TDCOSMO Millon, M; Galan, A; Courbin, F ...
Astronomy and astrophysics (Berlin),
07/2020, Volume:
639
Journal Article
Peer reviewed
Open access
Time-delay cosmography of lensed quasars has achieved 2.4% precision on the measurement of the Hubble constant, H0. As part of an ongoing effort to uncover and control systematic uncertainties, we ...investigate three potential sources: 1- stellar kinematics, 2- line-of-sight effects, and 3- the deflector mass model. To meet this goal in a quantitative way, we reproduced the H0LiCOW/SHARP/STRIDES (hereafter TDCOSMO) procedures on a set of real and simulated data, and we find the following. First, stellar kinematics cannot be a dominant source of error or bias since we find that a systematic change of 10% of measured velocity dispersion leads to only a 0.7% shift on H0 from the seven lenses analyzed by TDCOSMO. Second, we find no bias to arise from incorrect estimation of the line-of-sight effects. Third, we show that elliptical composite (stars + dark matter halo), power-law, and cored power-law mass profiles have the flexibility to yield a broad range in H0 values. However, the TDCOSMO procedures that model the data with both composite and power-law mass profiles are informative. If the models agree, as we observe in real systems owing to the “bulge-halo” conspiracy, H0 is recovered precisely and accurately by both models. If the two models disagree, as in the case of some pathological models illustrated here, the TDCOSMO procedure either discriminates between them through the goodness of fit, or it accounts for the discrepancy in the final error bars provided by the analysis. This conclusion is consistent with a reanalysis of six of the TDCOSMO (real) lenses: the composite model yields H0 = 74.0−1.8+1.7 km s−1 Mpc−1, while the power-law model yields 74.2−1.6+1.6 km s−1 Mpc−1. In conclusion, we find no evidence of bias or errors larger than the current statistical uncertainties reported by TDCOSMO.
Novel landmark-guided routes in ants Collett, T S; Graham, P; Harris, R A
Journal of experimental biology,
06/2007, Volume:
210, Issue:
Pt 12
Journal Article
Peer reviewed
Open access
We review studies in which ants familiar with fixed routes between their nest and a feeding site are displaced from one of these destinations to an unfamiliar site away from the route. Ants can reach ...their goal from such novel release sites guided by distant landmarks. We suggest that an ant's ability to take such novel landmark-guided routes after displacement is a by-product of the robustness of normal route-following and is unlikely to reflect the ant's use of a map-like knowledge of its surroundings.
Desert ants (Cataglyphis fortis) were trained to follow a fixed route around a barrier to a feeder. Their homeward trajectories were recorded on a test field containing a similar barrier, oriented ...either as in training or rotated through 22 or 45 . Under one set of experimental conditions, the homeward trajectories rotated with the orientation of the barrier, implying that the visual features of this extended landmark can determine the route independently of compass cues: the barrier provided a "visual scene" that controlled the trajectories of the ants. Under other conditions, the trajectories after rotation were a compromise between the habitual compass direction and the direction with respect to the rotated barrier. Trajectories were determined primarily by the visual scene when ants were allowed to return close to the nest before being caught and tested. The compromise trajectories were observed when ants were taken from the feeder. It seems that ants exhibit at least two separate learnt responses to the barrier: (i) a habitual compass direction triggered by the sight of the barrier and (ii) a visual scene direction that is compass-independent. We suggest that the weighting accorded to these different learnt responses changes with the state of the path integration system.
Insects are thought to pinpoint a place by using memorized “snapshots,” i.e., two-dimensional retinotopic views of the surrounding landmarks recorded when at the place (reviewed in 1). Insects then ...reach the place by moving until their current view matches their snapshot. To determine when snapshots are recalled, and how differences between view and snapshot are translated into appropriate movements, we analyzed the approaches of wood ants to a feeding site that was located in the center of an array of two or three cylinders. In ants, contrary to flying hymenopterans 2, body orientation and direction of travel are collinear, so that an ant approaching an object always looks at it with frontal visual field. On their way to a food site, ants fixated and approached a cylinder predominantly when its angular size was smaller than when viewed from the food site. This finding implies that ants store snapshots at this place while fixating landmarks with frontal retina, so simplifying the later alignment of snapshots with their current view. It also means that ants recall snapshots well in advance of reaching the place. Although snapshots are centered on a landmark, we show that they extend at least 120° into the periphery.
Spatial Memory in Insect Navigation Collett, Matthew; Chittka, Lars; Collett, Thomas S.
Current biology,
09/2013, Volume:
23, Issue:
17
Journal Article
Peer reviewed
Open access
A wide variety of insects use spatial memories in behaviours like holding a position in air or flowing water, in returning to a place of safety, and in foraging. The Hymenoptera, in particular, have ...evolved life-histories requiring reliable spatial memories to support the task of provisioning their young. Behavioural experiments, primarily on social bees and ants, reveal the mechanisms by which these memories are employed for guidance to spatial goals and suggest how the memories, and the processing streams that use them, may be organized. We discuss three types of memory-based guidance which, together, can explain a large part of observed insect spatial behaviour. Two of these, alignment image-matching and positional image-matching, are based on an insect's remembered views of its surroundings: The first uses views to keep to a familiar heading and the second to head towards a familiar place. The third type of guidance is based on a process of path integration by which an insect monitors its distance and direction from its nest through odometric and compass information. To a large degree, these guidance mechanisms appear to involve modular computational systems. We discuss the lack of evidence for cognitive maps in insects, and in particular the evidence against a map based on path integration, in which view-based and path integration memories might be combined. We suggest instead that insects have a collective of separate guidance systems, which cooperate and train each other, and together provide reliable guidance over a range of conditions.
