Sky and Telescope - December 2015 - 24

1OO Years of General Relativity: Einstein's Coming Golden Age

Next Step 3: Cosmic Structure

ALL-SEEING EYE The 8.4-meter Large Synoptic Survey Telescope (LSST) will be the most powerful survey telescope ever
built. Among other things, it will map billions of galaxies back
to the ﬁrst era of galaxy formation, helping to determine the
large-scale structure of spacetime throughout cosmic history.
Will relativity hold up or need a rewrite?

neutron stars or black holes orbit each other tightly at a
fair fraction of the speed of light. We know gravitational
waves are real because we see their indirect consequences. For the last 40 years we have been monitoring a particular pair of neutron stars, PSR B1913+16,
discovered by Russell Hulse and Joseph Taylor (for
which they won the 1993 Nobel Prize in Physics). Radio
astronomers continue to track the slow, inexorable decay
of the neutron stars' mutual orbit as gravitational waves
carry orbital energy away - at exactly the predicted rate.
This is compelling evidence, but, as with black holes, we
haven't yet seen gravitational waves directly.
That's also about to change. As described in the
following article, a consortium of gravitational wave
detectors around the world is virtually certain to see
the powerful bursts of ripples ﬂung out when pairs of
neutron stars as distant as 500 million light-years from
us ﬁnally spiral into each other and merge. Even more
exciting is the possibility that we will pick up the ﬁnal
inspirals of pairs of black holes. In this case the most
outlandish predictions of general relativity, black holes
and gravitational waves, would show us their most
extreme behavior combined. The signal from such an
event can be precisely modeled from GR theory. Will it
match what we see?
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December 2015 sky & telescope

LSST

L SS

Astronomers are also embarking on an ambitious
campaign to map out, over the next two decades, the 3D
positions of essentially all the large observable galaxies
within our cosmic horizon - and thereby reconstruct,
with unprecedented precision, the largest-scale structure of spacetime itself. By accurately understanding
how galaxies are laid out to form the cosmic web, and
how spacetime warps itself around the galaxy superclusters, ﬁlaments, and walls, we will try to understand
why, among other things, the expansion of the universe
started to accelerate about 7 billion years ago.
Earlier than that, the expansion of the universe was
slowing due to the gravitational pull of all matter on all
other matter. But as space expanded and matter thinned
out, some repulsive eﬀect, discovered only in 1998,
began to predominate over gravity. Is some kind of "dark
energy" doing the pushing? Or does GR itself need to be
reformulated or even discarded on cosmological scales?
For now, the leading opinion is that GR remains valid
but a weak repulsive force is structured into the fabric
of empty space itself. That's because the force seems to
work like Einstein's long-abandoned "cosmological constant," which he named Lambda (Λ). From what we can
tell so far, this mysterious space-pressure has remained
constant throughout cosmic history. That is, despite
space expanding, a cubic centimeter of space always contains the same amount of it. This is in contrast to things
that exist in space, such as particles and ﬁelds, which
thin out as space expands. But is that the true picture?
The Euclid satellite, supported by the European Space
Agency, should launch at the end of this decade. Euclid
will map large swaths of the universe, in particular galaxies as they existed when the universe was close to half
its current age (around redshift 0.8 ). That's about when
the cosmic expansion started speeding up. We'll get a
good read on how gravity worked at that time.
Euclid will harness another of GR's great predictions:
gravitational lensing. Gravitational lensing is the bending of light rays passing a massive object; it's the eﬀect
Eddington's expedition tested with the 1919 solar eclipse.

LOOKING AHEAD An architect's drawing (with a Hubble
Deep Field backdrop) of the ﬁnished LSST facility.

Sky and Telescope - December 2015

Table of Contents for the Digital Edition of Sky and Telescope - December 2015