Sky & Telescope - September 2016 - 35

Strong Prospects for

WEAK
LENSING

Astronomers are mapping tiny
distortions in the images of distant galaxies to study the invisible
- dark matter and dark energy.

Govert Schilling

Nothing is what it seems.
That's often the case in daily life. But it's also true in the
universe at large. Take a long-exposure photo of remote
galaxies, and you'll ﬁnd that none of them are what they
appear to be.
First of all, we see faraway galaxies as much younger
than they really are right now - their light took billions of years to reach us. We also see them as much
redder than they used to be. That's because, during the
long trip to Earth, the universe's expansion stretched
the galaxies' emitted light to longer wavelengths. It's as
though we look at an old greybeard and see a red-haired
schoolboy instead.
But that's not all. Even the observed shape of a distant
galaxy isn't real: it doesn't reﬂect the galaxy's true physical appearance. During the billions-year-long journey
the galaxy's light takes through the fabric of spacetime,
its path bends ever so slightly under the gravitational
inﬂuence of intervening matter. The result: a minute
distortion of the galaxy's shape, as if astronomers were
looking at the universe through a glass of water. In space
that eﬀect is known as gravitational lensing. And it has
become a powerful tool for cosmologists.
MORE THAN MEETS THE EYE A massive foreground cluster
rich with galaxies (and their invisible dark matter halos) smears
the light from bluer background galaxies into arcs. This eﬀect,
known as strong gravitational lensing, is obvious. The subtler
eﬀect of weak lensing is present too, but it's invisible to the eye.
Only computer algorithms can tease out the slight stretch in the
shape of background galaxies lying at the cluster's edge.
NASA / ESA / M. J. JEE & H. FORD (JOHNS HOPKINS UNIVERSITY)

Anything that exerts a gravitational force - not
just galaxies and clusters, but also the enigmatic dark
matter that makes up more than 80% of all mass in the
universe - can cause gravitational lensing. And therein
lies the technique's power. Astronomers know almost
nothing about invisible dark matter, but by measuring
the distortions it generates, they can outline the stuﬀ 's
cosmic distribution. These observations can then be
compared against cosmological simulations to better
understand the properties of dark matter.
Comprehensive lensing surveys might even shed
light on dark energy - the equally perplexing cosmic
constituent that's responsible for the present acceleration
of the expansion of the universe.

A Brief History of Lensing
Albert Einstein ﬁrst thought about gravity's ability to
bend light paths in 1912, three years before he penned
his general theory of relativity. Relativity's predictions
were famously conﬁrmed during the 1919 total solar
eclipse (S&T: Dec. 2015, p. 18). But it wasn't until 1936
that Einstein wrote a detailed paper in Science describing what we now call strong gravitational lensing. As he
explained, the light from a distant source can split into
multiple images or even bend into a ring when it passes
by another massive object on its way to us.
Einstein, who calculated the eﬀect for individual
stars, was convinced that not only would such a chance
alignment be rare, but it would also be far too small to
observe. But never say never in science. Although the
eﬀect of a chance alignment between two stars would
indeed be negligible, such alignments are far more likely
for galaxies, and the observable eﬀect is much larger,
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Sky & Telescope - September 2016

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