94 research outputs found
Outcome-Driven Reinforcement Learning via Variational Inference
While reinforcement learning algorithms provide automated acquisition of
optimal policies, practical application of such methods requires a number of
design decisions, such as manually designing reward functions that not only
define the task, but also provide sufficient shaping to accomplish it. In this
paper, we view reinforcement learning as inferring policies that achieve
desired outcomes, rather than as a problem of maximizing rewards. To solve this
inference problem, we establish a novel variational inference formulation that
allows us to derive a well-shaped reward function which can be learned directly
from environment interactions. From the corresponding variational objective, we
also derive a new probabilistic Bellman backup operator and use it to develop
an off-policy algorithm to solve goal-directed tasks. We empirically
demonstrate that this method eliminates the need to hand-craft reward functions
for a suite of diverse manipulation and locomotion tasks and leads to effective
goal-directed behaviors.Comment: Published in Advances in Neural Information Processing Systems 34
(NeurIPS 2021
Unveiling Dust-enshrouded Star Formation in the Early Universe: a Sub-mm Survey of the Hubble Deep Field
The advent of sensitive sub-mm array cameras now allows a proper census of
dust-enshrouded massive star-formation in very distant galaxies, previously
hidden activity to which even the faintest optical images are insensitive. We
present the deepest sub-mm survey of the sky to date, taken with the SCUBA
camera on the James Clerk Maxwell Telescope and centred on the Hubble Deep
Field. The high source density found in this image implies that the survey is
confusion-limited below a flux density of 2 mJy. However, within the central 80
arcsec radius independent analyses yield 5 reproducible sources with S(850um) >
2 mJy which simulations indicate can be ascribed to individual galaxies. We
give positions and flux densities for these, and furthermore show using
multi-frequency photometric data that the brightest sources in our map lie at
redshifts z~3. These results lead to integral source counts which are
completely inconsistent with a no-evolution model, and imply that massive
star-formation activity continues at redshifts > 2. The combined brightness of
the 5 most secure sources in our map is sufficient to account for 30 - 50% of
the previously unresolved sub-mm background, and we estimate statistically that
the entire background is resolved at about the 0.3 mJy level. Finally we
discuss possible optical identifications and redshift estimates for the
brightest sources. One source appears to be associated with an extreme
starburst galaxy at z~1, whilst the remaining four appear to lie in the
redshift range 2 < z < 4. This implies a star-formation density over this
redshift range that is at least five times higher than that inferred from the
ultraviolet output of HDF galaxies.Comment: 19 pages, 6 figures (to appear as a Nature Article
a trimolecular composition
Photochemical upconversion based on tripletâtriplet annihilation (TTA-UC) is
employed to enhance the short-circuit currents generated by two varieties of
thin-film solar cells, a hydrogenated amorphous silicon (a-Si:H) solar cell
and a dye-sensitized solar cell (DSC). TTA-UC is exploited to harvest
transmitted sub-bandgap photons, combine their energies and re-radiate
upconverted photons back towards the solar cells. In the present study we
employ a dual-emitter TTA-UC system which allows for significantly improved UC
quantum yields as compared to the previously used single-emitter TTA systems.
