How the universe is erasing evidence of its beginnings and moving faster toward its end
In 1917, on the third floor of an apartment building in the Wilmersdorf borough of wartime Berlin, an ailing tenant named Albert Einstein sat focused on a lofty subject: the universe. In February of that year, he published a paper that effectively launched the modern field of cosmology. In it, he suggested that the fabric of space and time contains an innate tension, an energy that seethes beneath the surface of every inch of the universe. This “cosmological constant” was the force that held gravity in check and kept the universe from collapsing on itself, he said.
In other words, the universe was in a holding pattern.
A dozen years later, however, the astronomer Edwin Hubble discovered that the universe was not standing still, as Einstein had suggested. Hubble found that the universe was instead expanding—forever moving outward—and didn’t need anything to keep itself from collapsing. Hubble’s discovery led Einstein to repudiate his own claim of a cosmological constant and to write the incident off as the “worst blunder” of his career.
In the years that followed, Einstein’s concept of a cosmological constant faded but never disappeared. Researchers continued to ask: If the universe is simply being carried out by its own momentum, does that necessarily mean that nothing, no tension is filling the vacuum of the universe?
It turns out Einstein’s conclusions might have been less farfetched than he thought. Cosmologists continued to research this theory, and what they discovered is shedding light on the future of the universe—while simultaneously erasing traces of its past.
A New Constant is Discovered
In 1995, physicists Lawrence Krauss, Ph.D., then at Case Western Reserve University, and Michael Turner, Ph.D., of Fermilab in Illinois, argued in the journal General Relativity and
Gravitation that the universe does, in fact, have a cosmological constant. It is a force that not only propels the expansion of the universe, but does so at ever-faster speeds, constantly accelerating, they said.
The scientists pieced together data, including X-ray telescope observations of faraway galaxies and Hubble Space Telescope distance measurements to nearby ones. They concluded that something seems to be pushing the expansion of the universe ever faster.
That force is dark energy, researchers say, and its existence means the universe will ultimately expand so far and so wide that the stars, planets and galaxies as we know them will disappear from view. Future astronomers will look skyward toward a barren universe that lacks any clues about its origins.
“There will be ever-diminishing evidence that there was a Big Bang,” says Glenn Starkman, Ph.D., a Case Western Reserve physicist and director of the university’s Origins Initiative. That could mean the end of cosmology as we know it. “Cosmologists in general are trying to answer big questions,” Starkman says. “Most of the questions we’ve been trying to answer are about the past. But I think the big questions about the future are, in many ways, just as interesting.”
Dark energy may indeed have a lot to say about the future, scientists are finding. In 1995, though, not everyone was on board with the concept of a new cosmological constant.
“The concept turned out to be right, and that was a very remarkable thing,” says Will Kinney, Ph.D., a physicist at the State University of New York at Buffalo. At the time, Kinney says, “I don’t know that a lot of people took the cosmological constant seriously.”
That changed in 1998, when an international coalition of astronomers released a sheaf of data in both the Astronomical Journal and the Astrophysical Journal that they said proved the universe is expanding at an increasingly rapid rate. Measuring the brightness of 102 exploding stars, or supernovae, in distant galaxies, the scientists found that these supernovae were often dimmer than expected. The findings fit a pattern that could only be explained by a universe whose expansion was accelerating over time.
The cosmic self-pressure that the scientists observed—dark energy—has since been confirmed by independent observations, including careful measurements by high-tech instruments such as NASA’s Wilkinson Microwave Anisotropy Probe, which launched in 2001.
Shedding Light on Dark Energy
No one knows for certain what dark energy is or what generates it, but one thing is clear: It is pressuring space to expand. That makes dark energy stand apart from everything else in the universe because every other form of matter or energy gravitationally tugs on other matter.
Dark energy’s peculiar feature is that it seems to fill any void or vacuum, including those created by the universe’s expansion. Even a patch of empty space that had been eradicated of all known forms of matter and energy still contains dark energy, Starkman says.
“So if you have twice as much vacuum as you had before, then you have twice as much of that energy,” he says. “That’s really peculiar. If you take a box and stretch it, you get something for free. That’s the property that accounts for the ability of the vacuum to expand at an accelerating rate. The more you expand it, the more of the [dark energy] you have, and the more that it pushes.”
If dark energy seems confusing, that’s because it is, Starkman says. The greatest minds in physics are baffled. Dark energy is one of the most perplexing unsolved mysteries in science today, and scientists’ best guess for what lies at the heart of dark energy and the cosmological constant lies in quantum physics, Starkman says.
