introduction to teleparallel and f(t) gravity and cosmology emmanuel n. saridakis physics...
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Introduction to Teleparallel and f(T) Gravity
and Cosmology
Emmanuel N. Saridakis
Physics Department, National and Technical University of Athens, Greece
Physics Department, Baylor University, Texas, USA
E.N.Saridakis – IAP, Sept 2014
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Goal We investigate cosmological
scenarios in a universe governed by torsional modified gravity
Note: A consistent or interesting cosmology is not a proof for
the consistency of the underlying gravitational theory
E.N.Saridakis – IAP, Sept 2014
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Talk Plan 1) Introduction: Gravity as a gauge theory, modified Gravity
2) Teleparallel Equivalent of General Relativity and f(T) modification
3) Perturbations and growth evolution
4) Bounce in f(T) cosmology
5) Non-minimal scalar-torsion theory
6) Teleparallel Equivalent of Gauss-Bonnet and f(T,T_G) modification
7) Black-hole solutions
8) Solar system and growth-index constraints
9) Conclusions-ProspectsE.N.Saridakis – IAP, Sept 2014
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Introduction
Einstein 1916: General Relativity: energy-momentum source of spacetime
Curvature Levi-Civita connection: Zero Torsion
Einstein 1928: Teleparallel Equivalent of GR:
Weitzenbock connection: Zero Curvature
Einstein-Cartan theory: energy-momentum source of Curvature, spin source of Torsion
[Hehl, Von Der Heyde, Kerlick, Nester Rev.Mod.Phys.48]
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Introduction
Gauge Principle: global symmetries replaced by local ones:
The group generators give rise to the compensating fields
It works perfect for the standard model of strong, weak and E/M interactions
Can we apply this to gravity?
)1(23 USUSU
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Introduction
Formulating the gauge theory of gravity (mainly after 1960): Start from Special Relativity Apply (Weyl-Yang-Mills) gauge principle to its Poincaré- group symmetries Get Poinaré gauge theory: Both curvature and torsion appear as field strengths
Torsion is the field strength of the translational group (Teleparallel and Einstein-Cartan theories are subcases of Poincaré theory)
[Blagojevic, Hehl, Imperial College Press, 2013]
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Introduction
One could extend the gravity gauge group (SUSY, conformal, scale, metric affine transformations)
obtaining SUGRA, conformal, Weyl, metric affine gauge theories of gravity
In all of them torsion is always related to the gauge structure.
Thus, a possible way towards gravity quantization would need to bring torsion into gravity description.
E.N.Saridakis – IAP, Sept 2014
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Introduction
1998: Universe acceleration Thousands of work in Modified Gravity
(f(R), Gauss-Bonnet, Lovelock, nonminimal scalar coupling,
nonminimal derivative coupling, Galileons, Hordenski, massive etc)
Almost all in the curvature-based formulation of gravity
[Copeland, Sami, Tsujikawa Int.J.Mod.Phys.D15], [Nojiri, Odintsov Int.J.Geom.Meth.Mod.Phys. 4]
E.N.Saridakis – IAP, Sept 2014
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Introduction
1998: Universe acceleration Thousands of work in Modified Gravity
(f(R), Gauss-Bonnet, Lovelock, nonminimal scalar coupling,
nonminimal derivative coupling, Galileons, Hordenski, massive etc)
Almost all in the curvature-based formulation of gravity
So question: Can we modify gravity starting from its torsion-based formulation?
torsion gauge quantization
modification full theory quantization
? ?
