At the beginning of time, the universe exploded into existence in an event known as the Big Bang. Fast forward to the present day, when the universe has been expanding for approximately 13.8 billion years. But now, the fate of the universe becomes uncertain. Will the universe continue to expand forever? Or will the universe end in a so-called Big Crunch, as gravity causes the mass in the universe to collapse together?

Throughout the 20th century, astronomers struggled over these questions. They hypothesized several possible scenarios: the first was the Big Crunch. The second was one in which there wasn’t quite enough matter to cause a re-collapse of the universe, but there was enough to slow down the expansion. 

Astronomers postulated the existence of dark energy, a mysterious substance that may be causing the universe’s accelerating expansion.

But in 1998, astronomers were analyzing distant supernovae when they discovered, to their great shock, that the universe appeared to actually be accelerating faster now than it was before. Some have compared this to tossing a ball, only to find that instead of dropping to the ground, it flies upward away from you!

In order to explain this phenomenon, astronomers postulated the existence of dark energy, a mysterious substance that may be causing the universe’s accelerating expansion. In fact, dark energy is thought to make up approximately 68% of the universe. Although dark energy is only very vaguely understood, the standard model to understand dark energy is as a cosmological constant, an idea that was first proposed by Einstein (although he later rejected it). This constant is thought to be related to the energy density of the vacuum — the amount of energy that naturally exists in a cubic meter of empty space — but of a form that acts opposite to gravity, so that it repels instead of attracts its surroundings. In other words, dark energy can be thought of as a constant because it may be an inherent property of space that creates a repulsive force and does not change when space is stretched.

However, there are also alternatives to dark energy to explain the accelerating expansion of the universe, such as the theory of quintessence. Rather than a fundamental part of space, quintessence explains the accelerating expansion of the universe as a new, fifth energy field that counters the effects of the four fundamental forces.


A graphic detailing the possible scenarios for the future of the universe based on yet-unknown, theoretical properties of dark matter and energy.

Graphic by: Scientific American

Today, the topic of dark energy remains only hazily understood. Some astronomers have even recently claimed that it is possible the universe is not accelerating in its expansion at all; instead, they proposed that observations of the universe can be explained by the fluctuations in the universe’s density, which equations predicting the evolution of the universe tend to ignore. Princeton professor and distinguished cosmologist Dr. Lyman Page favors the cosmological constant explanation, but he notes that he sees dark energy “as an indication that we are missing big things in fundamental physics and/or our model of the universe.” For now, however, we can look forward to the ever-faster expansion of the universe, forever.

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Connie Miao