torsional, teleparallel and f(t) gravity and cosmologyviavca.in2p3.fr/presentations/torsional...1)...
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Torsional, 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 – VIA web lecture, Feb 2014
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Goal
n We investigate cosmological scenarios in a universe governed by torsional modified gravity
n Note: A consistent or interesting cosmology
is not a proof for the consistency of the underlying gravitational theory
E.N.Saridakis – VIA web lecture, Feb 2014
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Talk Plan n 1) Introduction: Gravity as a gauge theory, modified Gravity
n 2) Teleparallel Equivalent of General Relativity and f(T) modification
n 3) Perturbations and growth evolution
n 4) Bounce in f(T) cosmology
n 5) Non-minimal scalar-torsion theory
n 6) Black-hole solutions
n 7) Solar system and growth-index constraints
n 8) Conclusions-Prospects
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Introduction
n Einstein 1916: General Relativity: energy-momentum source of spacetime Curvature Levi-Civita connection: Zero Torsion n Einstein 1928: Teleparallel Equivalent of GR: Weitzenbock connection: Zero Curvature
n 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
n 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
n Can we apply this to gravity?
( ) ( ) )1(23 USUSU ××
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Introduction
n Formulating the gauge theory of gravity (mainly after 1960): n 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
n 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
n One could extend the gravity gauge group (SUSY, conformal, scale, metric affine transformations)
obtaining SUGRA, conformal, Weyl, metric affine gauge theories of gravity
n In all of them torsion is always related to the gauge structure.
n Thus, a possible way towards gravity quantization would need to bring torsion into gravity description.
E.N.Saridakis – VIA web lecture, Feb 2014
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Introduction
n 1998: Universe acceleration Thousands of work in Modified Gravity
(f(R), Gauss-Bonnet, Lovelock, nonminimal scalar coupling,
nonminimal derivative coupling, Galileons, Hordenski, massive etc)
n 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]
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Introduction
n 1998: Universe acceleration Thousands of work in Modified Gravity
(f(R), Gauss-Bonnet, Lovelock, nonminimal scalar coupling,
nonminimal derivative coupling, Galileons, Hordenski, massive etc)
n Almost all in the curvature-based formulation of gravity
n 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)
n Let’s start from the simplest tosion-based gravity formulation, namely TEGR:
n Vierbeins : four linearly independent fields in the tangent space n Use curvature-less Weitzenböck connection instead of torsion-less
Levi-Civita one: n Torsion tensor:
)()()( xexexg BAAB νµµν η=
µAe
AA
W ee νµλλ
νµ ∂=Γ }{
( )AAA
WW eeeT µννµλλ
µνλνµ
λµν ∂−∂=Γ−Γ= }{}{ [Einstein 1928], [Pereira: Introduction to TG]
E.N.Saridakis – VIA web lecture, Feb 2014
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Teleparallel Equivalent of General Relativity (TEGR)
n Let’s start from the simplest tosion-based gravity formulation, namely TEGR:
n Vierbeins : four linearly independent fields in the tangent space n Use curvature-less Weitzenböck connection instead of torsion-less
Levi-Civita one: n Torsion tensor:
n Lagrangian (imposing coordinate, Lorentz, parity invariance, and up to 2nd order in torsion tensor)
)()()( xexexg BAAB νµµν η=
µAe
AA
W ee νµλλ
νµ ∂=Γ }{
( )AAA
WW eeeT µννµλλ
µνλνµ
λµν ∂−∂=Γ−Γ= }{}{
νµν
ρρµνµρ
ρµνρµν
ρµν ΤΤ−+=≡ TTTTTL21
41
[Einstein 1928], [Hayaski,Shirafuji PRD 19], [Pereira: Introduction to TG]
n Completely equivalent with GR at the level of equations
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f(T) Gravity and f(T) Cosmology
n f(T) Gravity: Simplest torsion-based modified gravity n Generalize T to f(T) (inspired by f(R))
n Equations of motion:
( )( ) }{1 4)]([41)(1 ΕΜ− Τ=+−∂+−+∂ ν
ρρν
µµνρ
ρνµρ
ρµλ
λµνρ
ρµ π AATTAATA GeTfTefTSeSTefSeee
[Bengochea, Ferraro PRD 79], [Linder PRD 82] [ ] mSTfTexdG
S ++= ∫ )(161 4
π
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f(T) Gravity and f(T) Cosmology
n f(T) Gravity: Simplest torsion-based modified gravity n Generalize T to f(T) (inspired by f(R))
n Equations of motion:
n f(T) Cosmology: Apply in FRW geometry:
(not unique choice)
n Friedmann equations:
[ ] mSTfTexdG
S ++= ∫ )(161 4
π
( )( ) }{1 4)]([41)(1 ΕΜ− Τ=+−∂+−+∂ ν
ρρν
µµνρ
ρνµρ
ρµλ
λµνρ
ρµ π AATTAATA GeTfTefTSeSTefSeee
[Bengochea, Ferraro PRD 79], [Linder PRD 82]
jiij
A dxdxtadtdsaaadiage δµ )(),,,1( 222 −=⇒=
22 26)(
38 HfTfGH Tm −−= ρπ
TTT
mm
fHfpGH 2121)(4
−++
−=ρπ!
