Cosmology and astroparticle physics have seen an avalanche of discoveries in the past decade (IceCube - high energy neutrinos, LIGO - gravitational waves, Fermi- gamma-ray telescope, Xenon-1T - dark matter detection, PLANCK- cosmic microwave radiation, EHT picture of black hole, SDSS -galaxy surveys), all of which require a multidisciplinary background for analyzing the phenomena. The arena for testing particle physics models is in the multimessenger astronomical observations and at the same time cosmology now requires a particle physics basis for explaining many phenomena. This book discusses the theoretical tools of particle physics and general relativity which are essential for understanding and correlating diverse astronomical observations.