The End of Time

How Will the Universe End?

The long history of astronomy and space exploration has been permeated with the drive to answer some of the biggest and deepest questions that we can ask: What is going on up there in the sky? Where did everything come from? How did life form? Are we alone? We are fortunate to live in a civilization and a time in human history when we have the luxury to actively seek the answers to such questions and the technology to try to do so.

A fitting place to end this journey through the major milestones of astronomy and space exploration is to come full circle from where we first began. In other words, the prevailing theory for the origin of our universe is that everything around us—all of space and time—was created 13.75 billion years ago in a gigantic, instantaneous explosion known as the Big Bang. The universe as we know it had a beginning. So an obvious big question to ask is, will the universe have an end? And, if so, when will it occur?

We know that the universe is expanding because we can measure the galaxies all receding from each other. Maybe this expansion will simply continue forever, perhaps fueled by the strangely repulsive and still almost completely mysterious force of dark energy accelerating the expansion of space, until the last of the stars fade away and even the black holes evaporate into the dark, quiet, cold of what astronomers call the heat death of the cosmos, perhaps 10¹⁰⁰ years from now. Maybe.

Some cosmologists believe, though, that there might be a more violent fate in store for the universe. If the mass of everything in the universe is large enough that dark energy ultimately can’t continue to accelerate the expansion of space, then the gravitational attraction between clusters of galaxies could slow and eventually reverse the expansion. Galaxies could begin moving toward each other, with all the mass of the cosmos eventually coming together again into one tiny, massive black hole singularity. What happens then? Another big bang? Or, perhaps, a big bounce?

The ultimate fate of the cosmos is of course unknown. Modern cosmologists are poking at the issue by actively trying to figure out if the universe is open (expanding forever), closed (destined to contract), or flat (in perfect balance). New observations and computer models may help figure out—to borrow a line from the modernist poet T. S. Eliot—whether the universe will end “not with a bang but a whimper.”

SEE ALSO Big Bang (c. 13.7 Billion BCE), Hubble’s Law (1929), Dark Matter (1933), Black Holes (1965), Dark Energy (1998) Age of the Universe (2001), Last of the Stars (~10¹⁴).

The galaxy cluster known as Abell S0740, photographed by the Hubble Space Telescope in 2005–2006. This cluster is about 450 million light-years away and contains a spectacular variety of galactic forms. What will happen to the universe’s 100 billion galaxies in the far, far future?