arXiv subjects

James Overduin

Publications and source records attributed to James Overduin.

At least 19 recordsLinked to original sources

arXiv2026

The Cosmic Ultraviolet Background at the Galactic Poles

We have used archival GALEX data to separate the cosmic ultraviolet background at the Galactic Poles into two components: the dust scattered light and an offset. We have modeled the dust-scattered light using a single- scattering model finding 1 sigma limits of 0.54 -- 0.71 for the albedo (a) and 0.74 -- 0.83 for the phase function asymmetry factor (g) at 1530 {\AA} and 0.66 -- 0.73 for a and 0.71 -- 0.77 for g at 2360 {\AA}, that is, the grains are moderately reflective and highly forward-scattering. The offsets are 277 -- 284 photon units at 1530 {\AA} and 513 -- 520 photon units at 2360 {\AA}. We have estimated other Galactic and extragalactic contributors to the offset finding that 161 +- 18 photon units is unaccounted for at 1530 {\AA} and 335 +- 38 at 2360 {\AA}. The offsets are constant over these regions with variances of 20 -- 30 photon units.

astro-ph.GA↗

arXiv2026

Observations of High Galactic Latitude Line and Continuum Emission (912 - 1600 \AA) with New Horizons

We present observations of the cosmic ultraviolet background (CUVB) from 912 - 1600 A using the Stem aperture of the Alice spectrograph on the New Horizons spacecraft at 56 AU from the Sun, providing a spectral resolution of 9 A for diffuse sources. We detect emission lines of CIII (977 A) and CIV (1548/1551 A) at the 3 sigma level with strengths of 4200 +/- 1500 and 4100 +/- 1200 ph cm(-2) s(-1) sr(-1), respectively, and a marginal detection of OVI (1032/1038 A) at 1400 +/- 1300 ph cm(-2) s(-1) sr(-1). We report a 3 sigma detection of an emission line at 1135 A, which we have identified with the N I resonance triplet. Although this line had earlier been observed in FUSE and SPEAR data, it had been attributed to airglow or instrumental effects. We confirm, for the first time, that it must originate in the Galaxy. The dust-scattered continuum is dominated by a small number (N < 100) of O9 - B2 stars and shows the deep absorption feature near 1000 A seen in the stellar spectrum. Our models suggest an albedo of a < 0.5 over most of the spectrum (950 -- 1550 A) for the dust grains with the phase function asymmetry of g < 0.6. We find an offset, comprising the extragalactic background light and halo contributors, consistent with our earlier results from the Box, including the decline in the offset near the Lyman limit. We confirm that much of the emission must be from an unidentified component of the CUVB.

astro-ph.GA↗

arXiv2023

Dust Scattered Radiation in the Galactic Poles

We have modeled the diffuse background at the Galactic Poles in the far-ultraviolet (FUV: 1536 \AA) and the near-ultraviolet (NUV: 2316 \AA). The background is well-fit using a single-scattering dust model with an offset representing the extragalactic light plus any other contribution to the diffuse background. We have found a dust albedo of 0.35 -- 0.40 (FUV) and 0.11 -- 0.19 in the NGP ($b > 70^{\circ}$) and 0.46 -- 0.56 (FUV) and 0.31 -- 0.33 (NUV) in the SGP ($b < 70^{\circ}$. The differences in the albedo may reflect changes in the dust-to-gas ratio over the sky or in the dust distribution. We find offsets at zero-reddening of 273 -- 286 and 553 -- 581 photons cm$^{-2}$ s$^{-1}$ sr$^{-1}$ \AA$^{-1}$ in the FUV and NUV, respectively, in the NGP with similar values in the SGP.

astro-ph.GA↗

arXiv2022

Einstein's Happiest Moment: The Equivalence Principle

Einstein's happiest thought was his leap from the observation that a falling person feels no gravity to the realization that gravity might be equivalent to acceleration. It affects all bodies in the same way because it is a property of spacetime -- its curvature -- not a force propagating through spacetime (like electromagnetic or nuclear forces). When expressed in a way that is manifestly independent of the choice of coordinates, this idea became General Relativity. But the ground for what is now known as the "equivalence principle" was laid long before Einstein, affording a fascinating example of the growth of a scientific idea through the continuous interplay between theory and experiment. That story continues through the present era, with improved techniques and measurements, and in the new environment of space. Theoretically, equivalence is now understood to rank with Lorentz invariance as one of nature's most fundamental principles. Violations of equivalence are generically predicted by attempts to unify gravity with "non-geometrical" fundamental interactions, but there is no consensus on the form that these violations will take. Experimental tests of equivalence thus face the challenge of striving simultaneously for the highest possible sensitivity and diversity in test materials.

