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 intro Astronomy class I taught at Willam Paterson University and CUNY Hunter. • Cosmological Principle: The universe is homogeneous and isotropic on the largest scales. • Redshift Survey Findings: Surveys suggest that the universe may not be perfectly homogeneous and isotropic on smaller scales. • Redshift Observation: Slipher observed redshift in spiral nebulae, suggesting they are moving away from us and are outside the Milky Way. • Hubble’s Discovery: Hubble discovered the systematic expansion of the universe in 1929. • Distance Measurement: Hubble used Cepheid variables as standard candles to measure distances to galaxies, based on Leavitt’s Period-Luminosity relationship. • Distance Measurement: Hubble used the periods of Cepheid variables and their light curves to determine the distances to M31 and M33. • Early Galaxy Observations: The image highlights early observations of galaxies like NGC 221, NGC 4473, and NGC 379, important for the cosmic distance debate. • Redshift Significance: Nearly every galaxy is redshifted, indicating they are moving away from us, which has profound implications for our understanding of the universe. • Redshift Definition: Redshift is a unitless measure of the difference between the observed wavelength of light and the emitted wavelength, indicating how much the light has been stretched or compressed. • Redshift Interpretation: The redshifted H&K lines observed in galaxies are consistent with the cosmological principle and indicate the presence of old stellar populations. • Hubble’s Law: Relates a galaxy’s distance to its recession velocity, indicating the universe’s expansion. • Hubble Parameter (H0): The current value of the universe’s expansion rate, approximately 72 km/s/Mpc. • Distance Measurement: Galaxies’ distances are determined by measuring their recession velocities and applying Hubble’s Law. • Hubble Parameter Significance: A measurement of the universe’s expansion rate at the current cosmic time. • Universe Age Estimation: The inverse of the Hubble parameter provides an approximation for the age of the universe, currently estimated at 14 billion years. • Distance Measurement: Accurate measurement of the Hubble parameter is crucial for determining distances to galaxies. • Universe Expansion: Proposed as a solution to the Einstein field equations, suggesting space expands to prevent celestial bodies from colliding. • Light’s Behavior: Light’s momentum is linked to its wavelength, and it travels along curved SpaceTime paths (null geodesics), with its wavelength changing as SpaceTime changes. • Redshift and Universe Expansion: Redshift, an indicator of the universe’s expansion, is directly related to the scale factor of the universe. • Redshift Origin: Redshift originates from the constant speed of light in an expanding universe, impacting photon wavelength and frequency. Overall, the segment emphasizes clear definitions, underlying geometry, and practical observing guidance so viewers can connect the concept to the real sky.