Collisions between galaxy clusters provide a test of the non-gravitational forces acting on dark matter. Dark matter's lack of deceleration in the 'bullet cluster collision' constrained its self-interaction cross-section σ DM /m < 1.25 cm 2 /g (68% confidence limit) for long-ranged forces. Using the Chandra and Hubble Space Telescopes we have now observed 72 collisions, including both 'major'and 'minor' mergers. Combining these measurements statistically, we detect the existence of dark mass at 7.6σ significance. The position of the dark mass has remained closely aligned within 5.8±8.2 kpc of associated stars: implying a self-interaction cross-section σ DM /m < 0.47 cm 2 /g (95% CL) and disfavoring some proposed extensions to the standard model.Many independent lines of evidence now suggest that most of the matter in the Universe is in a form outside the standard model of particle physics. A phenomenological model for cold dark matter (1) has proved hugely successful on cosmological scales, where its gravitational influence dominates the formation and growth of cosmic structure. However, there are several 1 arXiv:1503.07675v2 [astro-ph.CO] 13 Apr 2015 challenges on smaller scales: the model incorrectly predicts individual galaxy clusters to have more centrally concentrated density profiles (2), larger amounts of substructure (3, 4), and the Milky Way to have more satellites able to produce stars (5) than are observed. These inconsistencies could be resolved through astrophysical processes (6), or if dark matter particles are either warm (7) or self-interact with cross-section 0.1 ≤ σ DM /m ≤ 1 cm 2 /g (8-10). Following (11), we define the momentum transfer per unit mass σ DM /m, integrating over all scattering angles and assuming that individual dark matter particles are indistinguishable.Self-interaction within a hidden dark sector is a generic consequence of some extensions to the standard model. For example, models of mirror dark matter (12) and hidden sector dark matter (12-16) all predict anisotropic scattering with σ DM /m ≈ 1 barn/GeV = 0.6 cm 2 /g, similar to nuclear cross-sections in the standard model. Note that couplings within the dark sector can be many orders of magnitude larger than those between dark matter and standard model particles, which is at most of order picobarns (17).In terrestrial collider experiments, the forces acting on particles can be inferred from the trajectory and quantity of emerging material. Collisions between galaxy clusters, which contain dark matter, provide similar tests for dark sector forces. If dark matter's particle interactions are frequent but exchange little momentum (via a light mediator particle that produces a longranged force and anisotropic scattering), the dark matter will be decelerated by an additional drag force. If the interactions are rare but exchange a lot of momentum (via a massive mediator that produces a short-ranged force and isotropic scattering), dark matter will tend to be scattered away and lost (11,18,19).The dynamics of colliding dark matter...

The first half of this paper explores the origin of systematic biases in the measurement of weak gravitational lensing. Compared to previous work, we expand the investigation of PSF instability and fold in for the first time the effects of non-idealities in electronic imaging detectors and imperfect galaxy shape measurement algorithms. Together, these now explain the additive A( ) and multiplicative M( ) systematics typically reported in current lensing measurements. We find that overall performance is driven by a product of a telescope/camera's absolute performance, and our knowledge about its performance.The second half of this paper propagates any residual shear measurement biases through to their effect on cosmological parameter constraints. Fully exploiting the statistical power of Stage IV weak lensing surveys will require additive biases A < ∼ 1.8× 10 −12 and multiplicative biases M < ∼ 4.0 × 10 −3 . These can be allocated between individual budgets in hardware, calibration data and software, using results from the first half of the paper.If instrumentation is stable and well-calibrated, we find extant shear measurement software from GREAT10 already meet requirements on galaxies detected at S/N=40. Averaging over a population of galaxies with a realistic distribution of sizes, it also meets requirements for a 2D cosmic shear analysis from space. If used on fainter galaxies or for 3D cosmic shear tomography, existing algorithms would need calibration on simulations to avoid introducing bias at a level similar to the statistical error. Requirements on hardware and calibration data are discussed in more detail in a companion paper. Our analysis is intentionally general, but is specifically being used to drive the hardware and ground segment performance budget for the design of the European Space Agency's recently-selected Euclid mission.