physics.hist-ph↗

arXiv2021

The Scourge of Online Solutions and an Academic Hertzsprung-Russell Diagram

We report on preliminary results of a statistical study of student performance in more than a decade of calculus-based introductory physics courses. Treating average homework and test grades as proxies for student effort and comprehension respectively, we plot comprehension versus effort in an academic version of the astronomical Hertzsprung-Russell diagram (which plots stellar luminosity versus temperature). We study the evolution of this diagram with time, finding that the "academic main sequence" has begun to break down in recent years as student achievement on tests has become decoupled from homework grades. We present evidence that this breakdown is likely related to the emergence of easily accessible online solutions to most textbook problems, and discuss possible responses and strategies for maintaining and enhancing student learning in the online era.

physics.ed-ph↗

arXiv2020

Physics and the Pythagorean Theorem

Pythagoras' theorem lies at the heart of physics as well as mathematics, yet its historical origins are obscure. We highlight a purely pictorial, gestalt-like proof that may have originated during the Zhou Dynasty. Generalizations of the Pythagorean theorem to three, four and more dimensions undergird fundamental laws including the energy-momentum relation of particle physics and the field equations of general relativity, and may hint at future unified theories. The "pre-mathematical" nature of this theorem lends support to the Eddingtonian view that "the stuff of the world is mind-stuff."

physics.hist-ph↗

arXiv2020

The Diffuse Ultraviolet Background Close to the Galactic Plane

We have used Voyager and Galex observations to map the diffuse Galactic light near the Galactic equator. We find that most of the observations are relatively faint with surface brightnesses of less than 5,000 photon units. This is important because many ultraviolet telescopes have not observed at low Galactic latitudes because of the fear of a bright diffuse emission. Our data are consistent with emission from interstellar dust grains with albedo ($a$) of 0.2 -- 0.3 and phase function ($g$) $ < 0.7$ at 1100 Å; $0.2 < a < 0.5; g < 0.8$ at 1500 Å; and $0.4 < a < 0.6; g < 0.4$ at 2300 Å.

astro-ph.GA↗

arXiv2019

Components of the Diffuse Ultraviolet Radiation at High Latitudes

We have used data from the Galaxy Evolution Explorer to study the different components of the diffuse ultraviolet background in the region between the Galactic latitudes 70-80 degree. We find an offset at zero dust column density (E(B - V) = 0) of $240 \pm 18$ photon units in the FUV (1539A) and $394 \pm 37$ photon units in the NUV (2316A). This is approximately half of the total observed radiation with the remainder divided between an extragalactic component of $114 \pm 18$ photon units in the FUV and $194 \pm 37$ photon units in the NUV and starlight scattered by Galactic dust at high latitudes. The optical constants of the dust grains were found to be a=0.4$\pm$0.1 and g=0.8$\pm$0.1 (FUV) and a=0.4$\pm$0.1 and g=0.5$\pm$0.1 (NUV). We cannot differentiate between a Galactic or extragalactic origin for the zero-offset but can affirm that it is not from any known source.

astro-ph.GA↗

arXiv2018

Paul Wesson and Space-Time-Matter Theory

A short scientific biography is given of physicist Paul Wesson (1949-2015), who published over 300 works encompassing the fields of astrobiology, astrophysics, geophysics, cosmology, and relativity; and who was particularly associated with a fully covariant version of Kaluza-Klein theory known as Space-Time-Matter theory, in which matter and energy in four dimensions are induced from empty space in higher dimensions, thus realizing Einstein's dream and unifying the gravitational field with its source.

