The Pennsylvania State University The Graduate School PROBING THE DARK UNIVERSE WITH GRAVITATIONAL WAVES FROM SUBSOLAR MASS COMPACT OBJECTS A Dissertation in Physics by Ryan Magee © 2021 Ryan Magee Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy May 2021 The dissertation of Ryan Magee was reviewed and approved by the following: Chad Hanna Associate Professor of Physics & Astronomy and Astrophysics Dissertation Advisor Chair of Committee Yuexing Li Associate Professor of Astronomy B.S. Sathyaprakash Elsbach Professor of Physics & Professor of Astronomy and Astrophysics Sarah Shandera Associate Professor of Physics Richard Robinett Professor of Physics Associate Head for Undergraduate and Graduate Studies ii Abstract The detection of gravitational waves by Advanced LIGO in 2015 marked the start of a new era in astrophysics. These small ripples in space-time - first predicted in the early 20th century by Albert Einstein - encode properties of the progenitor system and provide a powerful new way to probe distant and extreme astrophysical environments. My dissertation focuses on contributions I have made in facilitating the multi-messenger detection of electromagnetically bright sources and using LIGO’s observations (or lack thereof) to constrain models of the dark matter. I describe the motivation for Advanced LIGO searches for sub-solar mass ultracompact binaries, as well as two recent searches I carried out with the LIGO-Virgo Scientific Collaboration. No confident detections were made in these searches, but the null result allowed us to place the tightest contraint to date on a particular model of the dark matter. I also discuss my contributions to efforts to detect binary neutron stars. Although the first BNS detection, GW170817, was a model multi-messenger discovery, there remains much to be learned about the extreme environment of the coalescence that can only be resolved by additional, prompt observations. I describe a subthreshold search for BNS that aims to increase our catalog of joint discoveries by facilitating searches for temporal or spatial coincidence, as well as recent attempts to detect BNS prior to merger to enable prompt electromagnetic followup. iii Table of Contents List of Figures vii List of Tables xv Acknowledgments xvi Chapter 1 Introduction1 1.1 Gravitational waves and their detection................ 1 1.2 Compact binaries............................ 6 1.3 Data analysis .............................. 8 1.4 Astrophysics and cosmology with gravitational waves . 14 Chapter 2 Disentangling the potential dark matter origin of LIGO’s black holes 17 2.1 Motivation................................ 17 2.2 Abstract................................. 19 2.3 Introduction............................... 19 2.4 An overview of constraints on primordial black hole dark matter . 21 2.5 Tension in the microlensing regime .................. 22 2.6 An extended primordial black hole mass function .......... 25 2.7 LIGO primordial black hole merger rates............... 28 2.8 Discussion................................ 30 2.9 Acknowledgements ........................... 30 Chapter 3 Methods for the detection of gravitational waves from sub-solar mass ultracompact binaries 32 iv 3.1 Abstract................................. 32 3.2 Introduction............................... 33 3.3 Analysis Techniques .......................... 33 3.3.1 Estimates of sensitivity..................... 34 3.3.2 Sensitive distance........................ 36 3.3.3 Approximation of the merger rate for null-results . 39 3.3.4 Non-spinning waveforms.................... 40 3.4 Potential constraints on primordial black hole abundance . 42 3.5 Future prospects and discussion.................... 47 3.6 Acknowledgments............................ 48 Chapter 4 Search for sub-solar mass ultracompact binaries in Advanced LIGO’s first observing run 49 4.1 Introduction............................... 50 4.2 Search.................................. 52 4.3 Constraint on binary merger rate ................... 54 4.4 Constraint on primordial black holes as dark matter......... 55 4.5 Conclusion................................ 57 4.6 Acknowledgments............................ 59 Chapter 5 Search for sub-solar mass ultracompact binaries in Advanced LIGO’s second observing run 61 5.1 Introduction............................... 62 5.2 Search.................................. 64 5.3 Constraint on binary merger rate ................... 66 5.4 General constraints on sub-solar mass black hole dark matter . 68 5.5 Conclusion................................ 70 5.6 Acknowledgments............................ 71 Chapter 6 Sub-threshold binary neutron star search in advanced LIGO’s first observing run 73 6.1 Motivation................................ 