Gravitational Wave Cosmology and Primordial Black Holes
Summary
Gravitational wave cosmology has emerged as a transformative approach to probing the early universe, supplementing traditional electromagnetic observations with ripples in spacetime generated by accelerating masses. Beyond black-hole and neutron-star mergers, a stochastic background of gravitational waves may carry imprints of primordial processes, including inflationary dynamics and phase transitions. Primordial black holes (PBHs), hypothetical compact objects formed from the collapse of extreme density fluctuations in the infant universe, occupy a central role in this endeavour. Their formation channels link to enhanced curvature perturbations, which themselves source secondary (scalar-induced) gravitational waves. By characterising the spectrum, amplitude and anisotropies of this background, cosmologists aim to constrain the primordial curvature power spectrum on scales inaccessible to the cosmic microwave background and large-scale structure surveys. Moreover, if PBHs constitute a non-negligible fraction of dark matter, their mass distribution and merger rate can leave distinctive signatures in both direct-detection observatories and pulsar timing arrays. Thus, gravitational waves offer a unique window onto the origin of structure, the nature of dark matter and the physics of the very early universe, uniting theoretical modelling, numerical simulation and novel observational strategies.
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Gravitational Wave Cosmology and Primordial Black Holes publication trend
The graph below shows the total number of articles in gravitational wave cosmology and primordial black holes across all publications each year (not limited to Nature Index journals).
Technical terms
Gravitational wave: A propagating perturbation in the fabric of spacetime generated by accelerating masses, observable as minute oscillations in distance between test masses.
Primordial black hole (PBH): A black hole formed in the early universe from the direct collapse of enhanced density fluctuations, potentially contributing to dark matter.
Stochastic gravitational-wave background: A superposition of unresolved gravitational-wave signals of varied origin, producing a persistent, broadband noise spectrum.
Scalar-induced gravitational waves: Tensor perturbations generated at second order by nonlinear interactions of primordial scalar (density) perturbations.
Curvature perturbation: A spatial fluctuation in the curvature of spacetime during inflation, which seeds the formation of large-scale structure and can collapse to form PBHs.
References
- Cosmological gravitational waves from isocurvature fluctuations. AAPPS Bulletin (2024).
- Constraints on primordial curvature spectrum from primordial black holes and scalar-induced gravitational waves. European Physical Journal C (2023).
- Implications of pulsar timing array data for scalar-induced gravitational waves and primordial black holes: Primordial non-Gaussianity fNL considered. Physical Review Research (2024).
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