Evidence for a Long-Range Dark Matter Self Interaction (“Fifth Force”) Glennys R. Farrar Center for Cosmology and Particle Physics New York University See GRF + Rachel Rosen, preprint in preparation
Long Range Interaction of ~Gravitational Strength is Generic! V(r ) = VN (r ) exp(-r/r5) • String theory and most extensions of the SM contain fields, e.g., moduli, which are massless at every order of perturbation theory => very long range. • Vevs are naturally of order MGUT or MPl => very weak coupling. ~ (MPl / vev)2 • May be Yukawa with r 5 ~ m-1 or r 5 ~ a (GF+Peebles) • May have more complicated dependence on distance, e.g., Chameleon models. => must test for fifth force on all scales.
IE 0657-67 “bullet cluster” Markevitch astro-ph/0511345 Z = 0.3 Mach 3.0 ± 0.4 vgas = 4740 ± 630 km/s (actually, - 550 km/s) Tclus = 14 keV (Tif quiescent ~ 10 keV)
NEWWeak + Strong Lensing Surface Density (red)X-ray Brightness (white) From Bradac et al astro-ph/0608408 DM proof: Clowe et al, astro-ph/0608
Furthermore… • Sub-cluster is moving in plane of sky (<~8o). • Sub-cluster is on its first pass; went directly through center of main cluster. • Gas bullet lags DM. (model-independent disproof of MOND) • DM sub-cluster velocity = 4740 +670- 550 km/s • If Keplerian, vplunging = 21/2 vcirc = 3360 km/s • How serious is this > 2 “discrepancy”?
More careful modelingGRF + Rachel Rosen, paper in preparation • Consider various density profiles for main DM cluster. • Use Mass Accretion History of Wechsler et al (2002) • Use actual MAH of 12 most massive simulated clusters • Fix initial infall velocity to 300 km/s (600 km/s) => initial position and time are not independent. • Predicted final velocity is insensitive to starting time. • “Fiducial model” predicts v = 2950 km/sec • Results range from 2360 km/s to ~ 3500 km/s. • Best for no-5th force: prolate mass distsribution with c/a = 0.65, aligned with direction of motion => v <~3500 km/sec • Observed: v = 4740 km/s; v-1 = 4190 km/s
MAH variations and other complications in modeling • Use actual Mass accretion history of 12 most massive halos in M. White& J. Cohn simulation, compared to WBPKD formula • Most extreme case has 10% bigger velocity • Other complications either increase discrepancy (shape of DM distribution, weak lensing bkg, …) or appear to be small • Statistical Appoach. E. Hayashida & S. White, astro-ph/0604443 -- used old and rounded parameter values. Updating their analysis with newest vsub and accurate mass values, gives • Prob( vsub/V200 > obs) < 10-6 • Prob(v-1sigma/V200 > obs < 10-3
Some Observational Benefits of a 5th Force • Helps reconcile 8 from WMAP (.75) and Large Scale Structure simulations (0.9-1.1) • Large scale, high precision simulations underway (GRF, V. Springel) • Helps explain factor-10 discrepancy between LCDM simulations and number of superclusters observed in SDSS (Einasto et al, astro-ph/06…) • Helps explain insufficiency of observed DM substructures in galaxies • Helps empty voids (?); reduces late accretion • (Nusser et al astro-ph/0412586: static case, rough statistics. GRF-Peebles qualitative argument, voids emptier, less late accretion)
Conclusions • Interpreting IE 0657-56 at face value as “5th” force: • r5 1 Mpc ~ 1/2 - 1 • Consistent with present constraints (Gradwohl&Frieman, ApJ 398, 1992) • Improving IE 0657-56 measurements: • Improve lensing: measure redshifts of more arcs • Model gas deceleration; reduce error on gas bullet velocity • Need large statistics studies in other systems (edges of voids, velocity dispersion vs weak lensing, …) SDSS…. • Smaller r5 accessible via Tidal Tails of dwarf galaxies in Milky Way. Kesden&Kamionkowski 2006 • IfDM experiences a 5th force, it may be hard to see in direct detection expts (loop corrections + EotWash)