Late last Friday, on the first cloudy night in a week of clear weather, Dark Energy Survey science team member James Annis took time out from his role in the $35 million project’s commissioning to answer a few questions from the Blanco 4-meter telescope.
By night, the route up to the Blanco. atop Chile's Cerro Tololo mountain, winds around some of the most desolate-looking stretches of two-lane blacktop imaginable.
Switchback after switchback finally leads to the telescope itself, where Annis, a Fermilab astrophysicist and colleagues, have been fine-tuning the attached digital camera that offers the next best hope in answering questions about the true nature of dark energy --- responsible for the inexplicable accelerating expansion of the observable universe.
Star trails are formed in this long exposure image of the Blanco 4-m Telescope on CTIO. Image... [+]
Though the five-year survey’s international collaboration is only in its first year of commissioning, the Dark Energy Camera may help understand the force that has already redefined the universe’s makeup as follows: 72 percent dark energy, 23 percent dark matter and roughly 5 percent ordinary matter.
“Modern telescopes are built light and fast and almost always have small fields of view,” said Annis. “But the Blanco is spectacular for this wide survey. However, the camera’s commissioning involves getting this 40 year-old telescope, whose 15-ton mirror sits on a huge piece of steel, to perform as if it were a new telescope.”
To that end, the 570-megapixel camera will eventually measure up to 300 million galaxies up to 8 billion light years distant, as well as 100,000 galaxy clusters, and 5,000 supernovae. The goal is to cover 5000 square degrees of the non-galactic plane of the sky as possible; or about an eight of the sky. Using four different probes of dark energy, Annis says the idea is that dark energy should reveal itself, in part, by just how it affects the growth of structure in the cosmos.
However, the first hints of dark energy were found in the late 1990s by astronomers looking for signs of a cosmos decelerating billions of years after the onset of the Big Bang’s inflationary expansion. Instead, by observing distant supernovae which appeared farther distant than expected, two independent astronomical teams concluded that the universe’s expansion rate had begun accelerating some 6.5 billion years after the Big Bang due to a heretofore unknown force dubbed dark energy.
This presents a real problem --- either dark energy is causing a repulsive anti-gravity-like acceleration of spacetime itself, or Einstein’s theories about gravity don’t apply on the largest scales.
At the time Einstein proposed his Theory of General Relativity, it had not been observationally established that the universe was in a state of expansion. Thus, Einstein introduced his cosmological constant as an ad hoc factor in his equations, in order to conform with observational cosmology of the era which called for a static universe. By 1932, however, there was observational data indicating that indeed the universe was expanding.
But Adam Riess, who along with Brian Schmidt and Saul Perlmutter, shared the 2011 Nobel Prize in Physics for their discovery of dark energy, ended up resurrecting Einstein’s cosmological constant as a possible explanation for this mysterious force. It fit perfectly.
“Even if the cosmological constant is the most natural explanation for dark energy, we still know nothing about it,” says Annis. “The force that dark energy exerts is just like anti-gravity; pushing stuff further apart.”
Question is, says Annis, why does matter end up being repulsed by nothing?
*Annis says, at the moment, one of the most attractive ideas to explain away dark energy is that on cosmological scales of a tenth of the size of the observable universe or more, gravity simply behaves differently.
That would also mean that Einstein’s Theory of General Relativity would need to significantly modified; something astrophysicists are extremely reluctant to do. Even so, at the moment, Annis leans toward dark energy simply being explained by a modified form of gravity.
In theory, dark energy should also manifest itself experientially in the laboratory. Thus, Christian Beck, an applied mathematician at Queen Mary, University of London, and colleagues used superconducting devices known as Josephson junctions to look for evidence of dark energy’s repulsive force at University College London’s Center for Nanotechnology. The results were inconclusive.
Beck hopes to eventually use entire arrays of Josephson junctions to better measure quantum noise spectra in the terahertz region, noting that any anomaly there that cannot be explained by conventional physics could be a hint of dark energy.
However, Riess, an astronomer at Johns Hopkins University, is still counting on observational astronomy to solve the conundrum in a four phase process.
“Stage I was finding it,” said Riess. “Stage II was reconnoitering it. And the Dark Energy Survey is a big part of Stage III. Hopefully, Stage III will involve discovering dark energy’s identity or else it’s on to stage IV --- where we try from space and likely concede if we fail after that.”
Annis says current plans call for the camera’s primary science observations to begin in September.
Schmidt, an astronomer at Australian National University, says that “if dark energy's impact on the cosmos is different than Einstein's prediction of a cosmological constant, and that difference is just out of reach of current experiments ---- then the Dark Energy Survey will quite likely find it.”
* Updated from an earlier version, to note that "on cosmological scales of a tenth of the size of the observable universe or more, gravity simply behaves differently."
