Observers watch the sun and the stars move across the sky. So, it's kind of a 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 exist outside of our solar system. But a hundred years ago back in the 1910's astronomer saw something that eventually led to one of the weirdest mind-blowing is realizations ever made about the universe and that is that the universe is expanding.
In 1912 and astronomer named Vesto Slipher discovered that the light from certain groups of stars which they later figured out we're galaxies was changing on its journey to their telescope.
Vesto Slipher - wikipedia.org
He was being redshifted meaning that the light's wavelength was being shifted toward the part of the spectrum with longer wavelengths.
Redshift - www.physicsforums.com
They could tell because certain elements like hydrogen a metal light in very specific parts of the spectrum they were all in the wrong places for instance like 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 us.
Andromeda galaxy - wikipedia.org
That had to mean that as other galaxies are emitting light they were moving away from us. Stretching out their wavelengths but then astronomers started measuring the Redshift and distances to lots of Galaxy 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 George Lemaitre realize that these changing redshift could only mean one thing the universe is expanding.
George Lemaitre - www.physicsoftheuniverse.com
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.
Astronomers eventually realized that those redshifting galaxies were speeding away from us through space, instead Space Itself is Stretching Out.
By describing the universe as a balloon covered in polka dots as the balloon or space inflate the dots or galaxies.
nature.com
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 it looks the same for all the dots the matter where they are on the balloon.
Let me propose that by measuring how much each Galaxy is red shifting based on its distance we could have been chilly figure out how quickly the universe was expanded.
Two years later American astronomer Edwin Hubble the guy that telescope is named after study.
Edwin Hubble - wikipedia.org
This some more and put a more accurate though it's 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 Lamesa researchers have discovered that not only is the universe expanding, but its expansion.
Actually getting faster the universe is accelerating thanks to a mysterious Force known simply as dark energy.
Dark Energy - www.interestingfacts.org
This knowledge is not only explain the weird observations that astronomers started making a hundred years ago. It is also taught us a lot about the history of the universe after all exclamation realized once you figure out that the universe is expanding. You can work backwards when it started expanding otherwise known as The Big Bang.
Wide Field - wfirst.gsfc.nasa.gov
So, thanks for the work done a hundred years ago by astronomers. 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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Summary and Created by @switapril