3,849 research outputs found
Enabling Electroweak Baryogenesis through Dark Matter
We study the impact on electroweak baryogenesis from a swifter cosmological
expansion induced by dark matter. We detail the experimental bounds that one
can place on models that realize it, and we investigate the modifications of
these bounds that result from a non-standard cosmological history. The
modifications can be sizeable if the expansion rate of the Universe increases
by several orders of magnitude. We illustrate the impact through the example of
scalar field dark matter, which can alter the cosmological history enough to
enable a strong-enough first-order phase transition in the Standard Model when
it is supplemented by a dimension six operator directly modifying the Higgs
boson potential. We show that due to the modified cosmological history,
electroweak baryogenesis can be realized, while keeping deviations of the
triple Higgs coupling below HL-LHC sensitivies. The required scale of new
physics to effectuate a strong-enough first order phase transition can change
by as much as twenty percent as the expansion rate increases by six orders of
magnitude
Tidal Love numbers of neutron stars
For a variety of fully relativistic polytropic neutron star models we
calculate the star's tidal Love number k2. Most realistic equations of state
for neutron stars can be approximated as a polytrope with an effective index
n~0.5-1.0. The equilibrium stellar model is obtained by numerical integration
of the Tolman-Oppenheimer-Volkhov equations. We calculate the linear l=2 static
perturbations to the Schwarzschild spacetime following the method of Thorne and
Campolattaro. Combining the perturbed Einstein equations into a single second
order differential equation for the perturbation to the metric coefficient
g_tt, and matching the exterior solution to the asymptotic expansion of the
metric in the star's local asymptotic rest frame gives the Love number. Our
results agree well with the Newtonian results in the weak field limit. The
fully relativistic values differ from the Newtonian values by up to ~24%. The
Love number is potentially measurable in gravitational wave signals from
inspiralling binary neutron stars.Comment: corrected Eqs. (20) and (23) and entries in Table (1
A correspondence between solution-state dynamics of an individual protein and the sequence and conformational diversity of its family.
Conformational ensembles are increasingly recognized as a useful representation to describe fundamental relationships between protein structure, dynamics and function. Here we present an ensemble of ubiquitin in solution that is created by sampling conformational space without experimental information using "Backrub" motions inspired by alternative conformations observed in sub-Angstrom resolution crystal structures. Backrub-generated structures are then selected to produce an ensemble that optimizes agreement with nuclear magnetic resonance (NMR) Residual Dipolar Couplings (RDCs). Using this ensemble, we probe two proposed relationships between properties of protein ensembles: (i) a link between native-state dynamics and the conformational heterogeneity observed in crystal structures, and (ii) a relation between dynamics of an individual protein and the conformational variability explored by its natural family. We show that the Backrub motional mechanism can simultaneously explore protein native-state dynamics measured by RDCs, encompass the conformational variability present in ubiquitin complex structures and facilitate sampling of conformational and sequence variability matching those occurring in the ubiquitin protein family. Our results thus support an overall relation between protein dynamics and conformational changes enabling sequence changes in evolution. More practically, the presented method can be applied to improve protein design predictions by accounting for intrinsic native-state dynamics
Constraining neutron star tidal Love numbers with gravitational wave detectors
Ground-based gravitational wave detectors may be able to constrain the
nuclear equation of state using the early, low frequency portion of the signal
of detected neutron star - neutron star inspirals. In this early adiabatic
regime, the influence of a neutron star's internal structure on the phase of
the waveform depends only on a single parameter lambda of the star related to
its tidal Love number, namely the ratio of the induced quadrupole moment to the
perturbing tidal gravitational field. We analyze the information obtainable
from gravitational wave frequencies smaller than a cutoff frequency of 400 Hz,
where corrections to the internal-structure signal are less than 10 percent.
For an inspiral of two non-spinning 1.4 solar mass neutron stars at a distance
of 50 Mpc, LIGO II detectors will be able to constrain lambda to lambda < 2.0
10^{37} g cm^2 s^2 with 90% confidence. Fully relativistic stellar models show
that the corresponding constraint on radius R for 1.4 solar mass neutron stars
would be R < 13.6 km (15.3 km) for a n=0.5 (n=1.0) polytrope.Comment: 4 pages, 2 figures, minor correction
Collaboration and competition:Market queens, trade unions and collective action of informal workers in Ghanaβs Makola Market
This paper focuses on informal workers in Makola Market, Accra (Ghana), and the ways in which they organize for collective action. Ghana has a long history of trade and this makes for well-developed and culturally embedded local institutions that have organized and represented the (informal) workersactive in markets. A prominent example is the market queens, who (cl)aim to oversee, protect and promote markets vis-a-vis the public and the (municipal) government. Yet, these social structures are not easily recognized as a kind of social movement by (inter)national trade unions. Hence trade unioninterventions and outreaches aimed at ameliorating the plight of informal workers tend to bypass and antagonize these existing formations, fuelling competition and division in the already fragmented and inherently competitive market space. Based on 2.5 year-long ethnographic research on strategic actors in inclusive development with a focus on informal workers, this article draws attention to empirical realities in Ghana. It demonstrates that bypassing culturally embedded groups is problematic because it feeds fragmentation and thus limits the possibilities for collective action
Book review:Cooking Data: Culture & Politics in an African Research World
status: publishe
Transient resonances in the inspirals of point particles into black holes
We show that transient resonances occur in the two body problem in general
relativity, in the highly relativistic, extreme mass-ratio regime for spinning
black holes. These resonances occur when the ratio of polar and radial orbital
frequencies, which is slowly evolving under the influence of gravitational
radiation reaction, passes through a low order rational number. At such points,
the adiabatic approximation to the orbital evolution breaks down, and there is
a brief but order unity correction to the inspiral rate. Corrections to the
gravitational wave signal's phase due to resonance effects scale as the square
root of the inverse of mass of the small body, and thus become large in the
extreme-mass-ratio limit, dominating over all other post-adiabatic effects. The
resonances make orbits more sensitive to changes in initial data (though not
quite chaotic), and are genuine non-perturbative effects that are not seen at
any order in a standard post-Newtonian expansion. Our results apply to an
important potential source of gravitational waves, the gradual inspiral of
white dwarfs, neutron stars, or black holes into much more massive black holes.
It is hoped to exploit observations of these sources to map the spacetime
geometry of black holes. However, such mapping will require accurate models of
binary dynamics, which is a computational challenge whose difficulty is
significantly increased by resonance effects. We estimate that the resonance
phase shifts will be of order a few tens of cycles for mass ratios , by numerically evolving fully relativistic orbital dynamics
supplemented with an approximate, post-Newtonian self-force.Comment: 4 pages, 1 figure, minor correction
- β¦