Local Void vs Dark Energy

1. Local Void vs Dark Energy Tirthabir Biswas IGC, Penn-State University with A Notari and R Mansouri, astro-ph/0606703 with A Notari, astro-ph/0702555 with A Notari, S Alexander & D Vaid, arXiv:0712.0370

2. Puzzles of Dark Energy • Smallness Problem • Coincidence or Why now Problem These puzzles are not unique to ΛCDM… … we are still very much in the dark Is dark energy real?

3. Concordant ΛCDM • Type Ia Supernovae • WMAP (CMB) • SDSS (galaxy power spectrum) • LRG (Baryon acoustic peak, BAO) • ISW effect • … Success of ΛCDM, failure of EdS How can we even contemplate not having Λ? What could we have missed?

4. The Inhomogeneous Universe Is homogeneity a good assumption?Celerier ‘97 • Back-reaction: “average expansion rate changes” [Brandenberger, Kolb…] Perturbative approach… not clear whether we can trust the results • Exact Models: What do we really measure Redshift Luminosity Distance Evolution history of universe effects DL(z) , but Inhomogeneities also effect light paths, or DL(z) Additional symmetries may lead to under-estimation of effect

5. Parameters governing the corrections • Amplitude of perturbations • Ratio between perturbation & cosmological length scale small quantity,L ~ 10-20 Mpc/h, RH~ 3000 Mpc/h How can it work? • Scenario I, Average or integrated effect: Light passes through hundreds of patches ~ 30 Mpc and pick up net correction • Does it adds coherrently? Yes. • How does it depend on L/ RH,, Doesn’t work, but could be a twist! • Scenario II, Local Effect: We are sitting inside a large ~ 300 Mpc void! Locally effect goes like Hubble expansion rate different inside and outside the patch

6. Lemaitre Tolman Bondi Model Spherically symmetric metric • Solution: • Constraints: • Matching to FLRW at boundary: k’(L) = 0  same average density • Consistency at origin: k’(0) = 0 • Flat FLRW  k(L) = 0 Both Analytical & Numerical progress can be made

7. Swiss-Cheese toy Model Integrated Effect: Place observer and source at two opposite ends • Redshift Correction • Luminocity Distance correction Photon flux conservation should yield zero average effect? [Tetradis et.al.] Can we study non-spherical configuration?

8. Minimal Void Model We are sitting inside a large ~ O(100) Mpc/h void! Non-linear evolution  Why we can still be optimistic? • We only need • We regularly observe voids ~ 50 Mpc/h, • & large structures, Great SLOAN wall ~ 400 Mpc/h • huge “hole” spanning 150 Mpc/h, and 25% underdensity! [Frith et al] • Observational evidence of a hole explaining cold spot in CMB [Rudnick, Brown & Williams] • How well do we understand nonlinear structure formation? Percolated voids ~ 100 Mpc/h[Shandarin, Sheth and Sahni] Having a large local void may not be as unlikely

9. Size of patch? Distant supernovae has z ~ 0.5-1 ~ O(1000) Mpc/h • Extreamely unlikely • Would effect CMB fluctuations • ruled out by distortion of black-body spectrum [Caldwell & Stebbins] We only need to modify upto z ~ 0.08 ~ O(200) Mpc/h “Currently” acceleration inferred through a mismatch of between Nearby, z < 0.08 & distant , z >0.4 supernovae data

10. Hout Hin Hout z<0.1, nearby supernova’s inside the patch, experience faster local Hubble expansion z>0.1outside the patch, see average expansion

13. Fitting WMAP Effect of void • Central Observer+Spherical symmetry  only monopole • Off-center observer  dipole ~ 10-3  within 10% of void-radius [Alnes & Amarzguioui] Higher multipoles are much suppressed • Non-spherical voids: Interresting possibilities, low multipole anomalies in CMB [Silk & Inoue] ? • Many large voids  interesting secondary effects to CMB anisotropies ~ 10-5 , Implications for ISW Assume spherical symmetry, EdS good enough for WMAP

14. Can EdS fit the WMAP? Yes! Provided we give up near scale-invariance [Sarkar et.al.,Souradeep et.al.] Include running instead of Λ

15. Best fit WMAP parameters • consistent with BBN • Bayons constitute 9% of matter, may be too low? • zrec ~ 13, broadly consistent • low spectral index & large running • seemingly consistent with BAO [Eisenstein et al] • 0.43< hout <0.47 low Hubble parameter Along with the jump, local h ~ 0.59 possible! HKP: h= .72 .08 supernovae: h= .59 .04 SZ effect: h= .54 .04

17. Baryon Acoustic Peak Galaxy power spectrum: • Primodial Spectrum • Scale at Matter radiation equality • Sound horizon at CMB: Baryons can move in a spherical wave around a dark matter over-density giving a peak in the galaxy power spectrum Two constraints: [Eisenstein et.al.] To fix, we possibly need higher hout • Include small curvature • Look at other spectral modifications, a peak [Sarkar et.al.] Enhances perturbations at the right scale!

18. Jobs to be done • SN Ia, WMAP, local h, BBN can work • SDSS, LRG (BAO), ISW Lyman-, to be done [Zibin] • Try to fit WMAP with h ~ .55 • Origin of void? • Can a peak in primordial spectrum help? • Study Non-linear aspects, such as percolation • Non-standard structure formation scenarios: Interacting dark matter?

19. Tests and Assessment • Optimistic it can work, but • Origin of void • Violation of Copernican principle, CMB dipole • Several tests: • D(z) curve differs from ΛCDM • direct observations of voids • correlated anisotropies in CMB and H measurements • Modifications in CMB black-body spectrum Who knows in 5 years we may be talking mystery of the void 