10,044 research outputs found
Single-color two-photon spectroscopy of Rydberg states in electric fields
Rydberg states of atomic helium with principal quantum numbers ranging from
n=20 to n=100 have been prepared by non-resonance-enhanced single-color
two-photon excitation from the metastable 2 {^3}S{_1} state. Photoexcitation
was carried out using linearly and circularly polarized pulsed laser radiation.
In the case of excitation with circularly polarized radiation, Rydberg states
with azimuthal quantum number |m_{\ell}|=2 were prepared in zero electric
field, and in homogeneous electric fields oriented parallel to the propagation
axis of the laser radiation. In sufficiently strong electric fields, individual
Rydberg-Stark states were resolved spectroscopically, highlighting the
suitability of non-resonance-enhanced multiphoton excitation schemes for the
preparation of long-lived high-|m_{\ell}| hydrogenic Rydberg states for
deceleration and trapping experiments. Applications of similar schemes for
Doppler-free excitation of positronium atoms to Rydberg states are also
discussed
Gravitational Waves from Mesoscopic Dynamics of the Extra Dimensions
Recent models which describe our world as a brane embedded in a higher
dimensional space introduce new geometrical degrees of freedom: the shape
and/or size of the extra dimensions, and the position of the brane. These modes
can be coherently excited by symmetry breaking in the early universe even on
``mesoscopic'' scales as large as 1 mm, leading to detectable gravitational
radiation. Two sources are described: relativistic turbulence caused by a
first-order transition of a radion potential, and Kibble excitation of
Nambu-Goldstone modes of brane displacement. Characteristic scales and spectral
properties are estimated and the prospects for observation by LISA are
discussed. Extra dimensions with scale between 10 \AA and 1 mm, which enter the
3+1-D era at cosmic temperatures between 1 and 1000 TeV, produce backgrounds
with energy peaked at observed frequencies in the LISA band, between
and Hz. The background is detectable above instrument and
astrophysical foregrounds if initial metric perturbations are excited to a
fractional amplitude of or more, a likely outcome for the
Nambu-Goldstone excitations.Comment: Latex, 5 pages, plus one figure, final version to appear in Phys.
Rev. Let
A Bayesian Estimate of the Primordial Helium Abundance
We introduce a new statistical method to estimate the primordial helium
abundance, Y_p from observed abundances in a sample of galaxies which have
experienced stellar helium enrichment. Rather than using linear regression on
metal abundance we construct a likelihood function using a Bayesian prior,
where the key assumption is that the true helium abundance must always exceed
the primordial value. Using a sample of measurements compiled from the
literature we find estimates of Y_p between 0.221 and 0.236, depending on the
specific subsample and prior adopted, consistent with previous estimates either
from a linear extrapolation of the helium abundance with respect to
metallicity, or from the helium abundance of the lowest metallicity HII region,
I Zw 18. We also find an upper limit which is insensitive to the specific
subsample or prior, and estimate a model-independent bound Y_p < 0.243 at 95%
confidence, favoring a low cosmic baryon density and a high primordial
deuterium abundance. The main uncertainty is not the model of stellar
enrichment but possible common systematic biases in the estimate of Y in each
individual HII region.Comment: 14 pages, latex, 3 ps figure
Scales of the Extra Dimensions and their Gravitational Wave Backgrounds
Circumstances are described in which symmetry breaking during the formation
of our three-dimensional brane within a higher-dimensional space in the early
universe excites mesoscopic classical radion or brane-displacement degrees of
freedom and produces a detectable stochastic background of gravitational
radiation. The spectrum of the background is related to the unification energy
scale and the the sizes and numbers of large extra dimensions. It is shown that
properties of the background observable by gravitational-wave observatories at
frequencies Hz to Hz contain information about
unification on energy scales from 1 to TeV, gravity propagating
through extra-dimension sizes from 1 mm to mm, and the dynamical
history and stabilization of from one to seven extra dimensions.Comment: 6 pages, Latex, 1 figure, submitted to Phys. Re
The rodent research animal holding facility as a barrier to environmental contamination
The rodent Research Animal Holding Facility (RAHF), developed by NASA Ames Research Center (ARC) to separately house rodents in a Spacelab, was verified as a barrier to environmental contaminants during a 12-day biocompatibility test. Environmental contaminants considered were solid particulates, microorganisms, ammonia, and typical animal odors. The 12-day test conducted in August 1988 was designed to verify that the rodent RAHF system would adequately support and maintain animal specimens during normal system operations. Additional objectives of this test were to demonstrate that: (1) the system would capture typical particulate debris produced by the animal; (2) microorganisms would be contained; and (3) the passage of animal odors was adequately controlled. In addition, the amount of carbon dioxide exhausted by the RAHF system was to be quantified. Of primary importance during the test was the demonstration that the RAHF would contain particles greater than 150 micrometers. This was verified after analyzing collection plates placed under exhaust air ducts and rodent cages during cage maintenance operations, e.g., waste tray and feeder changeouts. Microbiological testing identified no additional organisms in the test environment that could be traced to the RAHF. Odor containment was demonstrated to be less than barely detectable. Ammonia could not be detected in the exhaust air from the RAHF system. Carbon dioxide levels were verified to be less than 0.35 percent
Fast adaptation of cooperative channels engenders Hopf bifurcations in auditory hair cells
