Dark Energy in f(R) Gravity

Lecture by Nicodem Poplawski
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Дата публикации: 2012-02-07
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Dark Energy in f(R) Gravity Nikodem J. Popławski Indiana University 16th Midwest Relativity Meeting Washington University in St. Louis, St. Louis, MO 18 XI MMVI

Cosmic acceleration  Cosmological constant ΛCDM model 1 R  Rg   T  g  2 Agrees with observations NASA / WMAP We are living in an accelerating universe!   10 52 m 2 References: A. G. Riess et al., Astron. J. 116, 1009 (1998) S. Perlmutter et al., Astrophys. J. 517, 565 (1999)

Dark energy Hypothetical form of energy with strong negative pressure NATURE OF DARK ENERGY • homogeneous • not very dense • not known to interact nongravitationally EXPLANATIONS • Cosmological constant • Quintessence – dynamical field • Alternative gravity theories (talks of G. Mathews and G. J. Olmo)

Dark energy Dark Force = Hypothetical form of energy – Dark Energy with strong negative pressure NATURE OF DARK ENERGY • homogeneous • not very dense • not known to interact nongravitationally EXPLANATIONS • Cosmological constant • Quintessence – dynamical field • Alternative gravity theories

Variable cosmological constant Cosmological constant problem – why is it so small? No known natural way to derive it from particle physics Possible solution: dark energy decays  Cosmological constant is not constant (Bronstein, 1933) Λ energy matter  Dark energy interact with matter Current interaction rate very small Phenomenological models of decaying Λ relate it to: t-2, a-2, H2, q, R etc. (Berman, 1991; Ozer and Taha, 1986; Chen and Wu, 1990; Lima and Carvalho, 1994)  lack covariance and/or variational derivation

f(R) gravity • Lagrangian – function of curvature scalar R • R-1 or other negative powers of R → current acceleration • Positive powers of R → inflation Minimal coupling in Jordan (original) frame (JF)

f(R) gravity • Lagrangian – function of curvature scalar R • R-1 or other negative powers of R → current acceleration • Positive powers of R → inflation Minimal coupling in Jordan (original) frame (JF) • Fully covariant theory based on the principle of least action • f(R) usually polynomial in R • Variable gravitational coupling and cosmological term • Solar system and cosmological constraints  polynomial coefficients very small G. J. Olmo, W. Komp, gr-qc/0403092

Variational principles I • f(R) gravity field equations: vary total action for both the field & matter • Two approaches: metric and metric-affine

Variational principles I • f(R) gravity field equations: vary total action for both the field & matter • Two approaches: metric and metric-affine METRIC (Einstein–Hilbert) variational principle: • action varied with respect to the metric • affine connection given by Christoffel symbols (Levi-Civita connection)

Variational principles I • f(R) gravity field equations: vary total action for both the field & matter • Two approaches: metric and metric-affine METRIC (Einstein–Hilbert) variational principle: • action varied with respect to the metric • affine connection given by Christoffel symbols (Levi-Civita connection) METRIC–AFFINE (Palatini) variational principle: • action varied with respect to the metric and connection • metric and connection are independent • if f(R)=R  metric and metric-affine give the same field equations: variation with respect to connection  connection = Christoffel symbols E. Schrödinger, Space-time structure, Cambridge (1950)

Variational Principles: Metric METRIC variational principle: • connection: Christoffel symbols of metric tensor  metric compatibility • fourth-order differential field equations • mathematically equivalent to Brans–Dicke (BD) gravity with ω=0 • 1/R gravity unstable – but instabilities disappear with additional positive powers of R • potential inconsistencies with cosmological evolution • need to transform to the Einstein conformal frame to avoid violations of the dominant energy condition (DEC)  EF is physical

Variational Principles: Metric–Affine METRIC–AFFINE variational principle: • no a priori relation between metric and connection • second-order differential equations of field • mathematically equivalent to BD gravity with ω=−3/2 • field equations in vacuum reduce to GR with cosmological constant • no instabilities • no inconsistencies with cosmological evolution • both the Jordan and Einstein frame obey DEC Work presented here uses metric–affine formulation

Jordan frame Assume action for matter is independent of connection (good for cosmology) ~ ~ ~ Variation of connection    [ f ' ( R) g   g] 0  connection = Christoffel symbols of :   {}g

Jordan frame Assume action for matter is independent of connection (good for cosmology) ~ ~ ~ Variation of connection    [ f ' ( R) g   g] 0  connection = Christoffel symbols of :   {}g Variation of metric  Dynamical energy-momentum (EM) tensor generated by metric: ~ Writing   {} ~  ... and R ()  R ( g )    g allows interpretation of Θ as additional source and brings EOF into GR form

Helmholtz Lagrangian The action in the Jordan frame is dynamically equivalent to the Helmholtz action provided f " ( )  0 Scalar – tensor gravity (STG) GR limit and Solar System constraints under debate The scalar degree of freedom corresponding to nonlinear terms in the Lagrangian is transformed into an auxiliary nondynamical scalar field p (or φ) T. P. Sotiriou, Class. Quantum Grav. 23, 5117 (2006) V. Faraoni, Phys. Rev. D 74, 023529 (2006)

