Probing alternative theories of gravity with Planck

Transcription

1 Probing alternative theories of gravity with Planck Andrea Marchini Sapienza - University of Rome based on Updated constraints from the Planck experiment on modified gravity:prd88, In collaboration with V. Salvatelli January 16, 2014 Cosmology on the Beach 2014

2 Planck results The Planck experiment has provided new and precise measurements of CMB anisotropy. Angular scales covered up to multipole l The new data are in full agreement with the ΛCDM model......with some differences in the content of the Universe. 2 of 24 P. A. R. Ade et al. [ Planck Collaboration], arxiv: [astro-ph.co].

3 Tension with H 0 astrophysical measurements Discrepancy between H 0 measured by Planck compared with the values measured by independent cosmological probes. While systematics can be present, the discrepancy can be explained including new physical phenomena. The Planck determination of H 0 is model dependent. 3 of 24

4 Tension with H 0 astrophysical measurements Discrepancy between H 0 measured by Planck compared with the values measured by independent cosmological probes. While systematics can be present, the discrepancy can be explained including new physical phenomena. The Planck determination of H 0 is model dependent. 4 of 24

5 Anomalous lensing signal Planck+WP Planck+WP+BAO Planck+WP+HST A L scales the lensing power spectrum and modulates the lensing effect in the temperature power spectrum. Probability We expect A L = 1. There is an evidence for A L > A L A L = % c.l. A L = % c.l. A L = % c.l. 5 of 24

6 Hints of New Physics? How can we interpret the tensions? Systematic effects. Signature of New Physics. 6 of 24

7 Hints of New Physics? How can we interpret the tensions? Systematic effects. Signature of New Physics. The solution of the Planck tensions could arise from New Physics effects. We focus on: Alternative theories of gravity 6 of 24

8 The modern theory of gravitation General relativity is the theory of gravity The theory of gravitational fields, constructed on the basis of theory of relativity, is called general relativity. It was established by Einstein (and finally formulated by him 1915), and represents probably the most beautiful of all existing physical theories. Landau & Lifshits. The classical theory of fields. Vol. 2. GR has been tested directly in the Solar System in the weak-field limit. Indirect test in the same regime outside the Solar System from binary pulsar. Strong regime tests are missing and gravity is tested very poorly at the large scales. 7 of 24

9 Cosmology and General Relativity Can the success of ΛCDM be advocated as a clear confirmation that GR works properly at large scales? 8 of 24

10 Cosmology and general relativity Can the success of ΛCDM be advocated as a clear confirmation that GR works properly at large scales? Almost all theories of gravity admit the FLRW as solution of their field equations. Indeed cosmology could indicates that gravity is not exactly described by GR. 9 of 24 To explain the cosmic acceleration within the context of GR, one needs to introduce the dark energy, which is very exotic, comprises approximately 70% of the energy content of the universe, and is not detected in the laboratory.

11 Alternative theories of gravity: f (R) models Generalization of the Einstein-Hilbert action by making it more general function of the Ricci scalar. This choice leads to fourth-order field equations. Possible explanation for the observed late-time accelerating expansion of the Universe. General relativity f (R) S = d 4 x ( ) g f (R) 2κ + L m(ψ, g µν ) R µν 1 2 g µνr = κt µν 3H 2 = κ (ρ m + ρ rad ) 2Ḣ = κ ( ρ m ρ ) rad 10 of 24

12 Alternative theories of gravity: f (R) models Generalization of the Einstein-Hilbert action by making it more general function of the Ricci scalar. This choice leads to fourth-order field equations. Possible explanation for the observed late-time accelerating expansion of the Universe. f (R) theories S = d 4 x g ( ) f (R) 2κ + L m(ψ, g µν ) f R (R)R µν 1 2 f (R)g µν + (g µν µ ν ) f R (R) = κt µν 3f R (R)H 2 = κ (ρ m + ρ rad ) (f R(R)R f (R)) 3Hḟ R (R) 2f R (R)Ḣ = κ ( ρ m ρ rad) + f R (R) Hḟ R(R) 11 of 24

13 Parametrization Poisson and anisotropy equations for ΛCDM k 2 Ψ = 4πGa 2 {ρ + 3(ρ + P)σ} k 2 [Φ Ψ] = 12πGa 2 (ρ + P)σ 12 of 24

