Can Coupled Dark Energy Speed Up the Bullet Cluster?
  It has been recently shown that the observed morphological properties of the Bullet Cluster can be accurately reproduced in hydrodynamical simulations only when the infall pairwise velocity V_{c} of the system exceeds 3000km/s (or at least possibly 2500 km/s) at the pair separation of 2R_{vir}, where R_{vir} is the virial radius of the main cluster, and that the probability of finding such a bullet-like system is extremely low in the standard \Lambda CDM cosmology. We suggest here the fifth-force mediated by a coupled Dark Energy (cDE) as a possible velocity-enhancing mechanism and investigate its effect on the infall velocities of the bullet-like systems from the CoDECS (COupled Dark Energy Cosmological Simulations) public database. Five different cDE models are considered: three with constant coupling and exponential potential, one with exponential coupling and exponential potential, and one with constant coupling and supergravity potential. For each model, after identifying the bullet-like systems, we determine the probability density distribution of their infall velocities at the pair separations of (2-3)R_{vir}. Approximating each probability density distribution as a Gaussian, we calculate the cumulative probability of finding a bullet-like system with V_{c}>=3000 km/s or V_{c}>=2500 km/s. Our results show that in all of the five cDE models the cumulative probabilities increase compared to the \Lambda CDM case and that in the model with exponential coupling P(V_{c}>=2500 km/s) exceeds 10^{-4}. The physical interpretations and cosmological implications of our results are provided. 
Jounghun Lee (Seoul National U.), Marco Baldi (Excellence Cluster   Universe Garching)
astro-ph.CO