The Illusion of a Static Cosmos

Before the 1920s, the scientific consensus—even held by Albert Einstein—was that the universe was static, eternal, and relatively small. The Milky Way was thought to be the entirety of the cosmos. That changed drastically when Edwin Hubble began his observations using the 100-inch Hooker Telescope at Mount Wilson Observatory.
By studying the faint luminosity of Cepheid variable stars, Hubble first deduced that the faint "nebulae" seen in the night sky were not gas clouds within our galaxy, but entirely separate "island universes," or galaxies, separated by vast stretches of space (Kirshner, 2003).
Redshift and the Cosmic Speedometer
Once he established the immense distances to these galaxies, Hubble turned to another piece of the puzzle: their spectra. Building on the earlier work of astronomer Vesto Slipher, Hubble analyzed the light from these distant galaxies.
When an object moves away from an observer, its light waves are stretched, shifting them toward the red end of the electromagnetic spectrum—a phenomenon known as redshift. Hubble discovered a startling pattern: almost all galaxies were redshifted, meaning they were moving away from us. More importantly, he found a direct, linear relationship between a galaxy's distance and its recession velocity (Bagla, 2009).
Hubble's Law
In his landmark 1929 paper, Hubble mapped this relationship onto what is now famously known as the Hubble diagram. This linear relationship is mathematically expressed as Hubble's Law:
   V=H0d
Where:
v is the recession velocity of the galaxy.
H_0 is the Hubble constant (the rate of cosmic expansion).
d is the distance to the galaxy.
This equation demonstrated that the further a galaxy is from Earth, the faster it is receding (Bagla, 2009). A galaxy twice as far away is moving away twice as fast.
The Fabric of Space
Crucially, this discovery does not mean Earth is at the center of the universe. Instead, it indicates that the very fabric of space is stretching. Like raisins in a rising loaf of bread, every galaxy is moving away from every other galaxy.
Hubble's essential contribution was combining consistent distance measurements with velocity data to unveil this grand cosmic expansion (Kirshner, 2003). His observation ultimately discarded the static universe model and forced physics to embrace a dynamic, evolving cosmos—setting the stage for the Big Bang theory.
References
Bagla, J. S. (2009). Hubble, Hubble's law and the expanding universe. Resonance, 14(3), 216–225. https://doi.org/10.1007/s12045-009-0022-8
Cited by: 6
Kirshner, R. P. (2003). Hubble's diagram and cosmic expansion. Proceedings of the National Academy of Sciences, 101(1), 8–13. https://doi.org/10.1073/pnas.2536799100
Cited by: 145