The Cosmos

13
Aug

The Phantom Energy Menace: Is the Cosmos Doomed to a Big Rip?

First, the good news: Despite earlier doomsday prognoses, the cosmos is not fated to implode. If you stay up late at night worrying that the entirety of creation will cave in on you in a universal Big Crunch, rest assured. Astronomical evidence suggests that the Big Bang expansion will never reverse itself; the universe will not collapse back down to a point. Rather, the universe, like ever-sprawling suburbs, will burgeon forever, its galaxies receding farther and farther away from each other.

Now, the bad news: Cosmic expansion, discovered in 1998 through supernova observations conducted by Nobel Prize-winning physicists Saul Perlmutter, Brian Schmidt, and Adam Riess and their research teams, is not only continuing, it is picking up its pace. Though we don’t know what is causing the acceleration, one leading idea, called “phantom energy,” has ominous implications. If phantom energy continues to drive the universe faster and faster outward, it could literally tear space into shreds—a doomsday scenario called the “Big Rip.”

Phantom energy, proposed by Dartmouth physicist Robert Caldwell in 1999, is one possible variety of the mysterious stuff researchers call dark energy. Dark energy is the catchall phrase for the hidden agent of cosmic acceleration, and encompasses many different approaches. The simplest proposal adds an extra “antigravity” term, called a “cosmological constant,” to Einstein’s equations of general relativity. Einstein himself had proposed the term to stabilize his equations, but then hastily removed it after Edwin Hubble showed in 1929 that the universe was expanding—and not stable at all. Adding the term back into the equations, while assuming that the geometry of the universe is flat, leads to a prediction that the cosmic growth rate will gradually and steadily increase.

While a cosmological constant seems to model the accelerating universe nicely, at least according to current astronomical observations, many researchers are seeking a more tangible explanation. Instead of an extra term arbitrarily added to the gravitational equations, they are looking for an actual physical substance with repulsive properties. This substance must have a bizarre property called negative pressure. Though we never encounter negative pressure in everyday life, you can picture how it works by imagining poking a soap bubble and seeing it inflate rather than pop.

Gravitational researchers model the behavior of substances using an equation, called an equation of state, that relates the material’s pressure to its density. For conventional substances such as fluids and gases, both pressure and density are positive, so their ratio, usually called “w,” is positive as well. However, if a substance were to mimic a cosmological constant, its w ratio would be negative one. That is, if you try to squeeze it, it will get bigger, not smaller.

In 1998, shortly after the discovery of cosmic acceleration, Caldwell, along with astrophysicists Rahul Dave, then at Penn, and Paul Steinhardt of Princeton, argued for the greatest amount of flexibility in trying to describe dark energy. Borrowing the ancient Greek term for the “fifth essence,” they dubbed it “quintessence” and asserted that its equation of state could vary over time and space. Rather than assuming that w is negative one, they urged that its actual value be pinned down through astronomical observation.

This flexibility spurred Caldwell to think about the worst-case scenario imaginable for dark energy. He wondered what would happen if the amount of negative pressure exceeded the density—that is, if w was less than negative one. In our soap bubble analogy, larger negative pressure would mean that pressing on its exterior would cause it to puff up even faster. In the far future, he realized, gravity and all other forces would eventually lose potency, overwhelmed by the hulking menace of what he dubbed phantom energy. The reason is that unlike the steady density of the energy associated with a cosmological constant, the density of phantom energy would grow greater and greater, rising along with its negative pressure, building to a lethal crescendo—a Big Rip.

In Caldwell’s end game scenario, our distant descendants (relocated to other planets, perhaps) will notice the first sign of trouble billions of years from now as other galaxies recede beyond detection and disappear from the sky. (That would happen under the cosmological constant picture too, but is exacerbated in the case of phantom energy.) Eventually our local group of galaxies, including Andromeda, will become cosmic hermits. Then, tens of millions of years before the ultimate doomsday, the local group and then the Milky Way itself will break up like a dropped box of peanut brittle.

