Post-Newtonian Insights on the Origin and Gravitational Radiation of Massive Black Holes in Matter Backgrounds

This document is a cumulative thesis largely based on my doctoral work between September 2019 and the spring of 2023. It is structured into two main sections, consisting of an introductory part (Chapter 1) and several parts adapted from published scientific articles (Chapters 2, 3 and 4). Chapter 1 is intended to be read both as a stand-alone text and as a general introduction to the scientific articles showcased in later Chapters. It consists in a historical and conceptual overview of the various topics that motivated and enabled my scientific production over the last several years, flavoured by my own understanding and interest in the matter at hand. Crucially, it is not meant to be an exhaustive review of any particular subject. The style is therefore discursive, somewhere between a textbook and a general science outreach article, and is very sparsely referenced. A more rigorous approach is taken in later Chapters, which include detailed technical introductions and refer to relevant literature as customary in scientific articles. Section 1.1 summarises the history of General Relativity and its conceptual difficulties, while also setting up the idea and basic theory of the post-Newtonian expansion. Section 1.2 introduces the two main applications tackled in this thesis, i.e. the gravitational wave driven inspiral of massive black hole binaries and the collapse of hydrostatic structures via relativistic instabilities. Section 1.3 summarises the most important astrophysical sources of gravitational radiation for future spaceborne detectors and briefly touches on some data analysis techniques. Finally, Section 1.4 presents my scientific outlook for the next years. The bulk of this thesis is adapted from six articles, which trace my major topics of interest throughout the course of my doctoral studies. The articles are listed in their respective “Relevant Publications” sections, together with other related parts of my academic production. Chapter 2 is centered around two articles concerned with post Newtonian modifications to the inspiral timescale for binaries in the gravitational wave driven regime. Chapter 3 collects two articles tackling the issue of environmental effects on gravitational waveforms. Finally, Chapter 4 is based on a pair of articles regarding the formation and the multimessenger emission of the most massive black holes in our Universe. Chapters 2, 3 and 4 are supplemented by short preambles, written in the same tone and style as Chapter 1. They partly serve as an informal introduction to the more detailed text adapted from publications. They also reflect my own thoughts on the general state of the field, highlighting some interesting divides of attention and expertise in the communities involved in gravitational wave astronomy

Relativistic binary-disc dynamics and OJ-287's flares: New parameter posteriors and future timing predictions

We revisit the precessing black hole binary model, a candidate to explain the bizarre quasi-periodic optical flares in OJ-287's light curve, from first principles. We deviate from existing work in three significant ways: 1) Including crucial aspects of relativistic dynamics related to the accretion disc's gravitational moments. 2) Adopting a model-agnostic prescription for the disc's density and scale height. 3) Using monte-carlo Markhov-chain methods to recover reliable system parameters and uncertainties. We showcase our model's predictive power by timing the 2019 great Eddington flare within 40 hr of the observed epoch, exclusively using data available prior to it. Additionally, we obtain a novel direct measurement of OJ-287's disc mass and quadrupole moment exclusively from the optical flare timings. Our improved methodology can uncover previously unstated correlations in the parameter posteriors and patterns in the flare timing uncertainties. In contrast to the established literature, we predict the 26th optical flare to occur on the 21st of August 2023 $\pm$ 32 days, shifted by almost a year with respect to the alleged "missing" flare of October 2022.Comment: Submitted to MNRAS. Comments welcome

Multimessenger Astronomy with Black Holes: Extreme mass ratio inspirals

This text will appear as Section II of Chapter 5 of the book "Black Holes in the Era of Gravitational-Wave Astronomy". As a stand alone text, it serves as a brief overview of astrophysics and gravitational wave radiation of extreme mass ratio inspirals, or EMRIs. Topics covered consist of: dynamical and gas-assisted formation channels, basics of EMRI dynamics and gravitational radiation, and science potential for both astrophysics and fundamental physics.Comment: To appear in Chapter 5 in the book Black Holes in the Era of Gravitational Wave Astronomy, ed. Arca Sedda, Bortolas, Spera, pub. Elsevier. All authors equally contributed to the Chapter writin

