  Hyperbolic metamaterials were originally introduced to overcome the
diffraction limit of optical imaging. Soon thereafter it was realized that they
demonstrate a number of novel phenomena resulting from the broadband singular
behavior of their density of photonic states. These novel phenomena and
applications include microscopy, stealth technologies, enhanced
quantum-electrodynamic effects, thermal hyperconductivity, superconductivity,
and interesting gravitation theory analogues. Here we describe the behaviour of
cobalt nanoparticle-based ferrofluid in the presence of an external magnetic
field, and demonstrate that it forms a self-assembled hyperbolic metamaterial,
which may be described as an effective 3D Minkowski spacetime for extraordinary
photons. Moreover, such photons perceive thermal gradients in the ferrofluid as
analogue of gravitational field, which obeys the Newton law. If the magnetic
field is not strong enough, the effective Minkowski spacetime gradually melts
under the influence of thermal fluctuations. On the other hand, it may restore
itself if the magnetic field is increased back to its original value. Direct
microscopic visualization of such a Minkowski spacetime melting/crystallization
is presented, which is somewhat similar to hypothesized formation of the
Minkowski spacetime in loop quantum cosmology and may mimic various
cosmological Big Bang scenarios.
