This paper is organized as follows: In section II, we derive the background dynamic equations for the evolution of a universe with decaying dark matter. In section III, we describe the method to fit the CMB data. In the last section, we summarize the fitting results and conclusions. | Publications of the Korean Astronomical Society pISSN 1225-1534 30 315 319 2015 September eISSN 2287-6936 c 2015. The Korean Astronomical Society. All rights reserved. http FORMULATION AND CONSTRAINTS ON LATE DECAYING DARK MATTER Nguyen Q. Lan1 Nguyen A. Vinh1 and Grant J. Mathews2 1 Hanoi National University of Education 136 Xuan Thuy Cau Giay Hanoi Vietnam 2 Center for Astrophysics Department of Physics University of Notre Dame Notre Dame IN 46556 E-mail nquynhlan@ Received November 30 2014 Reviced May 31 2015 Aaccepted June 30 2015 ABSTRACT We consider a late decaying dark matter model in which cold dark matter begins to decay into relativistic particles at a recent epoch z 6 1 . A complete set of Boltzmann equations for dark matter and other relevant particles particles is derived which is necessary to calculate the evolution of the energy density and density perturbations. We show that the large entropy production and associated bulk viscosity from such decays leads to a recently accelerating cosmology consistent with observations. We determine the constraints on the decaying dark matter model with bulk viscosity by using a MCMC method combined with observational data of the CMB and type Ia supernovae. Key words cosmology dark matter - cosmology dark energy 1. INTRODUCTION shifts the dilemma of modern cosmology from that of explaining dark energy to one of explaining how an oth- For more than a decade modern cosmology has been erwise stable heavy particle might begin to decay at a faced with the dilemna that most of the mass-energy late epoch. in the universe is attributed to material of which we know almost nothing. It has been a difficult to under- stand and explain the nature and origin of both the That previous work however was limited in that it dark energy responsible for the present apparent accel- only dealt with the supernova-redshift constraint and eration and the cold dark matter responsible for most
