32 research outputs found
Causal Set Phenomenology
Central to the development of any new theory is the investigation of the
observable consequences of the theory. In the search for quantum gravity,
research in phenomenology has been dominated by models violating Lorentz
invariance (LI) -- despite there being, at present, no evidence that LI is
violated. Causal set theory is a LI candidate theory of QG that seeks not to
quantise gravity as such, but rather to develop a new understanding of the
universe from which both GR and QM could arise separately. The key hypothesis
is that spacetime is a discrete partial order: a set of events where the
partial ordering is the physical causal ordering between the events. This
thesis investigates Lorentz invariant QG phenomenology motivated by the causal
set approach. Massive particles propagating in a discrete spacetime will
experience diffusion in both position and momentum in proper time. This thesis
considers this idea in more depth, providing a rigorous derivation of the
diffusion equation in terms of observable cosmic time. The diffusion behaviour
does not depend on any particular underlying particle model. Simulations of
three different models are conducted, revealing behaviour that matches the
diffusion equation despite limitations on the size of causal set simulated. The
effect of spacetime discreteness on the behaviour of massless particles is also
investigated. Diffusion equations in both affine time and cosmic time are
derived, and it is found that massless particles undergo diffusion and drift in
energy. Constraints are placed on the magnitudes of the drift and diffusion
parameters by considering the blackbody nature of the CMB. Spacetime
discreteness also has a potentially observable effect on photon polarisation.
For linearly polarised photons, underlying discreteness is found to cause a
rotation in polarisation angle and a suppression in overall polarisation.Comment: PhD Thesis, 145 page
Energy-momentum diffusion from spacetime discreteness
We study potentially observable consequences of spatiotemporal discreteness
for the motion of massive and massless particles. First we describe some simple
intrinsic models for the motion of a massive point particle in a fixed causal
set background. At large scales, the microscopic swerves induced by the
underlying atomicity manifest themselves as a Lorentz invariant diffusion in
energy-momentum governed by a single phenomenological parameter, and we derive
in full the corresponding diffusion equation. Inspired by the simplicity of the
result, we then derive the most general Lorentz invariant diffusion equation
for a massless particle, which turns out to contain two phenomenological
parameters describing, respectively, diffusion and drift in the particle's
energy. The particles do not leave the light cone however: their worldlines
continue to be null geodesics. Finally, we deduce bounds on the drift and
diffusion constants for photons from the blackbody nature of the spectrum of
the cosmic microwave background radiation.Comment: 13 pages, 4 figures, corrected minor typos and updated to match
published versio
Model based automated cycling ergometer
Laboratory testing of cyclists is currently undertaken using turbo trainers or cycle ergometers. The benefits of laboratory testing are the ability to measure performance: (i) more accurately and repeatedly and (ii) under controlled conditions enabling, for example, video analysis to determine joint-specific power production or enable novel instrumentation to be applied to the bicycle, for example, to measure seat interface pressure. Influence of the bicycle fit on torque production have been presented by Irriberri et al(2008) and Peveler et al(2007). Market leading bicycle ergometer manufacturers, such as Lode and Monark, provide feedback on performance metrics including cadence and force measurement. However, neither ergometer provides real time adaptation of bicycle fit to the resolution (i.e. < mm precision) required by elite athletes or allows adjustments to position whilst cycling under simulated road / track conditions. The objective of the research presented in this paper is to demonstrate
and provide initial validation results for a novel, fully automated cycle ergometer that incorporates faster, repeatable and more
accurate adjustments to bicycle geometry. The ergometer also allows the cyclist to use their preferred handlebars and saddle to
accommodate the different cycling disciplines, e.g. track, road, mountain and BMX. The ergometer enables fitting adjustments to be controlled whilst cycling and aims to reduce initial set-up times for different athletes to about 30 seconds as opposed to 30 minutes (required by the end-users current ergometer instantiations). Instrumented cranks have been fitted to monitor the torque and force generated by the crank movements in 2-axes through 360 degrees of crank motion. The ergometer can be coupled (via a user selectable clutch mechanism) to an AC servo motor within the drive chain which supports the application of models of bicycle performance to the ergometer to enable torque versus position versus speed profiles as derived from road and / or track.