In doing so we achieve record photo-current enhancement values for both the
a-Si:H device and the DSC, surpassing 10â3 mA cmâ2 sunâ2 for the first time
for a TTA-UC system and marking a record for upconversion-enhanced solar cells
in general. We discuss pertinent challenges of the TTA-UC technology which
need to be addressed in order to achieve its viable device application
Increased upconversion performance for thin film solar cells: A trimolecular composition
Photochemical upconversion based on triplet-triplet annihilation (TTA-UC) is employed to enhance the short-circuit currents generated by two varieties of thin-film solar cells, a hydrogenated amorphous silicon (a-Si:H) solar cell and a dye-sensitized solar cell (DSC). TTA-UC is exploited to harvest transmitted sub-bandgap photons, combine their energies and re-radiate upconverted photons back towards the solar cells. In the present study we employ a dual-emitter TTA-UC system which allows for significantly improved UC quantum yields as compared to the previously used single-emitter TTA systems. In doing so we achieve record photo-current enhancement values for both the a-Si:H device and the DSC, surpassing 10-3 mA cm-2 sun-2 for the first time for a TTA-UC system and marking a record for upconversion-enhanced solar cells in general. We discuss pertinent challenges of the TTA-UC technology which need to be addressed in order to achieve its viable device application
Improving the light-harvesting of amorphous silicon solar cells with photochemical upconversion
Single-threshold solar cells are fundamentally limited by their ability to
harvest only those photons above a certain energy. Harvesting below-threshold
photons and re-radiating this energy at a shorter wavelength would thus boost
the efficiency of such devices. We report an increase in light harvesting
efficiency of a hydrogenated amorphous silicon (a-Si:H) thin-film solar cell
due to a rear upconvertor based on sensitized tripletâtriplet-annihilation in
organic molecules. Low energy light in the range 600â750 nm is converted to
550â600 nm light due to the incoherent photochemical process. A peak
efficiency enhancement of (1.0 ± 0.2)% at 720 nm is measured under irradiation
equivalent to (48 ± 3) suns (AM1.5). We discuss the pathways to be explored in
adapting photochemical UC for application in various single threshold devices
IR and UV Galaxies at z=0.6 -- Evolution of Dust Attenuation and Stellar Mass as Revealed by SWIRE and GALEX
We study dust attenuation and stellar mass of star-forming
galaxies using new SWIRE observations in IR and GALEX observations in UV. Two
samples are selected from the SWIRE and GALEX source catalogs in the
SWIRE/GALEX field ELAIS-N1-00 ( deg). The UV selected sample
has 600 galaxies with photometric redshift (hereafter photo-z) and NUV (corresponding to \rm L_{FUV} \geq 10^{9.6} L_\sun).
The IR selected sample contains 430 galaxies with mJy
(\rm L_{dust} \geq 10^{10.8} L_\sun) in the same photo-z range. It is found
that the mean ratios of the z=0.6 UV galaxies are
consistent with that of their z=0 counterparts of the same . For
IR galaxies, the mean ratios of the z=0.6 LIRGs (\rm
L_{dust} \sim 10^{11} L_\sun) are about a factor of 2 lower than local LIRGs,
whereas z=0.6 ULIRGs (\rm L_{dust} \sim 10^{12} L_\sun) have the same mean
ratios as their local counterparts. This is consistent
with the hypothesis that the dominant component of LIRG population has changed
from large, gas rich spirals at z to major-mergers at z=0. The stellar
mass of z=0.6 UV galaxies of \rm L_{FUV} \leq 10^{10.2} L_\sun is about a
factor 2 less than their local counterparts of the same luminosity, indicating
growth of these galaxies. The mass of z=0.6 UV lunmous galaxies (UVLGs: \rm
L_{FUV} > 10^{10.2} L_\sun) and IR selected galaxies, which are nearly
exclusively LIRGs and ULIRGs, is the same as their local counterparts.Comment: 27 pages, 8 figures, to be published in the Astrophysical Journal
Supplement series dedicated to GALEX result
Improving the light-harvesting of second generation solar cells with photochemical upconversion
Photovoltaics (PV) offer a solution for the development of sustainable energy
sources, relying on the sheer abundance of sunlight: More sunlight falls on
the Earthâs surface in one hour than is required by its inhabitants in a year.
However, it is imperative to manage the wide distribution of photon energies
available in order to generate more cost efficient PV devices because single
threshold PV devices are fundamentally limited to a maximum conversion
efficiency, the Shockley-Queisser (SQ) limit. Recent progress has enabled the
production of c-Si cells with efficiencies as high as 25%,1 close to the
limiting efficiency of âŒ30%. But these cells are rather expensive, and
ultimately the cost of energy is determined by the ratio of system cost and
efficiency of the PV device. A strategy to radically decrease this ratio is to
circumvent the SQ limit in cheaper, second generation PV devices. One
promising approach is the use of hydrogenated amorphous silicon (a-Si:H),
where film thicknesses on the order of several 100nm are sufficient.
