Detecting Massive Scalar Fields with Extreme Mass-Ratio Inspirals.

We study the imprint of light scalar fields on gravitational waves from extreme mass-ratio inspirals-binary systems with a very large mass asymmetry. We first show that, to leading order in the mass ratio, any effects of the scalar on the waveform are captured fully by two parameters: the mass of the scalar and the scalar charge of the secondary compact object. We then use this theory-agnostic framework to show that the future observations by LISA will be able to simultaneously measure both of these parameters with enough accuracy to detect ultralight scalars.

Detecting Subsolar-Mass Primordial Black Holes in Extreme Mass-Ratio Inspirals with LISA and Einstein Telescope.

Primordial black holes possibly formed in the early Universe could provide a significant fraction of the dark matter and would be unique probes of inflation. A smoking gun for their discovery would be the detection of a subsolar mass compact object. We argue that extreme mass-ratio inspirals will be ideal to search for subsolar-mass black holes not only with LISA but also with third-generation ground-based detectors such as Cosmic Explorer and the Einstein Telescope. These sources can provide unparalleled measurements of the mass of the secondary object at a subpercent level for primordial black holes as light as O(0.01) M_{⊙} up to luminosity distances around hundred megaparsec and few gigaparsec for LISA and Einstein Telescope, respectively, in a complementary frequency range. This would allow claiming, with very high statistical confidence, the detection of a subsolar-mass black hole, which would also provide a novel (and currently undetectable) family of sources for third-generation detectors.