Einstein frame Conformal transformation of metric: Effective potential Non-minimal coupling in Einstein frame (EF)

Einstein frame Conformal transformation of metric: Effective potential Non-minimal coupling in Einstein frame (EF) • If minimal coupling in Einstein frame  GR with cosmological constant • Both JF and EF are equivalent in vacuum • Coupling matter–gravity different in conformally related frames • Principle of equivalence violated in EF → constraints on f(R) gravity • Experiments should verify which frame (JF or EF) is physical G. Magnano, L. M. Sokołowski, Phys. Rev. D 50, 5039 (1994)

Equations of field and motion Variation of : Variation of : Structural equation   V

Equations of field and motion Variation of : Variation of : Structural equation   V • If T=0 (vacuum or radiation)  algebraic equation for φ → φ=const  GR with cosmological constant • Gravitational coupling and cosmological term vary • The energy-momentum tensor is not covariantly conserved • If the EM tensor generated by the EF metric tensor is physical  constancy of V(φ) → GR with cosmological constant NJP, Class. Quantum Grav. 23, 2011 (2006)

Dark energy–momentum tensor • Non-conservation of EM tensors for matter and DE separately • Total EM for matter + DE conserved  interaction

Dark energy–momentum tensor • Non-conservation of EM tensors for matter and DE separately • Total EM for matter + DE conserved  interaction Assume homogeneous and isotropic universe Continuity equation with interaction term Q: Interaction rate Γ=Q/εΛ Nondimensional rate γ=Γ/H NJP, Phys. Rev. D 74, 084032 (2006)

Cosmological parameters Hubble parameter Deceleration parameter Omega (L=f) Redshift  H(z) Higher derivatives of scale factor (jerk and snap) more complicated More nondimensional parameters: deceleration-to-acceleration transition redshift zt, dq/dz|0 etc. NJP, Class. Quantum Grav. 23, 4819 (2006); Phys. Lett. B 640, 135 (2006)

Cosmological term Palatini f(R) gravity in Einstein frame predicts (p=0)

Cosmological term Palatini f(R) gravity in Einstein frame predicts (p=0) Duh! ΛCDM model says so But: ΛCDM – constant Λ relates H and q f(R) gravity – variable Λ depends on H and q • Resembles simple phenomenological models of variable cosmological constant • Unlike them, it arises from least-action-principle based theory NJP, Phys. Rev. D 74, 084032 (2006)

R-1/R gravity   10 52 m 2 The simplest f(R) that produces current cosmic acceleration Deceleration-to-acceleration transition:

R-1/R gravity   10 52 m 2 Simplest f(R) that produces current cosmic acceleration Deceleration-to-acceleration transition: Unification of inflation and current cosmic acceleration    T=0  2 de Sitter phases: D. N. Vollick, Phys. Rev. D 68, 063510 (2003) S. M. Carroll, V. Duvvuri, M. Trodden, M. S. Turner, Phys. Rev. D 70, 043528 (2004) S. Nojiri, S. D. Odintsov, Phys. Rev. D 68, 123512 (2003); NJP, CQG 23, 2011 (2006)

R-1/R gravity   10 52 m 2 The simplest f(R) that produces current cosmic acceleration Deceleration-to-acceleration transition: Unification of inflation and current cosmic acceleration    β/α ~10120 ? T=0  2 de Sitter phases: D. N. Vollick, Phys. Rev. D 68, 063510 (2003) S. M. Carroll, V. Duvvuri, M. Trodden, M. S. Turner, Phys. Rev. D 70, 043528 (2004) S. Nojiri, S. D. Odintsov, Phys. Rev. D 68, 123512 (2003); NJP, CQG 23, 2011 (2006)

Compatibility with observations I  Use f(R) observations ΛCDM Zt=-0.56+0.07-0.04 j=1 SNLS X clusters A. G. Riess et al., Astrophys. J. 607, 665 (2004) Gold

Compatibility with observations II  Use f(R) observations ΛCDM Zt=-0.56+0.07-0.04 j=1 SNLS X clusters A. G. Riess et al., Astrophys. J. 607, 665 (2004) Gold

Compatibility with observations III Current interaction rate At deceleration-to-acceleration transition Interaction between matter and dark energy is weak ε ~ a3-n f(R): n=0.04 Observations  n<0.1 P. Wang, X. H. Meng, CQG 22, 283 (2005)

Conclusions • f(R) gravity provides possible explanation for present cosmic acceleration • Dark energy interacts with matter in EF – decaying Λ • R-1/R model is nice – simple, nondimensional cosmological parameters do not depend on α • We need stronger constraints from astronomical observations FUTURE WORK • Compare with JF • Generalize to p≠0 (inflation and radiation epochs) • Solar system constraints and Newtonian limit? THANK YOU!

Back-up Slides

Conservation of matter Bianchi identity Homogeneous and isotropic universe with no pressure (comoving frame) Time evolution of φ NJP, Class. Quantum Grav. 23, 2011 (2006)

Dark energy density in f(R) Matter energy density Dark energy density NJP, Phys. Rev. D 74, 084032 (2006)

More cosmological parameters Deceleration parameter slope dq |0 dz Jerk parameter NJP, Class. Quantum Grav. 23, 4819 (2006); Phys. Lett. B 640, 135 (2006)

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