[Copeland, Sami, Tsujikawa Int.J.Mod.Phys.D15], [Nojiri, Odintsov Int.J.Geom.Meth.Mod.Phys. 4]
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Teleparallel Equivalent of General Relativity (TEGR)
Let’s start from the simplest tosion-based gravity formulation, namely TEGR:
Vierbeins : four linearly independent fields in the tangent space
Use curvature-less Weitzenböck connection instead of torsion-less Levi-Civita one:
Torsion tensor:
)()()( xexexg BAAB
Ae
AA
W ee
}{
AAA
WW eeeT
}{}{ [Einstein 1928], [Pereira: Introduction to
TG]
E.N.Saridakis – IAP, Sept 2014
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Teleparallel Equivalent of General Relativity (TEGR)
Let’s start from the simplest tosion-based gravity formulation, namely TEGR:
Vierbeins : four linearly independent fields in the tangent space
Use curvature-less Weitzenböck connection instead of torsion-less Levi-Civita one:
Torsion tensor:
Lagrangian (imposing coordinate, Lorentz, parity invariance, and up to 2nd order in torsion tensor)
)()()( xexexg BAAB
Ae
AA
W ee
}{
AAA
WW eeeT
}{}{
TTTTTL2
1
4
1
[Einstein 1928], [Hayaski,Shirafuji PRD 19], [Pereira: Introduction to TG]
Completely equivalent with
GR at the level of equations
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f(T) Gravity and f(T) Cosmology
f(T) Gravity: Simplest torsion-based modified gravity Generalize T to f(T) (inspired by f(R))
Equations of motion:
}{1 4)]([4
1)(1
AATTAATA GeTfTefTSeSTefSeee
[Ferraro, Fiorini PRD 78], [Bengochea, Ferraro PRD 79]
mSTfTexdG
S )(16
1 4
[Linder PRD 82]
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f(T) Gravity and f(T) Cosmology
f(T) Gravity: Simplest torsion-based modified gravity Generalize T to f(T) (inspired by f(R))
Equations of motion:
f(T) Cosmology: Apply in FRW geometry:
(not unique choice)
Friedmann equations:
mSTfTexdG
S )(16
1 4
}{1 4)]([4
1)(1
AATTAATA GeTfTefTSeSTefSeee
jiij
A dxdxtadtdsaaadiage )(),,,1( 222
22 26
)(
3
8Hf
TfGH Tm
TTT
mm
fHf
pGH
2121
)(4
Find easily26HT
[Ferraro, Fiorini PRD 78], [Bengochea, Ferraro PRD 79][Linder PRD 82]
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f(T) Cosmology: Background
Effective Dark Energy sector:
Interesting cosmological behavior: Acceleration, Inflation etc At the background level indistinguishable from other dynamical DE models
TDE f
Tf
G 368
3
]2][21[
2 2
TTTT
TTTDE TffTff
fTTffw
[Linder PRD 82]
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f(T) Cosmology: Perturbations Can I find imprints of f(T) gravity? Yes, but need to go to perturbation level
Obtain Perturbation Equations:
Focus on growth of matter overdensity go to Fourier modes:
)1(,)1(00
ee ji
ij dxdxadtds )21()21( 222
SHRSHL ....
SHRSHL .... [Chen, Dent, Dutta, Saridakis PRD
83], [Dent, Dutta, Saridakis JCAP 1101]
m
m
0436)1)(/3(1213 42222 kmkTTTkTTT GfHfakHfHfH
[Chen, Dent, Dutta, Saridakis PRD 83]
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f(T) Cosmology: Perturbations
Application: Distinguish f(T) from quintessence 1) Reconstruct f(T) to coincide with a given quintessence
scenario:
with and
CHdHH
GHHf Q 216)(
)(2/2 VQ
6/
[Dent, Dutta, Saridakis JCAP 1101]
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f(T) Cosmology: Perturbations
Application: Distinguish f(T) from quintessence 2) Examine evolution of matter overdensity
[Dent, Dutta, Saridakis JCAP 1101]
m
m
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Bounce and Cyclic behavior
Contracting ( ), bounce ( ), expanding ( ) near and at the bounce
Expanding ( ), turnaround ( ), contracting near and at the turnaround