Since the pioneering work of Thomas Gold, published in 1948, it has been known that we owe our sensitive sense of hearing to a process in the inner ear that can amplify incident sounds on a cycle-by-cycle basis. Called the active process, it uses energy to counteract the viscous dissipation associated with sound-evoked vibrations of the ear’s mechanotransduction apparatus. Despite its importance, the mechanism of the active process and the proximate source of energy that powers it have remained elusive, especially at the high frequencies characteristic of amniote hearing. This is partly due to our insufficient understanding of the mechanotransduction process in hair cells, the sensory receptors and amplifiers of the inner ear. It has been proposed previously that cyclical binding of Ca2+ ions to individual mechanotransduction channels could power the active process. That model, however, relied on tailored reaction rates that structurally forced the direction of the cycle. Here we ground our study on our previous model of hair-cell mechanotransduction, which relied on cooperative gating of pairs of channels, and incorporate into it the cyclical binding of Ca2+ ions. With a single binding site per channel and reaction rates drawn from thermodynamic principles, the current model shows that hair cells behave as nonlinear oscillators that exhibit Hopf bifurcations, dynamical instabilities long understood to be signatures of the active process. Using realistic parameter values, we find bifurcations at frequencies in the kilohertz range with physiological Ca2+ concentrations. The current model relies on the electrochemical gradient of Ca2+ as the only energy source for the active process and on the relative motion of cooperative channels within the stereociliary membrane as the sole mechanical driver. Equipped with these two mechanisms, a hair bundle proves capable of operating at frequencies in the kilohertz range, characteristic of amniote hearing
Relating gravitational wave constraints from primordial nucleosynthesis, pulsar timing, laser interferometers, and the CMB: implications for the early universe
We derive a general master equation relating the gravitational-wave
observables r and Omega_gw(f). Here r is the tensor-to-scalar ratio,
constrained by cosmic-microwave-background (CMB) experiments; and Omega_gw(f)
is the energy spectrum of primordial gravitational-waves, constrained e.g. by
pulsar-timing measurements, laser-interferometer experiments, and Big Bang
Nucleosynthesis (BBN). Differentiating the master equation yields a new
expression for the tilt d(ln Omega_gw(f))/d(ln f). The relationship between r
and Omega_gw(f) depends sensitively on the uncertain physics of the early
universe, and we show that this uncertainty may be encapsulated (in a
model-independent way) by two quantities: w_hat(f) and nt_hat(f), where
nt_hat(f) is a certain logarithmic average over nt(k) (the primordial tensor
spectral index); and w_hat(f) is a certain logarithmic average over w_tilde(a)
(the effective equation-of-state in the early universe, after horizon
re-entry). Here the effective equation-of-state parameter w_tilde(a) is a
combination of the ordinary equation-of-state parameter w(a) and the bulk
viscosity zeta(a). Thus, by comparing constraints on r and Omega_gw(f), one can
obtain (remarkably tight) constraints in the [w_hat(f), nt_hat(f)] plane. In
particular, this is the best way to constrain (or detect) the presence of a
``stiff'' energy component (with w > 1/3) in the early universe, prior to BBN.
Finally, although most of our analysis does not assume inflation, we point out
that if CMB experiments detect a non-zero value for r, then we will immediately
obtain (as a free by-product) a new upper bound w_hat < 0.55 on the
logarithmically averaged effective equation-of-state parameter during the
``primordial dark age'' between the end of inflation and the start of BBN.Comment: v1: 12 + 6 pages (main text + appendices), 7 figures; v2: fonts fixed
in figure
Transcriptional networks specifying homeostatic and inflammatory programs of gene expression in human aortic endothelial cells.
Endothelial cells (ECs) are critical determinants of vascular homeostasis and inflammation, but transcriptional mechanisms specifying their identities and functional states remain poorly understood. Here, we report a genome-wide assessment of regulatory landscapes of primary human aortic endothelial cells (HAECs) under basal and activated conditions, enabling inference of transcription factor networks that direct homeostatic and pro-inflammatory programs. We demonstrate that 43% of detected enhancers are EC-specific and contain SNPs associated to cardiovascular disease and hypertension. We provide evidence that AP1, ETS, and GATA transcription factors play key roles in HAEC transcription by co-binding enhancers associated with EC-specific genes. We further demonstrate that exposure of HAECs to oxidized phospholipids or pro-inflammatory cytokines results in signal-specific alterations in enhancer landscapes and associate with coordinated binding of CEBPD, IRF1, and NFκB. Collectively, these findings identify cis-regulatory elements and corresponding trans-acting factors that contribute to EC identity and their specific responses to pro-inflammatory stimuli
Spontaneous Magnetization of Axion Domain Wall and Primordial Magnetic Field
We show that axion domain walls gain spontaneous magnetization in early
universe by trapping either electrons or positrons with their spins polarized.
The reason is that the walls produces an attractive potential for these
particles. We argue that the wall bounded by an axionic superconducting string
leaves a magnetic field after its decay. We obtain a field
Gauss on the scale of horizon at the recombination.Comment: 10 Pages, Revte
Geodesics in spacetimes with expanding impulsive gravitational waves
We study geodesic motion in expanding spherical impulsive gravitational waves
propagating in a Minkowski background. Employing the continuous form of the
metric we find and examine a large family of geometrically preferred geodesics.
For the special class of axially symmetric spacetimes with the spherical
impulse generated by a snapping cosmic string we give a detailed physical
interpretation of the motion of test particles.Comment: 12 pages, Revtex, final versio
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