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The Scale of Interstellar and Cosmological Waves

A petametre (Pm) equals 1,000 terametres (10¹⁵ metres), representing unimaginably vast distances that describe the longest electromagnetic wavelengths in the universe. These wavelengths correspond to frequencies in the attohertz (10⁻¹⁸ Hz) and lower ranges, which are mostly relevant in cosmology, astrophysics, and the study of gravitational waves and large-scale cosmic phenomena.

For context, a frequency of 1 attohertz (10⁻¹⁸ Hz) corresponds to a wavelength of approximately 300 petametres. This scale is far beyond any human-made signals and reflects waves that stretch across entire galaxies or even clusters of galaxies. Such waves help scientists study the cosmic microwave background (CMB) fluctuations, the large-scale structure of the universe, and primordial gravitational waves created shortly after the Big Bang.

Using petametres to measure wavelength allows researchers to grasp the vastness of these cosmic oscillations and the slowest processes influencing the universe’s evolution. These extreme wavelengths provide crucial insight into the origins, expansion, and ultimate fate of the cosmos.

The Realm of Extremely Slow Oscillations

The nanohertz (nHz) is a unit of frequency equal to 10⁻⁹ hertz, meaning one cycle occurs every 1 billion seconds—about 31.7 years. This incredibly low frequency is important in fields like astrophysics, cosmology, and geophysics, where slow periodic phenomena unfold over decades to centuries.

Nanohertz frequencies are often associated with gravitational waves produced by supermassive black hole binaries orbiting each other over many years. These ultra-low-frequency waves have immense wavelengths, spanning light-years across space. Pulsar timing arrays, which monitor the precise arrival times of pulsar signals, are used to detect such nanohertz gravitational waves, offering insights into galaxy evolution and cosmic structure.

On Earth, nanohertz frequencies can describe long-term oscillations in the geomagnetic field or climate cycles. Studying these slow frequencies helps scientists understand gradual changes in planetary environments and the universe.

Although nanohertz waves are far below everyday human perception and technological applications, they are critical for unraveling the universe's slowest dynamics. Using nanohertz as a unit helps researchers quantify and analyze these grand-scale processes, linking time scales from decades to cosmic evolution.