128 research outputs found
Hawking Radiation as a Mechanism for Inflation
The Friedman-Robertson-Walker (FRW) space-time exhibits particle creation
similar to Hawking radiation of a black hole. In this essay we show that this
FRW Hawking radiation leads to an effective negative pressure fluid which can
drive an inflationary period of exponential expansion in the early Universe.
Since the Hawking temperature of the FRW space-time decreases as the Universe
expands this mechanism naturally turns off and the inflationary stage
transitions to a power law expansion associated with an ordinary radiation
dominated Universe.Comment: 6 pages. Published version -- Awarded "Honorable Mention" for the
2012 Gravity Research Foundation Essay Contes
Inflation with a graceful exit and entrance driven by Hawking radiation
We present a model for cosmological inflation which has a natural "turn on"
and a natural "turn off" mechanism. In our model inflation is driven by the
Hawking-like radiation that occurs in Friedman-Robertson-Walker (FRW)
space-time. This Hawking-like radiation results in an effective negative
pressure "fluid" which leads to a rapid period of expansion in the very early
Universe. As the Universe expands the FRW Hawking temperature decreases and the
inflationary expansion turns off and makes a natural transition to the power
law expansion of a radiation dominated universe. The "turn on" mechanism is
more speculative, but is based on the common hypothesis that in a quantum
theory of gravity at very high temperatures/high densities Hawking radiation
will stop. Applying this speculation to the very early Universe implies that
the Hawking-like radiation of the FRW space-time will be turned off and
therefore the inflation driven by this radiation will turn off.Comment: 19 pages, 2 figures revtex, matches PRD published versio
Adversarial Perturbations Against Real-Time Video Classification Systems
Recent research has demonstrated the brittleness of machine learning systems
to adversarial perturbations. However, the studies have been mostly limited to
perturbations on images and more generally, classification that does not deal
with temporally varying inputs. In this paper we ask "Are adversarial
perturbations possible in real-time video classification systems and if so,
what properties must they satisfy?" Such systems find application in
surveillance applications, smart vehicles, and smart elderly care and thus,
misclassification could be particularly harmful (e.g., a mishap at an elderly
care facility may be missed). We show that accounting for temporal structure is
key to generating adversarial examples in such systems. We exploit recent
advances in generative adversarial network (GAN) architectures to account for
temporal correlations and generate adversarial samples that can cause
misclassification rates of over 80% for targeted activities. More importantly,
the samples also leave other activities largely unaffected making them
extremely stealthy. Finally, we also surprisingly find that in many scenarios,
the same perturbation can be applied to every frame in a video clip that makes
the adversary's ability to achieve misclassification relatively easy
Effective Values of Komar Conserved Quantities and Their Applications
We calculate the effective Komar angular momentum for the Kerr-Newman (KN)
black hole. This result is valid at any radial distance on and outside the
black hole event horizon. The effcetive values of mass and angular momentum are
then used to derive an identity () which relates the Komar
conserved charge () corresponding to the null Killing vector
() with the thermodynamic quantities of this black hole. As an
application of this identity the generalised Smarr formula for this black hole
is derived. This establishes the fact that the above identity is a local form
of the inherently non-local generalised Smarr formula.Comment: v3, minor modifications over v2; LaTex, 9 pages, no figures, to
appear in Int. Jour. Theo. Phy
Switchable X-ray Orbital Angular Momentum from an Artificial Spin Ice
Artificial spin ices (ASI) have been widely investigated as magnetic
metamaterials with exotic properties governed by their geometries. In parallel,
interest in X-ray photon orbital angular momentum (OAM) has been rapidly
growing. Here we show that a square ASI with a programmed topological defect, a
double edge dislocation, imparts OAM to scattered X-rays. Unlike single
dislocations, a double dislocation does not introduce magnetic frustration, and
the ASI equilibrates to its antiferromagnetic (AF) ground state. The
topological charge of the defect differs with respect to the structural and
magnetic order; thus, X-ray diffraction from the ASI produces photons with even
and odd OAM quantum numbers at the structural and AF Bragg conditions,
respectively. The magnetic transitions of the ASI allow the AF OAM beams to be
switched on and off by modest variations of temperature and applied magnetic
field. These results demonstrate ASIs can serve as metasurfaces for
reconfigurable X-ray optics that could enable selective probes of electronic
and magnetic properties.Comment: 7 pages, 4 figure
Lessons learned from shallow subglacial bedrock drilling campaigns in Antarctica
We review successes and challenges from five recent subglacial bedrock drilling campaigns intended to find evidence for Antarctic Ice Sheet retreat during warm periods in the geologic past. Insights into times when the polar ice sheets were smaller than present serve as guiding information for modeling efforts that aim to predict the rate and magnitude of future sea level rise that would accompany major retreat of the Antarctic Ice Sheet. One method to provide direct evidence for the timing of deglaciations and minimum extent of prior ice sheets is to extract subglacial bedrock cores for cosmogenic nuclide analysis from beneath the modern ice sheet surface. Here we summarize the lessons learned from five field seasons tasked with obtaining bedrock cores from shallow depths (<120 m beneath ice surface) across West Antarctica since 2016. We focus our findings on drilling efforts and technology and geophysical surveys with ground-penetrating radar. Shallow subglacial drilling provides a high risk, high reward means to test for past instabilities of the Antarctic Ice Sheet, and we highlight key challenges and solutions to increase the likelihood of success for future subglacial drilling efforts in polar regions