73 6.2 Abstract................................. 75 6.3 Introduction............................... 75 6.4 Search Description ........................... 77 6.4.1 Template bank ......................... 77 6.4.2 Estimating significance of events................ 79 v 6.4.3 Estimating the sensitivity of the search............ 79 6.5 Results.................................. 80 6.6 Discussion................................ 84 6.7 Acknowledgments............................ 85 Chapter 7 First demonstration of early warning gravitational wave alerts 86 7.1 Abstract................................. 86 7.2 Introduction............................... 87 7.3 Analysis................................. 89 7.4 Results.................................. 92 7.5 Looking ahead.............................. 94 7.6 Acknowledgments............................ 97 Appendix A GstLAL: A software framework for gravitational wave discovery 99 A.1 Motivation and significance ...................... 99 A.2 Software description ..........................102 A.2.1 Software Architecture......................104 A.2.2 gstlal package.........................105 A.2.3 gstlal-ugly package......................105 A.2.4 gstlal-inspiral package...................105 A.2.5 gstlal-burst package.....................107 A.2.6 gstlal-calibration package . 107 A.3 Illustrative Examples..........................108 A.3.1 Example Gstreamer pipeline with GstLAL . 108 A.3.2 Compact binary searches....................111 A.4 Impact..................................112 A.5 Conclusions ...............................114 A.6 Conflict of interest ...........................114 Appendix B Permissions 115 Bibliography 120 Bibliography 120 vi List of Figures 1.1 The impact of passing gravitational waves of + (top) and (middle) × polarization on a ring of freely falling test particles as time progresses. The bottom row shows how the strain changes in time, which is the quantity ground based interferometers aim to measure........ 3 1.2 A simplified schematic of the LIGO interferometers. The detectors are based on Michelson interferometry and make use of Fabry-Perot cavities and power recycling mirrors to increase sensitivity. This figure is reproduced from [1]....................... 5 1.3 The past and expected evolution of the noise spectra in LIGO- Livingston. LIGO is presently most sensitive from 100 300 ∼ − Hz. Note in particular the expected improvements at frequencies . 20Hz. These are crucial to realize early warning detection with this generation of interferometers; see Chapter7 for more discussion on future early warning prospects. The PSDs were obtained from the public LIGO DCC documents G1600151, G1801952, and T2000012.9 1.4 The template bank used by GstLAL in Advanced LIGO’s third observing run. Each dot represents a single waveform in the m1, m2 plane. The bank is a 4-dimensional object. For computational considerations, we only search for binaries with aligned component spins. Any two waveforms represented by coordinates within the 4-volume are at least 97%, and often 99%, similar by construction. The bounds of the bank are complicated and described in Table II of [2].................................... 11 vii 2.1 Several of the constraints on monochromatic PBH DM in the mass range detectable by LIGO. The red shows the range of most likely MACHO masses [3], while the light blue shows the range of compo- nent masses found by LIGO. The long and short dashed lines show the MACHO [4] and EROS2 [5] collaboration constraints on compact objects for the standard halo. The blue line shows a 99.9% confi- dence limit derived from mass segregation in Segue I [6] while the green lines are derived from the survival of a star cluster in Eridanus II [7,8] for velocity dispersions and dark-matter densities at the 1 3 galactic center of σ = 5; 10 km s− respectively and ρ = :1M pc− . The shaded regions are excluded by these constraints. Our mass function is designed to satisfy the constraints set by the blue curve for M > 1M . We allow it to break the constraint set by the lower dotted line (EROS2) while satisfying those set by the MACHO collaboration since MACHO claimed multiple detections of compact objects while EROS2 had a single candidate event. ......... 23 2.2 Allowed values of α and M ∗ for the PBH distribution (2.4). Within the LIGO allowed values of α, we consider the tightest constraints on LIGO binaries comprised of PBHs given by [6], which defines the blue region. The green regions show the Eridanus II constraints for the dispersions and density referenced in Fig. 2.1. The red region shows which values of α and M ∗ correspond to a mass function that peaks inside the 90% confidence region of the MACHO col- laboration’s detections across several different