Researchers suggest a practical experiment to test an unpopular theory.
Image: Abulic Monkey/Creative Commons
There are a few big placeholders in our understanding of the universe: dark energy, dark matter, gravitational waves. These are the names of things that should exist with certain general properties in order for our most cherished notion(s) of the universe not to collapse, but their precise nature is as yet undetermined.
It all feels a bit precarious, as proving the nonexistence of any of these concepts might put other, more certain notions into question, like the constancy of the gravitational force throughout the universe. And while we have these big blanks blacking out vast regions of understanding, we have something like the opposite in antimatter: we can see it readily in labs and in nature, but we don't have the immense cosmic blank to fit it into. One should exist out there, but we don't see it. The universe seems to be composed almost entirely of regular matter.
So, we might make a connection: there are vast blanks to which we give cryptic names, and at the same time we have a well-known concept that should have a great big cosmic blank but doesn't. According to a piece out this week in Physics World, a pair of researchers at Italy's Astrophysical Observatory of Turin are claiming a practical method to gather evidence for a whole new cosmological model that would erase those blanks in favor of a model involving antigravity.
Antigravity is a theorized property of antimatter in which it responds to a gravitational field in precisely the opposite way as we might expect normal matter to. The same thing should hold true for matter-matter in the presence of an antigravitational field: falling up. You can see the allure in that kind of symmetry.
In fact, the theory the pair is advancing is even more symmetrical. First developed by CERN physicist Dragan Hajdukovic, the general idea rests on the notion that there is no such thing as empty space. Instead, at the very base level of nothing, we would find a continuous foam of "virtual" particles being created and quickly destroyed. The Hajdukovic interpretation adds in antigravity. Within the quantum foam, we know that both a regular particle and an antiparticle are being created (that's just how any particle with mass is created), and if antigravity were real, those particles would repel each other as they'd have opposite gravitational "charges."
"On the assumption that particles and antiparticles forming the QV [quantum vacuum] have opposite gravitational charge, the QV energy could be compatible with that attributed to dark energy ."
Taken at very large scales, we would find a lot of repulsion in the universe's empty spaces, and this might be what we would otherwise think of as dark energy—the force accounting for the universe's accelerating outward expansion. It's a tantalizing thought, but we're just now getting to the stage where we can test how antimatter behaves in response to gravity; the going assumption is that it will behave just the same as regular matter, which is actually pretty reasonable. Expect confirmation either for or against the antigravity theory in the very near future, as experiments are currently underway at CERN setting "freefall limits" on trapped antihydrogen atoms.
In the meantime, however, physicists can get wild. The Hajdukovic idea also accounts for dark matter, in addition to dark energy, as the virtual particles foaming through quantum vacuums will exert some massive attractive gravitational pull as well, magnifying the normal gravitational effects of objects in space. The quantum vacuum is thus attractive and repulsive in just the right ways as to account for our "dark" problems.
Hajdukovic suggested that this could be tested using some very small stellar object that has an even smaller satellite traveling in an elliptical orbit. Now, the Turin researchers think they've found a suitable candidate in the dwarf planet UX25, which orbits the sun in the general region of Neptune and could be observed using current or near-current telescopes.
"On the assumption that particles and antiparticles forming the QV [quantum vacuum] have opposite gravitational charge, the QV energy could be compatible with that attributed to dark energy and the same hypothesis could be used to explain the eﬀects now attributed to dark matter," a recent paper from the Turin duo summarizes. "From the astronomical point of view, however, the consequences of attributing a gravitational eﬀect to QV should in principle be considered at all scales, not just at the cosmological ones needed to explain the dark energy eﬀect."
Dwarf planet UX25 seen through a telescope. Image: Wikimedia Commons/Kevin Heider
As for UX25, it's just far enough away from the sun to be viewed as a "closed" system, gravitationally speaking. "The additional gravity source represented by [quantum foam] would necessarily induce a deviation from the perfect symmetry of the gravity ﬁeld which is necessary to have closed orbits," the paper continues, "and therefore show itself up as an excess of the shift of the pericenter of a body orbiting around a central gravity source."
In simpler terms, given the relatively high sensitivity of the system to gravitational influence, "The properties of quantum vacuums described in Hajdukovic's theory would apply an additional [gravitational] force on UX25, perturbing the orbit of the system," Alberto Vecchiato, one half of the Turin team, told Physics World. The extra gravitational influence would induce an extra wobble in the dwarf-planet system, just large enough to be detectable by the Hubble Space Telescope and its successor, the James Webb Space Telescope. According to the paper, it would more likely be the latter, however, as detection will take a few years of continuous observation and the HST is on its way out.
The Hajdukovic theory isn't exactly a popular candidate within astrophysics and cosmology, and I asked Vecchiato and his study co-author Mario Gai what they thought the chances were of getting the needed years of telescope time to make the neccessary observations of UX25.
"Indeed the issue of getting observing time is critical," Gai responded. "For a risky subject, which may or not give results—which anyway in that case would be very important—it is possible, but far from granted, to get some resources. Institutes and agencies are ready to take some risks, after thorough evaluation, if the potential return is high. For [the James Webb Space Telescope] or large, ground-based AO [optical] telescopes, the issue could be acceptable. But we are not yet there." Nor, it should be noted, is the James Webb itself: it launches in 2018, hopefully.
Gai noted that the unique properties of UX25 make it worth observing for reasons well beyond antigravity theories. "UX25 is an interesting 'test particle,' quite sensitive to perturbations, whichever their source," he said. "Thus, it would make sense to monitor it, to check for deviations
from known physics."
And deviations from known physics are always just arrows toward new physics, whether they include antigravity or not.