3,753 research outputs found
High-redshift voids in the excursion set formalism
Voids are a dominant feature of the low-redshift galaxy distribution. Several
recent surveys have found evidence for the existence of large-scale structure
at high redshifts as well. We present analytic estimates of galaxy void sizes
at redshifts z ~ 5 - 10 using the excursion set formalism. We find that recent
narrow-band surveys at z ~ 5 - 6.5 should find voids with characteristic scales
of roughly 20 comoving Mpc and maximum diameters approaching 40 Mpc. This is
consistent with existing surveys, but a precise comparison is difficult because
of the relatively small volumes probed so far. At z ~ 7 - 10, we expect
characteristic void scales of ~ 14 - 20 comoving Mpc assuming that all galaxies
within dark matter haloes more massive than 10^10 M_sun are observable. We find
that these characteristic scales are similar to the sizes of empty regions
resulting from purely random fluctuations in the galaxy counts. As a result,
true large-scale structure will be difficult to observe at z ~ 7 - 10, unless
galaxies in haloes with masses less than ~ 10^9 M_sun are visible. Galaxy
surveys must be deep and only the largest voids will provide meaningful
information. Our model provides a convenient picture for estimating the
"worst-case" effects of cosmic variance on high-redshift galaxy surveys with
limited volumes.Comment: 12 pages, 9 figures, 1 table, accepted by MNRA
Consequences of Fine-Tuning for Fifth Force Searches
Light bosonic fields mediate long range forces between objects. If these
fields have self-interactions, i.e., non-quadratic terms in the potential, the
experimental constraints on such forces can be drastically altered due to a
screening (chameleon) or enhancement effect. We explore how technically natural
values for such self-interaction coupling constants modify the existing
constraints. We point out that assuming the existence of these natural
interactions leads to new constraints, contrary to the usual expectation that
screening leads to gaps in coverage. We discuss how screening can turn
fundamentally equivalence principle (EP)-preserving forces into EP-violating
ones. This means that when natural screening is present, searches for EP
violation can be used to constrain EP-preserving forces. We show how this
effect enables the recently discovered stellar triple system \textit{PSR
J03371715} to place a powerful constraint on EP-preserving fifth forces.
Finally, we demonstrate that technically natural cubic self-interactions modify
the vacuum structure of the scalar potential, leading to new constraints from
spontaneous and induced vacuum decay.Comment: 36 pages, 9 figures -- v3 reflects version published in JHE
Large Fluctuations in the High-Redshift Metagalactic Ionizing Background
Recent observations have shown that the scatter in opacities among coeval
segments of the Lyman-alpha forest increases rapidly at z > 5. In this paper,
we assess whether the large scatter can be explained by fluctuations in the
ionizing background in the post-reionization intergalactic medium. We find that
matching the observed scatter at z ~ 5.5 requires a short spatially averaged
mean free path of 3 shorter than direct
measurements at z ~ 5.2. We argue that such rapid evolution in the mean free
path is difficult to reconcile with our measurements of the global H I
photoionization rate, which stay approximately constant over the interval z ~
4.8 - 5.5. However, we also show that measurements of the mean free path at z >
5 are likely biased towards higher values by the quasar proximity effect. This
bias can reconcile the short values of the mean free path that are required to
explain the large scatter in opacities. We discuss the implications of this
scenario for cosmological reionization. Finally, we investigate whether other
statistics applied to the z > 5 Lyman-alpha forest can shed light on the origin
of the scatter. Compared to a model with a uniform ionizing background, models
that successfully account for the scatter lead to enhanced power in the
line-of-sight flux power spectrum on scales k < 0.1 h/Mpc. We find tentative
evidence for this enhancement in observations of the high-redshift Lyman-alpha
forest.Comment: Matches version published by MNRAS with clarifications and expanded
discussio
Simulation Analysis of High Velocity Maintenance for the B-1B
This thesis explores the impact that High Velocity Maintenance (HVM) will have on aircraft availability rates for the B-1B by examining the proposed changes to the field maintenance and supply processes for the two B-1B squadrons of the 28th Bombardment Wing located at Ellsworth AFB, SD. There is a significant restructuring of depot level maintenance planned with the implementation of HVM, and the impact that this will have on base-level operations is important to determine, for it will provide insight as to whether or not HVM will be a feasible program with a high probability of successfully improving B-1B aircraft availability rates. To examine the impact of HVM at the base level, discrete-event simulation is used. Two simulation models are created in ARENA 12. The first model captures the current state of operations for the base maintenance and supply processes, while the second model captures the processes as they are planned with the implementation of HVM. Comparisons of the two models reveal that HVM does have the potential to significantly improve aircraft availability rates, but the improvements that must occur with aircraft failure rates and base stockage effectiveness for HVM to operate as planned may not be feasible
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