English

Anisotropic separate universe and Weinberg's adiabatic mode

Cosmology and Nongalactic Astrophysics 2021-08-04 v1 General Relativity and Quantum Cosmology High Energy Physics - Phenomenology High Energy Physics - Theory

Abstract

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 δN\delta 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 δN\delta 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 δN\delta 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.

Keywords

Cite

@article{arxiv.2101.05707,
  title  = {Anisotropic separate universe and Weinberg's adiabatic mode},
  author = {Takahiro Tanaka and Yuko Urakawa},
  journal= {arXiv preprint arXiv:2101.05707},
  year   = {2021}
}

Comments

48 pages, 1 figure