Expansion of the Cosmos

   For about the first two decades of the 20th century, the universe was believed to consist of the Milky Way galaxy. The other galaxies observed at the time were believed to be nebulae, vast clouds of hydrogen and helium and other elements aglow from nearby stars and the protostars forming within. This was a long way off from the heliocentric model, and an even farther contrast to the geocentric universe. But the view of our universe changed when Edwin Hubble, for which the Hubble Space Telescope is now named, provided evidence that many of the nebulae discovered were actually galaxies like ours, or "island universes". Suddenly, the Triangulum nebula and the Andromeda nebula gained galactic status in the eyes of humanity, as well as many other nebulae. But this was only the beginning.

   The Doppler Effect is a well-known scientific principle to most people. To use a common example, it is demonstrated when an ambulance or some other vehicle with an activated siren passes you. As the vehicle moves towards you, its sound waves come at you in a manner where they are closer together, whereas when it travels away, the sound waves are stretched because their source emits the waves at a farther and farther distance away. Basically the relative motion of the emitter changes the apparent frequency of the sound waves, so an object getting closer appears to increase in frequency, reaches its peak when closest to you, and decreases in frequency when moving away. This concept is illustrated below:

The sound of the motorbike decreases as it moves away from the woman and increases as it moves towards the man

  The same principle is applicable for light. Light emitted from an object moving towards you increases in frequency, resulting in it being shifted to the blue end of the visible spectrum. The light emitted from an object moving away decreases in frequency, so the light is shifted towards the red end of the spectrum. This shift is largely imperceptible in everyday objects, but measurable for things moving very fast. Hubble applied this experiment to the newly discovered galaxies. When Hubble measured the light from these galaxies, he found that nearly all were red-shifted. This meant that galaxies were moving away from us, but not only moving away, but the farther they were away, the faster their rate of retreat. This lead to the conclusion that either our galaxy was somehow the center of a universal retreat from galaxies, or far more likely, the space itself between galaxies was expanding. Hubble measured this rate of expansion, now called Hubble's Constant. 

The Doppler Effect when applied to light

   So space was expanding. But not only was it expanding, its rate of expansion was increasing. This would lead science to a whole new understanding of our universe, an understanding that would culminate in nothing less than the very birth of our cosmos. So, next post will be about a big topic: the big bang theory. Feel free to ask questions and subscribe! Until then, salutations!