physics.hist-ph↗

arXiv2018

Discovery of an Ionizing Radiation Field in the Universe

We draw attention to observational evidence indicating that a substantial fraction of the well-known cosmic celestial diffuse ultraviolet background radiation field is actually due not to dust-scattered starlight, but rather---considering its spectral character at most locations in the sky---has an unknown physical origin. We arrive at this conclusion from re-examination of spectra of the diffuse ultraviolet background that were obtained---long ago---using the ultraviolet spectrometers aboard the two Voyager spacecraft, which were located far out in our solar system, and at very different locations. As there is neither a reasonable nor even an unreasonable conventional astrophysical source for this newly-identified fraction of the radiation, we are led to speculate that the photons that we observe have their origin in the very slow decay of the particles that make up the ubiquitous dark matter, which we know envelopes our Galaxy. Whether or not that actually is the source, this new radiation field extends somewhat below 912 Å, so we have found, at last, the radiation that re-ionized the universe.

astro-ph.GA↗

arXiv2018

The Diffuse Radiation Field at High Galactic Latitudes

We have used GALEX observations of the North and South Galactic poles to study the diffuse ultraviolet background at locations where the Galactic light is expected to be at a minimum. We find offsets of 230 -- 290 photon units in the FUV (1531 Å) and 480 -- 580 photon units in the NUV (2361 Å). Of this, approximately 120 photon units can be ascribed to dust scattered light and another 110 (190 in the NUV) photon units to extragalactic radiation. The remaining radiation is, as yet, unidentified and amounts to $120 -- 180$ photon units in the FUV and $300 -- 400$ photon units in the NUV. We find that molecular hydrogen fluorescence contributes to the FUV when the 100 $μ$m surface brightness is greater than 1.08 MJy sr$^{-1}$.

astro-ph.GA↗

arXiv2016

Supernova constraints on higher-dimensional cosmology with a phantom field

We use observational data on the magnitude-redshift relation for Type Ia supernovae together with constraints on the ages of the oldest stars to rule out a higher-dimensional extension of General Relativity with a negative kinetic-energy scalar field. This theory is of considerable physical interest because it produces accelerated expansion at both early and late times with a single new field, as in quintessential inflation scenarios. It is also of mathematical interest because it is characterized by an analytic expression for the macroscopic scale factor $a(t)$. We show that cosmological solutions of this theory can be usefully parametrized by a single quantity, the lookback time $τ_{\text{tr}}$ corresponding to the transition from deceleration to acceleration. Supernovae data from the recently released Supernova Cosmology Project Union 2.1 compilation single out a narrow range of values for $τ_{\text{tr}}$. In the context of the theory, however, these same values of $τ_{\text{tr}}$ imply that the universe is much older than the oldest observed stars.

gr-qc↗

arXiv2014

Cosmological Coincidence without Fine Tuning

We present a simple cosmological model in which a single, non-minimally coupled scalar field with a quartic potential is responsible for both inflation at early times and acceleration at late times. Little or no fine tuning is needed to explain why the present density of dark energy is comparable to that of pressureless matter. Dark energy is identified with the potential of the scalar field, which is sourced by the trace of the energy-momentum tensor. This becomes significant when matter has decoupled from radiation and become fully non-relativistic, so that $ϕ\proptoρ_m^{1/3}\proptoρ_{m,0}^{1/3}(a_0/a)\sim(10^{-120})^{1/3}(10^{10})\sim10^{-30}$ and $V\simϕ^4\sim10^{-120}$ in Planck units, as observed.

hep-th↗

arXiv2014

The Mystery of the Cosmic Diffuse Ultraviolet Background Radiation

The diffuse cosmic background radiation in the GALEX far ultraviolet (FUV, 1300 Å - 1700 Å) is deduced to originate only partially in the dust-scattered radiation of FUV-emitting stars: the source of a substantial fraction of the FUV background radiation remains a mystery. The radiation is remarkably uniform at both far northern and far southern Galactic latitudes, and it increases toward lower Galactic latitudes at all Galactic longitudes. We examine speculation that it might be due to interaction of the dark matter with the nuclei of the interstellar medium but we are unable to point to a plausible mechanism for an effective interaction. We also explore the possibility that we are seeing radiation from bright FUV-emitting stars scattering from a "second population" of interstellar grains---grains that are small compared with FUV wavelengths. Such grains are known to exist (Draine 2011) and they scatter with very high albedo, with an isotropic scattering pattern. However, comparison with the observed distribution (deduced from their $100\ μ$m emission) of grains at high Galactic latitudes shows no correlation between the grains' location and the observed FUV emission. Our modeling of the FUV scattering by small grains also shows that there must be remarkably few such "smaller" grains at high Galactic latitudes, both North and South; this likely means simply that there is very little interstellar dust of any kind at the Galactic poles, in agreement with Perry & Johnston (1982). We also review our limited knowledge of the cosmic diffuse background at ultraviolet wavelengths shortward of Lyman $α$---it could be that our "second component" of the diffuse far-ultraviolet background persists shortward of the Lyman limit, and is the cause of the re-ionization of the Universe (Kollmeier et al. 2014).