In the cosmic twilight times, all planetary systems will be torn apart and the stars and planets will explode. About thirty minutes later, all atoms will burst like fireworks. Then, in a mere fraction of a second, space itself will tear to shreds in the all-destroying Big Rip.1

If the devouring of reality by a ravenous shredder sounds revolting, you can console yourself with an alternative developed by physicist Pedro Gonzalez-Diaz of the Institute of Mathematics and Fundamental Physics in Madrid. He conjectures that phantom energy could feed one of the many wormholes that could exist in the fabric of spacetime. Over billions of years, the wormhole would swell up faster than the cosmic expansion rate and eventually engulf the material content of whole universe—galaxies and such—sparing it from a Big Rip. Theoretically, then, everything could pass unscathed out the other end of the wormhole, emerging in another sector of reality, wherever that would be. Not exactly a joy ride, but at least it wouldn’t leave you in tatters!

The jury is still out on what causes cosmic acceleration. It could well be the case that the culprit is not phantom energy, but rather a gentler form of dark energy—such as a type of quintessence with a w value greater than or equal to negative one. If so, the expansion of space would increase more gradually, leaving the local group and the Milky Way gravitationally bound together as other galaxies eventually recede from view. While our enclave of space would be lonely, it would remain intact. In short, we’d experience a “Big Stretch” rather than a “Big Rip.”

1A recent calculation by a team of researchers led by Chinese physicist XiaoDong Li uses current data to estimate that the Big Rip will take place 16.7 billion years from now. The dissolution of atoms would take place a mere 30 quintillionth of a second before the Big Rip.

Go Deeper
Editor’s picks for further reading

NBCNews.com: Will universe end in a ‘big rip’?
Space writer Robert Roy Britt talks with cosmologists about the possibility of a Big Rip.

Open Yale Courses: Dark Energy, the Accelerating Universe, and the Big Rip
In this video, Yale astronomer Charles Bailyn reviews the evidence for dark matter and dark energy and presents the possibility of a Big Rip.

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Paul Halpern

    Paul Halpern is Professor of Physics at the University of the Sciences in Philadelphia. A prolific author, he has written thirteen science books and dozens of articles. His interests range from space, time and higher dimensions to cultural aspects of science. The recipient of a Guggenheim Fellowship, Fulbright Scholarship, and an Athenaeum Literary Award, he has appeared on the History Channel, the Discovery Channel, the PBS series "Future Quest," and "The Simpsons 20th Anniversary Special." Halpern's books include "Time Journeys," "Cosmic Wormholes," "The Cyclical Serpent," "Faraway Worlds," "The Great Beyond," "Brave New Universe," "What's Science Ever Done for Us?," "Collider," and most recently "Edge of the Universe: A Voyage to the Cosmic Horizon and Beyond" (Wiley 2012). More information about his writings can be found at phalpern.com.

    • http://www.facebook.com/lauren.collins.7923 Lauren Collins

      How does the “Heat Death” hypothesis tie into this?

      • Paul Halpern

        Excellent question, Lauren.

        If the “Big Rip” scenario is true, the universe would be destroyed long before the Heat Death (when entropy is maximum, the stars have all burned out, and there is no more usable energy).

        On the other hand, if there is a “Big Stretch” with a less powerful form of dark energy, then our galaxy would persist for many billions more years until all its stars burn out, and its black holes slowly radiate away. Once everything has exactly the same low temperature, that would be the “Heat Death.” (Despite its name, the “Heat Death” would be extremely cold!)

        • BrianHSummerlin

          Would the existence of black holes show that the universe can also move toward an ordered state and isn’t limited to ever increasing entropy?

          • Paul Halpern

            Interesting comment. As shown by Jacob Bekenstein, black holes possess a kind of gravitational entropy that depends on the area of their event horizons. So, as black holes gobble up more material and expand, their entropy increases. Seems that you can’t avoid the law of entropy!

    • http://www.facebook.com/people/Eric-Jorgensen/807192655 Eric Jorgensen

      Question #1: At what point do black holes get ripped apart in the big rip?

    • http://www.facebook.com/people/Eric-Jorgensen/807192655 Eric Jorgensen

      Question #2,3: At what point does the magnitude of space exapansion during the big rip match the magnitude of inflation during the big bang? Could it be that the big bang is just a tiny piece of the last big rip?

      • Primordial

        ” Could it be that the big bang is just a tiny piece of the last big rip? “, very good point.

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