Ritasakano - Outubros
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Lentes gravitacionais.
What is Gravitational Lensing?
A gravitational lens is a distribution of matter (such as a cluster of galaxies) between a distant light source and an observer, that is capable of bending the light from the source as the light travels towards the observer. This effect is known as gravitational lensing, and the amount of bending is one of the predictions of Albert Einstein’s general theory of relativity.
This illustration shows how gravitational lensing works. The gravity of a large galaxy cluster is so strong, it bends, brightens and distorts the light of distant galaxies behind it. The scale has been greatly exaggerated; in reality, the distant galaxy is much further away and much smaller. Credit: NASA, ESA, L. Calcada
There are three classes of gravitational lensing:
1° Strong lensing: where there are easily visible distortions such as the formation of Einstein rings, arcs, and multiple images.
Einstein ring. credit: NASA/ESA&Hubble
2° Weak lensing: where the distortions of background sources are much smaller and can only be detected by analyzing large numbers of sources in a statistical way to find coherent distortions of only a few percent. The lensing shows up statistically as a preferred stretching of the background objects perpendicular to the direction to the centre of the lens. By measuring the shapes and orientations of large numbers of distant galaxies, their orientations can be averaged to measure the shear of the lensing field in any region. This, in turn, can be used to reconstruct the mass distribution in the area: in particular, the background distribution of dark matter can be reconstructed. Since galaxies are intrinsically elliptical and the weak gravitational lensing signal is small, a very large number of galaxies must be used in these surveys.
The effects of foreground galaxy cluster mass on background galaxy shapes. The upper left panel shows (projected onto the plane of the sky) the shapes of cluster members (in yellow) and background galaxies (in white), ignoring the effects of weak lensing. The lower right panel shows this same scenario, but includes the effects of lensing. The middle panel shows a 3-d representation of the positions of cluster and source galaxies, relative to the observer. Note that the background galaxies appear stretched tangentially around the cluster.
3° Microlensing: where no distortion in shape can be seen but the amount of light received from a background object changes in time. The lensing object may be stars in the Milky Way in one typical case, with the background source being stars in a remote galaxy, or, in another case, an even more distant quasar. The effect is small, such that (in the case of strong lensing) even a galaxy with a mass more than 100 billion times that of the Sun will produce multiple images separated by only a few arcseconds. Galaxy clusters can produce separations of several arcminutes. In both cases the galaxies and sources are quite distant, many hundreds of megaparsecs away from our Galaxy.
Gravitational lenses act equally on all kinds of electromagnetic radiation, not just visible light. Weak lensing effects are being studied for the cosmic microwave background as well as galaxy surveys. Strong lenses have been observed in radio and x-ray regimes as well. If a strong lens produces multiple images, there will be a relative time delay between two paths: that is, in one image the lensed object will be observed before the other image.
As an exoplanet passes in front of a more distant star, its gravity causes the trajectory of the starlight to bend, and in some cases results in a brief brightening of the background star as seen by a telescope. The artistic concept illustrates this effect. This phenomenon of gravitational microlensing enables scientists to search for exoplanets that are too distant and dark to detect any other way.Credits: NASA Ames/JPL-Caltech/T. Pyle
Explanation in terms of space–time curvature
Simulated gravitational lensing by black hole by: Earther
In general relativity, light follows the curvature of spacetime, hence when light passes around a massive object, it is bent. This means that the light from an object on the other side will be bent towards an observer’s eye, just like an ordinary lens. In General Relativity the speed of light depends on the gravitational potential (aka the metric) and this bending can be viewed as a consequence of the light traveling along a gradient in light speed. Light rays are the boundary between the future, the spacelike, and the past regions. The gravitational attraction can be viewed as the motion of undisturbed objects in a background curved geometry or alternatively as the response of objects to a force in a flat geometry.
A galaxy perfectly aligned with a supernova (supernova PS1-10afx) acts as a cosmic magnifying glass, making it appear 100 billion times more dazzling than our Sun. Image credit: Anupreeta More/Kavli IPMU.
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Why are some insects so shiny and iridescent?
First here’s a post I answered on HOW insects form shiny or iridescent exoskeletons. It’s always good to know the answer to how because it can give context to the why. So why are some insects iridescent or shiny?
This can be split into two main categories; communicative functions and non-communicative.
COMMUNICATION
1. Mate Selection
While insects tend to use chemical communication more as a means of attracting potential mates colouration can however play a significant role in mate selection in a number of ways.
Honest signalling - in which the colouration reveals the quality of the individual as these colourations are costly to produce. Therefore only individuals that are high-quality are able to afford the cost of producing these signals (in this case the iridescence or shiny colouration)
Example:
A study (Fitzstephens & Getty, 2000) found that male Calopteryx maculata (Black-winged damselfly) with higher fat stores had a much bluer iridescent colouration compared to males on low fat diets.
