Black holes, gravitational waves and fundamental physics: a roadmap

TL;DR

The paper surveys how gravitational waves from compact-object mergers open a multi-messenger window on fundamental physics, including tests of General Relativity, the nature of black holes, and the particle content of the Universe.It integrates astrophysical formation channels (stellar, dynamical, primordial) with numerical and analytical waveform modelling frameworks (PN, GSF, NR, EOB, Phenom) to outline a road-map for exploiting current and future GW detectors.Key contributions include a synthesis of BBH/BNS formation channels, the role of dynamical environments, the DM connection, and the prospects for standard sirens and cosmology with LIGO/Virgo, ET, LISA, and PTAs.The work emphasizes the need for accurate, fast waveform models and robust analysis tools to maximize the scientific return of next-generation GW observations and to test GR in the strong-field regime.

Abstract

The grand challenges of contemporary fundamental physics---dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem---all involve gravity as a key component. And of all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some of the most remarkable predictions of General Relativity: event horizons, singularities and ergoregions. The hitherto invisible landscape of the gravitational Universe is being unveiled before our eyes: the historical direct detection of gravitational waves by the LIGO-Virgo collaboration marks the dawn of a new era of scientific exploration. Gravitational-wave astronomy will allow us to test models of black hole formation, growth and evolution, as well as models of gravitational-wave generation and propagation. It will provide evidence for event horizons and ergoregions, test the theory of General Relativity itself, and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to radically reshape our understanding of the cosmos and of the laws of Nature. The purpose of this work is to present a concise, yet comprehensive overview of the state of the art in the relevant fields of research, summarize important open problems, and lay out a roadmap for future progress.

Figures (12)

• Figure 1: Cartoon illustrating the BH mass spectrum encompassing the whole astrophysical relevant range, from sBHs to SMBHs, through the unexplored (light-green) zone where BH seeds are expected to form and grow. Vertical black-lines denote the two sBH masses in GW150914, the mass $M_\bullet$ of RGG118 (the lightest SMBH known as of today in the dwarf galaxy RG118), of SgrA* in the Milky Way, and of J0100+2802 (the heaviest SMBH ever recorded). The mass distribution of sBHs, drawn from the observations of the Galactic sBH candidates, has been extended to account for the high-mass tail following the discovery of GW150914. The minimum (maximum) sBHs is set equal to $3{\rm ~M}_{\odot}$ ($60{\rm ~M}_{\odot}$), and the theoretically predicted pair-instability gap is depicted as a narrow darker-grey strip. The SMBH distribution has been drawn scaling their mass according to the local galaxy mass function and $M_\bullet$-$\sigma$ correlation. The decline below $\sim 10^5{\rm ~M}_{\odot}$ is set arbitrarily: BH of $\sim 10^{4-5}{\rm ~M}_{\odot}$ may not be ubiquitous in low-mass galaxies as often a nuclear star cluster is in place in these galaxies, which may or may not host a central IMBH Graham09. The black stars and dashed tracks illustrate the possibility that a SMBH at high redshift forms as sBH-only (born on the left side of the sBH gap) or as light seed (on the right of the gap) which then grows through phases of super-Eddington accretion Lupi16. The red circle and dotted track illustrates the possibility of a genetic divide between sBHs and SMBHs, and that a heavy seed forms through the direct collapse of a supermassive protostar in a metal free, atomic-hydrogen cooling, DM halo Latif13Schleicher13. The seed later grows via gas accretion and mergers with SMBHs in other black halos.
• Figure 2: Left: Redshifted total merger mass distribution for two population synthesis models Belczynski:2017gds: M10 (low BH natal kicks) and M23 (high BH natal kicks). The O2 LIGO sensitivity is marked; the most likely detections are expected when models are closest to the sensitivity curve. We also mark LIGO/Virgo BBH merger detections (vertical positions have no meaning), all of which fall within the most likely detection region between $20-100{\rm ~M}_{\odot}$. Right: Source frame BBH merger-rate density of several population synthesis models for the local Universe ($z=0$). The current LIGO O1/O2 BBH merger rate is $12$--$213{\rm ~Gpc}^{-3} {\rm ~yr}^{-1}$ (blue double-headed arrow). Note that the models with fallback-attenuated BH natal kicks (M10, M20) are at the LIGO upper limit, while models with high BH natal kicks are at the LIGO lower limit (M13, M23). Models with small (M26) and intermediate (M25) BH kicks fall near the middle of the LIGO estimate.
• Figure 3: Distribution of simulated double compact object binaries in the total mass– chirp mass plane for a metallicity of Z = 0.0002. Three islands of data are visible, corresponding to BBH, mixed BH-NS and BNS systems. The colour code indicates the merger rate per pixel for a Milky Way equivalent galaxy. The three solid grey lines indicate a constant mass ratio of 1, 3 and 10 (from top to bottom). Observed LIGO/Virgo sources are shown with black crosses and event names are given for the four most massive cases. The lowest mass BBH mergers can only be reproduced with a higher metallicity. Figure taken from Ref. Kruckow:2018slo.
• Figure 4: Summary of astrophysical constraints on PBHs in the mass range $M \in [10^{-2},\,10^5]\,M_\odot$. Details of the constraints are given in the main text and we plot here the most conservative. We emphasize that astrophysical constraints may have substantial systematic uncertainties and that the constraints shown here apply only for monochromatic mass functions.
• Figure 5: Cartoon illustrating the journey travelled by SMBHs of masses in the range $10^{6-8}{\rm ~M}_{\odot}$ during major galaxy-galaxy collisions. The $x-$ axis informs on the SMBH separation given in various panels, while the $y-$ informs on the timescale. The journey starts when two galaxies (embedded in their DM halos) collide on kpc scales (right-most plot). The inset shows a selected group of galaxies from the cosmological simulation described in Ref. Khan:2016vln. The inset in the second panel (from right to left), from Ref. 2015MNRAS.447.2123C, depicts the merger of two disc galaxies and their embedded SMBHs. Pairing occurs when the two SMBHs are in the midst of the new galaxy that has formed, at separations of a few kpc. The SMBHs sink under the action of star-gas-dynamical friction. In this phase, SMBHs may find themselves embedded in star forming nuclear discs, so that their dynamics can be altered by the presence of massive gas-clouds. Scattering off the clouds makes the SMBH orbit stochastic, potentially broadening the distribution of sinking times during the pairing phase Fiacconi132015MNRAS.446.1765L2017MNRAS.464.2952T2015MNRAS.453.3437L. Furthermore, feedback from supernovae and AGN triggering by one or both the SMBHs affect the dynamics as these processes alter the thermodynamics of the gas and its density distribution, which in turn affect the process of gas-dynamical friction on the massive BHs. It is expected that eventually the SMBHs form a Keplerian binary, on pc scales. Then, individual scattering off stars harden the binary down to the GW-driven domain. This is an efficient mechanism (and not a bottleneck) if the relic galaxy displays some degree of triaxiality and/or rotation. In this case, there exists a large enough number of stars on low-angular momentum orbits capable to interact with the binary and extract orbital energy 2015ApJ...810...49V. The binary in this phase can also be surrounded by a (massive) circum-binary disc 2012AA...545A.127R. In a process reminiscent to type II planet migration, the two SMBHs can continue to decrease their separation and they eventually cross the GW boundary. Then, GW radiation controls the orbital decay down to coalescence.