Chapter 2: The Expanding Universe Explained (Lecture 2)

Edwin Hubble's revolutionary observations established that the universe extends far beyond the Milky Way, containing innumerable galaxies distributed throughout space. Through spectroscopic analysis, Hubble identified the redshift phenomenon, wherein light from distant galaxies exhibits wavelength elongation toward the red portion of the spectrum as a consequence of the Doppler effect, revealing that galaxies recede from us at velocities proportional to their distance. This discovery fundamentally challenged prevailing assumptions about a stationary cosmos, contradicting even Einstein's initial cosmological framework, which included an artificial constant to counterbalance gravitational attraction and preserve a static configuration. Alexander Friedmann's mathematical treatment of General Relativity, grounded in the cosmological principle that the universe maintains uniform properties across all directions, had predicted expansion theoretically years before Hubble's empirical validation. The subsequent detection of cosmic microwave background radiation by Penzias and Wilson provided independent confirmation of the universe's homogeneous nature and thermal history. The chapter examines three possible evolutionary scenarios for cosmic fate derived from Friedmann's equations: recollapse driven by gravitational deceleration, perpetual expansion against weakening gravity, and a critical boundary condition between these outcomes. The discussion incorporates the mass density problem and introduces dark matter as an unseen but gravitationally consequential component that influences which scenario governs cosmic destiny. The superseded Steady State theory represents an alternative approach now abandoned in light of observational evidence. The chapter concludes with singularity theorems developed by Roger Penrose demonstrating rigorously that General Relativity mandates a Big Bang origin—a primordial state of infinite density and spacetime curvature representing the temporal beginning of the universe itself.