dark energy
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Symmetron: Dark energy
Symmetry should break:
Local gravity constraints
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Scalar tensor theories
-Einstein frame
-Jordon frame
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Local gravity constraints: Screening Mechanism
Chameleon Field
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VIABLE f(R) MODELS
Stability conditions:
Stable de Sitter:
Local gravity constraints:
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Problems of f(R): scalaron mass
Thongkool, MS, R. Gannouji, S.Jhingan,
Phys.Rev.D80:043523,2009 ;
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CURING THE SINGULARITY ?
Countering Term:
Thongkool, MS, S. Rai Choudhury,
arXiv:0908.1663
Thongkool, MS, R. Gannouji, S. Jhingan,
Phys.Rev.D80:043523,2009 ;
S A. Appleby,R. A. Battye, A. Starobinsky,
arXiv:0909.1737
arXiv: 1201.4546, Chung-Chi Lee, ChaoQiang Geng, Louis Yang
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SELF ACCELERATION:SCOPE OF CHAMELEON THEORIES
Self acceleration:
Screening:
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Chameleon/Symmetron theories-
No scope for self acceleration
arXiv:1284612: J. Wang, L. Hui and J. Khoury
arXiv: 1211.2289: K. Bamba, R.
Gannouji, MS
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Glimpses of massive gravity
(helicity 2 object) + (helicity 1 object)+ (helicity zero object)
Universally couples to all fields
a
b
Ghost:
Stuckelberging
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Best model for quintessential inflation
BEST MODEL OF INFLATION AND DARK ENERGY?
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NATURE OF DARK ENERGY
IT COULD BE ANYTHING
OR
IT COULD BE NOTHING
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