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Edmund BertschingerMIT 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

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Alessandra Silvestri Mustafa Amin Phillip Zukin

3Dekel, Bertschinger and Faber 1990 EB, 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

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A 1990s Cosmological test

Compare (v and g) or (div.v and rho) to determine =f()/b

Willick 2000 (astro-ph/0003232):“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.)

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A 15-year enigma: cosmic acceleration

Lambda?

Dynamical dark energy?

Modified gravity? (This option usually retains dark matter but abandons dark energy)

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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

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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

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Bulk flow direction is insensitive to modifications of GR

Magnitude is however sensitive

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Crook et al. 2009

What is the response of the CMB itself to modified gravity?

Can it produce misalignment?

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Three cosmic functions

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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 particlesE.B. arXiv:1111.4659

Test masses: (if H=0)

Photons:

Measure using orbits of planets, stars, galaxiesMeasure using light (deflection, energy shift)

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CMB dipole in GR and Modified GravityE.B. + A. Silvestri, in preparation

Integrated Sachs-Wolfe effect: vCMB = -u,

Modified gravity => gravitational slip(Daniel et al. 2008, 9)

Also, scale-dependent departures from GR

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EB + A. Silvestri, in preparation

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Designer f(R) gravity,fR0 = 103

low-l ISW from Designer f(R)

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Another way to change the potentials:Late-time quintessence dynamics in GR

arXiv:1108.1793 Amin, Zukin, EB

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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)

vgalaxies-vCMB also probes ISW effect, depends on /t

A new test of GR is underway (EB+Silvestri, in prep)

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