How The Universe Ends

How The Universe Ends

Somewhere between a second and a millennium from now, you will die. Your body and all of its parts will cease functioning and rejoin the Earth as regular, lifeless stuff. The Earth, too, will die, engulfed by an expanding, ageing Sun. The Sun will burn off all of its fuel and end up a white dwarf, then burn out and die. The Milky Way will collide with nearby Andromeda, and form a large, elliptical galaxy, which will die by losing all of its stars to intergalactic space. The corpses of those remaining stars will die, decaying into their constituent parts. The universe will age onward, until all matter is either stored in black holes or floating as free elementary particles. Those black holes will evaporate, and then universe will die. All that was will be an icy cold nothing, forever.

This is one of the happier possible endings—this “heat death,” as in, the death of heat, at least leaves us time to say goodbye. The truth is, the universe far predates humans, it will far outlast humans, and contemplating its death is a depressing effort that highlights our incredible insignificance. At least studying the physics of it all serves as a nice pastime while we wait to dissolve into nothingness.

“We can try to understand it, but there’s nothing we can do to affect it in any way,” Katie Mack, North Carolina State University assistant professor currently writing a book on the end of the universe, told Gizmodo. “We have no legacy in the cosmos, eventually. That’s an interesting concept.”

Heat death, or the “big freeze,” is generally considered the most likely future, based on how things are looking today. The universe is expanding, and will continue expanding. As things move farther apart, stars will form less frequently from the sparser dust and gas. The last black holes will have slowly evaporated into energy through the theoretical process proposed by Stephen Hawking, perhaps in a googol (that’s 10100) years. And maybe some time afterward, the remaining particles will decay, and the entire universe will assume an average temperature of nearly, but not exactly, absolute zero. Basically, the universe will be so big and sparse that the odds of finding anything at all will be effectively absolute zero.

None of this will happen any time soon. Provided humans survive our own self-destructive tendencies, the Earth’s atmosphere could have another billion years, and the Sun maybe 7 billion to 10 billion years before it grows into a red giant, ejects its outer layers, and remains just a glowing core around the same size as the Earth but packing far more mass, called a white dwarf, according to John Baez, a physicist at University of California, Riverside. Smaller red stars will stick around for perhaps a hundred trillion years—maybe humans can settle on a planet orbiting a nice red dwarf like Proxima Centauri to live out our days. These are timescales far beyond human comprehension—think of the amount of time it would take to walk across the universe at its present size, if you had to stop and count every atom in the universe after each step.

“I guess everyone tends to feel depressed about it,” Baez told Gizmodo. “People are future-thinking animals, and we always like to think about life in terms of a story, and hope it has a happy ending. It goes against our brain to imagine development other than things getting more interesting, but there’s no reason to think that this will last forever.”

The Big Bang and expansion of the universe. (Image: Sidharth Nikam, JPL, NASA)

Some more eventful theorised fates could arrive sooner, like the Big Rip. You might be aware that in 1998, scientists discovered the universe wasn’t just expanding, but that the expansion rate was increasing. They theorise that some energy seemingly innate to the vacuum of the universe, called dark energy, powers the accelerated expansion. There’s a possibility that in over 100 billion years, the dark energy will cause the universe to expand so quickly that it tears apart galaxies, solar systems, planets, and atoms before they can run out of energy on their own. The space between every individual point would grow infinitely large. Physics theory seems stacked in favour of heat death over the Big Rip, but who knows—observations don’t forbid it.

Then, there’s the chance that the very vacuum of space itself could change. Maybe the “Higgs field,” a field the permeates the universe that determines the mass of subatomic particles, isn’t in the lowest energy configuriation. Maybe it is actually “metastable” and there’s a lower-energy ground state it could decay to. Imagine spending your whole life living on a platform, thinking it was the solid ground—this platform is the metastable state. Maybe one day, the platform will collapse and reveal a true floor a hundred feet below. The laws of physics as you know them would no longer work, and you would fall and die. This is essentially what would happen if the universe snapped from a metastable state to a more stable state, if we were living on the platform all along. This would end the universe as we know it, since this new, lower-energy universe would not support the existence of the present-day Standard Model that governs the identities and interactions of particles that make up matter. It’s unlikely such an event would occur before the heat death does. But it would be a spectacular death.

“At some spot in the universe you’d create a bubble of true vacuum that expands at the speed of light and envelopes the universe, destroying everything,” said Mack. Its light speed means you wouldn’t see it coming—the death would arrive simultaneously with the warning that death was coming.

But not every possible cosmic conclusion is one of utter desolation and emptiness. Maybe in some distant, post-heat-death future, the energy in the universe’s vacuum could spontaneously jump back upward at a point, initiating inflation at that point in space from which entirely new universes form, Alan Guth, MIT physicist who invented the theory of cosmic inflation, told Gizmodo. Perhaps that’s how our universe formed, and perhaps there are an infinite number of universes forming in the same way, decaying out of an infinitely inflating grander universe. Maybe there are places beyond the reach of our own universe that won’t be impacted by the demise of our own.

“This is the most optimistic point of view among theories, because even though our part of the universe will die out, other parts that may be teeming with life would go on forever,” Guth said. Our universe dies, either way.

And maybe dark energy isn’t an innate, constant value to the universe. Maybe, as we previously reported, its strength is decreasing, which might lead to the universe’s expansion eventually slowing. Everything could then turn around under gravity’s force and collapse—that’s the “Big Crunch.”

There’s a lot we don’t know about the universe, so any or none of these ideas could be right. Any new discovery about the nature of dark energy, the Higgs boson, or spacetime itself could reveal a vastly different fate of the universe, where everything wastes away to an infinitely vast nothingness, everything collapses, or new universes spawn from the ashes of the old, or something else entirely happens. Regardless, humanity’s existence and legacy—and everything else, ever—will cease to exist or have meaning.

“Even though we all know that we personally are going to die, it still kinda hurts that so is everything else,” said Mack. “There’s nothing that’s going to live on.”