Binding the Diproton in Stars: Anthropic Limits on the Strength of Gravity

We calculate the properties and investigate the stability of stars that burn via strong (and electromagnetic) interactions, and compare their properties with those that, as in our Universe, include a rate-limiting weak interaction. It has been suggested that, if the diproton were bound, stars would burn ~10^{18} times brighter and faster via strong interactions, resulting in a universe that would fail to support life. By considering the representative case of a star in our Universe with initially equal numbers of protons and deuterons, we find that stable, "strong-burning" stars adjust their central densities and temperatures to have familiar surface temperatures, luminosities and lifetimes. There is no "diproton disaster". In addition, strong-burning stars are stable in a much larger region of the parameter space of fundamental constants, specifically the strength of electromagnetism and gravity. The strongest anthropic bound on stars in such universes is not their stability, as is the case for stars limited by the weak interaction, but rather their lifetime. Regardless of the strength of electromagnetism, all stars burn out in mere millions of years unless the gravitational coupling constant is extremely small, \alpha_G < 10^{-30}.Comment: 16 pages, 4 figures. Accepted for publication in JCA

Testing the Multiverse: Bayes, Fine-Tuning and Typicality

Theory testing in the physical sciences has been revolutionized in recent decades by Bayesian approaches to probability theory. Here, I will consider Bayesian approaches to theory extensions, that is, theories like inflation which aim to provide a deeper explanation for some aspect of our models (in this case, the standard model of cosmology) that seem unnatural or fine-tuned. In particular, I will consider how cosmologists can test the multiverse using observations of this universe.Comment: 19 pages, 3 figures. Conference proceedings: to appear in "The Philosophy of Cosmology", edited by Khalil Chamcham, Joseph Silk, John D. Barrow, and Simon Saunders. Cambridge University Press, 201

Producing the Deuteron in Stars: Anthropic Limits on Fundamental Constants

Stellar nucleosynthesis proceeds via the deuteron (D), but only a small change in the fundamental constants of nature is required to unbind it. Here, we investigate the effect of altering the binding energy of the deuteron on proton burning in stars. We find that the most definitive boundary in parameter space that divides probably life-permitting universes from probably life-prohibiting ones is between a bound and unbound deuteron. Due to neutrino losses, a ball of gas will undergo rapid cooling or stabilization by electron degeneracy pressure before it can form a stable, nuclear reaction-sustaining star. We also consider a less-bound deuteron, which changes the energetics of the $pp$ and $pep$ reactions. The transition to endothermic $pp$ and $pep$ reactions, and the resulting beta-decay instability of the deuteron, do not seem to present catastrophic problems for life.Comment: 19 pages, 5 figures. Accepted to JCAP. Revised to match the published version; corrected to better take into account free neutron

The bias of DLAs at z ~ 2.3: contraining stellar feedback in shallow potential wells

We discuss the recent Baryon Oscillation Spectroscopic Survey measurement of a rather high bias factor for the host galaxies/haloes of Damped Lyman-alpha Absorbers (DLAs), in the context of our previous modelling of the physical properties of DLAs within the $\Lambda$ cold dark matter paradigm. Joint modelling of the column density distribution, the velocity width distribution of associated low ionization metal absorption, and the bias parameter suggests that DLAs are hosted by galaxies with dark matter halo masses in the range $10 < \log M_v < 12$, with a rather sharp cutoff at the lower mass end, corresponding to virial velocities of 35 km/sec. The observed properties of DLAs appear to suggest efficient (stellar) feedback in haloes with masses/virial velocities below the cutoff and a large retained baryon fraction (> 35 %) in haloes above the cutoff.Comment: 10 pages, 9 figures. Published in MNRAS, May 21, 2014. 440 (3): 2313-2321. v3: Corrections in light of errata: MNRAS, 454(1), p. 218. Note, in particular, the changes to Figure 5 and the virial velocity cut-of