Prospects for localising Planet 9 with a future Uranus mission

Past years have seen various publications attempting to explain the apparent clustering features of trans-Neptunian objects, the most popular explanation being an unconfirmed "Planet 9". The recently proposed Uranus Orbiter and Probe mission by NASA's Planetary Science and Astrobiology Decadal Survey could offer the opportunity to precisely determine Planet 9's sky location and mass by carefully monitoring ranging data during the interplanetary cruise. We use Monte Carlo-Markov Chain methods to reconstruct simulated spacecraft trajectories in a simplified solar system model containing Planet 9, providing an estimate of the mission's localisation capacity depending on sky location, Earth-spacecraft Doppler link noise level and data collection rate. We characterise the noise via the Allan deviation $\sigma_{\rm A}$, scaled to the Cassini-era value $\sigma_{\rm A}^{\rm \scriptscriptstyle Cass} = 3 \times 10^{-15}$, finding that daily measurements of the spacecraft position can lead to $\sim$0.2 deg$^2$ localisation of Planet 9 (assuming $M_9 = 6.3 M_{\oplus}$, $d_9 = 460$AU). As little as a 3-fold improvement in $\sigma_{\rm A}$ drastically decreases the sky localisation area size to $\sim$0.01 deg$^2$. Thus, we showcase that a future Uranus mission carries a significant potential also for non-Uranian science.Comment: Submitted to MNRAS: Letters, 5 pages, 4 figure

Prospects for a local detection of dark matter with future missions to Uranus and Neptune

We investigate the possibility of detecting the gravitational influence of dark matter (DM) on the trajectory of prospective Doppler ranging missions to Uranus and Neptune. In addition, we estimate the constraints such a mission can provide on modified and massive gravity theories via extra-precession measurements using orbiters around the ice giants. We employ Monte Carlo-Markov Chain methods to reconstruct fictitious spacecraft trajectories in a simplified solar system model with varying amounts of DM. We characterise the noise on the Doppler link by the Allan deviation $\sigma_{\rm A}$, scaled on the Cassini-era value of $\sigma^{\rm{Cass}}_{\rm A}= 3 \times 10^{-15}$. Additionally, we compare the precision of prospective extra-precession measurements of Uranus and Neptune with the expected rates from simulations, in the context of modifications to the inverse square law. We estimate that the prospective mission will be sensitive to DM densities of the order of $\rho_{\rm{DM}} \sim 9 \times 10^{-20} \, (\sigma_{\rm A}/\sigma_{\rm A}^{\rm{Cass}})$ kg/m$^3$, while the $1\sigma$ bound on the expected galactic density of $\rho_{\rm{DM}} \sim 5 \times 10^{-22}$ kg/m$^3$ decreases as $1.0 \times 10^{-20} \, (\sigma_{\rm A}/\sigma^{\rm{Cass}}_{\rm A})^{0.8}$ kg/m$^3$. An improvement of two to three orders of magnitude from the baseline Allan deviation would guarantee a local detection of DM. Only a moderate reduction in ranging noise is required to rule out Milgrom's interpolating function with solar system based observations, and improve constraints the graviton mass beyond current local- or gravitational wave-based measurements. Our analysis also highlights the potential of future ranging missions to improve measurements of the standard gravitational parameters in the solar system.Comment: Published by A&

Beware of the employer: financial incentives for employees may fail to prolong old age employment

This paper shows that increasing the normal retirement age and introducing pension deductions for retirement before normal retirement age in Germany did not prolong employment of older men. The reason for this surprising result is that employers encouraged their employees to use the bridge options unemployment or partial retirement instead of the early retirement option for the long-term insured. Bridge options allowed employers to terminate employment considerably earlier than the pension for long-term insured. Employers however had to compensate their employees for the substantially higher costs of the bridge options. Therefore mainly employers with high employment adaption costs induced employees to use a bridge option during the implementation phase of the pension reform

