  Nuclear matter is considered to be inhomogeneous at subnuclear densities that
are realized in supernova cores and neutron star crusts, and the structures of
nuclear matter change from spheres to cylinders, slabs, cylindrical holes and
spherical holes as the density increases. In this letter, we discuss other
possible structures, that is, gyroid and double-diamond morphologies, which are
periodic bicontinuous structures discovered in a block copolymer. Utilizing the
compressible liquid drop model, we evaluate their surface and Coulomb energies
and show that there is a chance of gyroid appearance near the transition point
from a cylinder to a slab. This interesting analogy between nuclear and polymer
systems is not merely qualitative. The volume fraction at the phase transition
is also similar for the two systems. Although the five shapes listed initially
have been long thought to be the only major constituents of so-called nuclear
pasta at subnuclear densities, our findings imply that this may not be the case
and suggest that more detailed studies on nuclear pasta including the gyroid
phase are needed.
