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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.

Experimental Observation of Melting of the Effective Minkowski Spacetime in Cobalt-Based Ferrofluids