1 Edmund Bertschinger MIT Department of Physics and Kavli Institute for Astrophysics and Space Research Testing Gravity on Large Scales Dekel 1994 Ann. Rev. Ast. Ap. with Peculiar Velocities

Collaborators 2 Alessandra SilvestriMustafa AminPhillip Zukin

3 Dekel, Bertschinger and Faber 1990EB, AD, SMF, Dressler, Burstein 1990

POTENT Reconstruction Assumptions 1. Universal free-fall of galaxies and dark matter 2. Fluctuations grew from smooth initial conditions 3. Irrotational velocity field (potential flow) Radial velocities suffice 4

A 1990s Cosmological test Compare (v and g) or (div.v and rho) to determine =f()/b Willick 2000 (astro-ph/ ): “a preponderance of the evidence at present argues against very large scale (>100 Mpc/h) bulk flows, and favors  ≃ 0.4–0.5, corresponding to a low- density ( m ≃ 0.2–0.3) universe.” Davis et al. 2011: 0.24 <  < 0.43 (95% c.l.) 5

A 15-year enigma: cosmic acceleration Lambda? Dynamical dark energy? Modified gravity? (This option usually retains dark matter but abandons dark energy) 6

Testing GR on cosmological scales Supernovae and BAO determine expansion history Expansion history does not test GR Any H(z) can be fit without  or DE, if GR modified Cosmic acceleration does not test GR 7

A continuing controversy Very large scale bulk flows and their (mis)alignment with the CMB dipole Can modified gravity produce either? Crook et al 2009: Modified Poisson equation changes bulk flow magnitude 8

Bulk flow direction is insensitive to modifications of GR Magnitude is however sensitive 9 Crook et al. 2009

What is the response of the CMB itself to modified gravity? Can it produce misalignment? 10

Three cosmic functions 11 For a flat background, this is the most general first-order perturbed Robertson-Walker spacetime excluding vector and tensor modes. It does not assume that GR is valid.

Measuring two potentials with test particles E.B. arXiv: Test masses: (if H=0) Photons: Measure  using orbits of planets, stars, galaxies Measure  using light (deflection, energy shift) 12

CMB dipole in GR and Modified Gravity E.B. + A. Silvestri, in preparation Integrated Sachs-Wolfe effect: v CMB = -u , Modified gravity => gravitational slip (Daniel et al. 2008, 9) Also, scale-dependent departures from  GR 13

EB + A. Silvestri, in preparation 14 a Designer f(R) gravity, f R0 = 10 3

low-l ISW from Designer f(R) 15

Another way to change the potentials: Late-time quintessence dynamics in GR arXiv: Amin, Zukin, EB 16

Conclusions Galaxy peculiar velocities (combined with gravity or density) depend only on (k,t) and give measurement of growth rate parameter b. Even with modified gravity, v  g (unless decaying mode) v galaxies -v CMB also probes ISW effect, depends on /t A new test of GR is underway (EB+Silvestri, in prep) 17