n Find easily 26HT −=
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f(T) Cosmology: Background
n Effective Dark Energy sector:
n Interesting cosmological behavior: Acceleration, Inflation etc n At the background level indistinguishable from other dynamical DE models
⎥⎦
⎤⎢⎣
⎡ +−= TDE fTfG 3683π
ρ
]2][21[2 2
TTTT
TTTDE TffTff
fTTffw−++
+−−=
[Linder PRD 82]
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f(T) Cosmology: Perturbations
n Can I find imprints of f(T) gravity? Yes, but need to go to perturbation level
n Obtain Perturbation Equations:
n Focus on growth of matter overdensity go to Fourier modes:
)1(,)1(00 φαδψδ αµ
αµµµ −=+= ee ji
ij dxdxadtds δφψ )21()21( 222 −−+=⇒
SHRSHL .... =
SHRSHL .... =! [Chen, Dent, Dutta, Saridakis PRD 83],
[Dent, Dutta, Saridakis JCAP 1101]
m
m
ρδρ
δ ≡
( ) [ ] 0436)1)(/3(1213 42222 =+−+++−+ kmkTTTkTTT GfHfakHfHfH δρπφφ!
[Chen, Dent, Dutta, Saridakis PRD 83]
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f(T) Cosmology: Perturbations
n Application: Distinguish f(T) from quintessence n 1) Reconstruct f(T) to coincide with a given quintessence scenario:
with and
CHdHH
GHHf Q += ∫ 216)(ρ
π )(2/2 φφρ VQ += ! 6/Τ−=Η
[Dent, Dutta, Saridakis JCAP 1101]
E.N.Saridakis – VIA web lecture, Feb 2014
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f(T) Cosmology: Perturbations
n Application: Distinguish f(T) from quintessence n 2) Examine evolution of matter overdensity
[Dent, Dutta, Saridakis JCAP 1101]
m
m
ρδρ
δ ≡
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Bounce and Cyclic behavior
n Contracting ( ), bounce ( ), expanding ( ) near and at the bounce
n Expanding ( ), turnaround ( ), contracting near and at the turnaround
0<H 0=H 0>H0>H!
0>H 0=H 0<H0<H!
E.N.Saridakis – VIA web lecture, Feb 2014
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Bounce and Cyclic behavior in f(T) cosmology
n Contracting ( ), bounce ( ), expanding ( ) near and at the bounce
n Expanding ( ), turnaround ( ), contracting near and at the turnaround
0<H 0=H 0>H0>H!
0>H 0=H 0<H0<H!
