The Universe was born in the fire of the big bang and began to cool almost immediately. This process resembles the freezing of water: just as cracks appear in ice, so too did ruptures in fields, stable defects, emerge in the cooling cosmos. Among them are one-dimensional cosmic strings (threads of pure energy), two-dimensional domain walls (boundaries between different “varieties” of vacuum), and magnetic monopoles — particles with a single magnetic pole, whose existence was proposed by Gerard 't Hooft.

The walls were unstable. They attracted each other, collapsed, and wound around the strings, forming composite objects. Their fate depended on how much the Universe inflated in the first moments (an idea put forward by Alan Guth). In some scenarios the strings are nearly eternal, in others they slowly fade away, but they always have time for the main thing: they shake spacetime, giving birth to gravitational waves. Using computer simulations, scientists calculated exactly what “sound”—or rather, what pattern of waves, such events leave behind.

The calculations showed that the frequency of these waves matches the capabilities of modern detectors. If the string tension is low, the signal falls within the range of ground-based laser facilities like LIGO (one of whose creators was Rainer Weiss). If the strings are heavier, the waves become lower in frequency, and they may be spotted by pulsars — rapidly spinning stars that act as ultra-precise cosmic clocks. As the universe expands, the wave frequency could shift, much like the reddening of light from distant galaxies. Catching such a signal is like hearing the echo of the world's birth. From the “frost pattern” of gravitational waves, we can reconstruct how the young Universe froze.