Astroparticle Physics and Cosmology : Perspectives in the Multimessenger Era 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. Effective potential and phase transitions Gravitational Waves Black Holes High energy cosmic rays Dark Matter Supersymmetry Inflation Lorentz transformations of fields The FRW Universe Gauge Invariant Metric Perturbations.