Early in the 20 th century, astronomers discovered that most galaxies are moving away from the Milky Way (that is, they are redshifted.) a. What was the significance of this discovery? b. Edwin Hubble later made an even more important discovery: that the speed at which galaxies are receding is proportional to their distance. Why was this among the more important scientific discoveries of the 20 th century?

Redshift is a concept in observational cosmology where light from distant galaxies is observed to be shifted towards the red end of the spectrum. Think of light as a series of waves. When a galaxy moves away from us, the wavelength of light it emits gets stretched. This shift to longer wavelengths is what we call 'redshift'. Its occurrence is similar to how sound waves stretch and shift to a lower pitch when a siren moves away from us.
Redshift is crucial because it implies that the universe is expanding. The more distant a galaxy is, the faster it appears to move away, increasing its redshift. This relationship was key to propelling our understanding of the cosmos forward.

The concept of an expanding universe came from the observation that galaxies are moving away from us. Previously, it was believed that the universe was static. With the discovery of redshift, astronomers realized that the space between galaxies is stretching. This indicates that the universe itself is expanding.
Imagine an inflating balloon with dots on it. As the balloon inflates, the dots move away from each other. Similarly, galaxies move apart as the universe expands. This insight was groundbreaking because it changed our understanding of the cosmos from a static to a dynamic, ever-changing state.
Edwin Hubble's observations, which quantified this expansion, gave rise to Hubble's Law. This law mathematically describes the relationship between the distance of a galaxy and its recessional velocity, confirming that the universe is indeed expanding.

The Big Bang Theory is a foundational concept in cosmology that explains the origin of the universe. According to this theory, the universe began as a singular, extremely dense point approximately 13.8 billion years ago. From this point, it has been expanding ever since.
The observations of redshift and the expanding universe provided strong evidence for the Big Bang Theory. Hubble's Law, describing the relationship between a galaxy's distance and its speed, indicated that everything in the universe was once much closer together.
In essence, the Big Bang Theory positions the universe as starting from a hot, dense state and gradually expanding, cooling, and evolving to form stars, galaxies, and other cosmic structures over billions of years. This theory also predicts the existence of cosmic microwave background radiation, a remnant from the early universe that has been observed and measured, further supporting the Big Bang model.

Observational cosmology is a branch of astronomy that deals with studying and understanding the universe based on observational data. Discoveries like redshift, Hubble's Law, and the expanding universe have all stemmed from careful observation and measurement of cosmic phenomena.
Through observational cosmology, scientists use telescopes and other instruments to collect light and other electromagnetic radiation from celestial objects. Analyzing this data allows them to infer properties about the distance, composition, velocity, and behavior of those objects.
Hubble's work meticulously measuring the distances and velocities of galaxies laid the groundwork for modern observational cosmology. By establishing that the universe is expanding, he set the stage for further studies into the age, size, and ultimate fate of the universe.
In conclusion, observational cosmology connects directly with practical observations, allowing us to test theoretical frameworks like the Big Bang Theory and apply mathematical models to better understand our universe.