22 26)(
38 HfTfGH Tm −−= ρπ
TTT
mm
fHfpGH 2121)(4
−++
−=ρπ!
n Bounce and cyclicity can be easily obtained [Cai, Chen, Dent, Dutta, Saridakis CQG 28]
E.N.Saridakis – VIA web lecture, Feb 2014
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Bounce in f(T) cosmology
n Start with a bounching scale factor:
3/12
231)( ⎟
⎠
⎞⎜⎝
⎛ += tata B σ
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛ ++−−±=⇒ 2
43
34
32
34)(
σσσ
σ TT
TTt
⎥⎥⎦
⎤
⎢⎢⎣
⎡⎟⎟⎠
⎞⎜⎜⎝
⎛+
++
+=⇒ tsArcTan
ttM
tMtttf mB
pmB
p 236
326
)32(4)( 2
22
22 ρσσσρ
σ
E.N.Saridakis – VIA web lecture, Feb 2014
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Bounce in f(T) cosmology
n Start with a bounching scale factor:
n Examine the full perturbations:
with known in terms of and matter
n Primordial power spectrum: n Tensor-to-scalar ratio:
⎟⎟
⎠
⎞
⎜⎜
⎝
⎛ ++−−±=⇒ 2
43
34
32
34)(
σσσ
σ TT
TTt
⎥⎥⎦
⎤
⎢⎢⎣
⎡⎟⎟⎠
⎞⎜⎜⎝
⎛+
++
+=⇒ tsArcTan
ttM
tMtttf mB
pmB
p 236
326
)32(4)( 2
22
22 ρσσσρ
σ
02
222 =+++ kskkk akc φφµφαφ !!! 22 ,, scµα TTT fffHH ,,,, !
22288 pMP
πσ
ζ =
01123 2
2
=+
−⎟⎟⎠
⎞⎜⎜⎝
⎛ ∇−+ h
ffHHh
ahHh
T
TTijijij
!!!!!
3108.2 −×≈r
⇒⇒
[Cai, Chen, Dent, Dutta, Saridakis CQG 28]
3/12
231)( ⎟
⎠
⎞⎜⎝
⎛ += tata B σ
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Non-minimally coupled scalar-torsion theory
n In curvature-based gravity, apart from one can use n Let’s do the same in torsion-based gravity:
[Geng, Lee, Saridakis, Wu PLB704]
2^ϕξRR +)(RfR +
( ) ⎥⎦
⎤⎢⎣
⎡ +−+∂∂+= ∫ mLVTTexdS )(21
22
24 ϕϕξϕϕ
κµ
µ
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Non-minimally coupled scalar-torsion theory
n In curvature-based gravity, apart from one can use n Let’s do the same in torsion-based gravity:
n Friedmann equations in FRW universe: with effective Dark Energy sector:
n Different than non-minimal quintessence! (no conformal transformation in the present case)
2^ϕξRR +)(RfR +
( ) ⎥⎦
⎤⎢⎣
⎡ +−+∂∂+= ∫ mLVTTexdS )(21
22
24 ϕϕξϕϕ
κµ
µ
( )DEmH ρρκ+=
3
22
( )DEDEmm ppH +++−= ρρκ2
2!
222
3)(2
ϕξϕϕ
ρ HVDE −+=!
( ) 222
234)(2
ϕξϕϕξϕϕ HHHVpDE !!!
+++−=
[Geng, Lee, Saridakis,Wu PLB 704]
[Geng, Lee, Saridakis, Wu PLB704]
E.N.Saridakis – VIA web lecture, Feb 2014
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Non-minimally coupled scalar-torsion theory
n Main advantage: Dark Energy may lie in the phantom regime or/and experience the phantom-divide crossing
n Teleparallel Dark Energy:
[Geng, Lee, Saridakis, Wu PLB 704]
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Observational constraints on Teleparallel Dark Energy
n Use observational data (SNIa, BAO, CMB) to constrain the parameters of the theory
n Include matter and standard radiation: n We fit for various
ξ,0DEDE0M0 w,Ω,Ω )(ϕV
azaa rrMM /11,/,/ 40
30 =+== ρρρρ
E.N.Saridakis – VIA web lecture, Feb 2014
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Observational constraints on Teleparallel Dark Energy
Exponential potential
Quartic potential
[Geng, Lee, Saridkis JCAP 1201] E.N.Saridakis – VIA web lecture, Feb 2014
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Phase-space analysis of Teleparallel Dark Energy
n Transform cosmological system to its autonomous form:
n Linear Perturbations: n Eigenvalues of determine type and stability of C.P
),sgn(13
2222 ξ
ρ zyx=Hm
m +−−≡Ω⇒
[Xu, Saridakis, Leon, JCAP 1207] ( )ξ,,, zyxw=w DEDE
,f(X)=X'⇒QUU =⇒ 'UX=X C +
0' =|XCX=X
Q
κϕξϕκϕκ
=== zHVy
Hx ,
3)(,
6!
