The early Universe was like a rapidly cooling ocean. At colossal temperatures, everything was one — quarks, electrons, forces all blended into a symmetric 'steam.' But as it expanded and cooled, this steam condensed, phase transitions cascading one after another like water turning to ice. And just as a freezing pond develops cracks and bubbles, the cooling cosmos gave rise to topological defects — stable ruptures in fields, born from symmetry breaking.

That's the picture painted by new theoretical work. The authors explored one possible scenario: the sequential breaking of gauge symmetry SU(2) → U(1) → Z₂ → 1. First, when the Universe was merely 10⁻¹⁰ seconds old, the strong force decoupled, and magnetic monopoles (particles with a single magnetic pole, predicted by Gerard 't Hooft long ago) appeared in the big bang. A little later, one-dimensional cosmic strings formed — threads of pure energy as thin as an atomic nucleus but with a mass per meter comparable to a mountain range. These strings are intimately tied to string theory. Subsequently, two-dimensional domain walls emerged, dividing the vacuum into regions with different properties.

The walls couldn't last forever: they attracted each other and collapsed, winding themselves around the strings. A network of composite strings formed, whose fate hinged on subtle parameters: the inflationary expansion proposed by Alan Guth and quantum tunneling. In one version, the strings are almost eternal; in another, they decay slowly; in a third, they are short-lived but manage to radiate gravitational waves. Using numerical simulation and the Friedmann equations, the scientists calculated the gravitational waves background, ripples in spacetime curvature predicted by Einstein's equations, that arises in each scenario.

The results are encouraging. For a dimensionless string tension Gμ between 10⁻¹² and 10⁻¹⁰, the signal peak lies in the hundreds of hertz — right where ground-based LIGO laser interferometers (which first detected gravitational waves under the leadership of Rainer Weiss) and the future Einstein Telescope operate. For heavier strings (Gμ ∼ 10⁻⁷) and a network formation delay $d_s$ ≳ 10⁷, the background shifts into the nanohertz range, accessible to pulsar timing arrays like NANOGrav. These 'cosmic clocks'—rapidly rotating pulsar neutron stars — compare their rhythm with atomic standards, and the slightest irregularities betray a passing gravitational wave. In all three scenarios, the amplitude $Ω_{GW}$ h² ranges from 10⁻¹⁰ to 10⁻⁸ — strong enough to be captured.

This kind of work isn't just a mathematical exercise. If the signal is detected, it will become direct evidence of phase transitions in the first moments after the big bang and could potentially offer a glimpse beyond the Standard Model — into the realm where Grand Unification of forces hides. Gravitational waves are turning into a kind of 'fossil light' from an era inaccessible to particle accelerators. And it's also a story of how the most exotic objects (monopoles, strings, walls) can leave behind a quiet but insistent hum, trembling in the fabric of the Universe. And for the first time, we are learning to hear it.