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Is the universe expanding? About a hundred years ago astronomers made a discovery that helped us unravel the mystery of the history of the universe!

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If you've been hanging around here long enough, you've learned about plenty of astronomical discoveries that have completely changed the way we think about the universe and our place in it.  Like for thousands of years, observers watched the sun and the stars move across the sky, so it was kind of natural for them to assume that the Earth was the center of the universe, and until about 25 years ago, we had no idea that other planets existed outside of our solar system, but 100 years ago, back in the 1910s, astronomers saw something that eventually led to one of the weirdest, mind-blowingest realizations ever made about the universe, and that is that the universe is expanding.

In 1912, an astronomer named Vesto Slipher discovered that the light from certain groups of stars, which they'd later figure out were galaxies, was changing on its journey to their telescopes.  It was being red-shifted, meaning that the light's wavelength was being shifted toward the part of the spectrum with longer wavelengths.  They could tell, because certain elements, like hydrogen, emit light in very specific parts of the spectrum, but they were all in the wrong places.  For instance, light that left the nearby Andromeda galaxy in the orange part of the spectrum would look ever so slightly redder by the time it got to Earth.  That had to mean that as other galaxies were emitting light, they were moving away from us, stretching out their wavelengths, but then astronomers started measuring the red-shifts and distances to lots of galaxies.  That's when they discovered something weird.  The farther away the galaxy was, the greater the red-shift, so the faster it was moving away from us.  

In 1927, a Belgian astronomer named Georges Lemaitre realized that these changing red-shifts could only mean one thing--the universe is expanding.  Now, this is commonly misunderstood to mean that everything is flying away from us, or from everything else, which is understandable but wrong, because the reality is even more complicated and, if you ask me, cooler.  

Astronomers eventually realized that those red-shifting galaxies weren't speeding away from us through space, instead, space itself is stretching out.  Whoa, right?!  Astronomers often explain this phenomenon by describing the universe as a balloon covered in polka dots.  As the balloon, or space, inflates, the dots, or galaxies, all move away from each other, but because in our example all the parts of the balloon are getting bigger at the same rate, the farther two dots are from each other, the more the distance between them will increase, and the effect looks the same for all the dots, no matter where they are on the balloon.  

Lemaitre proposed that by measuring how much each galaxy was red-shifting based on its distance, we could eventually figure out how quickly the universe was expanding.  Two years later, American astronomer Edwin Hubble, the guy that telescope is named after, studied this some more, and put a more accurate, though still not exactly right, number on how fast the universe was expanding.  This rate of expansion is called the Hubble Constant.  

Since the day of Hubble and Lemaitre, researchers have discovered that not only is the universe expanding, but its expansion is actually getting faster.  The universe is accelerating, thanks to a mysterious force known simply as dark energy.  This knowledge has not only explained the weird observations that astronomers started making a hundred years ago, it has also taught us a lot about the history of the universe.  After all, as Lemaitre realized, once you figure out that the universe is expanding, you can work backward to where it started expanding, otherwise known as the Big Bang.  

So thanks to the work done a hundred years ago by astronomers who were willing to believe that things that didn't make sense must be true, we've discovered a lot about the universe and its history, just by studying our little spot on that giant expanding balloon that is space.  

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