Amplifier traits - iridescent / shiny colours may be used to amplify the differences in the signals of quality (however no studies have directly focused if this is a function of iridescence)
Sensory drives - iridescent colouration was favoured due to being more effective in signalling in certain ecological environments
Receiver Bias - females (or in rare cases the males or hermaphrodites) as the receivers may have favoured certain iridescent colouration due to being better received by their sensory system therefore resulting in the evolution of this colouration
2. Species Recognition
Man species use iridescent / shiny colouration in order to identify members of their own species! As simple as that!
Example:
Colias eurytheme (orange sulfur butterfly) use UV iridescence to communication with conspecifics
3.Agonistic interactions
Iridescense / shiny colouration may be used in intrasexual encounters; mostly male-male agonistic encounters and can be used as a territorial signal.
4.flocking behaviour
Can help insects that school together facilitate their orientation or direction within their schools / flocks. This is seen in many aquatic species of fish and even squid.
5. Predation avoidance
Iridescense / shiny colouration can be used to order to actually avoid predation! Although at first glance you’d assume this would make them more visible, this isn’t always the case.
Mimicry / camouflage - insects can avoid predation by mimicking objects, leaves, raindrops, other harmful species or even by blending into the background
Examples:
A number of iridescent jumping spider species (Salticidae); such as Brettus adonis in this study (Jackson & Hallas 1986) were found to mimic raindrops to avoid predation
The iridescent green leaf beetles such as the (Dogbane Leaf Beetle, Chrysochus auratus) similarity use their iridescence colouration to mimic dew on leaves.
Species like tiger beetles ( Cicindelinae) even use their iridescence to create an unsaturated appearance that allow them to blend into their envrionment (Schultz 1986, 2001)
Warning colouration - insects may use their colouration to communicate their toxicity or unpalatability serving as aposematic warning.
Example:
Panamanian tortoise beetle (Charidotella egregia) that change from gold to red when disturbed by predators(Vigneron et al. 2007).
Startle displays - some species will use their iridescence colouration to create a flash that may startle potential predators long enough for them to escape due to the way in which the iridescence reflects light.
Example:
The tiger beeltes again! Some of them have bright colouration like below that they use as startle defenses against predators (Sargent 1990).
NON-COMMUNICATION
1. Thermoregulation
There’s much debate over whether Iridescense / shiny colouration has any function in either heat absorption or dispersion.
Some found evidence that the structures used to created iridescense / shiny colouration acted as heat collectors, like in the wings of butterflies ( Miaoulis & Heilman 1998). However other’s have found no evidence of thermoregulation in tiger beetles Schultz & Hadley (1987).
2.Friction reduction
iridescense structures may reduce the friction in burrowing insects
Example: Carabid Beetles (Brachininae) (Seago et al. 2009).
These are just some of the proposed and studied functions of iridescense and shiny colouration in insects, there are more that aren’t as well studied or understood just yet!
More reading:
Doucet S, Meadows M 2009, ‘Iridescence: a functional perspective’, Journal of the Royal Society Interface, vol.6, no.2, pp.115–132
Meadows M, Butler M, Morehouse N, Taylor L, Toomey M, McGraw K, Rutowski R 2009, ‘Iridescence: views from many angles’, Journal of the Royal Society Interface, vol.6, pp.203–211
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The Island Named After a Satellite
It is so small that you cannot see it on Google maps. It measures 25 by 45 meters (27 by 49 yards), about half the size of a football field. This barren bit of rock off the coast of Canada also has an unusual namesake: the Landsat 1 satellite. The small size is actually what made the island notable in 1973, when it was initially discovered. Well, that, and the polar bear trying to eat one of the surveyors.
Betty Fleming, a researcher with the Topographic Survey of Canada, was hunting for uncharted islands and rocks amidst data from the new Landsat 1 satellite. She was particularly interested in the new satellite’s ability to find small features. Working with the Canadian Hydrographic Service, Fleming scanned images of the Labrador coast, an area that was poorly charted. About 20 kilometers (12 miles) offshore, the satellite detected a tiny, rocky island. Surveyors were sent to verify the existence of the island and encountered a hungry polar bear on the island. The surveyor quickly retreated. Eventually, the island became known as “Landsat Island,” after the satellite that discovered it. Watch the video to learn more about Betty Fleming and how Landsat Island was discovered by satellite and ground surveyors.
For more details about Landsat Island, read the full stories here:
The Island Named After a Satellite
The Unsung Woman Who Discovered an Unknown Island
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