0H 0H 0H0H
0H 0H 0H0H
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Bounce and Cyclic behavior in f(T) cosmology
Contracting ( ), bounce ( ), expanding ( ) near and at the bounce
Expanding ( ), turnaround ( ), contracting near and at the turnaround
0H 0H 0H0H
0H 0H 0H0H
22 26
)(
3
8Hf
TfGH Tm
TTT
mm
fHf
pGH
2121
)(4
Bounce and cyclicity can be easily obtained
[Cai, Chen, Dent, Dutta, Saridakis CQG 28]
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Bounce in f(T) cosmology
Start with a bounching scale factor: 3/1
2
2
31)(
tata B
2
43
3
4
3
2
3
4)(
T
T
TTt
t
sArcTan
t
tM
tMt
ttf mB
pmB
p 2
36
32
6
)32(
4)(
2
22
22
E.N.Saridakis – IAP, Sept 2014
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Bounce in f(T) cosmology
Start with a bounching scale factor:
Examine the full perturbations:
with known in terms of and matter
Primordial power spectrum: Tensor-to-scalar ratio:
2
43
3
4
3
2
3
4)(
T
T
TTt
t
sArcTan
t
tM
tMt
ttf mB
pmB
p 2
36
32
6
)32(
4)(
2
22
22
02
222 kskkk a
kc
22 ,, sc TTT fffHH ,,,,
22288 pMP
01
123
2
2
h
f
fHHh
ahHh
T
TTijijij
3108.2 r
[Cai, Chen, Dent, Dutta, Saridakis CQG 28]
3/12
2
31)(
tata B
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Non-minimally coupled scalar-torsion theory
In curvature-based gravity, apart from one can use Let’s do the same in torsion-based gravity:
[Geng, Lee, Saridakis, Wu PLB 704]
2^RR )(RfR
mLVTT
exdS )(2
1
22
24
E.N.Saridakis – IAP, Sept 2014
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Non-minimally coupled scalar-torsion theory
In curvature-based gravity, apart from one can use Let’s do the same in torsion-based gravity:
Friedmann equations in FRW universe:
with effective Dark Energy sector:
Different than non-minimal quintessence! (no conformal transformation in the present case)
2^RR )(RfR
mLVTT
exdS )(2
1
22
24
DEmH
3
22
DEDEmm ppH 2
2
22
2
3)(2
HVDE
222
234)(2
HHHVpDE
[Geng, Lee, Saridakis,Wu PLB 704]
[Geng, Lee, Saridakis, Wu PLB 704]
E.N.Saridakis – IAP, Sept 2014
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Non-minimally coupled scalar-torsion theory
Main advantage: Dark Energy may lie in the phantom regime or/and experience the phantom-divide crossing
Teleparallel Dark Energy:
[Geng, Lee, Saridakis, Wu PLB 704]
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Observational constraints on Teleparallel Dark Energy
Use observational data (SNIa, BAO, CMB) to constrain the parameters of the theory
Include matter and standard radiation: We fit for various
,0DEDE0M0 w,Ω,Ω )(V
azaa rrMM /11,/,/ 40
30
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Observational constraints on Teleparallel Dark Energy
Exponential potential
Quartic potential
[Geng, Lee, Saridkis JCAP 1201]E.N.Saridakis – IAP, Sept 2014
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Phase-space analysis of Teleparallel Dark Energy
Transform cosmological system to its autonomous form:
Linear Perturbations: Eigenvalues of determine type and stability of C.P
),sgn(13
2222
zyx=
Hm
m
[Xu, Saridakis, Leon, JCAP 1207]
,,, zyxw=w DEDE
,f(X)=X'QUU 'UX=X C
0' =|XCX=X
Q
z
H
Vy
Hx ,
3
)(,
6
)sgn(3
2222
zyx=
HDE
DE
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Phase-space analysis of Teleparallel Dark Energy
Apart from usual quintessence points, there exists an extra stable one for corresponding to
[Xu, Saridakis, Leon, JCAP 1207]
2 1,1,1 qwDEDE
At the critical points however during the evolution it can lie in quintessence or phantom regimes, or experience the phantom-divide crossing!
