Introduction to gravitational lenses: what are they and how are they formed?

According to general relativity, Einstein’s theory that has the gravitational field as a central element, the presence of matter bends space-time, and this curvature is responsible for gravity. The greater the amount of mass, the greater the curvature of space-time and, consequently, the greater the gravity.

Gravitational lensing is a phenomenon caused by the effect of gravity on light from distant objects, and has become popular as a tool for studying the Universe. They allow, for example, to observe distant and ancient objects that existed in the early stages of the formation of the Universe or to estimate the distribution of dark matter.

What is a gravitational lens?

The concept of gravitational lensing is quite simple: mass produces gravitational attraction, and if an object is massive enough, it can bend even in passing light and fall under the influence of its gravitational field.

The gravitational field of a massive object, such as a star, galaxy, black hole, or even a planet, pulls closer objects harder. When light from a distant and behind source passes nearby, it will be affected by the gravitational field, bent and refocused.

In fact, light is not curved. Gravity bends the fabric of spacetime, light just passes through that curved space.

The image produced by a gravitational lens is usually a distorted image of bodies behind the object created by the lens. For example, in the image below you can see a gravitational lens produced by the red galaxy seen at the center of the lens (LRG 3-757); this galaxy distorted light from a much more distant blue galaxy; the distortion is such that the light from the blue galaxy forms almost a complete ring.

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This ring-shaped distortion was predicted by Einstein, which is why they are known as einstein rings. Around 1912-1916, Einstein was able to calculate how light would be deflected as it passed through the Sun’s gravitational field. The deflection angle of light would answer the following equation:

α = 4GM/bctwo

Einstein’s idea was observed in 1919 during a total solar eclipse by several astronomers who concluded that gravitational lensing was real. With these facts, it’s fair to say that gravitational lensing was discovered in the early 20th century.

Currently, gravitational lensing is used to study numerous phenomena and objects in the distant Universe. The strongest gravitational lensing is produced by galaxies, galaxy clusters or black holes, but it can also be produced by dark matter as it also creates a gravitational field.

Types of gravitational lenses

Gravitational lenses can be classified into three groups, strong glasses, weak lenses and microlenses:

Strong gravitational lenses are the easiest to understand as they are detected directly with the naked eye in an image, for example in telescope images. They are seen in the form of Einstein rings, arcs, or multiple images of the same object. They are created by very massive objects such as galaxies, galaxy clusters and black holes. Weak gravitational lenses are more difficult to detect. All distant galaxies are somehow affected by weak gravitational lensing, as dark matter is everywhere and creates gravitational lensing. Weak lenses require the study of various background objects for detection. Gravitational microlensing they do not appear to show distortion, but detailed analysis can detect faint variations in the light intensity of background objects.
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Strong lenses make it possible to see distant objects, especially distant galaxies, by intensifying their light. By observing the light from the most distant galaxies, some so old that their formation dates back to the early expansion phases of the Universe, astronomers can study conditions billions of years ago, and even what happened during the formation of these galaxies. That is, strong lenses are used in a similar way to how telescopes are used.

Weak lenses are used to estimate the amount of dark matter present in space and its distribution. They also allow estimating the mass of the object produced by the lens.

Microlensing is usually formed by the passage of a star or planet in front of another star or other more distant object. The light from the distant object is not as distorted as in strong lenses, not even reaching the distortion of weak lenses; Image distortion may be barely noticeable, but decreasing light intensity enhances microlenses. Thanks to microlensing, countless extrasolar planets.

While gravitational lensing is often associated with visible light, it actually affects the entire spectrum of electromagnetic waves, including radio, infrared, visible and ultraviolet light.


Massimo Meneghetti. Introduction to Gravitational Lensing.

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