495 research outputs found
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 and reactions. The transition to endothermic and
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 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 , 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
Recommended from our members
Studying galaxy formation through Lyman alpha in emission and absorption
Galaxy formation is one of the central problems of Physical Cosmology. Neutral hydrogen plays an important role, linking the collapse of cooling gas into haloes with the formation of stars. Lyman alpha, hydrogen’s strongest spectral line, can directly probe neutral hydrogen in the high redshift Universe. Lyα can be observed in absorption in Damped Lyman Alpha systems (DLAs): high Hi column density regions that dominate the neutral gas content of the Universe between z ∼ 0 − 5. Lyα in emission is an important signature of early, starforming galaxies. Both populations, however, present significant theoretical challenges. As part of my thesis, I have developed a Monte Carlo Lyα radiative transfer code to investigate models of early galaxies.
Rauch et al. (2008) performed an ultra-deep spectroscopic survey and discovered a new
population of very faint, spatially extended Lyα emitters, which they claimed to be the
long-sought host galaxies of DLAs at z ∼ 3.
I show here that a simple analytical model, which reproduces the incidence rate and
kinematics of DLAs in the context of [Lambda]CDM models for structure formation, also reproduces the size distribution of the faint Lyα emitters for plausible parameters, which supports their identification as DLA host galaxies. The model suggests that galaxies in haloes with vc ~ 100−150 km s−1 account for the majority of DLA host galaxies, and that these galaxies at z ~ 3 are the building blocks of typical present-day galaxies like our Milky Way.
I further use my newly developed Lyα code to perform detailed 1D radiative transfer
calculations, investigating the spatial and spectral distribution of Lyα emission due to star formation at the centre of DLAs, and its dependence on the spatial and velocity structure of the gas. The modelling reproduces the observed properties of both DLAs and the faint Lyα emitters, including the velocity width and column density distribution of DLAs and the large observed spatial extent of the faint emitters. In the model, haloes hosting DLAs retain up to 20% of the cosmic baryon fraction in the form of neutral hydrogen. The scattering of Lyα photons at the observed radii, which can be as large as 50 kpc, requires the bulk velocity of the gas at the centre of the haloes to be moderate.
I furthermore perform 3D Lyα radiative transfer simulations, building on numerical simulations of galaxy formation that include galactic winds and gas infall. The Lyα emission region is shown to be larger and smoother than the cross-section for damped absorption by ~ 50%, with Lyα photons scattered effectively by gas with column densities >~ 1017 cm−2.
The spectra typically show two peaks, with the relative strength of the red (blue) peak being a reflection of the relative contribution of outflow (inflow) in the velocity profile. There is considerable variation in the observed line profile and spectral intensity with viewing angle.
These more realistic models support many of the simplifying assumptions of my previous
models, and have the potential to probe the important role of galactic winds in protogalaxies.
The main conclusion is that the faint population of Lyα emitters are indeed the longsought host population of DLAs. Ultra-faint observations of Lyα emission have exceptional potential to directly probe the spatial distribution and kinematics of neutral hydrogen in early galaxies
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
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