Yashar Akrami COSMO 2013 / Cambridge / September 5, 2013 Postdoctoral Fellow Institute of Theoretical Astrophysics University of Oslo, Norway with: Tomi S. Koivisto, David F. Mota & Marit Sandstad Cosmology of Ghost-free Bimetric Gravity: Late-time Acceleration and Double-coupling Generalization Accelerated expansion from ghost-free bigravity: a statistical analysis with improved generality JHEP 1303 (2013) 099 [arXiv:1209.0457] & Bimetric gravity doubly coupled to matter: theory and cosmological implications arXiv:1306.0004, to be submitted to JCAP
14
Embed
Cosmology of Ghost-free - Centre for Theoretical CosmologyCOSMO 2013 / Cambridge / September 5, 2013 Postdoctoral Fellow Institute of Theoretical Astrophysics University of Oslo, Norway
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
Yashar Akrami
COSMO 2013 / Cambridge / September 5, 2013
Postdoctoral Fellow Institute of Theoretical Astrophysics University of Oslo, Norway
with: Tomi S. Koivisto, David F. Mota & Marit Sandstad
Cosmology of Ghost-free Bimetric Gravity:
Late-time Acceleration and Double-coupling Generalization
Accelerated expansion from ghost-free bigravity: a statistical analysis with improved generality JHEP 1303 (2013) 099 [arXiv:1209.0457]
& Bimetric gravity doubly coupled to matter: theory and cosmological implications
arXiv:1306.0004, to be submitted to JCAP
Fierz & Pauli (1939):
Demand for Massive Gravity
+2
The only ghost-free mass term:
(linearized metric fluctuations around flat space-time)
1. Luminosity distances from Type Ia SNe: 580 points (Union 2.1)
2. Position of the 1st CMB power-spectrum peak (angular scale
of sound horizon at recombination era): 1 point (WMAP 7)
3. BAO: Ratio of sound horizon scale at drag epoch to dilation scale: 6 points (2dFGRS, 6dFGS, SDSS and WiggleZ)
4. Constraints on H0: 1 point
(purely background)
Yashar Akrami / Institute of Theoretical Astrophysics, University of Oslo / Cambridge, September 5, 2013 5
Only B1 and Ωm0 nonzero
+2
Yashar Akrami / Institute of Theoretical Astrophysics, University of Oslo / Cambridge, September 5, 2013 6
Only B1 and Ωm0 nonzero
Graviton Mass: m2 = 1.45 H0
2 ± 0.25 H02
+2
Yashar Akrami / Institute of Theoretical Astrophysics, University of Oslo / Cambridge, September 5, 2013 7
Only B1 and Ωm0 nonzero
+2
Yashar Akrami / Institute of Theoretical Astrophysics, University of Oslo / Cambridge, September 5, 2013 8
Only B1 and Ωm0 nonzero
+2
ΩΛ = B1y
Yashar Akrami / Institute of Theoretical Astrophysics, University of Oslo / Cambridge, September 5, 2013 9
Full Results
+2
• No exact ΛCDM without explicit cosmological constant (vacuum energy) • Dynamical dark energy • Degeneracies between parameters
Yashar Akrami / Institute of Theoretical Astrophysics, University of Oslo / Cambridge, September 5, 2013 10
Generalization: doubly coupled bigravity
+2
Action
Dust-dominated universe: Shown to be theoretically consistent
Yashar Akrami / Institute of Theoretical Astrophysics, University of Oslo / Cambridge, September 5, 2013 11
• So far, by proper semantics: “gravity coupled to matter and a symmetric 2-tensor” • We want a “truly bimetric theory of gravity” • Different from two copies of GR (we have proven)
Novel Features
+2
Ø Metrics conformally related.
Ø Now possible to get exact ΛCDM without vacuum energy: o Only B1 and Ωm
0 nonzero:
o Only B2 and Ωm0 nonzero:
o Only B3 and Ωm0 nonzero:
Ø Partially massless gravity:
o Fierz-Pauli mass is fully determined in terms of Λ:
o Potentially dangerous, helicity-zero component of spin-2 field decouples, leaving 4 healthy propagating mode.
o PM treats the 2 metrics on an equal footing and introduces a symmetry between them. The symmetry protects Λ.
o But no viable cosmological solution in Hassan-Rosen theory.
Our doubly coupled bimetric theory results in a natural PM gravity with viable cosmology:
Now viable!
Yashar Akrami / Institute of Theoretical Astrophysics, University of Oslo / Cambridge, September 5, 2013 12
① A comprehensive statistical analysis of Hassan-Rosen ghost-free, massive, bimetric gravity (for cosmology at background level) + several analytical explanations of results has been done.
② The theory can give “accelerated expansion” with no explicit cosmological constant.
③ The generalization of the theory to a truly (doubly coupled) theory of gravity has been shown to be theoretically viable.
④ It gives exact ΛCDM without vacuum energy (at background level) and a PM theory of gravity.
⑤ Some conceptual issues in this setup need to be addressed: physical metric, equivalence principle.
⑥ Perturbative analysis is needed to further test both theories and break the degeneracies between parameters.
⑦ Local gravitational experiments and astrophysical observations are essential.
Summary, Conclusions and Perspectives +2
Yashar Akrami / Institute of Theoretical Astrophysics, University of Oslo / Cambridge, September 5, 2013 13