4,464 research outputs found
Toward solving the cosmological constant problem by embedding
The typical scalar field theory has a cosmological constant problem. We
propose a generic mechanism by which this problem is avoided at tree level by
embedding the theory into a larger theory. The metric and the scalar field
coupling constants in the original theory do not need to be fine-tuned, while
the extra scalar field parameters and the metric associated with the extended
theory are fine-tuned dynamically. Hence, no fine-tuning of parameters in the
full Lagrangian is needed for the vacuum energy in the new physical system to
vanish at tree level. The cosmological constant problem can be solved if the
method can be extended to quantum loops.Comment: published versio
The construction and evaluation of an instrument to determine the sports knowledge of boys from grades nine through twelve,
Thesis (M.A.)--Boston Universit
Nuclear Quasi-Elastic Electron Scattering Limits Nucleon Off-Mass Shell Properties
The use of quasi-elastic electron nucleus scattering is shown to provide
significant constraints on models of the proton electromagnetic form factor of
off-shell nucleons. Such models can be constructed to be consistent with
constraints from current conservation and low-energy theorems, while also
providing a contribution to the Lamb shift that might potentially resolve the
proton radius puzzle in muonic hydrogen. However, observations of quasi-elastic
scattering limit the overall strength of the off-shell form factors to values
that correspond to small contributions to the Lamb shift.Comment: 11 pages, 2 figures. Resubmission to improve the clarity, and correct
possible misconception
Post-Newtonian expansion for Gauss-Bonnet Gravity
The Parametrized Post-Newtonian expansion of gravitational theories with a
scalar field coupled to the Gauss-Bonnet invariant is performed and
confrontation of such theories with Solar system experiments is discussed.Comment: 4 pages; typos corrected, published versio
The Effect of On-Line Videos on Learner Outcomes in a Mechanics of Materials Course
The Mechanics of Materials course is one of the core engineering courses included in the curriculum of mechanical, civil, mining, petroleum, marine, aeronautical, and several other engineering disciplines. As a core course, the Mechanics of Materials course typically has large enrollment. Initiatives aimed at improving the effectiveness of the engineering core courses can have a major impact on engineering education by virtue of the large number of students affected.
Computers afford opportunities for creative instructional activities that are not possible in the traditional lecture-and-textbook class format. The study described in this paper examines the effectiveness of asynchronous online video that has been used in various ways in a Mechanics of Materials course over the past four years. The content delivered via the Internet included concept videos, problem-solving videos, and videos of demonstrations and laboratory activities.
In this study, four differing approaches to present the Mechanics of Materials course to approximately 1000 students in 17 course sections over a four-year period were compared. The first approach involved traditional, face-to-face lectures. The second approach completely replaced the face-to-face lectures with videos recorded by the instructor outside of the classroom, but covering the same topics as the classroom lectures, and then posted to a class web site. The instructor was available in his office during class time to answer questions. The third approach combined face-to-face lectures with videos. The fourth approach was an inverted format where students watched videos at home and worked on homework during class.
