arXiv subjects

K. A. S. Croker

Publications and source records attributed to K. A. S. Croker.

4 recordsLinked to original sources

arXiv2019

Maximum likelihood fits to a cored halo enclosing finite mass

The hybrid Ricci scalar $R_{μν}η^{μν}$ formed from a Robertson-Walker derived Ricci tensor and a distinct flat space metric becomes a dark, pseudo-isothermal density under asymptotic constraint. While extension of this constraint to more general scenarios is unclear, it is model independent and suggestive of a radical premise: all galactic dark matter halos are sourced by very many distinct Robertson-Walker spacetimes. We explore this premise and present a cored galactic halo, which encloses finite mass over all space. We compute fit parameters to published data on the Milky Way dwarf spheroidal galaxies, and find consistency with a predicted reciprocal square scaling relation. Spherical symmetry permits treatment of each individual halo as a point particle interacting under a modified gravitational force. This force is Newtonian at large separation with a leading dipole correction, and naturally softens to zero at small separation. Taken as cold dark matter, ensembles of such halos present a possible resolution to tension between large-scale $N$-body simulations and small-scale astrophysical data.

astro-ph.GA↗

arXiv2017

Cosmological tests of the gravastar hypothesis

Gravitational vacuum stars (gravastars) have become a viable theoretical alternative to the black hole (BH) end-stage of stellar evolution. These objects gravitate in vacuum like BHs, yet have no event horizons. In this paper, we present tests of the gravastar hypothesis within flat Friedmann cosmology. Such tests are complementary to optical and gravitational wave merger signatures, which have uncertainties dominated by the poorly constrained gravastar crust. We motivate our analysis directly from the action principle, and show that a population of gravastars must induce a time-dependent dark energy (DE). The possibility of such a contribution has been overlooked due to a subtlety in the de facto definition of the isotropic and homogeneous stress. Conservation of stress-energy directly relates the time evolution of this DE contribution to the measured BH comoving mass function and a gravastar population crust parameter $η$. We show that a population of gravastars formed between redshift $8 \lesssim z \lesssim 20$ readily produces the present-day DE density over a large range of $η$, providing a natural resolution to the coincidence problem. We compute an effective DE equation of state $w_\mathrm{eff}(a)$ as a function of present-epoch BH population observables and $η$. Using a BH population model developed from the cosmic star formation history, we obtain $w_0$ and $w_a$ consistent with Planck best-fit values. In summary, the gravastar hypothesis leads to an unexpected correlation between the BH population and the magnitude and time-evolution of DE.

astro-ph.CO↗

arXiv2016

A Smolin-like branching multiverse from multiscalar-tensor theory

We implement a Smolin-like branching multiverse through a directed, acyclic graph of $N$ metrics. Our gravitational and matter actions are indistinguishable from $N$ decoupled statements of General Relativity, if one varies with respect to metric degrees of freedom. We replace $N-1$ metrics with scalar fields by conformally relating each metric to its unique graph predecessor. Varying with respect to the $N-1$ scalar fields gives a multiscalar-tensor model which naturally features dark matter candidates. Building atop an argument of Chapline and Laughlin, branching is accomplished with the emergence of order parameters during gravitational collapse: we bootstrap a suitably defined $N$ scalar field model with initial data from an $N-1$ field model. We focus on the nearest-neighbour approximation, determine conditions for dynamical stability, and compute the equations of motion. The model features a novel screening property where the scalar fields actively adjust to decouple themselves from the stress, oscillating about the requisite values. In the Newtonian limit, these background values for the scalar fields exactly reproduce Newton's law of gravitation.

gr-qc↗

arXiv2016

ngravs: Distinct gravitational interactions in GADGET-2

We discuss an extension of the massively parallel cosmological simulation code GADGET-2, which now enables investigation of multiple and distinct gravitational force laws, provided they are dominated by a constant scaling of the Newtonian force. In addition to simplifying investigations of a universally modified force law, the ngravs extension provides a foundation for state-of-the-art collisionless cosmological simulations of exotic gravitational scenarios. We briefly review the algorithms used by GADGET-2, and present our extension to multiple gravities, highlighting additional features that facilitate consideration of exotic force laws. We discuss the accuracy and performance of the ngravs extension, both internally and with an unaltered GADGET-2, in the relevant operational modes. The ngravs extension is publicly released to the research community.

astro-ph.IM↗