Dark Days for Dark Energy?

Supernova sluthing
Left:A small portion of the original Hubble Deep Field image, which the Hubble Space Telescope acquired in late 1995. Right: The same field, imaged 6½ years later with Hubble's Advanced Camera for Surveys. ACS serendipitously captured a supernova explosion (SN2002dd) that took place when the universe was slightly less than half its current age. Such supernovae have shown that the universe's expansion has been speeding up in the last few billion years.
Courtesy NASA and John P. Blakeslee (Johns Hopkins University).
Most astrophysics papers sink into the ponds of academia and hardly make a ripple. Others spawn tidal waves. The latest cosmological tsunami was triggered on March 14th, when Edward ("Rocky") Kolb (Fermilab), Sabino Matarrese (University of Padova, Italy), Alessio Notari (McGill University, Canada), and Antonio Riotto (National Institute of Nuclear Physics, Italy) posted a terse, four-page paper on arxiv.org, an online bulletin board that's rapidly taking the place of established journals. The paper invokes the 25-year-old inflation theory to explain away dark energy — a mysterious force that astronomers have recently embraced to explain why the universe's expansion is accelerating.

cosmic pie chart
In today's conventional view of cosmology, baryonic (atomic) matter amounts to only 4.4 percent of the total cosmic matter-and-energy budget. The other 95.6 percent comprises dark matter and dark energy — both complete mysteries.
Courtesy NASA / WMAP Science Team.

Dark energy has become widely accepted as the main ingredient in today's cosmic soup. Arguing that "there is no dark energy," as Kolb says, is as heretical today as was advocating for its existence (in the form of Albert Einstein's cosmological constant) a decade ago. Thus it's no surprise that weblogs and university hallways are buzzing with the news of dark energy's possible demise.

Some welcome the news. "It would be nice to think we could slip out of this dark-energy thing in some way," says astrophysicist Anthony Aguirre (University of California, Santa Cruz). After all, at face value the mystery force is 120 orders of magnitude weaker than predicted by the leading theory that aims to account for its existence. If Kolb and his colleagues are right, Aguirre says, "it would be really exciting." But most cosmologists are betting that Kolb's team has either made a mathematical error or run afoul of immutable physical laws.

How space has expanded
This graph shows how much space lies between galaxies on average throughout the universe's history. A dark-matter-dominated deceleration put on the brakes shortly after the Big Bang, while a more recent period of cosmic acceleration — supposedly powered by dark energy — has dominated in recent eons.
Sky & Telescope illustration by Steven Simpson. Source: Space Telescope Science Institute.

No one knows what dark energy is. But, as University of Chicago theorist Sean Carroll explains in the March 2005 issue of Sky & Telescope, astronomers have concluded that some kind of energy occupies every cubic centimeter of space and pushes galaxies away from one another at ever-increasing speeds. The most dramatic evidence in its favor comes courtesy of Type Ia supernovae. When spotted in distant galaxies, these exploding white dwarfs appear 10 or 20 percent dimmer than expected, on average. This implies that their home galaxies had been pushed away from our Milky Way by an "extra" 5 or 10 percent in the last few billion years.

To most astronomers, the too-faint supernovae demonstrate that the universe's expansion is accelerating, as if it were stepping on the gas pedal after applying the brakes for billions of years. Kolb and his colleagues don't take issue with that interpretation. But when it comes to what's in the gas tank, they stand apart from the crowd. They attribute the universe's accelerating expansion not to dark energy, but to ripples in the fabric of space and time that dwarf the entire visible universe.

Like notes sounded by a piano with endless row of keys, ripples of all sizes were spawned during the Big Bang's first fraction of a nanosecond by quantum-mechanical fluctuations. Imprinted on the primordial soup, these ripples went along for the ride when the fabric of space ballooned abruptly, doubling in size dozens of times over. Part and parcel of the quarter-century-old inflation theory, some of the ripples are visible today in the lacy structures traced by galaxies. Others speckle images of the cosmic microwave background — radiation that shows how matter was distributed just 400,000 years after the Big Bang.

cosmic microwave background sky
This all-sky map shows myriad ripples in the early universe as reflected in the cosmic microwave background radiation. Acquired by the WMAP satellite, it convinced most astronomers that our observable universe is essentially flat — powerful if indirect evidence for the dark energy that supernova hunters discovered in 1998.
Courtesy NASA / WMAP Science Team.

But the ripples that Kolb's team invokes are unusual. Inflation stretched these bass notes so dramatically that not even one of their wave crests can fit within our Hubble horizon — the region of space-time from which any force can reach us traveling at light speed.

And therein lies a fatal flaw, says Princeton University cosmologist Mustapha Ishak-Boushaki. Ever since Einstein put forth his special theory of relativity 100 years ago, modern physics has been predicated on the notion that no physical force can outpace light. "Nothing can affect you if it's outside your [past] light cone," as physicists call the region of space and time from which light can reach an observer today, says Ishak-Boushaki. "How can something that is not in causal contact with us influence local physics?"

Some cosmologists admit that a super-sized wave's presence could slightly skew our physical universe's overall curvature. But precise measurements of the cosmic microwave background already have eliminated that possibility, say several critics, since they show that the visible universe is flat to within a few percent. Finally, a March 27th preprint by two Princeton physicists implies that Kolb and his colleagues misled themselves by oversimplifying the equations they used to analyze the universe's expansion.

Though Physical Review Letters hasn't yet agreed to publish his team's paper, Kolb is defiant. "We have an alternate explanation for the expansion history of the universe," he says, one that relies solely on the relatively well-established quantum-mechanical processes that kick-started inflation. And while it has plenty of critics, that explanation "looks very testable," says supernova hunter Adam Riess (Space Telescope Science Institute). "If you can measure supernova distances to 5 percent at two widely discrepant redshifts," Riess concludes, "you can measure this."

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