Cosmic Strings in a Nanocomposite: How to See the Echo of the Big Bang ✛
After the Big Bang, according to inflationary cosmology, the Universe underwent rapid expansion and a series of phase transitions. During this period, theorists believe, topological defects could have frozen into the fabric of spacetime — cosmic strings, domain walls, monopoles. Their search has been going on for decades, but direct evidence is still missing. It's amazing, however, that physics allows us to glimpse this 'moment of creation' on a laboratory bench. When a substance undergoes a symmetry-breaking phase transition (for example, turning into a superconductor or a Bose-Einstein condensate) similar defects spontaneously arise. The only difference is the scale: instead of light-years, we operate with microns. Such a transition was studied by a team of physicists using a nanocomposite based on MgB₂, a superconductor with a well-understood pairing nature, discovered by John Bardeen and his colleagues within the framework of BCS theory.
The key feature of the experiment is the cooling regime. Usually, to erase traces of non-equilibrium and see defects 'frozen' by the Kibble–Zurek mechanism, a sharp temperature quench is required. Here, however, vortices were observed with very gradual cooling — about 0.2 K/s. The reason lies in the unusual morphology of the composite: superconducting MgB₂ grains, occupying about 30% of the volume, are surrounded by a dielectric but electrically conductive (due to oxygen vacancies) MgO matrix. The grains are connected by Josephson junctions, so that macroscopic phase coherence emerges, but before it settles, each domain already possesses its own random phase of the order parameter. When these 'islands' join into a single network, the phase cannot completely equalize, generating vortex loops with a huge, up to 10 µm, field penetration depth. Scanning interferometric SQUID microscopy, acting as a supersensitive sensor for quantum measurements of magnetic flux, allowed for the first time seeing these structures directly. Numerical simulation of current distributions confirmed that the vortices literally trace the tortuous fractal geometry of channels carved by supercurrent among the grains.
This work is not just a beautiful image, but a breakthrough in understanding phase transitions in disordered media. It transforms a piece of superconducting nanocomposite into a programmable simulator of the early Universe: by changing the cooling rate or grain morphology, one can investigate how topological defects are born and evolve in conditions far from perfect homogeneity. The near-term prospect is using SQUID-on-tip with nanometer resolution to examine a spontaneous vortex in minute detail and directly extract the phase correlation length. This will not only test the predictions of the Kibble–Zurek theory in a new, 'slow' limit, but also potentially find approaches to control superconducting properties through the controlled creation of vortex structures. Ultimately, we are getting closer to reading the earliest imprints of symmetry breaking — the frozen magnetic echoes of the inflationary epoch, preserved in a piece of ceramic at liquid helium temperatures.
One flux quantum Φ₀ is so small that to create a magnetic field of 1 tesla (like a good magnet) over an area of 1 mm², you'd need to pack about 50 million of these quanta. A SQUID microscope can detect a field a billion times weaker!
The idea of cosmic strings, gigantic wrinkles in spacetime left over from the Big Bang, has long migrated from theoretical physics into science fiction. In the novel 'Cosmic Strings' and the 'Star Trek' series, they serve as a source of unimaginable energy and even portals to other universes. Experiments with nanocomposites show that such defects can be studied without leaving the lab.