Sky & Telescope - September 2016 - 38

Covert Cosmology
a cluster, but by the lightpath-bending power of a single,
massive foreground galaxy.
"Galaxy weak lensing tells us about the dark matter halos surrounding visible galaxies," Mandelbaum
explains. Current models of galaxy evolution predict that
huge dark matter halos surround large galaxies like our
own Milky Way. And the Atacama Cosmology Telescope
(ACT) conducted a recent weak lensing study involving
the cosmic microwave background radiation (the Big
Bang's afterglow) that supports this scenario.
The ACT is surveying microwaves emitted 370,000
years after the Big Bang. Intervening galaxies, imaged
by the Sloan Digital Sky Survey, smear this light. These
sharp, small-scale changes in the background radiation
intensity reveal massive dark matter halos around individual galaxies. So far, these measurements match what's
expected from popular cosmological simulations, where
galaxies begin their lives as concentrations of dark matter. Eventually, with more data, the team of astronomers
hopes to trace the growth of dark matter halos over time.
The third and subtlest form of weak lensing, called
cosmic shear, is caused when background light passes
through the uneven distribution of matter across space,
as Gunn predicted in 1967. The universe's large-scale
structure - the clusters, superclusters, and great walls
of galaxies, as well as the vast, largely empty voids -
exerts a quiet inﬂuence on the trajectory of every single
light ray passing through the depths of space. It's like an
uneven ﬂoor that causes a marble to roll along a wigSUBTLE STRETCH While the eﬀect of weak lensing is impossible to measure for an individual galaxy, it can be teased out of
larger samples. As the photons from background galaxies skirt
through the outer edges of a cluster's mass, the galaxy shapes
are stretched ever so slightly along a circle around the lensing
mass. The subtle eﬀect is magniﬁed here to be clearly visible.

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September 2016 sky & telescope

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gling path instead of a straight line. By the time a distant
galaxy's light arrives at our telescope, even if there's no
galaxy or cluster directly in front of it, its shape is nonetheless slightly distorted.
First observed by no less than four separate teams of
astronomers in 2000, cosmic shear sheds light on the
growth of large-scale structure. It could also teach us
about dark energy's role in cosmic evolution.
Right now, no one knows what dark energy is. The
most popular scenario suggests that dark energy is a
constant property of empty space. But other scenarios
leave room for dark energy to ﬂuctuate over time. By
probing the universe's large-scale structure, whose form
depends on dark energy's properties, studies of cosmic
shear can test these alternate scenarios.
For these studies to work, Hoekstra explains, astronomers must study many millions of background galaxies
over large swaths of sky. A technique known as cosmic
tomography - comparable to a 3D MRI scan of all the
mass in the universe - then enables astronomers to
examine the cosmic web at diﬀerent distances, corresponding to diﬀerent look-back times. The process is a
bit like reconstructing the evolution of life on Earth by
studying fossil evidence from various epochs.

Wide and Deep
Not that studying the universe's fossil record makes for
an easy task. Lensing maps require photographic sky
surveys that are both very wide and very deep - sensitive to faint sources across large portions of the celestial
sphere. Observations must also be conducted at multiple
wavelengths to enable measurements of photometric
redshifts, which provide distance estimates to faint
galaxies based on their brightness levels at diﬀerent
wavelengths (see box on page 40).
Then observations need to be corrected for all kinds
of artiﬁcial eﬀects that might mimic weak lensing,
such as systematic distortions introduced by the telescope and detector, as well as the eﬀects of atmospheric
turbulence. Resolution is also an issue: if two galaxies lie
close together on the sky, they might blend into a single
elongated smudge of light.
Finally, there's another very real eﬀect called intrinsic
alignment. The collective formation history of a group of
galaxies can align galaxies in a non-random way that has
nothing to do with cosmic shear. According to Hoekstra,
"this is one of the biggest problems in the ﬁeld." One
straightforward way to mitigate this issue is to carry out
extremely large surveys, to improve the statistics.
That's exactly what astronomers are working on right
now, using wide-ﬁeld telescopes, sensitive detectors, and
powerful data-analysis software. Some of the ﬁrst weak
lensing surveys have already been carried out as part
of the Sloan Digital Sky Survey, based in New Mexico,
as well as by the 3.6-meter Canada-France-Hawaii

Sky & Telescope - September 2016

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