Edwin Hubble discovered the expansion of the Cosmos by seeking the distance to the Andromeda Galaxy. What is the cosmic redshift and how was it found? This is part of my complete intro Astronomy class that I taught at Willam Paterson University and CUNY Hunter. • Hubble’s Law: The discovery of the relationship between a galaxy’s distance and its recession velocity, indicating the universe’s expansion. • Distance Measurement: Hubble used Cepheid variables to determine the distance to galaxies like Andromeda, revealing their vast distances and the universe’s size. • Hubble’s Law: The farther away a galaxy is, the faster it is receding from us, implying an expanding universe. • Redshift Observation: The redshift of absorption features in elliptical galaxies’ spectra provided evidence for their increasing radial velocity and distance from us. • Hubble Parameter (H0): Represents the rate of expansion of the universe, currently estimated to be around 68 kilometers per second per megaparsec. • Redshift Definition: Redshift is measured using the ratio of the observed wavelength to the emitted wavelength, with higher redshifts indicating faster recession. • Redshift and Distance: Redshift is proportional to the distance divided by the speed of light, and can be used to estimate the distance to galaxies. • Cosmological Redshift: The observed redshift of galaxies is primarily attributed to the expansion of the universe rather than individual galaxy motion. • Hubble’s Law Confirmation: Observations of Type Ia supernovae confirm that the universe’s expansion rate increases with distance. • Hubble Constant Value: The most accurate measurement of the Hubble constant is approximately 67.7 kilometers per second per megaparsec. • Nature of Expansion: Space-time itself expands, carrying galaxies along with it, rather than galaxies moving through space. • Light Wave Stretching: As the universe expands, the wavelength of light traveling through it gets stretched, similar to how a wave on an expanding balloon stretches. • Cosmological Redshift: The stretching of light waves as they travel through the expanding universe causes a redshift, which is proportional to the expansion of the universe. • Distance Measurement: The redshift of light from a galaxy can be used to determine its distance from the observer, as the stretched wavelength is directly proportional to the distance traveled. • Redshift and Expansion: Redshift measures the stretching of light due to the universe’s expansion, indicating how the universe’s size has changed over time. • Hubble Constant and Expansion Rate: The Hubble constant characterizes the rate of universe expansion, connecting observed redshift to this rate. • Hubble Parameter and Time: The Hubble parameter, relating velocity and distance, is a function of time, showing that the universe’s expansion rate is not constant. • Universe Expansion: The velocity and distance of objects are functions of time, reflecting the dynamic expansion of the universe. Overall, the segment emphasizes clear definitions, underlying geometry, and practical observing guidance so viewers can connect the concept to the real sky.