Redshift and Hubble's Law Show an Expanding Universe

Let me show you something strange I spotted in starlight — something that changed how we understand the entire universe. When I spread a galaxy's light into a spectrum, I can pick out dark absorption lines left by specific elements — hydrogen, helium, calcium. Every element has a fixed fingerprint of line positions. But look at what happens with distant galaxies: those fingerprint lines are all shifted toward the red end of the spectrum. This is called redshift. Now, you might first think of the Doppler effect — the way a siren drops in pitch as it races away. And that analogy helps as a starting point. But here is the critical twist: cosmological redshift is NOT galaxies moving through space like rockets. What is actually happening is that space itself is stretching. A photon leaves a distant galaxy, and while it is traveling toward my telescope, the universe expands. The wavelength of that photon literally stretches along with space. By the time the photon arrives, its wavelength is longer — redder — not because the galaxy moved through space, but because space expanded during the photon's journey. In 1929, Edwin Hubble compared the redshifts of dozens of galaxies with their independently measured distances and discovered a clean pattern: the farther a galaxy is, the greater its redshift. This is Hubble's Law: v = H₀ × d where v is the recessional velocity in km/s, d is the distance in megaparsecs (one megaparsec equals about 3.26 million light-years, abbreviated Mpc), and H₀ is the Hubble constant — approximately 70 km/s/Mpc today. So a galaxy 100 Mpc away recedes at roughly 7,000 km/s. Here is what I find most mind-bending: no galaxy is the center of this expansion. Think of dots painted on the surface of a balloon being inflated — every dot moves away from every other dot, and the farther apart two dots are, the faster they separate. An observer on any galaxy would see the same expanding pattern in all directions. There is no special center; the expansion is the same everywhere. Finally, running the expansion backward gives cosmologists a useful timescale called the Hubble time: 1/H₀ comes out to roughly 14 billion years. I want to be precise here — the Hubble time is not the same thing as the age of the universe. The actual age requires integrating over the full expansion history, including the contributions of matter, radiation, and dark energy across cosmic time. For our particular universe (described by the Lambda-CDM model), that calculation gives about 13.8 billion years — which happens to be close to the Hubble time, but the two quantities are measuring different things. Think of the Hubble time as a characteristic timescale that tells you the rough order of magnitude of how long the universe has been expanding.