1DEw
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Non-minimally matter-torsion coupled theory
In curvature-based gravity, one can use coupling Let’s do the same in torsion-based gravity:
mLRf )(
mLTfTfTexdS )(1)(2
121
42
[Harko, Lobo, Otalora, Saridakis, PRD 89]
E.N.Saridakis – IAP, Sept 2014
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Non-minimally matter-torsion coupled theory
In curvature-based gravity, one can use coupling Let’s do the same in torsion-based gravity:
Friedmann equations in FRW universe:
with effective Dark Energy sector:
Different than non-minimal matter-curvature coupled theory
mLRf )(
mLTfTfTexdS )(1)(2
121
42
DEmH
3
22
DEDEmm ppH 2
2
22
212
121212
2
1fHffHf mDE
22
221
21
22
22
12
1
22
2 122
12
122121
121fHf
fHf
fHffHf
fHfpp m
mmmDE
[Harko, Lobo, Otalora, Saridakis, PRD 89]
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Non-minimally matter-torsion coupled theory
Interesting phenomenology
,)( 211 TTf
[Harko, Lobo, Otalora, Saridakis, PRD 89]
212 )( TTf ,)(1 Tf 2
112 )( TTTf
E.N.Saridakis – IAP, Sept 2014
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Non-minimally matter-torsion coupled theory
In curvature-based gravity, one can use coupling Let’s do the same in torsion-based gravity:
),( Rf
mLTfTexdS ),(2
1 42 [Harko, Lobo, Otalora, Saridakis, 1405.0519, to appear in
JCAP]
E.N.Saridakis – IAP, Sept 2014
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Non-minimally matter-torsion coupled theory
In curvature-based gravity, one can use coupling Let’s do the same in torsion-based gravity:
Friedmann equations in FRW universe ( ):
with effective Dark Energy sector:
Different from gravity
),( Rf
mLTfTexdS ),(2
1 42
DEmH
3
22
DEDEmm ppH 2
2
mmTDE pffHf
2122
1 22
mmTTmmmTTT
mmDE pffHffddpdHdHfHf
fpp
2122
11
/31/121
/1 222
2
[Harko, Lobo, Otalora, Saridakis, 1405.0519, to appear in JCAP]
mm p3
),( Rf
E.N.Saridakis – IAP, Sept 2014
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Non-minimally matter-torsion coupled theory
Interesting phenomenology
[Harko, Lobo, Otalora, Saridakis, 1405.0519, to appear in JCAP]
2),( TTf nTTf ),(
E.N.Saridakis – IAP, Sept 2014
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Teleparallel Equivalent of Gauss-Bonnet and f(T,T_G) gravity
In curvature-based gravity, one can use higher-order invariants like the Gauss-Bonnet one
Let’s do the same in torsion-based gravity: Similar to we construct with
RRRRRG 42
,2 eTeTRe divergtoteTGe G .
E.N.Saridakis – IAP, Sept 2014
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Teleparallel Equivalent of Gauss-Bonnet and f(T,T_G) gravity
In curvature-based gravity, one can use higher-order invariants like the Gauss-Bonnet one
Let’s do the same in torsion-based gravity: Similar to we construct with
gravity:
Different from and gravities
RRRRRG 42
abcdaaaa
fdc
eaf
aeb
aaa
fcd
eaf
aeb
aaa
fad
efc
aeb
aaa
fad
afc
eab
aeaG KKKKKKKKKKKKKKKKT
4321
4321432143214321
2 ,222
[Kofinas, Saridakis, 1404.2249 to appear in PRD]
),( GTf
,2 eTeTRe divergtoteTGe G .
