Rodrigo Vicente, Theophanes K. Karydas and Gianfranco Bertone from the UvA Institute of Physics and the GRAPPA centre of excellence for Gravitation and Astroparticle Physics Amsterdam report their results in Physical Review Letters. In the paper they present an improved way to calculate how dark matter near black holes affects orbital motion and the corresponding gravitational waves.
The study concentrates on extreme mass-ratio inspirals, or EMRIs, where a relatively small compact object such as a stellar-mass black hole orbits and gradually spirals into a much more massive black hole typically located at a galactic centre. As the smaller object moves inward it emits a long gravitational-wave signal that can span hundreds of thousands to millions of orbits.
Upcoming space missions including the European Space Agency's LISA antenna, scheduled for launch in 2035, are expected to capture EMRI signals for months to years, resolving very large numbers of orbital cycles. With accurate modelling these signals can act as fingerprints that map how matter, including dark matter thought to make up most of the Universe's matter content, is distributed around massive black holes.
Ahead of such observations, theorists must predict in detail what types of waveforms are expected and how to extract information from them. Earlier work generally used simplified descriptions of the EMRI environment and often treated gravity with Newtonian approximations.
The new study closes this gap for a broad class of environments by providing a fully relativistic framework based on Einstein's theory of gravity. It describes how the surroundings of a massive black hole modify an EMRI's orbit and leave distinctive signatures in the gravitational waves.
The authors pay particular attention to dense dark-matter concentrations, often termed spikes or mounds, that may develop around massive black holes. By embedding their relativistic description into state-of-the-art waveform models, they show that these dark-matter structures would leave measurable imprints in signals recorded by future detectors.
This modelling forms a key step in using gravitational waves to chart dark matter in galactic nuclei. It contributes to a broader programme that seeks to use gravitational-wave astronomy to probe the distribution and fundamental nature of dark matter in the Universe.
Research Report:Fully Relativistic Treatment of Extreme Mass-Ratio Inspirals in Collisionless Environments
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