Dark energy may be distorting long-sought gravitational wave signals from distant sources in the universe, according to a paper posted to the arXiv pre-print server by astrophysicists at Penn State University. The waves may still be visible to us, the researchers say, but because of these still poorly understood effects, they could look very different from expected. Blame the expansion of the universe.
A gravitational wave detection is currently one of astrophysics' white whales. Despite an impressive number of detection experiments currently underway worldwide, we've yet to actually bag a gravitational wave observation. For the past couple of weeks, the astrophysics rumor mill has been buzzing with the prospect of a detection at the United States' massive LIGO experiment—which is based on an interferometer stretching from Washington state to Louisiana—but no announcement has been made, nor does one seem to be forthcoming.
The difficulty in detecting gravitational waves has to do with their relative weakness. They arise as the result of some massive body, with a very large amount of gravitational attractiveness, accelerating through space and leaving faint ripples in the fabric of space-time. This is general relativity: gravity deforms space-time itself. An acceleration, a change in velocity, will cause a change in this deformation, which we should be able to experience as gravitational waves.
Gravity is an almost unimaginably weak force, however, compared to the other fundamental forces, so it should take a very, very, very massive stellar event—like a pair of big honkin' black holes colliding—to result in gravitational waves with sufficient magnitudes to be detectable. And, according to the Penn State paper, the situation may be muddied even further by another astrophysics mystery: dark energy, or vacuum energy.
LIGO detector at Hanford, Washington. Image: LIGO
Dark energy is the thing currently shoving our universe apart. It's described by what's known as the cosmological constant (which is represented as the symbol Λ), a set value for the base level energy of empty space. There is no such thing as emptiness really, just this peculiar fizz of spontaneously created and then immediately destroyed particles. Until it was discovered in 1998 that the universe is not just expanding but accelerating in its expansion, this constant was assumed to be 0. Now we know that that's not the case.
What does the vacuum energy of space have to do with missing gravitational waves? It's not hiding them, but it may be making them appear different, according to the Penn State group. This is the case, at least, for gravitational waves traveling across very large distances.
The reason is intuitive enough. The effect of dark energy is expanding space. If you were to track an ocean wave across the surface of an expanding ocean, it would probably look different when it arrived than if it had traveled across the surface of a static ocean.
"Given the early confusion on whether gravitational waves are physical and the fact that the gravitational wave observatories are now on the threshold of opening a new window on the universe, it is important that the theoretical foundations of the subject be solid," the Penn State group writes. "One cannot be certain that the effects of Λ would be necessarily negligible because, irrespective of how small its value is, its mere presence introduces several conceptual complications requiring a significant revision of the standard framework."
Because the dark energy effect should only be significant for waves traveling huge distances, cosmologically speaking, not all detection experiments should be affected. As noted in Physics World, the LIGO detector searches for gravitational waves traveling distances of less than a few hundred thousand light-years away, which is a small enough distance to not be affected, but experiments like the Laser Interferometer Space Antenna (LISA) and the Einstein Telescope hunt for waves from billions of light-years away. These projects may need to revise their approach.