Unfortunately, the optical threshold of a-Si:H is rather high (1.7-1.8 eV) and
the material suffers from light-induced degradation. Thinner absorber layers
in a-Si:H devices are generally more stable than thicker films due to the
better charge carrier extraction, but at the expense of reduced conversion
efficiencies, especially in the red part of the solar spectrum (absorption
losses). Hence for higher bandgap materials, which includes a-Si as well as
organic and dye-sensitized cells, the major loss mechanism is the inability to
harvest low energy photons
Diminazene resistance in Trypanosoma congolense is not caused by reduced transport capacity but associated with reduced mitochondrial membrane potential
Trypanosoma congolense is a principal agent causing livestock trypanosomiasis in Africa, costing developing economies billions of dollars and undermining food security. Only the diamidine diminazene and the phenanthridine isometamidium are regularly used, and resistance is widespread but poorly understood. We induced stable diminazene resistance in T. congolense strain IL3000 in vitro. There was no crossâresistance with the phenanthridine drugs, melaminophenyl arsenicals, oxaborole trypanocides, or with diamidine trypanocides, except the close analogues DB829 and DB75. Fluorescence microscopy showed that accumulation of DB75 was inhibited by folate. Uptake of [3H]âdiminazene was slow, low affinity and partly but reciprocally inhibited by folate and by competing diamidines. Expression of T. congolense folate transporters in diminazeneâresistant T. b. brucei significantly sensitized the cells to diminazene and DB829, but not to oxaborole AN7973. However, [3H]âdiminazene transport studies, whole genome sequencing and RNAâseq found no major changes in diminazene uptake, folate transporter sequence or expression. Instead, all resistant clones displayed a moderate reduction in the mitochondrial membrane potential. We conclude that diminazene uptake in T. congolense proceed via multiple low affinity mechanisms including folate transporters; while resistance is associated with a reduction in Κm it is unclear whether this is the primary cause of the resistance
Effect of a back reflector
Photochemical upconversion is applied to a hydrogenated amorphous silicon
solar cell in the presence of a back-scattering layer. A custom-synthesized
porphyrin was utilized as the sensitizer species, with rubrene as the emitter.
Under a bias of 24 suns, a peak external quantum efficiency (EQE) enhancement
of ~2 % was observed at a wavelength of 720 nm. Without the scattering layer,
the EQE enhancement was half this value, indicating that the effect of the
back-scatterer is to double the efficacy of the upconverting device. The
results represent an upconversion figure of merit of 3.5 Ă 10â4 mA cmâ2 sunâ2,
which is the highest reported to date
The X-Ray Derived Cosmological Star Formation History and the Galaxy X-Ray Luminosity Functions in the Chandra Deep Fields North and South
The cosmological star formation rate in the combined Chandra Deep Fields
North and South is derived from our X-Ray Luminosity Function for Galaxies in
these Deep Fields. Mild evolution is seen up to redshift order unity with SFR ~
(1 + z)^{2.7}. This is the first directly observed normal star-forming galaxy
X-ray luminosity function (XLF) at cosmologically interesting redshifts (z>0).
This provides the most direct measure yet of the X-ray derived cosmic
star-formation history of the Universe. We make use of Bayesian statistical
methods to classify the galaxies and the two types of AGN, finding the most
useful discriminators to be the X-ray luminosity, X-ray hardness ratio, and
X-ray to optical flux ratio. There is some residual AGN contamination in the
sample at the bright end of the luminosity function. Incompleteness slightly
flattens the XLF at the faint end of the luminosity function. The XLF has a
lognormal distribution and agrees well with the radio and infrared luminosity
functions. However, the XLF does not agree with the Schechter luminosity
function for the H-alpha LF indicating that additional and different physical
processes may be involved in the establishment of the lognormal form of the
XLF. The agreement of our star formation history points with the other star
formation determinations in different wavebands (IR, Radio, H-alpha) gives an
interesting constraint on the IMF, and X-rays may be measuring directly the
binary star formation history of the Universe. X-ray studies will continue to
be useful for probing the star formation history of the universe by avoiding
problems of obscuration. Star formation may therefore be measured in more
detail by deep surveys with future x-ray missions.Comment: Accepted for publication in ApJ. 19 pages with 10 figures formatted
with emulateapj. Version with B/W only figures available at
http://www.pha.jhu.edu/~ptak/paper
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