A dielectric logging tool, electromagnetic propagation tool (EPT), was deployed in 2007 in the BPXA-DOE-USGS Mount Elbert Gas Hydrate Stratigraphic Test Well (Mount Elbert Well), North Slope, Alaska. ...The measured dielectric properties in the Mount Elbert well, combined with density log measurements, result in a vertical high-resolution (cm-scale) estimate of gas hydrate saturation. Two hydrate-bearing sand reservoirs about 20 m thick were identified using the EPT log and exhibited gas-hydrate saturation estimates ranging from 45% to 85%. In hydrate-bearing zones where variation of hole size and oil-based mud invasion are minimal, EPT-based gas hydrate saturation estimates on average agree well with lower vertical resolution estimates from the nuclear magnetic resonance logs; however, saturation and porosity estimates based on EPT logs are not reliable in intervals with substantial variations in borehole diameter and oil-based invasion.
EPT log interpretation reveals many thin-bedded layers at various depths, both above and below the thick continuous hydrate occurrences, which range from 30-cm to about 1-m thick. Such thin layers are not indicated in other well logs, or from the visual observation of core, with the exception of the image log recorded by the oil-base microimager. We also observe that EPT dielectric measurements can be used to accurately detect fine-scale changes in lithology and pore fluid properties of hydrate-bearing sediments where variation of hole size is minimal. EPT measurements may thus provide high-resolution in-situ hydrate saturation estimates for comparison and calibration with laboratory analysis.
We conducted numerical modeling of coupled multiphase fluid-flow, thermal, and geomechanical processes during gas production from an oceanic hydrate deposit to study the geomechanical performance and ...wellbore stability. We investigated two alternative cases of depressurization-induced gas production: (1) production from horizontal wells in a Class 3 deposit (a hydrate layer sandwiched between two low-permeability layers); and (2) production from vertical wells in a Class 2 deposit (a hydrate layer with an underlying zone of mobile water). The analysis showed that geomechanical responses around the wellbore are driven by reservoir-wide pressure depletion, which in turn, depends on production rate and pressure decline at the wellbore. The calculated vertical compaction of the relatively soft sediments and increased shear stress caused local yielding of the formation around the well assembly for both the horizontal and vertical well cases. However, the analysis also showed that the extent of the yield zone can be reduced if using overbalanced drilling (at an internal well pressure above the formation fluid pressure) and well completion that minimizes any annular gap between the well assembly and the formation. Our further analysis indicated that the most extensive yield zone would occur around the perforated production interval of a vertical well, where the pressure gradient is the highest. In the field, such yielding and shearing of the sediments could lead to enhanced sand production if not prevented with appropriate sand control technology. Moreover, our analysis shows that the vertical compaction of the reservoir can be substantial, with subsidence on the order of several meters and vertical compaction strain locally exceeding 10%. In the field, such substantial compaction strain will require appropriate well design (such as slip joints or heavy wall casing) to avoid tensile or buckling failure of the well assembly.
► We modeled wellbore stability during gas production from an oceanic hydrate deposit. ► Vertical compaction and increased shear stress caused yielding around the well assembly. ► Such yielding and shearing of the sediments could lead to enhanced sand production. ► Yielding can be reduced by overbalanced drilling and appropriate well completion. ► Compaction strain of up to 10% could significantly impact the well assembly.
It is often suggested that animals may link landmark memories to a global coordinate system provided by path integration, thereby obtaining a map-like representation of familiar terrain. In an ...attempt to discover if desert ants form such associations we have performed experiments that test whether desert ants recall a long-term memory of a global path integration vector on arriving at a familiar food site. Ants from three nests were trained along L-shaped routes to a feeder. Each route was entirely within open-topped channels that obscured all natural landmarks. Conspicuous artificial landmarks were attached to the channelling that formed the latter part of the route. The homeward vectors of ants accustomed to the route were tested with the foodward route, either as in training, or with the first leg of the L shortened or extended. These ants were taken from the feeder to a test area and released, whereupon they performed a home vector. If travelling the latter part of a familiar route and arriving at a familiar food site triggers the recall of an accustomed home vector, then the home vector should be the same under both test conditions. We find instead that the home vector tended to reflect the immediately preceding outward journey. In conjunction with earlier work, these experiments led us to conclude in the case of desert ants that landmark memories do not prime the recall of long-term global path integration memories. On the other hand, landmark memories are known to be linked to local path integration vectors that guide ants along a segment of a route. Landmarks thus seem to provide procedural information telling ants what action to perform next but not the positional information that gives an ant its location relative to its nest.
Under some circumstances, Diptera and Hymenoptera learn visual shapes retinotopically, so that they only recognize the shape when it is viewed by the same region of retina that was exposed to it ...during learning,. One use of such retinotopically stored views is in guiding an insect's path to a familiar site. Because the retinal image of an object changes with viewing distance and (sometimes) direction, a single stored view may be insufficient to guide an insect from start to goal. Little, however, is known about the number of views that insects store. Here we show that wood ants take several 'snapshots' of a familiar beacon from different vantage points. An ant leaving a newly discovered food source at the base of a landmark performs a tortuous walk back to its nest during which it periodically turns back and faces the landmark. The ant, on revisiting the familiar landmark, holds the edges of the landmark's image steady at several discrete positions on its retina. These preferred retinal positions tend to match the positions of landmark edges that the ant captured during its preceding 'learning walks'.
Animals typically have several navigational strategies available to them. Interactions between these strategies can reduce navigational errors and may lead to the emergence of new capacities.
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