2021/01/14 by Takahiro Tanaka, Yuko Urakawa
Physics and Astronomy · #Adiabatic process #Anisotropy #Cosmology #Cosmology and Gravitation Theories #De Sitter universe #Ekpyrotic universe #Formalism (music) #Galaxies: Formation, Evolution, Phenomena #Homogeneous #Metric expansion of space #Pulsars and Gravitational Waves Research #Universe #astro-ph.CO #gr-qc #hep-ph #hep-th
paper · pdf · doi:10.1088/1475-7516/2021/07/051
48 pages, 1 figure
arxiv created 2021/01/14 · openalex created_date 2021/01/18 · openalex publication_date 2021/07/01 · arxiv updated 2021/08/04 · openalex updated_date 2026/08/05
In the separate universe approach, an inhomogeneous universe is rephrased as a set of glued numerous homogeneous local patches. This is the essence of the gradient expansion and the δ N formalism, which have been widely used in solving a long wavelength evolution of the universe. In this paper, we show that the separate universe approach can be generically used, as long as a theory under consideration is local and preserves the spatial diffeomorphism invariance. Focusing on these two conditions, we also clarify the condition for the existence of the so-called Weinberg's adiabatic mode. Remarkably, the separate universe approach and the δ N formalism turn out to be applicable also to models with shear on large scales and also to modified theories of gravity, accepting violation of four-dimensional diffeomorphism invariance. The generalized δ N formalism enables us to calculate all the large scale fluctuations, including gravitational waves. We also argue several implications on anisotropic inflation and ultra slow-roll inflation.