No Evidence of a Common DNA Variant Profile Specific to World Class Endurance Athletes
There are strong genetic components to cardiorespiratory fitness and its
response to exercise training. It would be useful to understand the
differences in the genomic profile of highly trained endurance athletes of
world class caliber and sedentary controls. An international consortium
(GAMES) was established in order to compare elite endurance athletes and
ethnicity-matched controls in a case-control study design. Genome-wide
association studies were undertaken on two cohorts of elite endurance athletes
and controls (GENATHLETE and Japanese endurance runners), from which a panel
of 45 promising markers was identified. These markers were tested for
replication in seven additional cohorts of endurance athletes and controls:
from Australia, Ethiopia, Japan, Kenya, Poland, Russia and Spain. The study is
based on a total of 1520 endurance athletes (835 who took part in endurance
events in World Championships and/or Olympic Games) and 2760 controls. We
hypothesized that world-class athletes are likely to be characterized by an
even higher concentration of endurance performance alleles and we performed
separate analyses on this subsample. The meta-analysis of all available
studies revealed one statistically significant marker (rs558129 at GALNTL6
locus, p = 0.0002), even after correcting for multiple testing. As shown by
the low heterogeneity index (I2 = 0), all eight cohorts showed the same
direction of association with rs558129, even though p-values varied across the
individual studies. In summary, this study did not identify a panel of genomic
variants common to these elite endurance athlete groups. Since GAMES was
underpowered to identify alleles with small effect sizes, some of the
suggestive leads identified should be explored in expanded comparisons of
world-class endurance athletes and sedentary controls and in tightly
controlled exercise training studies. Such studies have the potential to
illuminate the biology not only of world class endurance performance but also
of compromised cardiac functions and cardiometabolic diseases
Integrative Genomics Reveals Novel Molecular Pathways and Gene Networks for Coronary Artery Disease
The majority of the heritability of coronary artery disease (CAD) remains unexplained, despite recent successes of genome-wide association studies (GWAS) in identifying novel susceptibility loci. Integrating functional genomic data from a variety of sources with a large-scale meta-analysis of CAD GWAS may facilitate the identification of novel biological processes and genes involved in CAD, as well as clarify the causal relationships of established processes. Towards this end, we integrated 14 GWAS from the CARDIoGRAM Consortium and two additional GWAS from the Ottawa Heart Institute (25,491 cases and 66,819 controls) with 1) genetics of gene expression studies of CAD-relevant tissues in humans, 2) metabolic and signaling pathways from public databases, and 3) data-driven, tissue-specific gene networks from a multitude of human and mouse experiments. We not only detected CAD-associated gene networks of lipid metabolism, coagulation, immunity, and additional networks with no clear functional annotation, but also revealed key driver genes for each CAD network based on the topology of the gene regulatory networks. In particular, we found a gene network involved in antigen processing to be strongly associated with CAD. The key driver genes of this network included glyoxalase I (GLO1) and peptidylprolyl isomerase I (PPIL1), which we verified as regulatory by siRNA experiments in human aortic endothelial cells. Our results suggest genetic influences on a diverse set of both known and novel biological processes that contribute to CAD risk. The key driver genes for these networks highlight potential novel targets for further mechanistic studies and therapeutic interventions
Positional Cloning of “Lisch-like”, a Candidate Modifier of Susceptibility to Type 2 Diabetes in Mice
In 404 Lepob/ob F2 progeny of a C57BL/6J (B6) x DBA/2J (DBA) intercross, we mapped a DBA-related quantitative trait locus (QTL) to distal Chr1 at 169.6 Mb, centered about D1Mit110, for diabetes-related phenotypes that included blood glucose, HbA1c, and pancreatic islet histology. The interval was refined to 1.8 Mb in a series of B6.DBA congenic/subcongenic lines also segregating for Lepob. The phenotypes of B6.DBA congenic mice include reduced β-cell replication rates accompanied by reduced β-cell mass, reduced insulin/glucose ratio in blood, reduced glucose tolerance, and persistent mild hypoinsulinemic hyperglycemia. Nucleotide sequence and expression analysis of 14 genes in this interval identified a predicted gene that we have designated “Lisch-like” (Ll) as the most likely candidate. The gene spans 62.7 kb on Chr1qH2.3, encoding a 10-exon, 646–amino acid polypeptide, homologous to Lsr on Chr7qB1 and to Ildr1 on Chr16qB3. The largest isoform of Ll is predicted to be a transmembrane molecule with an immunoglobulin-like extracellular domain and a serine/threonine-rich intracellular domain that contains a 14-3-3 binding domain. Morpholino knockdown of the zebrafish paralog of Ll resulted in a generalized delay in endodermal development in the gut region and dispersion of insulin-positive cells. Mice segregating for an ENU-induced null allele of Ll have phenotypes comparable to the B.D congenic lines. The human ortholog, C1orf32, is in the middle of a 30-Mb region of Chr1q23-25 that has been repeatedly associated with type 2 diabetes
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