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Back then, our galaxy was the universe, and nothing was thought to exist beyond it.
They got the size of our galaxy right, but they were very wrong about everything else — our galaxy is not the whole universe.
Edwin Hubble’s discoveries in the 1920s sproved that the Milky Way was just one of countless galaxies and that universe itself was expanding. Fast forward to today, and we know that the Milky Way is just one of an estimated 2 trillion galaxies.
And how big is our observable universe? Well, about 93 billion light-years across!
How we know the size of the universe
Since the observable universe spans an estimated 93 billion light-years in diameter, the farthest objects we can detect are about 46 billion light-years away in any direction. This measurement is based on the universe's age — 13.8 billion years — and the expansion of space itself.
A light-year is the distance light can travel in one Earth year. The estimated age of the universe since the Big Bang is 13.8 billion years, so the light emitted by objects in space that humans can see has been traveling toward Earth for no more than 13.8 billion years. Given this information, you might assume that the observable universe is 13.8 billion light-years in any direction from Earth and 27.6 billion light-years in diameter.
However, the ongoing expansion of the cosmos, as described by Hubble’s law, has stretched those distances much farther, leading to the staggering scale we estimate today. Calculations of the expansion show that objects that emitted light 13.8 billion years ago, from a distance of 13.8 billion light-years, are now even farther away from Earth — 46 billion light-years away, approximately. That's what gives us the 93 billion light-year diameter.
The deeper we look into space, the more it expands before our eyes. And yet, what we see today is likely just a tiny fraction of the whole. Beyond the edge of the observable universe, space could stretch on infinitely. There might even be other universes out there.
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