Prospects for Localizing Planet 9 with a Future Uranus Mission

Past years have seen various publications attempting to explain the apparent clustering features of trans-Neptunian objects, the most popular explanation being an unconfirmed ‘Planet 9’. The recently proposed Uranus Orbiter and Probe mission by NASA’s Planetary Science and Astrobiology Decadal Survey could offer the opportunity to precisely determine Planet 9’s sky location and mass by carefully monitoring ranging data during the interplanetary cruise. We use Monte Carlo Markov chain methods to reconstruct simulated spacecraft trajectories in a simplified Solar system model containing Planet 9, providing an estimate of the mission’s localization capacity depending on sky location, Earth-spacecraft Doppler link noise level and data collection rate. We characterize the noise via the Allan deviation σA, scaled to the Cassini-era value $\sigma _{\rm A}^{\rm \scriptscriptstyle Cass} = 3 \times 10^{-15}$, finding that daily measurements of the spacecraft position can lead to ∼0.2 deg2 localization of Planet 9 (assuming M9 = 6.3 M⊕, d9 = 460 au). As little as a three-fold improvement in σA drastically decreases the sky localization area size to ∼0.01 deg2. Thus, we showcase that a future Uranus mission carries a significant potential also for non-Uranian science

Priorities in gravitational waveforms for future space-borne detectors: vacuum accuracy or environment?

In preparation for future space-borne gravitational-wave (GW) detectors, should the modelling effort focus on high-precision vacuum templates or on the astrophysical environment of the sources? We perform a systematic comparison of the phase contributions caused by 1) known environmental effects in both gaseous and stellar matter backgrounds, or 2) high-order post-Newtonian {(PN)} terms in the evolution of mHz GW sources {during the inspiral stage of massive binaries}. We use the accuracy of currently available analytical waveform models as a benchmark {value, finding} the following trends: the largest unmodelled phase contributions are likely environmental rather than PN for binaries lighter than $\sim 10^7/(1+z)^2$~M$_{\odot}$, where $z$ is the redshift. Binaries heavier than $\sim 10^8/(1+z)$~M$_{\odot}$ do not require more accurate {inspiral} waveforms due to low signal-to-noise ratios (SNRs). For high-SNR sources, environmental {phase contributions} are relevant at low redshift, while high-order vacuum templates are required at $z > 4$. Led by these findings, we argue that including environmental effects in waveform models should be prioritised in order to maximize the science yield of future mHz detectors.Comment: Accepted in MNRA

Money also is sunny in a retiree's world

This paper shows that labor income plays an important positive role for the decision to work after retirement. Especially individuals who have the chance to substantially supplement their pension entitlements have a higher earnings elasticity. Men are more attracted by earnings incentives than women. Also individuals who work until retirement can easier be attracted by financial incentives to work after retirement than those with bridge options. Our analysis is based on a representative and large administrative individual career data set that includes employer information. We use an endogeneity correction model to estimate labor and non-labor financial determinants of labor market participation after retirement

Direct collapse of exceptionally heavy black holes in the merger-driven scenario

We revisit the conditions present in supermassive discs (SMDs) formed by the merger of gas-rich, metal-enriched galaxies at red-shift z ∼ 10. We find that SMDs naturally form hydrostatic cores which go through a rapidly accreting supermassive star phase, before directly collapsing into massive black holes via the general relativistic instability. The growth and collapse of the cores occurs within ∼5 × 105 yr from the formation of the SMD, producing bright electromagnetic, neutrino and gravitational wave transients with a typical duration of a few minutes and, respectively, a typical flux and a typical strain amplitude at Earth of ∼10−8 erg s−1 cm−2 and ∼4 × 10−21. We provide a simple fitting formula for the the resulting black hole masses, which range from a few 106 M⊙ to 108 M⊙ depending on the initial SMD configuration. Crucially, our analysis does not require any specific assumption on the thermal properties of the gas, nor on the angular momentum loss mechanisms within the SMD. Led by these findings, we argue that the merger-driven scenario provides a robust pathway for the rapid formation of supermassive black holes at z > 6. It provides an explanation for the origin of the brightest and oldest quasars without the need of a sustained growth phase from a much smaller seed. Its smoking gun signatures can be tested directly via multi-messenger observations