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The earth is a large place. The solar system that contains this world is even larger. The Milky Way galaxy is larger still. The entire universe is larger than all the rest. All that is common knowledge. But despite the universe’s massive size already, it’s still growing. And that growth is getting ever quicker as time goes on. The expansion rate of the universe has been steadily increasing since the time of the Big Bang. This expansion is caused by something called dark energy, an unknown form of energy that’s hypothesized to fill all the so-called “empty” space throughout the entire universe. This energy is causing the entire universe to expand outwards, growing larger and larger, and increasing the expansion rate as time goes on, until the Big Crunch finally comes to pass, and everything that was released from the Big Bang comes back together.

As the universe gets older, the speed at which it’s expanding continues to raise and raise, thanks to a force called dark energy. Dark energy is a force that repels gravity, pushing the universe outwards, growing stronger as time goes on (Dark Matter and Dark Energy). This growth in strength is the event that causes the outwards expansion of the universe to accelerate with time. But once the universe ceases to expand any further, what’s going to happen? Will what remains just stay the same size for eternity? Scientists have guessed that, upon running out of dark energy to push the universe outwards, the universe will then begin to contract in upon itself, resulting in an event currently known to scientists as the “Big Crunch.” It was named this because it will be the crunching together of the entire universe, and thus the opposite of the Big Bang. All the matter in the universe will be condensed into one point, just like how it all started. Perhaps the cycle of the universe will restart: Big Bang, creation of the universe, expansion, contraction, Big Crunch, repeat. Although, the universe might never run out of dark energy to keep it expanding, faster and faster. There’s no way to know for sure until the time comes, and humans most likely won’t be there to see that happen.

The universe’s expansion is accelerating due to a mysterious force known as dark energy. This force repels gravity, pushing all the matter in the universe outwards. Dark energy gets stronger as time goes on, causing the acceleration itself. The dark energy takes up 70% of the universe, with dark matter taking up 25% and the observable universe taking up the remaining 5%. That small portion of the universe, that twentieth that’s visible to humans, is so seemingly vast to humanity, but in comparison to the remaining 95%, what people could see is a minute amount. There’s just so much dark energy that the expansion of the universe will almost definitely continue to accelerate long after humanity- and the solar system- have been wiped from the universe.

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"Accelerating Expansion of the Universe ." Wikipedia. N.p., n.d. Web. 15 Apr. 2017.

"Accelerating Universe and Dark Energy." The Physics of the Universe. N.p., n.d. Web. 16 Apr. 2017.

"Why is the Universe Accelerating?" IFL Science. N.p., n.d. Web. 19 Apr. 2017.

"Dark Matter and Dark Energy." National Geographic. N.p., n.d. Web. 18 Apr. 2017.

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The first evidence for acceleration came from the observation of Type Ia supernovae, which are exploding white dwarfs that have exceeded their stability limit. Because they all have similar masses, their luminosity can be standardized. Repeated imaging of selected areas of sky is used to discover the supernovas, then follow-up observations give their peak brightness, which is converted into a quantity called luminosity distance. Spectral lines of their light can be used to determine their redshift.

For supernovae at redshift less than around 0.1, or light travel time less than 10% of the age of the universe, this gives a nearly linear distance–redshift relation due to Hubble's law. At larger distances, since the expansion rate of the universe has changed over time, the distance-redshift relation deviates from linearity, and this deviation depends on how the expansion rate has changed over time. The full calculation requires integration of the Friedmann equation, but a simple derivation can be given as follows: the redshift z directly gives the cosmic scale factor at the time the supernova exploded.

So a supernova with a measured redshift z = 0.5 implies the universe was 1/1 + 0.5 = 2/3 of its present size when the supernova exploded. In an accelerating universe, the universe was expanding more slowly in the past than it is today, which means it took a longer time to expand from two thirds its present size to its present size compared to a non-accelerating universe. This results in a larger light-travel time, larger distance and fainter supernovae, which corresponds to the actual observations. Adam Riess found that "the distances of the high-redshift SNe Ia were, on average, 10% to 15% farther than expected in a low mass density ΩM = 0.2 universe without a cosmological constant" This means that the measured high-redshift distances were too large, compared to nearby ones, for a decelerating universe.

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The accelerating expansion  of the universe was discovered in 1998, when two independent projects found results suggesting the acceleration. They had been expecting a deceleration, and were surprised to see that the opposite had occurred.

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Universal expansion is the scientific theory that the universe is expanding. But the thing is, it's not only expanding: the expansion process is getting faster. It's expanding because of dark energy. Dark energy is an unknown form of energy which is hypothesized to permeate all of space, accelerating the expansion of the universe.