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Transcript of 1 Edmund Bertschinger MIT Department of Physics and Kavli Institute for Astrophysics and Space...
1
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
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
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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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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