Lyman Alpha and MgII as Probes of Galaxies and their Environments

Ly{\alpha} emission, Ly{\alpha} absorption and MgII absorption are powerful tracers of neutral hydrogen. Hydrogen is the most abundant element in the universe and plays a central role in galaxy formation via gas accretion and outflows, as well as being the precursor to molecular clouds, the sites of star formation. Since 21cm emission from neutral hydrogen can only be directly observed in the local universe, we rely on Ly{\alpha} emission, and Ly{\alpha} and MgII absorption to probe the physics that drives galaxy evolution at higher redshifts. Furthermore, these tracers are sensitive to a range of hydrogen densities that cover the interstellar medium, the circumgalactic medium and the intergalactic medium, providing an invaluable means of studying gas physics in regimes where it is poorly understood. At high redshift, Ly{\alpha} emission line searches have discovered thousands of star-forming galaxies out to z = 7. The large Ly{\alpha} scattering cross-section makes observations of this line sensitive to even very diffuse gas outside of galaxies. Several thousand more high-redshift galaxies are known from damped Ly{\alpha} absorption lines and absorption by the MgII doublet in quasar and GRB spectra. MgII, in particular, probes metal-enriched neutral gas inside galaxy haloes in a wide range of environments and redshifts (0.1 < z < 6.3), including the so-called redshift desert. Here we review what observations and theoretical models of Ly{\alpha} emission, Ly{\alpha} and MgII absorption have told us about the interstellar, circumgalactic and intergalactic medium in the context of galaxy formation and evolution.Comment: 59 Pages, 19 Figures, 1 Table. Accepted for publication in Publications of the Astronomical Society of the Pacifi

Geodesics, General Relativity and Spacetime

General Relativity (GR) is founded on the revolutionary idea that space and time are merely parts of a greater, unified whole: spacetime. Furthermore, the force we know as gravity results from the bending and stretching of the geometry of spacetime by its energetic contents. GR is notorious for its mathematical complexity and subtlety, meaning that an intuitive understanding of a spacetime is difficult. One of the best approaches to studying the properties of a given spacetime is to consider its geodesic structure—that is, to consider the motion of unaccelerated, “free-falling” particles. This report presents the results of such a study into two important spacetimes — the Kerr solution for a rotating black hole, and the Robertson-Walker solution for a homogeneous universe

Geodesics, General Relativity and Spacetime

General Relativity (GR) is founded on the revolutionary idea that space and time are merely parts of a greater, unified whole: spacetime. Furthermore, the force we know as gravity results from the bending and stretching of the geometry of spacetime by its energetic contents. GR is notorious for its mathematical complexity and subtlety, meaning that an intuitive understanding of a spacetime is difficult. One of the best approaches to studying the properties of a given spacetime is to consider its geodesic structure—that is, to consider the motion of unaccelerated, “free-falling” particles. This report presents the results of such a study into two important spacetimes — the Kerr solution for a rotating black hole, and the Robertson-Walker solution for a homogeneous universe

The Matanuska-Susitna Borough Community Survey, 2014 and Trends 2009–2014: A Sourcebook of Community Attitudes

The Matanuska-Susitna Borough Community Survey (Mat-Su Survey), conducted annually since 2006, is a cooperative research effort between the Justice Center at University of Alaska Anchorage (UAA) and the Matanuska-Susitna Borough. The survey asks Mat-Su Borough residents to evaluate the quality of Borough services, provide opinions about Borough decision-making, and sum up their perceptions about a range of issues relevant to the present and future of the Mat-Su community. The 2014 survey was distributed to 2,491 adult heads-of-household in the Mat-Su Borough in the winter and spring of 2014; a total of 1,003 surveys were returned, for a response rate of 40.3%. This sourcebook presents both the results from the 2014 Mat-Su Survey and trends from 2009–2014 in five major areas: (1) evaluation of current borough services; (2) use of borough facilities; (3) life in Mat-Su neighborhoods; (4) local government access, policies, and practices; and (5) respondent background information. A set of additional questions focusing on salmon and the environment was added to the 2014 Mat-Su Survey at the request of the Nature Conservancy. Additionally, findings from a derived importance-performance analysis of the survey data are presented, as is a compilation of respondent comments.Matanuska-Susitna BoroughIntroduction / Organization of Sourcebook / Methods / Executive Summary / 2014 RESULTS AND 2009–2014 TRENDS / Part I. Evaluation of Current Borough Services / Part II. Use of Borough Facilities / Part III. Life in Matanuska-Susitna Borough Neighborhoods / Part IV. Local Government: Access, Policies and Practices / Part V. Open Space and Salmon / Part VI. Sample Characteristics / Part VII. Derived Importance-Performance Analysis / Part VIII. Respondents’ Comments / Appendix: Questionnair