1,982 research outputs found
Why anthropic reasoning cannot predict Lambda
We revisit anthropic arguments purporting to explain the measured value of
the cosmological constant. We argue that different ways of assigning
probabilities to candidate universes lead to totally different anthropic
predictions. As an explicit example, we show that weighting different universes
by the total number of possible observations leads to an extremely small
probability for observing a value of Lambda equal to or greater than what we
now measure. We conclude that anthropic reasoning within the framework of
probability as frequency is ill-defined and that in the absence of a
fundamental motivation for selecting one weighting scheme over another the
anthropic principle cannot be used to explain the value of Lambda, nor, likely,
any other physical parameters.Comment: 4 pages, 1 figure. Discussion slighlty expanded, refs added,
conclusions unchanged. Matches published versio
The Virtues of Frugality - Why cosmological observers should release their data slowly
Cosmologists will soon be in a unique position. Observational noise will
gradually be replaced by cosmic variance as the dominant source of uncertainty
in an increasing number of observations. We reflect on the ramifications for
the discovery and verification of new models. If there are features in the full
data set that call for a new model, there will be no subsequent observations to
test that model's predictions. We give specific examples of the problem by
discussing the pitfalls of model discovery by prior adjustment in the context
of dark energy models and inflationary theories. We show how the gradual
release of data can mitigate this difficulty, allowing anomalies to be
identified, and new models to be proposed and tested. We advocate that
observers plan for the frugal release of data from future cosmic variance
limited observations.Comment: 5 pages, expanded discussion of Lambda and of blind anlysis, added
refs. Matches version to appear in MNRAS Letter
Should we doubt the cosmological constant?
While Bayesian model selection is a useful tool to discriminate between
competing cosmological models, it only gives a relative rather than an absolute
measure of how good a model is. Bayesian doubt introduces an unknown benchmark
model against which the known models are compared, thereby obtaining an
absolute measure of model performance in a Bayesian framework. We apply this
new methodology to the problem of the dark energy equation of state, comparing
an absolute upper bound on the Bayesian evidence for a presently unknown dark
energy model against a collection of known models including a flat LambdaCDM
scenario. We find a strong absolute upper bound to the Bayes factor B between
the unknown model and LambdaCDM, giving B < 3. The posterior probability for
doubt is found to be less than 6% (with a 1% prior doubt) while the probability
for LambdaCDM rises from an initial 25% to just over 50% in light of the data.
We conclude that LambdaCDM remains a sufficient phenomenological description of
currently available observations and that there is little statistical room for
model improvement.Comment: 10 pages, 2 figure
Directional Detection of Dark Matter with MIMAC
Directional detection is a promising search strategy to discover galactic
Dark Matter. We present a Bayesian analysis framework dedicated to Dark Matter
phenomenology using directional detection. The interest of directional
detection as a powerful tool to set exclusion limits, to authentify a Dark
Matter detection or to constrain the Dark Matter properties, both from particle
physics and galactic halo physics, will be demonstrated. However, such results
need highly accurate track reconstruction which should be reachable by the
MIMAC detector using a dedicated readout combined with a likelihood analysis of
recoiling nuclei.Comment: 4 pages, 2 figures, to appear in the proceedings of the TAUP 2011
conference held in Munich (5 - 9 September, 2011
Hunting Down the Best Model of Inflation with Bayesian Evidence
We present the first calculation of the Bayesian evidence for different
prototypical single field inflationary scenarios, including representative
classes of small field and large field models. This approach allows us to
compare inflationary models in a well-defined statistical way and to determine
the current "best model of inflation". The calculation is performed numerically
by interfacing the inflationary code FieldInf with MultiNest. We find that
small field models are currently preferred, while large field models having a
self-interacting potential of power p>4 are strongly disfavoured. The class of
small field models as a whole has posterior odds of approximately 3:1 when
compared with the large field class. The methodology and results presented in
this article are an additional step toward the construction of a full numerical
pipeline to constrain the physics of the early Universe with astrophysical
observations. More accurate data (such as the Planck data) and the techniques
introduced here should allow us to identify conclusively the best inflationary
model.Comment: 12 pages, 2 figures, uses RevTeX. Misprint corrected, references
added. Matches published versio
The effect of local optically thick regions in the long-wave emission of young circumstellar disks
Multi-wavelength observations of protoplanetary disks in the sub-millimeter
