Sky & Telescope - September 2016 - 40

Covert Cosmology

G O IN G THE D I S TAN C E:
S PE C TR O S C O PI C VS .
PH OT O ME TR I C RE D S HIF T
As light traverses the universe, spacetime's expansion
stretches the wavelength. For relatively nearby objects,
this is most easily measured by the redward shift of
emission (or absorption) lines in an object's spectrum.
But measuring spectroscopic redshifts becomes impractical for millions of faint, faraway galaxies - collecting a
spectrum requires a bright target and lots of time. So
astronomers resort to photometric redshifts: rather than
gathering a galaxy's light into the narrow buckets that
make up a spectrum, ﬁlters split light into broader buckets. Then astronomers compare this rough approximation of a spectrum to galaxy templates until they ﬁnd
the best match. Although photometric redshifts aren't
as precise as spectroscopic measurements, they work
well for large samples of faint objects.

Telescope on Mauna Kea in Hawai'i. While these early
surveys were generally shallow and/or limited in scope,
bigger projects are on the horizon.
One ongoing survey is the Kilo-Degree Survey
(KIDS), featuring the 268-megapixel OmegaCAM on the
European Southern Observatory's 2.6-meter VLT Survey
Telescope at Cerro Paranal. Another project is the Dark
Energy Survey (DES), which employs the 570-megapixel
Dark Energy Camera on the 4-meter Victor M. Blanco
telescope at the Cerro Tololo Inter-American Observatory, also in Chile.
Since 2011 KIDS has focused on two regions in the sky
totaling some 1,500 square degrees - slightly bigger than
the area covered by Ursa Major. DES started almost two
years later and aims to cover no less than 5,000 square
degrees - almost one-eighth of the celestial globe.
"In the end, the Dark Energy Survey will probably have
better statistics," says Hoekstra, "but KIDS provides better
photometric redshifts, because observations are carried
out both at optical and infrared wavelengths." Both surveys released preliminary maps mid-year in 2015.
There's also the Hyper Suprime-Cam (HSC) Survey:
it goes even deeper than DES or KIDS and covers 1,400
square degrees. The relative newcomer started in March
2014 and makes use of the giant, 870-megapixel ultrawide-ﬁeld HSC camera on the 8.2-meter Subaru telescope on Mauna Kea. "That's going to be phenomenal,"
says Hoekstra.
"There's certainly competition going on between the
various teams," Hoekstra adds, "but it's all relatively
friendly." Mandelbaum agrees. "We need multiple data
sets to carry out all kinds of crosschecks," she says. (For
a side-by-side comparison of the three surveys' complementary capabilities, see page 39.)
40

September 2016 sky & telescope

But Wait, There's More
KIDS, DES, and the HSC Survey are just scratching the
surface of cosmologists' new, versatile tool. Scientists
expect them to provide a useful proof of principle, but
for weak lensing to achieve its full potential, explorations
will need to go even wider and deeper.
That's why two huge weak lensing surveys will commence several years from now. One will be carried out
on the ground: the 8.4-meter Large Synoptic Survey
Telescope (LSST) is currently under construction at
Cerro Pachón in Chile and is scheduled to start science
operations in 2022 (see p. 14). The other survey comes
courtesy of the European Space Agency's Euclid mission, poised for launch in late 2020. Though its 1.2-meter
mirror is much smaller than LSST's giant eye, from its
vantage point in space (at the L2 Lagrangian point, 1.5
million kilometers from Earth in the anti-sunward direction) it won't suﬀer from atmospheric turbulence: it will
provide Hubble-quality imagery of the whole sky.
The two surveys are complementary. LSST goes
deeper - thanks to its larger mirror size - but its
lower angular resolution may blend some smaller galaxy
images. Euclid doesn't suﬀer so much from blending,
but its observations combine visible light along a quite
broad wavelength range. While LSST has multiple wavelength ﬁ lters, good for estimating photometric redshifts
and hence distances, Euclid will instead need supplemental observations from ground-based telescopes.
Another important diﬀerence is that weak lensing
is just one of the many science drivers for LSST: when
completed, the wide-ﬁeld telescope will also hunt for
supernovae, asteroids, and many other kinds of variable, transient, and moving objects. In contrast, Euclid's
development has been optimized for cosmology.
No one really knows what to expect from these surveys. Astronomers will need a wide variety of cosmological probes to truly understand the nature of dark energy
and the way it has governed the evolution of the largescale structure of the universe. And the minute distortions of multitudes of tiny galaxies, imperceptible to the
human eye, is one probe that will play a decisive role in
understanding today's cosmic mysteries.
And then what? "Diﬃcult to say," muses Mandelbaum. If everything is consistent with the current cosmological model, astronomers may delve deeper into the
details to better understand the nature of the universe's
main components, dark matter and dark energy.
But if the weak lensing results turn out to be incompatible with current thinking - if nothing is what it seems
- then we may be in for a cosmological revolution. ✦
Contributing Editor Govert Schilling writes about astronomy from his hometown in Amersfoort, The Netherlands.
His new book on gravitational waves will be published next
year by Harvard University Press.

Sky & Telescope - September 2016

Table of Contents for the Digital Edition of Sky & Telescope - September 2016