Sky and Telescope - May 2018 - 21

FER MIL A B

and distorted images of galaxies. NASA's Wide Field Infrared
Survey Telescope (WFIRST) is slated to launch soon after to
combine its own, similar observations with supernovae, but
its funding is in jeopardy. The Large Synoptic Survey Telescope
(LSST), anticipated to begin operating in 2023 from Chile,
will study dark energy using all four techniques.
With the ﬂood of new and improved data, cosmologists
anticipate they will ﬁnally be able to answer some of the most
basic questions about dark energy. Is it truly the cosmological
constant with w = −1? Does w change over time?
But measuring w is just the beginning. A single number
like w can't encapsulate the complexity of dark energy and
these large-scale observations. Even with a precise value of w
in hand, cosmologists won't have dark energy totally ﬁgured
out. "What you really care about is the rich detail that comes
out of these surveys," Trodden says. "W is a useful placeholder. It's a useful shorthand for talking about some features
of dark energy, but depending on what dark energy is, there
could be a lot more to it."
And there's the rub. At this point, despite a well-deﬁned
observational strategy, dark energy can be almost anything.
While theorists trying to understand it have learned a lot
about the subtleties of gravity and particle physics, they
haven't come to a consensus, Trodden says. Whether it's
the cosmological constant, quintessence, modiﬁed gravity,
a combination of these, or anything in between, the ﬁeld of
theories remains wide open.

p SURVEY ON THE SKY Over five years the Dark Energy Survey,
working at the Blanco telescope on Cerro Tololo in Chile, is mapping
several hundred million galaxies, as well as other cosmic expansion
signposts, to clarify the nature of dark energy. In yellow is roughly half
of its 5,000-square-degree footprint on the Southern Hemisphere sky.

Although future observations will almost certainly make
important discoveries that guide theorists, don't expect any
conclusive answers right away. In fact, Albrecht says, regardless of what next-generation telescopes like LSST ﬁnd, it's
hard to imagine that current theories will adequately explain
the data. "Our theoretical picture feels incomplete," he says.
Yet, being the scientists they are, cosmologists - and
especially theorists like Albrecht and Trodden - embrace this
ignorance. Dark energy is a hard problem, and taking the
long view is required. "It's one of the deepest things about the
universe we've ever learned, and we've only known it for 20
years," Trodden says.
The pursuit of dark energy demands an oxymoronic mixture of hubris and humility, a balance between deference to
nature's mysteries and the conﬁdence to conquer them. "I
hope that I'm the one who ﬁgures out how to understand it,"
Albrecht says, before adding a second point: "I'm skeptical
anyone will do that in my lifetime."

¢ MARCUS WOO is a freelance science journalist based in the
San Francisco Bay Area.
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Sky and Telescope - May 2018

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