284,522 research outputs found
The Hands-On Universe: Making Sense of the Universe with All Your Senses
No full textFor the past four years, the Hands-On Universe public engagement programme has explored unconventional, interactive and multi-sensorial ways of communicating complex ideas in cosmology and astrophysics to a wide variety of audiences. The programme lead, Roberto Trotta, has reached thousands of people through food-based workshops, art and science collaborations and a book written using only the 1000 most common words in the English language. In this article, Roberto reflects in first person on what has worked well in the programme, and what has not
All-optical switching and filtering based on liquid crystals and photosensitive composite organic materials
No full textIntegrated optic devices using low driving power are crucial to develop new photonic systems
both for optical communications and for sensing. The large electro-optic effect and nonlinear
optical properties of liquid crystals (LC) and doped LC novel materials allow the realization of low
cost optoelectronic devices. We present our recent experimental and theoretical results on optical
channel waveguides based on LC made on both glass and silicon substrates. Devices made of
waveguides using a LC core can operate both in linear [1] and nonlinear optical regime at 1550 nm
wavelength spectral region [2]. As linear waveguides they can behave both as variable optical
attenuators and as optical switches by exploiting the electrooptic effect in a nematic LC (NLC). An
applied voltage lower than just 5 V was required to drive an electro-optic switch with an on-off
contrast of more than 40 dB. Optical Freedericks transition was also experimentally observed when
an input optical power of just 25 mW was fiber coupled to the same NLC waveguide. Modelling of
the waveguide was obtained by minimizing the LC total free energy, including the dielectric energy
at optical frequencies by using finite elements. Preliminary simulations of waveguides using azodye
doped LC indicate that a further reduction of driving power is possible. All-optical tunable
filters can also be obtained by using photosensitive composite materials.
Recent experimental findings will be presented on an integrated optic filter, which combines the
simple and low cost ion-exchange waveguide technology with a composite LC methyl red (MR)
azo-dye photosensitive compound to obtain full optical tuneability. The structure of the integrated
optical filter recalls a previous POLICRYPS based electro-optic tuneable filter [3]. The prototype
showed a pass-band with more than 20 dB signal suppression at the Bragg wavelength with a
bandwidth of the transmitted notch of about 3 nm. A tuning range of 6.6 nm was observed by
applying a pump signal of 45 mW [4]. Full consistency between experiment and simulation results
was found. The measured shift is nearly double from previous results achieved in electro-optically
tunable POLICRYPS based filters, because of the nematic liquid crystals methyl red properties.
These devices can trigger the development of a new generation of low power, compact and low cost
all-optical components for next generation fiber optic telecom and sensor systems.
References
[1] D. Donisi, B. Bellini, R. Beccherelli, R. Asquini, G. Gilardi, M. Trotta, and A. d’Alessandro, “A switchable liquidcrystal
optical channel waveguide on silicon”, IEEE J. Quantum Electron. 46, 762–768 (2010).
[2] A. d’Alessandro, R. Asquini, M. Trotta, G. Gilardi, R. Beccherelli and I. C. Khoo, “All optical intensity modulation
of near infrared light in a liquid crystal channel waveguide”, Appl. Phys. Lett. 97, 093302 (2010).
[3] A. d’Alessandro, D. Donisi, L. De Sio, R. Beccherelli, R. Asquini, R. Caputo, and C. Umeton, “Tunable integrated
optical filter made of a glass ion-exchanged waveguide and an electro-optic composite holographic grating”, Opt.
Express 16, 9254–9260 (2008), Patent US 7925124 B2 (2011).
[4] G. Gilardi, L. De Sio, R. Beccherelli, R. Asquini, A. d’Alessandro, and C. Umeton, “Observation of tunable optical
filtering in photosensitive composite structures containing liquid crystals”, Opt. Lett. 36, 4755–4757 (2011)
Forecasting the Bayes factor of a future observation
No full textI present a new procedure to forecast the Bayes factor of a future observation by
computing the Predictive Posterior Odds Distribution (PPOD). This can assess the
power of future experiments to answer model selection questions and the probability
of the outcome, and can be helpful in the context of experiment design.
As an illustration, I consider a central quantity for our understanding of the cosmological
concordance model, namely the scalar spectral index of primordial perturbations,
nS. I show that the Planck satellite has over 90% probability of gathering
strong evidence against
n
S = 1, thus conclusively disproving a scale–invariant spectrum.
This result is robust with respect to a wide range of choices for the prior on nS
Applications of Bayesian model selection to cosmological parameters
No full textBayesian model selection is a tool for deciding whether the introduction of a new parameter is warranted by the data. I argue that the usual sampling statistic significance tests for a null hypothesis can be misleading, since they do not take into account the information gained through the data, when updating the prior distribution to the posterior. In contrast, Bayesian model selection offers a quantitative implementation of Occam's razor.
I introduce the Savage–Dickey density ratio, a computationally quick method to determine the Bayes factor of two nested models and hence perform model selection. As an illustration, I consider three key parameters for our understanding of the cosmological concordance model. By using Wilkinson Microwave Anisotropy Probe (WMAP) 3-year data complemented by other cosmological measurements, I show that a non-scale-invariant spectral index of perturbations is favoured for any sensible choice of prior. It is also found that a flat universe is favoured with odds of 29:1 over non-flat models, and that there is strong evidence against a cold dark matter isocurvature component to the initial conditions which is totally (anti)correlated with the adiabatic mode (odds of about 2000:1), but that this is strongly dependent on the prior adopted.