mG STTfTexdS ),(2
1 42
),( GRf )(Tf
[Kofinas, Saridakis, 1408.0107 to appear in PRD][Kofinas, Leon, Saridakis, CQG 31]
E.N.Saridakis – IAP, Sept 2014
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Teleparallel Equivalent of Gauss-Bonnet and f(T,T_G) gravity
Cosmological application:
[Kofinas, Saridakis, 1404.2249 to appear in PRD]
[Kofinas, Saridakis, 1408.0107 to appear in PRD]
[Kofinas, Leon, Saridakis, CQG 31]
GG TTGTDE fHfTfHf 32
22412
2
1
GGG TTGTGTTDE fHfTH
fTfHfHHfp 222
83
2434
2
1
26HT
2224 HHHTG
GG TTTTTf 22
1),( GG TTTTf 2
21),(
E.N.Saridakis – IAP, Sept 2014
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Exact charged black hole solutions
Extend f(T) gravity in D-dimensions (focus on D=3, D=4):
Add E/M sector: with
Extract field equations:
2)(2
1TfTexdS D
FFLF
2
1 dxAAdAF ,
SHRSHL .... [Gonzalez, Saridakis, Vasquez, JHEP 1207]
[Capozzielo, Gonzalez, Saridakis, Vasquez, JHEP 1302]
E.N.Saridakis – IAP, Sept 2014
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Exact charged black hole solutions
Extend f(T) gravity in D-dimensions (focus on D=3, D=4):
Add E/M sector: with
Extract field equations:
Look for spherically symmetric solutions:
Radial Electric field: known
2)(2
1TfTexdS D
FFLF
2
1 dxAAdAF ,
SHRSHL ....
2
1
2222
222
)(
1)(
D
idxrdrrG
dtrFds
,,,,)(
1,)( 2
31
210 rdxerdxedrrG
edtrFe
2Dr r
QE
22 )(,)( rGrF[Gonzalez, Saridakis, Vasquez, JHEP 1207], [Capozzielo, Gonzalez, Saridakis, Vasquez, JHEP 1302]
E.N.Saridakis – IAP, Sept 2014
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Exact charged black hole solutions
Horizon and singularity analysis:
1) Vierbeins, Weitzenböck connection, Torsion invariants: T(r) known obtain horizons and singularities
2) Metric, Levi-Civita connection, Curvature invariants: R(r) and Kretschmann known obtain horizons and singularities
[Gonzalez, Saridakis, Vasquez, JHEP1207], [Capozzielo, Gonzalez, Saridakis, Vasquez, JHEP 1302]
)(rRR
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Exact charged black hole solutions
[Capozzielo, Gonzalez, Saridakis, Vasquez, JHEP 1302]
E.N.Saridakis – IAP, Sept 2014
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Exact charged black hole solutions
More singularities in the curvature analysis than in torsion analysis!
(some are naked) The differences disappear in the f(T)=0 case, or in the uncharged
case.
Going to quartic torsion invariants solves the problem.
f(T) brings novel features.
E/M in torsion formulation was known to be nontrivial (E/M in Einstein-Cartan and Poinaré theories)
E.N.Saridakis – IAP, Sept 2014
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Solar System constraints on f(T) gravity
Apply the black hole solutions in Solar System: Assume corrections to TEGR of the form
)()( 32 TOTTf
rc
GM
rr
rc
GMrF
222
22 46
63
21)(
2222
22
22 8
82
224
3
8
3
21)(
rrc
GMr
rr
rc
GMrG
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Solar System constraints on f(T) gravity
Apply the black hole solutions in Solar System: Assume corrections to TEGR of the form
Use data from Solar System orbital motions:
T<<1 so consistent
f(T) divergence from TEGR is very small This was already known from cosmological observation constraints up to
With Solar System constraints, much more stringent bound.
)()( 32 TOTTf
rc
GM
rr
rc
GMrF
222
22 46
63
21)(
2222
22
22 8
82
224
3
8
3
21)(
rrc
GMr
rr
rc
GMrG
10)( 102.6 TfU
[Iorio, Saridakis, Mon.Not.Roy.Astron.Soc 427)
)1010( 21 O [Wu, Yu, PLB 693], [Bengochea PLB 695]
E.N.Saridakis – IAP, Sept 2014
Perturbations: , clustering growth rate:
γ(z): Growth index.
45
Growth-index constraints on f(T) gravity
)(ln
lna
ad
dm
m
)('1
1
TfGeff
mmeffmm GH 42
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Growth-index constraints on f(T) gravity
)(ln
lna
ad
dm
m
)('1
1
TfGeff
[Nesseris, Basilakos, Saridakis, Perivolaropoulos, PRD 88]
mmeffmm GH 42 Perturbations: , clustering growth rate:
γ(z): Growth index.