)sgn(3
2222 ξ
ρ zyx=HDE
DE −+≡Ω
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Phase-space analysis of Teleparallel Dark Energy
n Apart from usual quintessence points, there exists an extra stable one for corresponding to
[Xu, Saridakis, Leon, JCAP 1207]
ξλ <2 1,1,1 −=−==Ω qwDEDE
n At the critical points however during the evolution it can lie in quintessence or phantom regimes, or experience the phantom-divide crossing!
1−≥DEw
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Exact charged black hole solutions
n Extend f(T) gravity in D-dimensions (focus on D=3, D=4):
n Add E/M sector: with
n Extract field equations:
[ ]Λ−+= ∫ 2)(21 TfTexdS D
κ
FFLF∗∧−=
21 µ
µdxAAdAF ≡= ,
SHRSHL .... =[Gonzalez, Saridakis, Vasquez, JHEP 1207]
[Capozzielo, Gonzalez, Saridakis, Vasquez, JHEP 1302]
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Exact charged black hole solutions
n Extend f(T) gravity in D-dimensions (focus on D=3, D=4):
n Add E/M sector: with
n Extract field equations:
n Look for spherically symmetric solutions:
n Radial Electric field: known
[ ]Λ−+= ∫ 2)(21 TfTexdS D
κ
FFLF∗∧−=
21 µ
µdxAAdAF ≡= ,
SHRSHL .... =
∑−
−−=⇒2
1
2222
222
)(1)(
D
idxrdrrG
dtrFds
!,,,,)(
1,)( 23
1210 rdxerdxedr
rGedtrFe ====
2−= Dr rQE 22 )(,)( rGrF⇒
[Gonzalez, Saridakis, Vasquez, JHEP 1207], [Capozzielo, Gonzalez, Saridakis, Vasquez, JHEP 1302] E.N.Saridakis – VIA web lecture, Feb 2014
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Exact charged black hole solutions
n Horizon and singularity analysis: n 1) Vierbeins, Weitzenböck connection, Torsion invariants: T(r) known obtain horizons and singularities
n 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 µνρσµνρσ
⇒
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Exact charged black hole solutions
[Capozzielo, Gonzalez, Saridakis, Vasquez, JHEP 1302] E.N.Saridakis – VIA web lecture, Feb 2014
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Exact charged black hole solutions
n More singularities in the curvature analysis than in torsion analysis! (some are naked)
n The differences disappear in the f(T)=0 case, or in the uncharged case.
n Should we go to quartic torsion invariants?
n f(T) brings novel features.
n E/M in torsion formulation was known to be nontrivial (E/M in Einstein-Cartan and Poinaré theories)
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Solar System constraints on f(T) gravity
n Apply the black hole solutions in Solar System: n Assume corrections to TEGR of the form
)()( 32 TOTTf +=α
⎥⎦
⎤⎢⎣
⎡ Λ−−Λ−+
Λ−−=⇒
rcGM
rr
rcGMrF 22
22
2 4663
21)( α
⎥⎦
⎤⎢⎣
⎡⎟⎠
⎞⎜⎝
⎛ −Λ−Λ−−Λ
+Λ
−−=⇒ 2222
22
22 882224
38
321)(
rrcGMr
rr
rcGMrG α
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Solar System constraints on f(T) gravity
n Apply the black hole solutions in Solar System: n Assume corrections to TEGR of the form
n Use data from Solar System orbital motions:
T<<1 so consistent
n f(T) divergence from TEGR is very small n This was already known from cosmological observation constraints up to n With Solar System constraints, much more stringent bound.
