11,118 research outputs found
Detecting Early Galaxies Through Their 21-cm Signature
New observations over the next few years of the emission of distant objects
will help unfold the chapter in cosmic history around the era of the first
galaxies. These observations will use the neutral hydrogen emission or
absorption at a wavelength of 21-cm as a detector of the hydrogen abundance. We
predict the signature on the 21-cm signal of the early generations of galaxies.
We calculate the 21-cm power spectrum including two physical effects that were
neglected in previous calculations. The first is the redistribution of the UV
photons from the first galaxies due to their scattering off of the neutral
hydrogen, which results in an enhancement of the 21-cm signal. The second is
the presence of an ionized hydrogen bubble near each source, which produces a
cutoff at observable scales. We show that the resulting clear signature in the
21-cm power spectrum can be used to detect and study the population of galaxies
that formed just 200 million years after the Big Bang.Comment: 5 pages, 3 figures, submitted to MNRAS Let
Wave and Particle Scattering Properties of High Speed Black Holes
The light-like limit of the Kerr gravitational field relative to a distant
observer moving rectilinearly in an arbitrary direction is an impulsive plane
gravitational wave with a singular point on its wave front. By colliding
particles with this wave we show that they have the same focussing properties
as high speed particles scattered by the original black hole. By colliding
photons with the gravitational wave we show that there is a circular disk,
centered on the singular point on the wave front, having the property that
photons colliding with the wave within this disk are reflected back and travel
with the wave. This result is approximate in the sense that there are observers
who can see a dim (as opposed to opaque) circular disk on their sky. By
colliding plane electromagnetic waves with the gravitational wave we show that
the reflected electromagnetic waves are the high frequency waves.Comment: Latex file, 22 pages, 1 figure, accepted for publication in Classical
and Quantum Gravit
A Characterisation of Strong Wave Tails in Curved Space-Times
A characterisation of when wave tails are strong is proposed. The existence
of a curvature induced tail (i.e. a Green's function term whose support
includes the interior of the light-cone) is commonly understood to cause
backscattering of the field governed by the relevant wave equation. Strong
tails are characterised as those for which the purely radiative part of the
field is backscattered. With this definition, it is shown that electromagnetic
waves in asymptotically flat space-times and fields governed by tail-free
propagation have weak tails, but minimally coupled scalar fields in a
cosmological scenario have strong tails.Comment: 17 pages, Revtex, to appear in Classical and Quantum Gravit
Magnetic order and the electronic ground state in the pyrochlore iridate Nd2Ir2O7
We report a combined muon spin relaxation/rotation, bulk magnetization,
neutron scattering, and transport study of the electronic properties of the
pyrochlore iridate Nd2Ir2O7. We observe the onset of strongly hysteretic
behavior in the temperature dependent magnetization below 120 K, and an abrupt
increase in the temperature dependent resistivity below 8 K. Zero field muon
spin relaxation measurements show that the hysteretic magnetization is driven
by a transition to a magnetically disordered state, and that below 8 K a
complex magnetically ordered ground state sets in, as evidenced by the onset of
heavily damped spontaneous muon precession. Our measurements point toward the
absence of a true metal-to-insulator phase transition in this material and
suggest that Nd2Ir2O7 lies either within or on the metallic side of the
boundary of the Dirac semimetal regime within its topological phase diagram.Comment: 21 pages, 7 figure
Positive impact of low-dose, high-energy radiation on bone in partial- and/or full-weightbearing mice
Astronauts traveling beyond low Earth orbit will be exposed to galactic cosmic radiation (GCR); understanding how high energy ionizing radiation modifies the bone response to mechanical unloading is important to assuring crew health. To investigate this, we exposed 4-mo-old female Balb/cBYJ mice to an acute space-relevant dose of 0.5 Gy 56Fe or sham (n = ~8/group); 4 days later, half of the mice were also subjected to a ground-based analog for 1/6 g (partial weightbearing) (G/6) for 21 days. Microcomputed tomography (µ-CT) of the distal femur reveals that 56Fe exposure resulted in 65-78% greater volume and improved microarchitecture of cancellous bone after 21 d compared to sham controls. Radiation also leads to significant increases in three measures of energy absorption at the mid-shaft femur and an increase in stiffness of the L4 vertebra. No significant effects of radiation on bone formation indices are detected; however, G/6 leads to reduced % mineralizing surface on the inner mid-tibial bone surface. In separate groups allowed 21 days of weightbearing recovery from G/6 and/or 56Fe exposure, radiation-exposed mice still exhibit greater bone mass and improved microarchitecture vs. sham control. However, femoral bone energy absorption values are no longer higher in the 56Fe-exposed WB mice vs. sham controls. We provide evidence for persistent positive impacts of high-LET radiation exposure preceding a period of full or partial weightbearing on bone mass and microarchitecture in the distal femur and, for full weightbearing mice only and more transiently, cortical bone energy absorption values
Inhomogeneous High Frequency Expansion-Free Gravitational Waves
We describe a natural inhomogeneous generalization of high frequency plane
gravitational waves. The waves are high frequency waves of the Kundt type whose
null propagation direction in space-time has vanishing expansion, twist and
shear but is not covariantly constant. The introduction of a cosmological
constant is discussed in some detail and a comparison is made with high
frequency gravity waves having wave fronts homeomorphic to 2-spheres.Comment: 18 pages, Latex file, accepted for publication in Physical Review
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
Comparison of Exact and perturbative Results for Two Metrics
We compare the exact and perturbative results in two metrics and show that
the spurious effects due to the perturbation method do not survive for
physically relevant quantities such as the vacuum expectation value of the
stress-energy tensor.Comment: 12 page
Credit scores, cardiovascular disease risk, and human capital.
Credit scores are the most widely used instruments to assess whether or not a person is a financial risk. Credit scoring has been so successful that it has expanded beyond lending and into our everyday lives, even to inform how insurers evaluate our health. The pervasive application of credit scoring has outpaced knowledge about why credit scores are such useful indicators of individual behavior. Here we test if the same factors that lead to poor credit scores also lead to poor health. Following the Dunedin (New Zealand) Longitudinal Study cohort of 1,037 study members, we examined the association between credit scores and cardiovascular disease risk and the underlying factors that account for this association. We find that credit scores are negatively correlated with cardiovascular disease risk. Variation in household income was not sufficient to account for this association. Rather, individual differences in human capital factors—educational attainment, cognitive ability, and self-control—predicted both credit scores and cardiovascular disease risk and accounted for ∼45% of the correlation between credit scores and cardiovascular disease risk. Tracing human capital factors back to their childhood antecedents revealed that the characteristic attitudes, behaviors, and competencies children develop in their first decade of life account for a significant portion (∼22%) of the link between credit scores and cardiovascular disease risk at midlife. We discuss the implications of these findings for policy debates about data privacy, financial literacy, and early childhood interventions
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