An Etymological Dictionary of Astronomy and Astrophysics
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The gravitational effect of the intervening large-scale potentials on the → cosmic microwave background radiation (CMBR). This effect smoothes out the temperature peaks and alters the statistics of the CMB.

→ CMB; → lensing.

The act of producing or the state of a → gravitational lens.

→ gravitational; → lensing.

The difference in light travel times along the various light paths from the source to the observer when the source image is divided into several images because of → gravitational lensing. According to the theory of → general relativity, light rays are deflected in the vicinity of massive objects. If the light source and the deflector are sufficiently well aligned with the observer, and obey some conditions on their distances (→ Einstein radius), we can observe several (generally distorted and magnified) images of the source. A property of → strong lensing is that the light travel time from the source to the observer is generally not identical for the different images. In other words, we not only see several images of one same object, but we also see this object, in each image, at different times. This means, in one image the lensed object will be observed before the other image. Given a physical model of the gravitational lens, the light travel time for each image can be computed. The expression giving the time delay has two components: a term is called → geometric delay, and the second term, known as the → Shapiro time delay. The latter is due to time dilation by the gravitational field of the lens, a direct consequence of general relativity. See also → time delay distance.

→ gravitational; → lensing; → time; → delay.

1) The act or effect produced by a lens, in particular a → gravitational lens.
2) Pertaining to the object that produces the lensing effect. → lensing galaxy.

From → lens + → -ing.

1) Lenzeš, verbal noun of lenzidan, verb formed from E. lens + -idan infinitive suffix.
2) Lenzandé, verbal adj. from lenzidan, as above.

Effect created by a → gravitational lens.

→ lensing; → effect.

A galaxy that acts as a → gravitational lens. The effect can also be due to a cluster of galaxies.

→ lensing; → galaxy.

An astronomical object that creates → gravitational lensing. See also → lensing galaxy.

→ lensing; → object.

An important quantity in the characterization of → gravitational lensing. The lensing potential is obtained by projecting the three-dimensional Newtonian potential on the lens plane and by properly re-scaling it. It is a two-dimensional analog to the → gravitational potential.

→ lensing; → potential.

A type of → gravitational lens, where the foreground → lensing object is of low mass, and the multiple images produced are too close together on the sky to be observed as separate images. Gravitational microlensing occurs when a foreground star happens to lie very close to our line of sight to a more distant background star. The foreground star acts as a lens, splitting the light from the background source star into two or more images, which are typically unresolved. However, these images of the source are magnified, by an amount that depends on the angular separation between the lens and source. If with the passage of time the lens moves across the Earth-source, the amount of brightening changes. Typically the source will appear to brighten, reach a maximum and then fade symmetrically back to normal over the course of a few weeks or months; this is called a → microlensing event. If the foreground star happens to host a planet with projected separation near the paths of these images, the planet will also act as a lens, further perturbing the images and resulting in a characteristic, short-lived signature of the planet. Microlensing is used in the search for → dark matter in the → Milky Way galaxy and its nearest neighbours, as well as for → extrasolar planets (e.g. B. S. Gaudi, 2010, arXiv:1002.0332).

→ micro-; → lensing.

Determining the three various parameters of a microlensing event (the lens-source relative parallax and proper motion, and the mass of the lens) from only one physical parameter (the event time scale). Currently the microlensing degeneracy affects the vast majority of events and makes any individual event impossible to interpret with certainty.

→ microlensing; → degeneracy.

The effect arising whenever a source star and lens star pass each other at an angular separation involving the → Einstein radius (R_E) of the lens. The time-scale for such an event is defined as t_E = R_E/v, where v is the magnitude of the relative transverse velocity between source and lens projected onto the lens plane.

→ microlensing; → event.

A → gravitational lensing phenomenon in which the image distortion is strong enough to be readily recognized, such as in the case of the → Einstein cross or when giant luminous arcs show up in → galaxy clusters (e.g. Abell 2218). Opposite to → weak gravitational lensing.

→ strong; → gravitational; → lensing.

A situation where the mass concentration in the central regions of → galaxy clusters exceeds the → critical density required for lensing, resulting in multiple images of background objects.

→ strong; → lensing.

A gravitational bending of light by structures in the Universe that distorts the images of distant galaxies. The distortion allows the distribution of → dark matter and its evolution with time to be measured, thereby probing the influence of → dark energy on the growth of structures. Weak gravitational lensing is generally difficult to identify in individual images, in contrast to → strong gravitational lensing (see, e.g., Bartelmann & Peter Schneider, 2001, Phys. Rept. 340, 291).

→ weak; → gravitational; → lensing.

The → gravitational lensing in which the images are only weakly distorted, and do not form wide arcs or multiple image systems. This happens if the → gravitational lens mass in front of a source is not concentrated enough to form multiple images. The resulting small distortions cannot be seen on individual sources, as we do not know their unlensed, "intrinsic" shape. However, if an entire population of background sources is available, the distortions can be revealed, either statistically or by local averaging. See also → strong lensing.

→ weak; → lensing.