The universe may end in a 'Big Freeze,' holographic model of the universe suggests

Researchers have found that a hypothetical form of dark energy may lead to a grim fate for the universe: a "long freeze" where everything just…slows down.

In this scenario, the universe would expand to a finite size, but everything would grow so cold that all activity would essentially cease.

Dark energy is the mysterious force responsible for accelerating the expansion of the universe. It was discovered in the 1990s, but despite over two decades of research, it still remains the central mystery of modern cosmology. Over the years, scientists have presented some fascinating research into what it is and how it works.

One idea is known as holographic dark energy. In this scenario, gravity — and space itself — is an illusion. Our universe is really only two-dimensional, and exotic quantum forces on that surface give rise to the appearance of gravity and the structure of 3D space.

A consequence of this theory is a natural accelerated expansion of the universe, which we identify as dark energy.

Related: Our universe is merging with 'baby universes', causing it to expand, new theoretical study suggests

While many researchers have studied holographic dark energy models and ways to test it, a pair of astrophysicists examined the long-term fate of the universe if it is indeed ruled by holographic dark energy. They published their results Sept. 30 to the preprint database arXiv. (It has not been peer-reviewed.)

Dark energy takes up roughly 70% of the energy density of the entire cosmos. As the universe expands, the density of both regular and dark matter drops, while more and more dark energy manifests. To study the ultimate long-term fate of the universe, the researchers ignored matter and focused solely on the evolution of holographic dark energy.

They found that, as expected, holographic dark energy will continue to expand the universe. But, over time, its influence will slowly peter out and slow acceleration. The universe's expansion rate will steadily decrease until the cosmos reaches a nearly static value, essentially locking it to a final size.

But as the universe's expansion slows down, the density of holographic dark energy dwindles along with it. And since the density of matter also shrinks as the universe expands, the universe grinds to a halt. The researchers dub this scenario "the long freeze," in contrast to other commonly known fates of the universe like the "Big Freeze" (the accelerated expansion continues unabated) and the "Big Crunch" (something causes the universe to contract back toward the Big Bang).

The long freeze isn't a rosy scenario. While the universe's expansion will eventually stop, there won’t be any new sources of energy for all the matter inside of it. This means that eventually all the stars will wink out and decay, and all the subatomic particles will drift away from each other in the cold.

Unfortunately, even in their most exotic theories, cosmologists can't come up with a way to give the universe a happy ending.