)()( 32 TOTTf +=α
⎥⎦
⎤⎢⎣
⎡ Λ−−Λ−+
Λ−−=⇒
rcGM
rr
rcGMrF 22
22
2 4663
21)( α
⎥⎦
⎤⎢⎣
⎡⎟⎠
⎞⎜⎝
⎛ −Λ−Λ−−Λ
+Λ
−−=⇒ 2222
22
22 882224
38
321)(
rrcGMr
rr
rcGMrG α
10)( 102.6 −×≤Δ TfU
[Iorio, Saridakis, Mon.Not.Roy.Astron.Soc 427)
)1010( 21 −− −O [Wu, Yu, PLB 693], [Bengochea PLB 695]
E.N.Saridakis – VIA web lecture, Feb 2014
n Perturbations: , clustering growth rate:
n γ(z): Growth index.
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Growth-index constraints on f(T) gravity
)(lnln
aad
dm
m γδΩ=
)('11Tf
Geff +=
mmeffmm GH δρπδδ 42 =+ !!!
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Growth-index constraints on f(T) gravity
)(lnln
aad
dm
m γδΩ=
)('11Tf
Geff +=
[Nesseris, Basilakos, Saridakis, Perivolaropoulos, PRD 88]
mmeffmm GH δρπδδ 42 =+ !!!n Perturbations: , clustering growth rate:
n γ(z): Growth index.
n Viable f(T) models are practically indistinguishable from ΛCDM.
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Open issues of f(T) gravity
n f(T) cosmology is very interesting. But f(T) gravity and nonminially coupled teleparallel gravity has many open issues
n For nonlinear f(T), Lorentz invariance is not satisfied n 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]
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Open issues of f(T) gravity
n f(T) cosmology is very interesting. But f(T) gravity and nonminially coupled teleparallel gravity has many open issues
n For nonlinear f(T), Lorentz invariance is not satisfied n Equivalently, the vierbein choices corresponding to the same metric are
not equivalent (extra degrees of freedom)
n Black holes are found to have different behavior through curvature and torsion analysis
n Thermodynamics also raises issues
n 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)
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Gravity modification in terms of torsion?
n So can we modify gravity starting from its torsion formulation?
n The simplest, a bit naïve approach, through f(T) gravity is interesting, but has open issues
n Additionally, f(T) gravity is not in correspondence with f(R)
n 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?
n What about higher-order corrections, but using torsion invariants (similar to Gauss Bonnet, Lovelock, Hordenski modifications)?
n Can we modify gauge theories of gravity themselves? E.g. can we modify Poincaré gauge theory?
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Conclusions n i) Torsion appears in all approaches to gauge gravity, i.e to the first step
of quantization. n ii) Can we modify gravity based in its torsion formulation? n iii) Simplest choice: f(T) gravity, i.e extension of TEGR n iv) f(T) cosmology: Interesting phenomenology. Signatures in growth
structure. n v) We can obtain bouncing solutions n vi) Non-minimal coupled scalar-torsion theory : Quintessence,
phantom or crossing behavior. n vii) Exact black hole solutions. Curvature vs torsion analysis. n viii) Solar system constraints: f(T) divergence from T less than n ix) Growth Index constraints: Viable f(T) models are practically
indistinguishable from ΛCDM. n x) Many open issues. Need to search for other torsion-based
modifications too.
2ξTT ϕ+
1010−
E.N.Saridakis – VIA web lecture, Feb 2014
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Outlook n Many subjects are open. Amongst them:
n i) Examine thermodynamics thoroughly.
n ii) Understand the extra degrees of freedom and the extension to non-diagonal vierbeins.
n iii) Try to modify TEGR using higher-order torsion invariants.
n iv) Try to modify Poincaré gauge theory (extremely hard!)
n v) What to quantize? Metric, vierbeins, or connection?
n vi) Convince people to work on the subject!
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THANK YOU!
E.N.Saridakis – VIA web lecture, Feb 2014