Cosmic Spacetime.pdf

Cosmology is faced with several puzzles and coincidences, but re-examining the Cosmological principle and Weyl postulate suggests a simpler and more elegant Universe. The Cosmological principle assumes the Universe is homogeneous and isotropic, while the Weyl postulate proposes an orderly expansion. Together, they allow for a special time coordinate, cosmic time, to represent change since the big bang. The Weyl postulate reduces to a mathematical expression, enabling the interpretation of separation between points in the cosmic flow. A galaxy's proper distance from us determines its recession speed, accounting for Hubble's observations. Understanding the Universe's beginnings is hindered by the limitations of General Relativity and quantum mechanics. The Compton wavelength and Schwarzschild radius are used to estimate the Planck mass, length, and time, which constitute the starting point for discussing cosmic spacetime. However, the Universe's infinity raises questions about the relevance of the Schwarzschild radius. The Birkhoff theorem and its corollary help understand the difference between zero net acceleration and relative acceleration between an observer and other points in the cosmos.