Sky and Telescope - October 2018 - 9

COSMOLOGY

Best Long-range Test of
General Relativity

Pulsar Test Limits Existence
of "Fifth Force"

ASTRONOMERS HAVE CONDUCTED

SCIENTISTS RECENTLY STUDIED a
pulsar binary system to constrain the
existence of a hypothetical ﬁfth fundamental force of nature.
We already know about four fundamental forces: gravity, electromagnetism, and the strong and weak nuclear
forces. Some scientists attempting to
explain anomalous experimental results
have speculated about the existence of
a ﬁfth force of nature, one that could
work on dark matter.
Relativity predicts that normal
matter should fall freely toward dark
matter. But a ﬁfth force that has the
ability to interact
with both normal
and dark matter
could strengthen
or diminish dark
matter's gravitaArtist's concept of
tional pull. Lijing
pulsar-white dwarf pair
Shao (Max Planck
Institute for Radio
Astronomy, Germany) and colleagues
tested for this effect using the binary
system PSR J1713+0747.
This pulsar and its white dwarf companion, which are in a relatively wide
68-day orbit, lie 3,800 light-years from
Earth in the direction of the galactic
center. The pulsar, whose atoms have
been compacted into neutrons, is so
dense that its extreme gravitational ﬁeld
could enhance any possible interactions
with dark matter. The white dwarf isn't
nearly so compact. If a ﬁfth force did
exist, the Milky Way's dark matter halo,
whose density peaks in the galactic
center, would pull on the neutron star
and the white dwarf in different ways,
slightly altering their mutual orbit.
Drawing from more than 20 years
of radio observations of this system,
the researchers conclude that if a ﬁfth
force does exist, it must have less than
1% of gravity's strength - and gravity is
already the weakest of the four known
forces. The results appear in the June
15th Physical Review Letters.

ORBITS OF JUPITER M OONS: CA R NEGIE INSTIT U TION FOR SCIENCE / ROBERTO
M OL A R CA NDA NOSA; ENCEL A DUS: N ASA / JPL; EINSTEIN RING: ESO / ESA /
HUBBLE / N ASA; PULSA R: ESO / L. CA LÇA DA

the best galaxy-scale test of general relativity yet, and it rules out some (but not
all) theories of modiﬁed gravity. These
theories provide the main alternative to
the existence of dark matter.
General relativity - which describes
gravity as the curvature that mass
induces on spacetime - has passed
extensive tests within the conﬁnes of
our solar system. But fewer tests exist
on scales of thousands or millions of
light-years. On larger scales, theories
of modiﬁed gravity predict that gravity
behaves differently than it does in our
solar system. While astronomers have
conducted some tests on galactic scales,
none of them has put strong limits on
modiﬁed gravity.
Now, a study led by Thomas Collett
(University of Portsmouth, UK) in the
June 22nd Science has provided such a
test. The team investigated Hubble Space
Telescope and Very Large Telescope
observations of a so-called Einstein ring,
where a nearby galaxy's gravity has bent
light from a distant star-forming galaxy
into a blue circle around itself.
Collett and colleagues ﬁrst calculated the mass of the intervening
galaxy by measuring the movements
of stars within it. Then they measured
the mass the intervening galaxy would

saturated" with hydrogen and rich in
other atoms, such as oxygen or nitrogen.
Comets contain lots of organic material and likely contributed to Enceladus's
makeup, but study coauthor Christopher
Glein (Southwest Research Institute,
San Antonio) says those small bodies
exhibit a greater diversity and abundance of prebiotic compounds than
what's seen at Enceladus. One possible
explanation is that hydrothermal processes in the moon's interior drastically
altered the primordial inventory, so only
the most resilient molecules remain.
Or perhaps life arose in the ocean
of Enceladus and has overwritten any

p The nearby ESO 325-G004 (large elliptical
galaxy in foreground) acts as a cosmic lens to
distort the light from a more distant galaxy (inset). The so-called Einstein ring becomes visible
after subtracting the lensing galaxy's light.

have to have in order to bend the
background galaxy's light into a ring.
The mass inferred by spacetime curvature matches the mass as measured by
the stars' motions - exactly as general
relativity predicts.
Unlike other lensing tests of relativity, Collett's team relied less on assumptions about the nature of the intervening galaxy. So this test is relatively free
of systematic uncertainties that have
plagued previous studies, says Lucas
Lombriser (University of Geneva), who
was not involved in the study. He calls
the measurements "the most robust
test of gravity of this type and on these
length scales to date."
This study shows that gravity, as
described by relativity, behaves as
expected on scales less than 6,500 lightyears. Additional tests will be needed to
test modiﬁed gravity more generally.
■ MONICA YOUNG

organic signatures of its cometary
heritage. A group led by Ruth-Sophie
Taubner (University of Vienna, Austria)
recently identiﬁed a strain of bacteria
that can survive in Enceladus-like conditions by feasting on methane.
Still, Glein cautions that the default
hypothesis is that Enceladus lacks any
kind of life. With Cassini gone and no
ﬁrm plans to send another spacecraft
to Saturn's system, planetary scientists
might wait decades to ﬁnd out whether
Enceladus harbors its own biology.
■ ELIZABETH HOWELL

* Visit https://is.gd/enceladusorganics
for more information.

■ ELIZABETH HOWELL
sk yandtelescope.com * OCTOBE R 2 018

https://www.is.gd/enceladusorganics http://www.skyandtelescope.com

Sky and Telescope - October 2018

Table of Contents for the Digital Edition of Sky and Telescope - October 2018