### David Nichols

*Ph.D., 2012, California Institute of Technology *

*Assistant Professor*

*Theoretical Astrophysics, Gravity and Cosmology*

#### Research Interests

Gravitational waves from the mergers of several stellar-mass black-hole and neutron-star binaries have now been detected by the LIGO and Virgo collaborations. These observations have opened a new method for observing the Universe. With them, and the many detections that are expected to follow in the upcoming years, one can now study the properties of strongly gravitating objects and rapidly changing space and time. Determining the predictions of Einstein's theory of relativity for these systems and understanding how these predictions can be extracted from gravitational-wave measurements are the main areas of my research. In the past few years, I have focused in three more specific directions connected to this broader goal, which I list below:

- I am interested in a class of gravitational-wave effects that persist even after a burst of gravitational waves have passed. Called "gravitational-wave memory effects," these phenomena are closely connected to the symmetries and conserved quantities of spacetimes that become flat asymptotically. I have worked on understanding these effects, computing their magnitudes, and forecasting when they could be measured by gravitational-wave detectors.
- I have also been investigating when the presence of baryonic or dark matter around intermediate-mass or supermassive black holes can influence the inspiral of a second small compact object (a so-called intermediate- or extreme-mass-ratio inspiral). My collaborators and I have found scenarios in which these environmental effects can be significant.
- Lastly, I am interested in combining information from gravitational-wave mergers of neutron stars with the complementary information from their associated electromagnetic counterparts. I have focused primarily on the radio and x-ray observations of short-gamma-ray-burst afterglows and kilonova afterglows, although I am interested more generally in counterparts of any wavelength.

My webpage contains more information about my past and current research.

#### Selected Publications

- E. E. Flanagan, A. M. Grant, A. I. Harte, D. A. Nichols. “Persistent gravitational-wave observables: General framework.” Phys. Rev. D 99, 084004 (2019), arXiv:1901.00021.
- T. Hinderer, et al. “Discerning the binary neutron star or neutron star-black hole nature of GW170817 with Gravitational Wave and Electromagnetic Measurements.” arXiv:1808.03836.
- D. A. Nichols. “Center-of-mass angular momentum and memory effect in asymptotically flat spacetimes.” Phys. Rev. D 98, 064032 (2018), arXiv:1807.08767.
- D. A. Nichols. “Spin memory effect for compact binaries in the post-Newtonian approximation.” Phys. Rev. D 95, 084048 (2017), arxiv:1702.03300.
- E. E. Flanagan and D. A. Nichols. “Conserved charges of the extended Bondi-Metzner-Sachs algebra.” Phys. Rev. D 95, 044002 (2017), arxiv:1510.03386.