A giant drum has just taken a hit. Its membrane is still trembling, but besides the main boom, a fine ripple spreads across it — someone ran a finger over the taut skin. In much the same way, in the first moments after the Big Bang, spacetime vibrated not only with the "roar" of expansion but also with the tension of suddenly emerging magnetic fields.
In the new work, physicists considered a scenario where the strength of the magnetic interaction was not constant but changed as the universe cooled. Such a change acts like an alternating current in a speaker: it makes magnetic fields oscillate and emit gravitational waves: that very ripple in the geometry of the world. At the same time, the same processes cause compressions and rarefactions of primordial matter, which also become a source of waves. Comparing the two mechanisms showed that the magnetic "string" sounds louder: it is responsible for the highest peak of the signal, while matter adds a low hum on large scales.
Interestingly, for certain conditions, this signal naturally falls into the nanohertz range — the frequencies at which pulsar timing arrays operate. Today astronomers are already searching for such an ultra-low-frequency hum, and future data could not only confirm its existence but also, from the shape of the spectrum, distinguish the magnetic contribution from that of matter inhomogeneities. This would offer a rare opportunity to peer into an epoch when the universe was less than a second old and to understand how it heated up after its rapid inflation.
Pulsars are rotating neutron stars that emit radio pulses with the precision of atomic clocks. The slightest delay in their signals can reveal a passing gravitational wave with a wavelength of light-years.
In Carl Sagan's novel "Contact," an alien signal is disguised as a pulsar, but in reality, it is precisely the glitches in pulsar rhythm that might bring a message from the dawn of time.