continuum have measured spectral indices values which are significantly lower
than what is found in the diffuse interstellar medium. Under the assumption
that mm-wave emission of disks is mostly optically thin, these data have been
generally interpreted as evidence for the presence of mm/cm-sized pebbles in
the disk outer regions. In this work we investigate the effect of possible
local optically thick regions on the mm-wave emission of protoplanetary disks
without mm/cm-sized grains. A significant local increase of the optical depth
in the disk can be caused by the concentration of solid particles, as predicted
to result from a variety of proposed physical mechanisms. We calculate the
filling factors and implied overdensities these optically thick regions would
need to significantly affect the millimeter fluxes of disks, and we discuss
their plausibility. We find that optically thick regions characterized by
relatively small filling factors can reproduce the mm-data of young disks
without requesting emission from mm/cm-sized pebbles. However, these optically
thick regions require dust overdensities much larger than what predicted by any
of the physical processes proposed in the literature to drive the concentration
of solids. We find that only for the most massive disks it is possible and
plausible to imagine that the presence of optically thick regions in the disk
is responsible for the low measured values of the mm spectral index. For the
majority of the disk population, optically thin emission from a population of
large mm-sized grains remains the most plausible explanation. The results of
this analysis further strengthen the scenario for which the measured low
spectral indices of protoplanetary disks at mm wavelengths are due to the
presence of large mm/cm-sized pebbles in the disk outer regions.Comment: 13 pages, 2 figures, A&A in pres
Phonon-Assisted Two-Photon Interference from Remote Quantum Emitters
Photonic quantum technologies are on the verge offinding applications in everyday life with quantum cryptography andquantum simulators on the horizon. Extensive research has beencarried out to identify suitable quantum emitters and single epitaxialquantum dots have emerged as near-optimal sources of bright, on-demand, highly indistinguishable single photons and entangledphoton-pairs. In order to build up quantum networks, it is essentialto interface remote quantum emitters. However, this is still anoutstanding challenge, as the quantum states of dissimilarâartificialatomsâhave to be prepared on-demand with highfidelity and thegenerated photons have to be made indistinguishable in all possibledegrees of freedom. Here, we overcome this major obstacle and show an unprecedented two-photon interference (visibility of 51±5%) from remote strain-tunable GaAs quantum dots emitting on-demand photon-pairs. We achieve this result by exploiting forthefirst time the full potential of a novel phonon-assisted two-photon excitation scheme, which allows for the generation ofhighly indistinguishable (visibility of 71±9%) entangled photon-pairs (fidelity of 90±2%), enables push-button biexciton statepreparation (fidelity of 80±2%) and outperforms conventional resonant two-photon excitation schemes in terms of robustnessagainst environmental decoherence. Our results mark an important milestone for the practical realization of quantum repeatersand complex multiphoton entanglement experiments involving dissimilar artificial atom
A modified proximity approach in the fusion of heavy-ions
By using a suitable set of the surface energy coefficient, nuclear radius,
and universal function, the original proximity potential 1977 is modified. The
overestimate of the data by 4 % reported in the literature is significantly
reduced. Our modified proximity potential reproduces the experimental data
nicely compared to its older versions.Comment: 9 pages, 5 figures, Chin. Phys. lett.(2010) in pres
Fast Acceleration of Transrelativistic Electrons in Astrophysical Turbulence
Highly energetic, relativistic electrons are commonly present in many
astrophysical systems, from solar flares to the intra-cluster medium, as
indicated by observed electromagnetic radiation. However, open questions remain
about the mechanisms responsible for their acceleration, and possible
re-acceleration. Ubiquitous plasma turbulence is one of the possible universal
mechanisms. We study the energization of transrelativistic electrons in
turbulence using hybrid particle-in-cell, which provide a realistic model of
Alfv\'{e}nic turbulence from MHD to sub-ion scales, and test particle
simulations for electrons. We find that, depending on the electron initial
energy and turbulence strength, electrons may undergo a fast and efficient
phase of energization due to the magnetic curvature drift during the time they
are trapped in dynamic magnetic structures. In addition, electrons are
accelerated stochastically which is a slower process that yields lower maximum
energies. The combined effect of these two processes determines the overall
electron acceleration. With appropriate turbulence parameters, we find that
superthermal electrons can be accelerated up to relativistic energies. For
example, with heliospheric parameters and a relatively high turbulence level,
rapid energization to MeV energies is possible.Comment: Accepted for publication in The Astrophysical Journa
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