astro-ph.GA↗

arXiv2014

STEP and fundamental physics

The Satellite Test of the Equivalence Principle (STEP) will advance experimental limits on violations of Einstein's Equivalence Principle from their present sensitivity of 2 parts in $10^{13}$ to 1 part in $10^{18}$ through multiple comparison of the motions of four pairs of test masses of different compositions in a drag-free earth-orbiting satellite. We describe the experiment, its current status, and its potential implications for fundamental physics. Equivalence is at the heart of general relativity, our governing theory of gravity, and violations are expected in most attempts to unify this theory with the other fundamental interactions of physics, as well as in many theoretical explanations for the phenomenon of dark energy in cosmology. Detection of such a violation would be equivalent to the discovery of a new force of nature. A null result would be almost as profound, pushing upper limits on any coupling between standard-model fields and the new light degrees of freedom generically predicted by these theories down to unnaturally small levels.

gr-qc↗

arXiv2013

Expanded solar-system limits on violations of the equivalence principle

Most attempts to unify general relativity with the standard model of particle physics predict violations of the equivalence principle associated in some way with the composition of the test masses. We test this idea by using observational uncertainties in the positions and motions of solar-system bodies to set upper limits on possible differences $Δ$ between the gravitational and inertial mass of each body. For suitable pairs of objects, it is possible to constrain three different linear combinations of $Δ$ using Kepler's third law, the migration of stable Lagrange points, and orbital polarization (the Nordtvedt effect). Limits of order $10^{-10}-10^{-6}$ on $Δ$ for individual bodies can then be derived from planetary and lunar ephemerides, Cassini observations of the Saturn system, and observations of Jupiter's Trojan asteroids as well as recently discovered Trojan companions around the Earth, Mars, Neptune, and Saturnian moons. These results can be combined with models for elemental abundances in each body to test for composition-dependent violations of the universality of free fall in the solar system. The resulting limits are weaker than those from laboratory experiments, but span a larger volume in composition space.

gr-qc↗

arXiv2009

The Science Case for STEP

STEP (the Satellite Test of the Equivalence Principle) will advance experimental limits on violations of Einstein's equivalence principle (EP) from their present sensitivity of 2 parts in 10^13 to 1 part in 10^18 through multiple comparison of the motions of four pairs of test masses of different compositions in an earth-orbiting drag-free satellite. Dimensional arguments suggest that violations, if they exist, should be found in this range, and they are also suggested by leading attempts at unified theories of fundamental interactions (e.g. string theory) and cosmological theories involving dynamical dark energy. Discovery of a violation would constitute the discovery of a new force of nature and provide a critical signpost toward unification. A null result would be just as profound, because it would close off any possibility of a natural-strength coupling between standard-model fields and the new light degrees of freedom that such theories generically predict (e.g., dilatons, moduli, quintessence). STEP should thus be seen as the intermediate-scale component of an integrated strategy for fundamental physics experiments that already includes particle accelerators (at the smallest scales) and supernova probes (at the largest). The former may find indirect evidence for new fields via their missing-energy signatures, and the latter may produce direct evidence through changes in cosmological equation of state--but only a gravitational experiment like STEP can go further and reveal how or whether such a field couples to the rest of the standard model. It is at once complementary to the other two kinds of tests, and a uniquely powerful probe of fundamental physics in its own right.

gr-qc↗

arXiv2006

Wolfgang Priester: from the big bounce to the Lambda-dominated universe

Wolfgang Priester was one of Germany's most versatile and quixotic astrophysicists, re-inventing himself successively as a radio astronomer, space physicist and cosmologist, and making a lasting impact on each field. We focus in this personal account on his contributions to cosmology, where he will be most remembered for his association with quasars, his promotion of the idea of a nonsingular "big bounce" at the beginning of the current expansionary phase, and his recognition of the importance of dark energy (Einstein's cosmological constant Lambda) well before this became the standard paradigm in cosmology.

astro-ph↗