14 Parametrization Poisson and anisotropy equation for alternative theories of gravity k 2 Ψ = µ(a, k) 4πGa 2 {ρ + 3(ρ + P)σ} k 2 [Φ γ(a, k) Ψ] = µ(a, k) 12πGa 2 (ρ + P)σ The background evolution is fixed to ΛCDM but the evolution of matter perturbations can be different. Bertschinger-Zukin parametrization µ(a, k) = 1+β 1λ 2 1 k2 a s, γ(a, k) = 1+β 2λ 2 1+λ 2 1 k2 a s 2 k2 a s 1+λ 2 2 k2 a s (B. & Z., PRD 78, , 2008) The only free parameter for f (R) is λ 2 1. Usually it is expressed as the present length-scale in units of the horizon scale: B 0 = 2λ 2 1 H2 0 /c2. 13 of 24

15 Effect on the Cosmic Microwave Background 14 of 24

16 Constraints PLANCK PLANCK+BAO PLANCK+HST Parameters 68% limit 68% limit 68% limit Ω b h ± ± ± Ω ch ± ± ± θ ± ± ± τ ± ± ± n s ± ± ± log(10 10 A s) ± ± ± B 0 < (95% c.l.) < (95% c.l.) < (95% c.l.) Ω m ± ± ± Ω Λ ± ± ± z re 10.6 ± ± ± 1.1 H 0 [km/s/mpc] 69.1 ± ± ± 1.1 Age/Gyr ± ± ± f (R) models are not favored but we obtain an upper limit on B 0. f (R) theories alleviate the H 0 tension between Planck and HST. 15 of 24

17 Constraints: varying the lensing amplitude PLANCK PLANCK+BAO PLANCK+HST Parameters 68% limit 68% limit 68% limit Ω b h ± ± ± Ω ch ± ± ± θ ± ± ± τ ± ± ± n s ± ± ± log(10 10 A s) ± ± ± B 0 < (95% c.l.) < (95% c.l.) < (95% c.l.) A L Ω m ± ± ± Ω Λ ± ± ± z re 10.8 ± ± ± 1.1 H 0[km/s/Mpc] 68.7 ± ± ± 1.3 Age/Gyr ± ± ± The bimodal behavior of the B 0 posterior distribution appears for all the data sets combinations. In f (R) models A L is in much better agreement with one than in ΛCDM. 16 of 24

18 Conclusions Some tensions can arise comparing Planck results with others independent astrophysical probes. While the systematic effects can be important it is worth to investigate if this discrepancy can be explained by new physics. f (R) theories are compatible but not favored and we obtain a tight upper limit on the length-scale B 0. In this scenario the tensions are alleviated indicating that new physics can be a proper explanation. 17 of 24

19 fffff 18 of 24

20 Why the bimodality? 19 of 24

21 Why the bimodality? Varying A L has the same effect of varying B 0 at the high multipoles. At low multipoles instead increasing B 0 lowers the integrated Sachs-Wolfe effect plateau, contrary to the effect of an increased H 0 value. This favors the match between theory and data even in presence of large H 0 values. The competition between these effects creates the local maximum in the posterior distribution. 19 of 24

22 Conformal transformation We can perform the conformal transformation g µν = e 2β κφ g E µν. We can recast f (R) theories as GR with a scalar field coupled to the matter sector. Action in the Einstein frame S = d 4 x ( ) g E R E 2κ 3 g E β µν µ φ ν φ V (φ) + L 2 m (ψ, e 2β κφ gµν) E V (φ) = Rf R (R) f (R) 2κf 2 R (R) The theory is well-defined as long as V (φ) is bounded from below. The scalar field is defined as φ 1 2β κ ln f R(R). The coupling for f (R) theories is β = 1 6 to obtain a canonical kinetic term (in general f R (R) > 0 to avoid ghost). 20 of 24

23 Coupling with matter The mass of the scalar for viable model is m 0 H Gev. L = κβφm ψ0 ψψ δφ N = 2β 2 G Nm 2 ψ0 r Constrained by Cassini measurements: β (B. Bertotti, L. Iess, and P. Tortora, Nature 425 (2003) 374) f (R) theories violate this condition. Screening mechanisms: chameleons, dilatons, symmetrons, of 24

24 B 0 vs Ω c h 2 and B 0 vs H 0 PLANCK PLANCK+HST PLANCK+BAO Ω c h B 0 H B 0 22 of 24

25 B 0 vs A L PLANCK PLANCK+HST PLANCK+BAO 23 of 24

26 Effect of the potentials 24 of 24