Have dark forces been messing with the cosmos?

Dennis Overbye

There was, you might say, a disturbance in the Force.

Long, long ago, when the universe was only about 100,000 years old — a buzzing, expanding mass of particles and radiation — a strange new energy field switched on. That energy suffused space with a kind of cosmic antigravity, delivering a not-so-gentle boost to the expansion of the universe.

Then, after another 100,000 years or so, the new field simply winked off, leaving no trace other than a speeded-up universe.

So goes the strange-sounding story being promulgated by a handful of astronomers from Johns Hopkins University. In a bold and speculative leap into the past, the team has posited the existence of this field to explain an astronomical puzzle: the universe seems to be expanding faster than it should be.

The cosmos is expanding only about 9 percent more quickly than theory prescribes. But this slight-sounding discrepancy has intrigued astronomers, who think it might be revealing something new about the universe.

And so, for the last couple of years, they have been gathering in workshops and conferences to search for a mistake or loophole in their previous measurements and calculations, so far to no avail.

“If we’re going to be serious about cosmology, this is the kind of thing we have to be able to take seriously,” said Lisa Randall, a Harvard theorist who has been pondering the problem.

At a recent meeting in Chicago, Josh Frieman, a theorist at the Fermi National Accelerator Laboratory in Batavia, Ill., asked: “At what point do we claim the discovery of new physics?”

Now ideas are popping up. Some researchers say the problem could be solved by inferring the existence of previously unknown subatomic particles. Others, such as the Johns Hopkins group, are invoking new kinds of energy fields.

Adding to the confusion, there already is a force field — called dark energy — making the universe expand faster. And a new, controversial report suggests that this dark energy might be getting stronger and denser, leading to a future in which atoms are ripped apart and time ends.

Thus far, there is no evidence for most of these ideas. If any turn out to be right, scientists may have to rewrite the story of the origin, history and, perhaps, fate of the universe.

Or it could all be a mistake. Astronomers have rigorous methods to estimate the effects of statistical noise and other random errors on their results; not so for the unexamined biases called systematic errors.

As Wendy L. Freedman, of the University of Chicago, said at the Chicago meeting, “The unknown systematic is what gets you in the end.” 

Hubble trouble

Generations of great astronomers have come to grief trying to measure the universe. At issue is a number called the Hubble constant, named after Edwin Hubble, the Mount Wilson astronomer who in 1929 discovered that the universe is expanding.

As space expands, it carries galaxies away from each other like the raisins in a rising cake. The farther apart two galaxies are, the faster they will fly away from each other. The Hubble constant simply says by how much.

But to calibrate the Hubble constant, astronomers depend on so-called standard candles: objects, such as supernova explosions and certain variable stars, whose distances can be estimated by luminosity or some other feature. This is where the arguing begins.

Until a few decades ago, astronomers could not agree on the value of the Hubble constant within a factor of two: either 50 or 100 kilometers per second per megaparsec. (A megaparsec is 3.26 million light years.)

But in 2001, a team using the Hubble Space Telescope, and led by Dr. Freedman, reported a value of 72. For every megaparsec farther away from us that a galaxy is, it is moving 72 kilometers per second faster.

More recent efforts by Adam G. Riess, of Johns Hopkins and the Space Telescope Science Institute, and others have obtained similar numbers, and astronomers now say they have narrowed the uncertainty in the Hubble constant to just 2.4 percent.

But new precision has brought new trouble. These results are so good that they now disagree with results from the European Planck spacecraft, which predict a Hubble constant of 67.

The discrepancy — 9 percent — sounds fatal but may not be, astronomers contend, because Planck and human astronomers do very different kinds of observations.

Planck is considered the gold standard of cosmology. It spent four years studying the cosmic bath of microwaves left over from the end of the Big Bang, when the universe was just 380,000 years old. But it did not measure the Hubble constant directly. Rather, the Planck group derived the value of the constant, and other cosmic parameters, from a mathematical model largely based on those microwaves.

In short, Planck’s Hubble constant is based on a cosmic baby picture. In contrast, the classical astronomical value is derived from what cosmologists modestly call “local measurements,” a few billion light-years deep into a middle-aged universe.

What if that baby picture left out or obscured some important feature of the universe? 

‘Cosmological Whac-a-Mole’

And so cosmologists are off to the game that Lloyd Knox, an astrophysicist from the University of California, Davis, called “cosmological Whac-a-Mole” at the recent Chicago meeting: attempting to fix the model of the early universe, to make it expand a little faster without breaking what the model already does well.

One approach, some astrophysicists suggest, is to add more species of lightweight subatomic particles, such as the ghostlike neutrinos, to the early universe. (Physicists already recognize three kinds of neutrinos, and argue whether there is evidence for a fourth variety.) These would give the universe more room to stash energy, in the same way that more drawers in your dresser allow you to own more pairs of socks. Thus invigorated, the universe would expand faster, according to the Big Bang math, and hopefully not mess up the microwave baby picture.

A more drastic approach, from the Johns Hopkins group, invokes fields of exotic anti-gravitational energy. The idea exploits an aspect of string theory, the putative but unproven “theory of everything” that posits that the elementary constituents of reality are very tiny, wriggling strings.

String theory suggests that space could be laced with exotic energy fields associated with lightweight particles or forces yet undiscovered. Those fields, collectively called quintessence, could act in opposition to gravity, and could change over time — popping up, decaying or altering their effect, switching from repulsive to attractive.

The team focused in particular on the effects of fields associated with hypothetical particles called axions. Had one such field arisen when the universe was about 100,000 years old, it could have produced just the right amount of energy to fix the Hubble discrepancy, the team reported in a paper late last year. They refer to this theoretical force as “early dark energy.”