trials to be readily mapped into the laboratory environmen
Polarization Diffusion from Spacetime Uncertainty
A model of Lorentz invariant random fluctuations in photon polarization is
presented. The effects are frequency dependent and affect the polarization of
photons as they propagate through space. We test for this effect by confronting
the model with the latest measurements of polarization of Cosmic Microwave
Background (CMB) photons.Comment: 4 pages, 1 figur
Observations of Extreme ICME Ram Pressure Compressing Mercury's Dayside Magnetosphere to the Surface
Mercury's magnetosphere is known to be affected by enhanced ram pressures and
magnetic fields inside interplanetary coronal mass ejections (ICMEs). Here we
report detailed observations of an ICME compressing Mercury's dayside
magnetosphere to the surface. A fast CME launched from the Sun on November 29
2013 impacted first MESSENGER, which was orbiting Mercury, on November 30 and
later STEREO-A near 1 AU on December 1. Following the ICME impact, MESSENGER
remained in the solar wind as the spacecraft traveled inwards and northwards
towards Mercury's surface until it reached and passed its closest approach to
the planet (at 371 km altitude) without crossing into the magnetosphere. The
magnetospheric crossing finally occurred 1 minute before reaching the planet's
nightside at 400 km altitude and 84N latitude, indicating the lack of
dayside magnetosphere on this orbit. In addition, the peak magnetic field
measured by MESSENGER at this time was 40% above the values measured in the
orbits just prior to and after the ICME, a consequence of the magnetospheric
compression. Using both a proxy method at Mercury and measurements at STEREO-A,
we show that the extremely high ram pressure associated with this ICME was more
than high enough to collapse Mercury's weak magnetosphere. As a consequence,
the ICME plasma likely interacted with Mercury's surface, evidenced by enhanced
sodium ions in the exosphere. The collapse of Mercury's dayside magnetosphere
has important implications for the habitability of close-in exoplanets around M
dwarf stars, as such events may significantly contribute to planetary
atmospheric loss in these systems
Extending the Planetary Mass Function to Earth Mass by Microlensing at Moderately High Magnification
A measurement by microlensing of the planetary mass function of planets with
masses ranging from 5M_E to 10M_J and orbital radii from 0.5 to 10 AU was
reported recently. A strategy for extending the mass range down to (1-3)M_E is
proposed here. This entails monitoring the peaks of a few tens of microlensing
events with moderately high magnifications with 1-2m class telescopes. Planets
of a few Earth masses are found to produce deviations of ~ 5% to the peaks of
microlensing light curves with durations ~ (0.7-3)hr in events with
magnification ~ 100 if the projected separation of the planet lies in the
annular region (0.85-1.2)r_E. Similar deviations are produced by Earth mass
planets in the annular region (0.95-1.05)r_E. It is possible that sub-Earths
could be detected very close to the Einstein ring if they are sufficiently
abundant, and also planetary systems with more than one low mass planet.Comment: 12 pages, 20 figures (in press) MNRAS (2013
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Dephosphorylation of the Proneural Transcription Factor ASCL1 Re-Engages a Latent Post-Mitotic Differentiation Program in Neuroblastoma.
Pediatric cancers often resemble trapped developmental intermediate states that fail to engage the normal differentiation program, typified by high-risk neuroblastoma arising from the developing sympathetic nervous system. Neuroblastoma cells resemble arrested neuroblasts trapped by a stable but aberrant epigenetic program controlled by sustained expression of a core transcriptional circuit of developmental regulators in conjunction with elevated MYCN or MYC (MYC). The transcription factor ASCL1 is a key master regulator in neuroblastoma and has oncogenic and tumor-suppressive activities in several other tumor types. Using functional mutational approaches, we find that preventing CDK-dependent phosphorylation of ASCL1 in neuroblastoma cells drives coordinated suppression of the MYC-driven core circuit supporting neuroblast identity and proliferation, while simultaneously activating an enduring gene program driving mitotic exit and neuronal differentiation. IMPLICATIONS: These findings indicate that targeting phosphorylation of ASCL1 may offer a new approach to development of differentiation therapies in neuroblastoma. VISUAL OVERVIEW: http://mcr.aacrjournals.org/content/molcanres/18/12/1759/F1.large.jpg.Work was supported by Cancer Research UK Programme Grant RG91505 (AP), Wellcome Trust Investigator Award 212253/Z/18/Z (AP), MRC Research Grant MR/L021129/1 (F.A, A.P); Neuroblastoma UK (D.M, T.P, A.P), CRUK Cambridge Centre Paediatric Programme (L.P), The Terry Fox Foundation (FA), MBRU College of Medicine Internal grant award
MBRU-CM-RG2019-14 (FA), MBRU-ALMAHMEED Collaborative Research Award ALM1909 (FA) and core support from the Wellcome Trust and the MRC Cambridge Stem Cell Institute (F.A, D.M, J.D., A.P.) and Cancer Research UK Cambridge Insititute (I.C, J.C)
Modular model for Mercury's magnetospheric magnetic field confined within the average observed magnetopause
Accurate knowledge of Mercury's magnetospheric magnetic field is required to understand the sources of the planet's internal field. We present the first model of Mercury's magnetospheric magnetic field confined within a magnetopause shape derived from Magnetometer observations by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft. The field of internal origin is approximated by a dipole of magnitude 190 nT RM 3, where RM is Mercury's radius, offset northward by 479 km along the spin axis. External field sources include currents flowing on the magnetopause boundary and in the cross‐tail current sheet. The cross‐tail current is described by a disk‐shaped current near the planet and a sheet current at larger (≳ 5 RM ) antisunward distances. The tail currents are constrained by minimizing the root‐mean‐square (RMS) residual between the model and the magnetic field observed within the magnetosphere. The magnetopause current contributions are derived by shielding the field of each module external to the magnetopause by minimizing the RMS normal component of the magnetic field at the magnetopause. The new model yields improvements over the previously developed paraboloid model in regions that are close to the magnetopause and the nightside magnetic equatorial plane. Magnetic field residuals remain that are distributed systematically over large areas and vary monotonically with magnetic activity. Further advances in empirical descriptions of Mercury's magnetospheric external field will need to account for the dependence of the tail and magnetopause currents on magnetic activity and additional sources within the magnetosphere associated with Birkeland currents and plasma distributions near the dayside magnetopause
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MESSENGER observations of induced magnetic fields in Mercury's core
Orbital data from the Magnetometer on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft allow investigation of magnetic fields induced at the top of Mercury's core by time‐varying magnetospheric fields. We used 15 Mercury years of observations of the magnetopause position as well as the magnetic field inside the magnetosphere to establish the presence and magnitude of an annual induction signal. Our results indicate an annual change in the internal axial dipole term, g10, of 7.5 to 9.5 nT. For negligible mantle conductivity, the average annual induction signal provides an estimate of Mercury's core radius to within ±90 km, independent of geodetic results. Larger induction signals during extreme events are expected but are challenging to identify because of reconnection‐driven erosion. Our results indicate that the magnetopause reaches the dayside planetary surface 1.5–4% of the time