Quantum theory predicts that empty space will wiggle with low-level vibrations, even when all the energy in that space is depleted. It says that the simplest kind of motion conceivable, subatomic particles moving back and forth like miniature springs, will be present even when no other energy is present and they will never not move. Imagine a universe filled with simple quantum particles. Now rob the universe of every ounce of energy it contains. What quantum theory says is that, powered by nothing whatsoever, the universe will still vibrate with what is sometimes called “vacuum energy” or “zero-point energy.”
Quantum vacuum energy is “the simplest explanation for the origin of [dark] energy,” Starkman says. But the explanation remains murky. Starkman holds out hope that in Geneva, Switzerland, the CERN laboratory’s Large Hadron Collider, the world’s most powerful particle accelerator, may uncover precious clues about dark energy. The accelerator, which began operating in September, will allow scientists to analyze high-energy beam collisions and possibly reveal a new world of unknown particles.
The experiments could ultimately explain why those particles exist and behave as they do. They could reveal the origins of mass, shed light on dark matter, uncover hidden symmetries of the universe, and possibly find extra dimensions of space.
In the meantime, the observed existence of dark energy—whatever its origins—is producing real consequences for the universe’s future.
The Universe’s Beginning and End
In 1999, Starkman co-authored a paper with fellow Case Western Reserve physicist Tanmay Vachaspati, Ph.D., and Mark Trodden, Ph.D., of Syracuse University. The research, which appeared in Astrophysical Journal, linked cosmic acceleration to a decidedly bleak future. The universe had entered an extended period of rapid growth, they said, and, eventually, the objects in it would move away so rapidly from our world that they would fall away from view.
The evidence came from observations of supernovae, they said, which measurements showed were not only moving away, but moving away at ever faster speeds. Traditional Big Bang theory runs counter to this notion. It predicts that cosmic expansion will slow or even halt over time. Think of a fireworks explosion: an initial blast, streamers shooting out from the core at great speed, then a gradual slowing until the lights of the fireworks collapse and fade.
If the universe’s expansion continues to speed up, not slow down, then light from distant galaxies will fade for a different reason: It eventually will be unable to keep up. “We realized that things were going to start disappearing,” Starkman says. “The longer you wait, the less you’ll see.”
However, he adds, it will take scores of billions of years to lose sight of the universe’s landscape as we know it. Today, the universe is just a teenager, a spry 14 billion years young. The cosmic end-state comes when the universe nears 100 billion years old.
As that faraway birthday approaches, cosmic expansion will have created vast stretches of void between galaxies. Today’s visible universe, with its hundreds of billions of galaxies stretchingfar into the great beyond, will have sunk below the Earth’s horizon. Our sun and solar system will be long gone, having fizzled somewhere near the 19 billion-year mark.
If civilizations exist in other galaxies at such a late date, their conclusions about the universe will be incomplete. Light from neighboring galaxies will be unable to reach them because the expansion of space will have quickened beyond the lowly photon’s ability to keep up. Cosmology, particularly the study of the universe’s origins, will by then have reached an end. The science launched by Einstein’s notion of a cosmological constant will be destroyed by that very same constant.
But scientists are not only considering questions of the past; they are also considering future prospects for life in the universe.
In 1979, physicist Freeman Dyson, Ph.D., of the Institute for Advanced Study at Princeton University published a paper in the journal Reviews of Modern Physics that argued life could survive indefinitely in a universe that also expanded indefinitely. In Dyson’s view, biology could ultimately win the battle with a hostile universe.
Of course, appearing 19 years before the discovery of accelerating cosmic expansion, Dyson’s paper did not consider dark energy or a cosmological constant. In 2004, Starkman co-wrote another paper with Lawrence Krauss that delivered the bad news: Life is eventually doomed. Einstein’s greatest blunder ultimately, after hundreds of billions of years, wrenches the universe apart. And with it goes the prospect for biology.
“The universe is going to have a long, slow end,” Starkman says. “It will first begin with ignorance. And if we are right, it will end with death.”
Kinney, of the University at Buffalo, expands on that argument. In a paper written with physicist Katherine Freese, Ph.D., of the University of Michigan, Kinney points out that no one knows for certain whether the cosmological constant is, in fact, constant. It could be that the acceleration of the universe’s expansion will change over time. In some scenarios, in which the amount of dark energy exponentially diminishes over time, they find that doom and gloom may not prevail. Under such circumstances, the universe and biological processes in it could, theoretically at least, continue far into the future.
The question is, how far into the future?
“We all agree that life can last longer if the cosmological constant isn’t constant,” Starkman says. “What we’re arguing over here is how long. The evidence doesn’t seem to suggest that it will last forever. But maybe the certainty of our continued existence isn’t the most important thing—maybe it’s the understanding that we gain while we’re here.”