Using common final exam scores as a quantitative measure of effectiveness, results showed that overall student performance was maintained as class sizes and instructor workloads increased. Additionally, there was some indication that the inverted approach was better suited for higher-ability students
The nearly Newtonian regime in Non-Linear Theories of Gravity
The present paper reconsiders the Newtonian limit of models of modified
gravity including higher order terms in the scalar curvature in the
gravitational action. This was studied using the Palatini variational principle
in [Meng X. and Wang P.: Gen. Rel. Grav. {\bf 36}, 1947 (2004)] and
[Dom\'inguez A. E. and Barraco D. E.: Phys. Rev. D {\bf 70}, 043505 (2004)]
with contradicting results. Here a different approach is used, and problems in
the previous attempts are pointed out. It is shown that models with negative
powers of the scalar curvature, like the ones used to explain the present
accelerated expansion, as well as their generalization which include positive
powers, can give the correct Newtonian limit, as long as the coefficients of
these powers are reasonably small. Some consequences of the performed analysis
seem to raise doubts for the way the Newtonian limit was derived in the purely
metric approach of fourth order gravity [Dick R.: Gen. Rel. Grav. {\bf 36}, 217
(2004)]. Finally, we comment on a recent paper [Olmo G. J.: Phys. Rev. D {\bf
72}, 083505 (2005)] in which the problem of the Newtonian limit of both the
purely metric and the Palatini formalism is discussed, using the equivalent
Brans--Dicke theory, and with which our results partly disagree.Comment: typos corrected, replaced to match published versio
Bayesian Analysis of the Polarization of Distant Radio Sources: Limits on Cosmological Birefringence
A recent study of the rotation of the plane of polarization of light from 160
cosmological sources claims to find significant evidence for cosmological
anisotropy. We point out methodological weaknesses of that study, and reanalyze
the same data using Bayesian methods that overcome these problems. We find that
the data always favor isotropic models for the distribution of observed
polarizations over counterparts that have a cosmological anisotropy of the type
advocated in the earlier study. Although anisotropic models are not completely
ruled out, the data put strong lower limits on the length scale (in
units of the Hubble length) associated with the anisotropy; the lower limits of
95% credible regions for lie between 0.43 and 0.62 in all anisotropic
models we studied, values several times larger than the best-fit value of
found in the earlier study. The length scale is not
constrained from above. The vast majority of sources in the data are at
distances closer than 0.4 Hubble lengths (corresponding to a redshift of
0.8); the results are thus consistent with there being no significant
anisotropy on the length scale probed by these data.Comment: 8 pages, 3 figures; submitted to Phys. Rev.
Prospectus, April 9, 1983
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f(R) Gravity and scalar-tensor theory
In the present paper we will investigate the relation between scalar-tensor
theory and theories of gravity. Such studies have been performed in the
past for the metric formalism of gravity; here we will consider mainly
the Palatini formalism, where the metric and the connections are treated as
independent quantities. We will try to investigate under which circumstances
theories of gravity are equivalent to scalar-tensor theory and examine
the implications of this equivalence, when it exists.Comment: minor changes to match published version, references adde
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Characterization of Retinal Structure in ATF6-Associated Achromatopsia.
PurposeMutations in six genes have been associated with achromatopsia (ACHM): CNGA3, CNGB3, PDE6H, PDE6C, GNAT2, and ATF6. ATF6 is the most recent gene to be identified, though thorough phenotyping of this genetic subtype is lacking. Here, we sought to test the hypothesis that ATF6-associated ACHM is a structurally distinct form of congenital ACHM.MethodsSeven genetically confirmed subjects from five nonconsanguineous families were recruited. Foveal hypoplasia and the integrity of the ellipsoid zone (EZ) band (a.k.a., IS/OS) were graded from optical coherence tomography (OCT) images. Images of the photoreceptor mosaic were acquired using confocal and nonconfocal split-detection adaptive optics scanning light ophthalmoscopy (AOSLO). Parafoveal cone and rod density values were calculated and compared to published normative data as well as data from two subjects harboring CNGA3 or CNGB3 mutations who were recruited for comparative purposes. Additionally, nonconfocal dark-field AOSLO images of the retinal pigment epithelium were obtained, with quantitative analysis performed in one subject with ATF6-ACHM.ResultsFoveal hypoplasia was observed in all subjects with ATF6 mutations. Absence of the EZ band within the foveal region (grade 3) or appearance of a hyporeflective zone (grade 4) was seen in all subjects with ATF6 using OCT. There was no evidence of remnant foveal cone structure using confocal AOSLO, although sporadic cone-like structures were seen in nonconfocal split-detection AOSLO. There was a lack of cone structure in the parafovea, in direct contrast to previous reports.ConclusionsOur data demonstrate a near absence of cone structure in subjects harboring ATF6 mutations. This implicates ATF6 as having a major role in cone development and suggests that at least a subset of subjects with ATF6-ACHM have markedly fewer cellular targets for cone-directed gene therapies than do subjects with CNGA3- or CNGB3-ACHM
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