“I was surprised how it came out,” said Marc Kamionkowski, a Johns Hopkins cosmologist who was part of the study. “This works.”

The jury is still out. Dr. Riess said that the idea seems to work, which is not to say that he agrees with it, or that it is right. Nature, manifest in future observations, will have the final say.

Dr. Knox called the Johns Hopkins paper “an existence proof” that the Hubble problem could be solved. “I think that’s new,” he said.

Dr. Randall, however, has taken issue with aspects of the Johns Hopkins calculations. She and a trio of Harvard postdocs are working on a similar idea that she says works as well and is mathematically consistent. “It’s novel and very cool,” Dr. Randall said.

So far, the smart money is still on cosmic confusion. Michael Turner, a veteran cosmologist at the University of Chicago and the organizer of a recent airing of the Hubble tensions, said, “Indeed, all of this is going over all of our heads. We are confused and hoping that the confusion will lead to something good!” 

Doomsday? Nah, nevermind

Early dark energy appeals to some cosmologists because it hints at a link to, or between, two mysterious episodes in the history of the universe. As Dr. Riess said, “This is not the first time the universe has been expanding too fast.”

The first episode occurred when the universe was less than a trillionth of a trillionth of a second old. At that moment, cosmologists surmise, a violent ballooning propelled the Big Bang; in a fraction of a trillionth of a second, this event — named “inflation” by the cosmologist Alan Guth, of M.I.T. — smoothed and flattened the initial chaos into the more orderly universe observed today. Nobody knows what drove inflation.

The second episode is unfolding today: cosmic expansion is speeding up. But why? The issue came to light in 1998, when two competing teams of astronomers asked whether the collective gravity of the galaxies might be slowing the expansion enough to one day drag everything together into a Big Crunch.

To great surprise, they discovered the opposite: the expansion was accelerating under the influence of an anti-gravitational force later called dark energy. The two teams won a Nobel Prize.

Dark energy comprises 70 percent of the mass-energy of the universe. And, spookily, it behaves very much like a fudge factor known as the cosmological constant, a cosmic repulsive force that Einstein inserted in his equations a century ago thinking it would keep the universe from collapsing under its own weight. He later abandoned the idea, perhaps too soon.

Under the influence of dark energy, the cosmos is now doubling in size every 10 billion years — to what end, nobody knows.

Early dark energy, the force invoked by the Johns Hopkins group, might represent a third episode of antigravity taking over the universe and speeding it up. Perhaps all three episodes are different manifestations of the same underlying tendency of the universe to go rogue and speed up occasionally. In an email, Dr. Riess said, “Maybe the universe does this from time-to-time?”

If so, it would mean that the current manifestation of dark energy is not Einstein’s constant after all. It might wink off one day. That would relieve astronomers, and everybody else, of an existential nightmare regarding the future of the universe. If dark energy remains constant, everything outside our galaxy eventually will be moving away from us faster than the speed of light, and will no longer be visible. The universe will become lifeless and utterly dark.

But if dark energy is temporary — if one day it switches off — cosmologists and metaphysicians can all go back to contemplating a sensible tomorrow.

“An appealing feature of this is that there might be a future for humanity,” said Scott Dodelson, a theorist at Carnegie Mellon who has explored similar scenarios. 

The phantom cosmos

But the future is still up for grabs.

Far from switching off, the dark energy currently in the universe actually has increased over cosmic time, according to a recent report in Nature Astronomy. If this keeps up, the universe could end one day in what astronomers call the Big Rip, with atoms and elementary particles torn asunder — perhaps the ultimate cosmic catastrophe.

This dire scenario emerges from the work of Guido Risaliti, of the University of Florence in Italy, and Elisabeta Lusso, of Durham University in England. For the last four years, they have plumbed the deep history of the universe, using violent, faraway cataclysms called quasars as distance markers.

Quasars arise from supermassive black holes at the centers of galaxies; they are the brightest objects in nature, and can be seen clear across the universe. As standard candles, quasars aren’t ideal because their masses vary widely. Nevertheless, the researchers identified some regularities in the emissions from quasars, allowing the history of the cosmos to be traced back nearly 12 billion years. The team found that the rate of cosmic expansion deviated from expectations over that time span.

One interpretation of the results is that dark energy is not constant after all, but is changing, growing denser and thus stronger over cosmic time. It so happens that this increase in dark energy also would be just enough to resolve the discrepancy in measurements of the Hubble constant.

The bad news is that, if this model is right, dark energy may be in a particularly virulent and — most physicists say — implausible form called phantom energy. Its existence would imply that things can lose energy by speeding up, for instance. Robert Caldwell, a Dartmouth physicist, has referred to it as “bad news stuff.”

As the universe expands, the push from phantom energy would grow without bounds, eventually overcoming gravity and tearing apart first Earth, then atoms.

The Hubble-constant community responded to the new report with caution. “If it holds up, this is a very interesting result,” said Dr. Freedman.

Astronomers have been trying to take the measure of this dark energy for two decades. Two space missions — the European Space Agency’s Euclid and NASA’s Wfirst — have been designed to study dark energy and hopefully deliver definitive answers in the coming decade. The fate of the universe is at stake.

In the meantime, everything, including phantom energy, is up for consideration, according to Dr. Riess.

“In a list of possible solutions to the tension via new physics, mentioning weird dark energy like this would seem appropriate,” he wrote in an email. “Heck, at least their dark energy goes in the right direction to solve the tension. It could have gone the other way and made it worse!”