Pulsars period and any changes thereof. Pulsars are thought

Pulsars
— one observational
manifestation of neutron stars, the
ultra-
dense remnants of certain supernova
explosions — emit a
beam of electromagnetic
radiation along their magnetic axes. If the
beam sweeps across the point of view as
seen from the Earth, we
observe a pulse of
radiation; this enables the highly accurate
measurement of the star’s spin period and
any changes thereof.

Pulsars are thought to
spin rapidly at birth, on average much more
than once per second. They subsequently
spin down because of the
torque exerted
on them by their colossal magnetic fields.

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But
observations of young to middle-aged
(thousand- to million-year-old)
X-ray
pulsars, with periods of up to about 10 s and
relatively high
surface magnetic fields (10 13 G
and above), pose a problem. Such
pulsars
should spin down rapidly to periods of up to
100 s, still
detectable in the X-ray band, on
timescales of around 10,000 years.

But we see
no such population of middle-aged, long period pulsars
with high magnetic fields:
where are they?

It
had been presumed that the problem
could be solved were the magnetic
field to
decay on timescales of 10 6 years, thus limiting
the
efficacy of the field in increasing the spin
period of the pulsar.

The contribution of
Jose Pons and co-workers 1 in Nature Physics
details the first consistent simulations of the
magnetic, thermal
and rotational evolution
of pulsars to test this scenario. Pons et
al.

find that fast enough magnetic field decay
occurs only if a
deep layer of the neutron star
crust consists of a highly disordered
lattice
and therefore possesses a high electrical
resistivity. This
places intriguing constraints
on the nuclear and condensed-matter
physics
of the neutron star crust and hints at the
existence of a
long-hypothesized set of exotic
geometries of nuclei known as
nuclear ‘pasta’.

Neutron
stars are remnants of supernova
explosions, resulting from the core
collapse
of stars in the region of 8–25 solar masses.

Containing
between one and two solar
masses of material compressed into a ball
20 km across, they are among the most exotic
objects in the
Universe. Discovered originally
as radio pulsars, they now are
manifest in a
large variety of astronomical systems and are
visible
in every band of the electromagnetic
spectrum 2 . Using a wealth of
observations,
astrophysicists are able to use neutron stars
to
probe gravitational, nuclear and high-

energy
particle physics in extreme conditions. Neutron stars are also strong
candidates
for the first sources that might be directly
detected by
their gravitational-wave emission.

Although
the name suggests a giant ball
of neutrons, theory predicts neutron
stars to
have quite a complex structure, as depicted
in Fig. 1a.

Their observed dipolar magnetic
fields are anchored in the solid
outer
crust. Laboratory knowledge of nuclear
physics constrains the
properties of the
outer part of the crust reasonably well, but
matter in the inner crust (Fig. 1b) exists at
pressures far beyond
those that can be stably
reproduced in terrestrial experiments, and
presents a fascinating theoretical problem
in condensed-matter and
nuclear physics.

Furthermore, it has been predicted that
in the
deepest layers of the inner crust,
matter becomes frustrated and can
undergo
a series of transitions between different
geometries of
nuclear structures. Noting the
resemblance of some of these
geometries to
spaghetti and lasagne, the community has
coined the
term ‘nuclear pasta’ to collectively
refer to them. Such phases
of matter are
analogous to those observed in terrestrial
soft-condensed-matter systems such as
surfactants and
microemulsions.

Whether
the inner crust is composed of a
relatively pure crystal made
predominantly
from one species of nucleus at a given density
with
few impurities (a homogeneous, ordered
lattice) or has a wide
distribution of nuclear
species arranged irregularly (a
heterogeneous,
disordered lattice) has an important bearing
on the
electrical and thermal conductivity of
the lattice. The theorized
nuclear pasta phases  

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