Viable f(T) models are practically indistinguishable from ΛCDM.
E.N.Saridakis – IAP, Sept 2014
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Open issues of f(T) gravity
f(T) cosmology is very interesting. But f(T) gravity and nonminially coupled teleparallel gravity has many open issues
For nonlinear f(T), Lorentz invariance is not satisfied Equivalently, the vierbein choices corresponding to the same
metric are not equivalent (extra degrees of freedom)
[Li, Sotiriou, Barrow PRD 83a],
[Li,Sotiriou,Barrow PRD 83c], [Li,Miao,Miao JHEP 1107]
[Geng,Lee,Saridakis,Wu PLB 704]
E.N.Saridakis – IAP, Sept 2014
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Open issues of f(T) gravity
f(T) cosmology is very interesting. But f(T) gravity and nonminially coupled teleparallel gravity has many open issues
For nonlinear f(T), Lorentz invariance is not satisfied Equivalently, the vierbein choices corresponding to the same
metric are not equivalent (extra degrees of freedom)
Black holes are found to have different behavior through curvature and torsion analysis
Thermodynamics also raises issues
Cosmological, Solar System and Growth Index observations constraint f(T) very close to linear-in-T form
[Li, Sotiriou, Barrow PRD 83a],
[Li,Miao,Miao JHEP 1107]
[Capozzielo, Gonzalez, Saridakis, Vasquez JHEP 1302]
[Bamba, Capozziello, Nojiri, Odintsov ASS 342], [Miao,Li,Miao JCAP 1111]
[Geng,Lee,Saridakis,Wu PLB 704]
[Nesseris, Basilakos, Saridakis, Perivolaropoulos, PRD 88]
[Iorio, Saridakis, Mon.Not.Roy.Astron.Soc 427)
E.N.Saridakis – IAP, Sept 2014
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Gravity modification in terms of torsion?
So can we modify gravity starting from its torsion formulation?
The simplest, a bit naïve approach, through f(T) gravity is interesting, but has open issues
Additionally, f(T) gravity is not in correspondence with f(R)
Even if we find a way to modify gravity in terms of torsion, will it be still in 1-1 correspondence with curvature-based modification?
What about higher-order corrections, but using torsion invariants (similar to Lovelock, Hordenski modifications)?
Can we modify gauge theories of gravity themselves? E.g. can we modify Poincaré gauge theory?
E.N.Saridakis – IAP, Sept 2014
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Conclusions i) Torsion appears in all approaches to gauge gravity, i.e to the first step of
quantization. ii) Can we modify gravity based in its torsion formulation? iii) Simplest choice: f(T) gravity, i.e extension of TEGR iv) f(T) cosmology: Interesting phenomenology. Signatures in growth
structure. v) We can obtain bouncing solutions vi) Non-minimal coupled scalar-torsion theory : Quintessence,
phantom or crossing behavior. Similarly in torsion-matter coupling and TEGB.
vii) Exact black hole solutions. Curvature vs torsion analysis. viii) Solar system constraints: f(T) divergence from T less than ix) Growth Index constraints: Viable f(T) models are practically
indistinguishable from ΛCDM. x) Many open issues. Need to search for other torsion-based modifications
too.
2ξTT
1010
E.N.Saridakis – IAP, Sept 2014
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Outlook Many subjects are open. Amongst them:
i) Examine thermodynamics thoroughly.
ii) Understand the extra degrees of freedom and the extension to non-diagonal vierbeins.
iii) Try to modify TEGR using higher-order torsion invariants.
iv) Try to modify Poincaré gauge theory (extremely hard!)
v) What to quantize? Metric, vierbeins, or connection?
vi) Convince people to work on the subject!
E.N.Saridakis – IAP, Sept 2014
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THANK YOU!
E.N.Saridakis – IAP, Sept 2014