Gravitational Lensing
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| Gravitational Lensing Visualization |
In the early 1900s the great German physicist Albert Einstein gave his groundbreaking theory of relativity. In the theory, he predicted that space and time is not what we had always perceived, rather, it is a 4-dimensional net-a fabric of spacetime. This fabric bent around any object existing in the fabric. The distortion increased near massive objects and was negligible around tiny bodies like us. This gave rise to a new concept of gravity. For generations gravity was considered to be the weakest of all the fundamental forces and was considered to be an attractive force between two masses. Now according to Einstein’s theory, gravity was causing a spacetime distortion, which was the new reason to explain the revolution of planets around massive bodies like sun. Light has been a mystery since forever, but some properties were absolutely right then and are true even now. For example: light travels in straight line. This property was the reason we could see the candle from a straight pipe and not from a bent one- one of the most exciting experiments we did as a kid. Now since light is a form of energy with 0 mass, it usually reflects, refracts etc., when it faces any hindrance. So, if the path that the light is traveling through is distorted, it has no choice but to deflect along with these distortions.
You might be wondering why I am discussing two quite different topics in one single blog. Well, let us discuss an experiment performed by Sir Arthur Eddington in 1919. Sir Arthur wanted a proof to the new concept of gravity predicted by Einstein. In his paper published in the 1911, Einstein had predicted the values of the amount of gravitational deflection of starlight which was a proof for his theory of general relativity, published in 1915. Thus, taking the advantage of a full solar eclipse and the property that light travels in straight lines, Sir Arthur started observing a star located on the edge of the solar disc. The experiment was carried out by two expeditions. One to the West African island of Principe and the other one to the Brazilian town of Sobral. If Einstein’s theory were right, the star would be deflected radially away from the sun by about 1.75 arc seconds. The deflection coefficient that came led between 1.80 and 2.16 arc seconds. Hence proving the bending of light in the bent spacetime net.
Now, the above experiment we experience a phenomenon we now refer to as THE GRAVITATIONAL LENSING. The light emitted by distant galaxies and stars when pass through the gravitational fields of massive objects, they tend to bend and get distorted. Since a similar property is exhibited by convex lenses, this phenomenon was named as gravitational lensing.
One amazing example of gravitational lensing was observed in 1979, when scientist discovered a double quasar. It later turned out to be one single quasar whose light was deflected by the intervening galaxies. Therefore, we got two images of one single quasar. This property of galaxies acting as a lens was postulated by an astronomer Fritz Zwicky.
Light from the object which bends around the massive bodies in front of it, sends the light from all the sides of the object, forward. This helps us gain a bigger and brighter than original. Now since we know what Gravitational lensing actually is, let us see how it is the most brilliant too to study the history of the universe, dark matter and the things which were unobservable before.
Gravitational lensing is classified into three broad categories:
STRONG LENSING
It is a type of gravitational lensing that is strong enough to produce multiple images, arcs or even Einstein’s ring. For point like objects we usually get multiple images and for extended objects, it acts like a magnifying glass. So, we usually get an arc or a ring. The strong lensing requires the projected lens mass density to be greater than the critical density (the average density of matter required for the universe to just halt its expansion). The strong lensing was predicted by Albert Einstein and hence the ring that we observe is called the Einstein’s ring. The ring usually appears when the quasar and the lens are aligned perfectly. The light from the quasar forms this perfect ring. If the perfect alignment is not possible, we observe arcs. Galaxies produce a heavenly bright, smeared, and magnified image, thus helping scientists determine the distance, masses etc. of these unforeseen galaxies.
WEAK LENSING
The weak lensing helps us prove the existence of dark matter. Here the scattered mass acts as our lens. Dark matter has mass and it can act as a lens for other objects. Thus, we can observe the dark matter in the foreground. Galaxy clusters have approximately 80% of its content in the form of dark matter. The fields of these clusters deflect light traveling near them. The observer for the earth sees a dramatic distortion of a background source such as multiple images and arcs. The cluster mass estimates determined by the lensing is valuable because it potentially reveal dark clusters which contains over dense concentration of dark matter. One of the most successful experiments in studying dark matter is the Dark Energy Survey.
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| Img 2. Weak vs Strong Lensing |
MICROLENSING
Microlensing is a phenomenon which can used to detect objects that range from the mass of a planet to the mass of a star, regardless of the light they emit. It happens only when small objects pass in front of a background source or lens. This technique has been used to detect planets which are orbiting the lens rather than the object which emits light. It was also used to detect Massive Compact Halo Objects or MACHOs. It was the dark matter candidate which was the size of our Jupiter. It enables the study of the population of Faint or dark objects like, brown dwarfs, red dwarfs and even black holes. It works upon all wavelengths, magnifies distant source objects that emit any kind of electromagnetic radiation.
Who knew, when we combined gravity and light, we could actually make the most powerful lenses and we could see what was lurking behind and even forward in the whole universe. The dark matter, the cosmic wave background, we can detect them all by this magnificent GRAVITATIONAL LENSING. It is one of the most promising method to discover the history of universe. The future awaits when this gravitational lensing will tear the limitations put up by us and introduce us with the universe lurking behind those limitation. Author - Ayushi Mishra, Mechanical Engineering
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