These results are contrasted with the analysis of WMAP 1-year data, which were not informative enough to allow a conclusion as to the status of the spectral index. In a companion paper, a new technique to forecast the Bayes factor of a future observation is presented
Palliduphantes petruzzielloi Bosmans & Trotta 2021, sp. n.
No full textPalliduphantes petruzzielloi sp. n. urn:lsid:zoobank.org:act:CEC1197F-6077-44E2-9AF8- 624E66BF830E (Figs 1a–j, 2a–e) Type material examined. Holotype ♂, ITALY: Campania, Salerno province, Acerno, Grotta Strazzatrippa [Strazzatrippa - - the two dorsal spines; metatarsi I with 1 dorsal spine; spines long, twice as long as the diameter of the tibia; position of trichobothrium on metatarsus I at 0.17–0.19. Male palp (Figs 1d–g, 2 a–b): Patella and tibia each with one large dorsal spine, the one on the tibia somewhat longer; proximal branch of paracymbium with 1 median and 1 basal tooth, median part with one small tooth; lamella caracteristica with two prongs, the ventral one terminally bifid, the dorsal one on its tour split into two prongs, the dorsal one ending in a sharp tooth, the ventral one terminally bifid (fig. 1e) Epigyne (Figs 1h–j, 2 c–e): Strongly raised, with curved scape accompanied by rounded lateral lobes, terminally with rounded stretcher; dorsal plate rectangular, elongated, two times wider than long. Geographic distribution. Only known from the type locality. • •Published as part of Bosmans, Robert & Trotta, Alessio, 2021, On two rare Italian Palliduphantes, including the description of a new species (Araneae: Linyphiidae), pp. 9-12 in Fragmenta entomologica 53 (1) on pages 9-10, DOI: 10.13133/2284-4880/454, http://zenodo.org/record/814802
How flat can you get ? A model comparison perspective on the curvature of the Universe
No full textThe question of determining the spatial geometry of the Universe is of greater relevance than ever, as precision cosmology promises to verify inflationary predictions about the curvature of the Universe. We revisit the question of what can be learnt about the spatial geometry of the Universe from the perspective of a three-way Bayesian model comparison. By considering two classes of phenomenological priors for the curvature parameter, we show that, given the current data, the probability that the Universe is spatially infinite lies between 67 and 98 per cent, depending on the choice of priors. For the strongest prior choice, we find odds of the order of 50:1 (200:1) in favour of a flat Universe when compared with a closed (open) model. We also report a robust, prior-independent lower limit to the number of Hubble spheres in the Universe, N U ≳ 5 (at 99 per cent confidence). We forecast the accuracy with which future cosmic microwave background (CMB) and baryonic acoustic oscillation (BAO) observations will be able to constrain curvature, finding that a cosmic variance-limited CMB experiment together with an Square Kilometer Array (SKA)-like BAO observation will constrain curvature independently of the equation of state of dark energy with a precision of about σ ∼ 4.5 × 10 -4. We demonstrate that the risk of 'model confusion' (i.e. wrongly favouring a flat Universe in the presence of curvature) is much larger than might be assumed from parameter error forecasts for future probes. We argue that a 5σ detection threshold guarantees a confusion- and ambiguity-free model selection. Together with inflationary arguments, this implies that the geometry of the Universe is not knowable if the value of the curvature parameter is below |Ω κ| ∼ 10 -4. This bound is one order of magnitude larger than what one would naively expect from the size of curvature perturbations, ∼10 -5. © 2009 RAS
Monolithic or hierarchical star formation? A new statistical analysis
No full textWe consider an analytic model of cosmic star formation which incorporates supernova feedback, gas accretion and enriched outflows, reproducing the history of cosmic star formation, metallicity, Type II supernova rates and the fraction of baryons allocated to structures. We present a new statistical treatment of the available observational data on the star formation rate and metallicity that accounts for the presence of possible systematics. We then employ a Bayesian Markov Chain Monte Carlo method to compare the predictions of our model with observations and derive constraints on the seven free parameters of the model. We find that the dust-correction scheme one chooses to adopt for the star formation data is critical in determining which scenario is favoured between a hierarchical star formation model, where star formation is prolonged by accretion, infall and merging, and a monolithic scenario, where star formation is rapid and efficient. We distinguish between these modes by defining a characteristic minimum mass, M≳ 1011M⊙, in our fiducial model, for early-type galaxies where star formation occurs efficiently. Our results indicate that the hierarchical star formation model can achieve better agreement with the data, but that this requires a high efficiency of supernova-driven outflows. In a monolithic model, our analysis points to the need for a mechanism that drives metal-poor winds, perhaps in the form of supermassive black hole induced outflows. Furthermore, the relative absence of star formation beyond z∼ 5 in the monolithic scenario requires an alternative mechanism to dwarf galaxies for re-ionizing the universe at z∼ 11, as required by observations of the microwave background. While the monolithic scenario is less favoured in terms of its quality-of-fit, it cannot yet be excluded
Letter from J. R. Eakin to Arthur G. Ringland
No full textLetter (copy) from J. R. Eakin to Arthur C. Ringland about the alignment of 40 acres near the Buggeln ranch
Letter from Arno B. Cammerer to J. R. Eakin
No full textLetter from Arno B. Cammerer to J. R. Eakin describing the procedure for purchasing Bright Angel Trail
Letter from J. R. Eakin to Carl Hayden
No full textLetter from J. R. Eakin to Carl T. Hayden concerning access to Rowe Well and the canyon
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