3,702 research outputs found
Soliton Solutions to the Einstein Equations in Five Dimensions
We present a new class of solutions in odd dimensions to Einstein's equations
containing either a positive or negative cosmological constant. These solutions
resemble the even-dimensional Eguchi-Hanson--(anti)-de Sitter ((A)dS) metrics,
with the added feature of having Lorentzian signatures. They provide an
affirmative answer to the open question as to whether or not there exist
solutions with negative cosmological constant that asymptotically approach
AdS, but have less energy than AdS. We present
evidence that these solutions are the lowest-energy states within their
asymptotic class.Comment: 9 pages, Latex; Final version that appeared in Phys. Rev. Lett; title
changed by journal from original title "Eguchi-Hanson Solitons
Standing on the Shoulders of Giants: The Cleft Palate-Craniofacial Journal (1964-1989)Electronic Archive
Current research and clinical practice in cleft palate and craniofacial disorders “stands on the shoulders of giants” who came before us. To enable thirty years of seminal research articles to become digitally available to a worldwide community of students, scholars, and clinicians, a collaboration was forged in 2004 between University of Pittsburgh’s Digital Research Library (DRL) and ACPA, (with the agreement of Allen Press), to create an electronic archive of the first thirty years of the Cleft Palate Craniofacial Journal . The work was performed pro bono, by all parties
On the Gannon-Lee Singularity Theorem in Higher Dimensions
The Gannon-Lee singularity theorems give well-known restrictions on the
spatial topology of singularity-free (i.e., nonspacelike geodesically
complete), globally hyperbolic spacetimes. In this paper, we revisit these
classic results in the light of recent developments, especially the failure in
higher dimensions of a celebrated theorem by Hawking on the topology of black
hole horizons. The global hyperbolicity requirement is weakened, and we expand
the scope of the main results to allow for the richer variety of spatial
topologies which are likely to occur in higher-dimensional spacetimes.Comment: 13 pages, no figures, to appear in Class. Quantum Gra
Uniqueness of de Sitter space
All inextendible null geodesics in four dimensional de Sitter space dS^4 are
complete and globally achronal. This achronality is related to the fact that
all observer horizons in dS^4 are eternal, i.e. extend from future infinity
scri^+ all the way back to past infinity scri^-. We show that the property of
having a null line (inextendible achronal null geodesic) that extends from
scri^- to scri^+ characterizes dS^4 among all globally hyperbolic and
asymptotically de Sitter spacetimes satisfying the vacuum Einstein equations
with positive cosmological constant. This result is then further extended to
allow for a class of matter models that includes perfect fluids.Comment: 22 pages, 2 figure
High N, dry: Experimental nitrogen deposition exacerbates native shrub loss and nonnative plant invasion during extreme drought.
Hotter, longer, and more frequent global change-type drought events may profoundly impact terrestrial ecosystems by triggering widespread vegetation mortality. However, severe drought is only one component of global change, and ecological effects of drought may be compounded by other drivers, such as anthropogenic nitrogen (N) deposition and nonnative plant invasion. Elevated N deposition, for example, may reduce drought tolerance through increased plant productivity, thereby contributing to drought-induced mortality. High N availability also often favors invasive, nonnative plant species, and the loss of woody vegetation due to drought may create a window of opportunity for these invaders. We investigated the effects of multiple levels of simulated N deposition on a Mediterranean-type shrubland plant community in southern California from 2011 to 2016, a period coinciding with an extreme, multiyear drought in the region. We hypothesized that N addition would increase native shrub productivity, but that this would increase susceptibility to drought and result in increased shrub loss over time. We also predicted that N addition would favor nonnatives, especially annual grasses, leading to higher biomass and cover of these species. Consistent with these hypotheses, we found that high N availability increased native shrub canopy loss and mortality, likely due to the higher productivity and leaf area and reduced water-use efficiency we observed in shrubs subject to N addition. As native shrub cover declined, we also observed a concomitant increase in cover and biomass of nonnative annuals, particularly under high levels of experimental N deposition. Together, these results suggest that the impacts of extended drought on shrubland ecosystems may be more severe under elevated N deposition, potentially contributing to the widespread loss of native woody species and vegetation-type conversion
Non-Existence of Black Holes in Certain Spacetimes
Assuming certain asymptotic conditions, we prove a general theorem on the
non-existence of static regular (i.e., nondegenerate) black holes in spacetimes
with a negative cosmological constant, given that the fundamental group of
space is infinite. We use this to rule out the existence of regular negative
mass AdS black holes with Ricci flat scri. For any mass, we also rule out a
class of conformally compactifiable static black holes whose conformal infinity
has positive scalar curvature and infinite fundamental group, subject to our
asymptotic conditions. In a limited, but important, special case our result
adds new support to the AdS/CFT inspired positive mass conjecture of Horowitz
and Myers.Comment: 17 pages, Latex. Typos corrected, minor changes to the text. Accepted
for publication in Classical and Quantum Gravit
What’s in It for Me? Reciprocal Exchanges between Underwriters and Venture Capitalists
In this article, we examine the impact of repeat interactions between VCs and underwriters. Past research has suggested that such interactions build trust and may contribute to more equitable treatment of issuing firms. We adopt an alternative perspective and suggest that these repeat interactions are characterized by reciprocal exchanges facilitated by opportunistic behavior from the VC. Our analysis demonstrates that VCs and underwriters interact in order to appropriate greater value from the IPO. This article provides a more complete understanding of repeat interactions between the VC and the underwriter by identifying characteristics of the relationship that have an impact on the value of the IPO
Calf pre-weaning traits and immunoglobulin response to bovine viral diarrhea virus vaccination
Calfhood vaccination for bovine viral diarrhea virus (BVDV) is a relatively new concept, and protocols are evolving. Our objective was to determine effects of BVDV type I vaccination protocol, calf behavior (chute score, and chute exit velocity), and gender on calf gain and immunoglobulin (Ig) response. Crossbred calves (n = 64) were randomly allotted to one of two vaccination protocols. In protocol 1, calves were vaccinated at 60 d of age (d 0) and at weaning (d 147). Calves assigned to protocol 2 were vaccinated against BVDV type I at 21 d prior to (d 126) and at weaning (d 147). Blood samples were collected from half of the calves in each protocol group on d 0 (60 days of age), d 21, d 126 (21 days prior to weaning), and d 147 (at weaning); serum was harvested and Ig titers were determined. Titers for BVDV type I were transformed (log base 2) and analyzed using a mixed model procedure. Calves vaccinated at d 0 and weaning had larger (P \u3c 0.0001) titers than calves vaccinated at d 126 and weaning (7.5 ± 0.36 and 5.1 ± 0.36, respectively). Mean BVDV titers were larger (P \u3c 0.0001) on d 147 when compared with d 126, d 21, and d 0 (8.3 ± 0.39, 5.1 ± 0.40, 5.9 ± 0.39 and 5.7 ± 0.39, respectively). A treatment × day interaction (P \u3c 0.0001) also affected BVDV titers. However, BVDV titers were not affected (P \u3e 0.05) by calf gender, chute score, or chute exit velocity. Weaning weight and pre-weaning average daily gain (ADG) were not related to BVDV type I titers. This study indicated that vaccinating beef calves against BVDV was effective in triggering an Ig response. Furthermore, our results suggest that calves should be vaccinated against BVDV type I at 60 d of age for greater disease resistance
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