The expansion of the universe is steadily accelerating

Cosmologists have known for almost a century that our universe isn’t static. In 1919, Edwin Hubble gazed up at the universe through his powerful telescope and made two ground-breaking observations: there exist galaxies outside our own, and those galaxies are flying away from us.
It is the second observation that revealed the true, expanding nature of our universe. Hubble arrived at this realization after he witnessed a strange phenomenon — all distant galaxies are red-shifted.

A red-shift occurs when an object is traveling away from an observer at speeds comparable to the speed of light. To the observer, it seems as though light waves emitted from the object are stretched out, increasing their wavelength. This makes the object appear red (red light has the longest wavelength). Hubble also noticed that the red-shift effect was stronger the further out he looked.

In every direction scientists looked, they saw galaxies zooming away from the Earth and one another. Naturally, theoretical physicists like Stephen Hawking wondered what would happen if we ran the clock backwards. The galaxies would come closer together! In fact, if we go back far enough in time, all the galaxies would be at the same point in space.

This large amount of mass at a single point creates something called singularity, a point in space with so much mass and gravity that it distorts space tremendously. When all the mass in the universe is compressed at a point, the resulting singularity wraps all space in on itself.
This is how the big bang theory was developed. At the beginning of the universe, 13 billion years ago, all matter was at a single point, then began to expand apart. Scientists are still unsure of what initially caused the expansion, however. The cosmic background radiation gives them a glimpse into the early universe.

Today, the universe is still expanding and at an accelerating rate. Space expands at 74 kilometers per second per megaparsec (one megaparsec is about three million light years). It is important to note that this expansion only occurs at large scales, such as in the space between galaxies; it cannot be observed here on Earth.

Understanding the nature of the universe’s expansion informs us about the future fate of our universe.

If the rate of expansion continues to accelerate, galaxies will continue to fly apart at such a high velocity that their light will not be able to reach us anymore, never to be seen again. When the stars in our galaxy die, the nighttime sky will be completely dark. This is know as the big freeze.
Another possibility is that there is just enough mass in the universe to halt the expansion. The universe will simply remain in a stationary state, neither expanding nor contracting.

The final probability is that the mass in the universe is just enough to reverse expansion. The universe will begin to contract, eventually returning it to its singularity state. This is known as the big crunch. Such a universe oscillates between states of singularity and expansion for eternity. All our current data suggests that the big freeze is the most likely future, because there simply isn’t enough mass in our universe to slow down its accelerating expansion.

As sad as this seems, it’s very, very far in the future. For dozens of billions of years, we can still enjoy the view of our neighboring galaxies — that is, if we’re still around.