BY: Arjit Mishra
Infinitely dispersed this Universe has number of mysteries, to remove the counts from these mysteries we created a powerful Telescope renowned as Hubble Telescope, which not only just enable us to observe distant and complicated Astronomical events but also we can peep into Past and observe the Past time of our Universe.
It is not completely true to say that only we(Humans) have to make efforts and build powerful machines for solving the mysteries of this Universe, sometimes nature also help us to do so. Gravitational Lensing is one amongst the natural phenomenons which stands for the above given fact.
The concept of Gravitational Lensing completely depend upon Properties of Space. It help us to see those Galaxies, Quasars and Stars which are billion of Light years away from us but we can't observe them directly. It is Gravitational Lensing which reveal the concept of Dark Energy.
This Blog will reveal the basic understanding behind Gravitational Lensing along with How Dark Matter was discovered with the help of Gravitational Lensing.
"what is Gravitational Lensing? and how does it work?" before answering these questions I would like to tell you the concept of lenses in normal Life:
Atually Lenses have the tendency to bend the path of Light.In case we have a Convex lens on which a beam of Light is focused it will converge the beam of light at a particular point.but if we have a Concave lens it will diverge the beam of light.
However Gravitational Lensing works similar to a Magnifying Glass. Let us understand how a simple Magnifying glass works:
We could see anything around us only if it's Light is reaching our EYE.When we see an object with our eyes it appears of same size as it is actually but if we see it through a magnifying glass it appears bigger in size.It is because beam of Light coming from the object got converged by the convex lens present in the magnifying glass and if we extend this converged path of light in it's backward direction we would be able to see that object apparently Bigger than it's original size; this is because our human brain can only perceive Light as a straight line.
In 1915 Albert Einstein proposed his general theory of Relativity which truly define gravitational force. He proposed that every mass warps the space-time continuum, and this warping phenomenon shows direct proportionality to the mass of an object. which means higher mass object would distort space-time more as compare to smaller one and because of this smaller mass object feel attraction towards the curve formed by higher mass object bcoz of the shape of space-time at that position, and this attraction is so called Gravitational Force.
We know that Light always travel in a straight in vacuum, but if Einstein's general theory of Relativity was true then it is obvious that higher mass objects like: Stars, galaxies, clusters, etc. would also warp the space-time according to their masses.which means when light passes near to these objects,it also changes it's original path because of shape of space-time at that position indicating to a fact that Gravity will act as a Lens similar to a Magnifying Glass, if this theory is true then we should get a natural Magnifying Glass witch would magnify the far present Galaxies, clusters, etc. along with we would be able to observe those stars, quasars, etc. which are hidden behind another big galaxies.but these are only theoretical considerations therefore there was need for experimental verification.
It was the year of 1919, when Eddington decided to prove this theory. He observed sun in Total Solar Eclipse through Telescope at that time a star was present behind the sun which we can't observe as sun was present just before it resisting it's vision. But if Einstein was right and sun warps space-time due to it's mass then due to Gravitational Lensing phenomenon we can observe that star, but you won't believe that Predictions of EINSTEIN'S theory were true and Eddington observed that star.
Originally light from that star shouldn't get to Earth by any means but as sun has bend the space-time by some extent so when light from that star passes through the sun it also gets bended by some extent and due to this virtual position of star changes and we can observe the star (hereby we can't see the original position of star but its apparent position).
Strong Lensing: This occurs when Gravitational lens is powerful or more specifically the light source behind it is situated very close it. For example- if a galaxy is present behind the galaxy cluster which is situated very close to galaxy cluster then the Galaxy cluster present in front of us act as a strong lens and the galaxy present behind it appears bigger in size as compare to it's original size as similar to magnifying glass. As I have already said we can observe an object only if it's light is reaching us. The light of hidden galaxy takes different paths to reach us due to strong lensing, therefore we can observe multiple images of same galaxy and if position of galaxy somehow changes the positions of multiple images though changes because these multiple images are of same galaxy but it can happen that all images don't change their positions at a same TIME because Light has taken different paths to reach us more specifically the image formed by short path taken by light changes first as compare to images formed by other paths.
Strong Lensing reveals one more wonderful phenomenon called Einstein Ring, when source of Light, observer and lens are present in a same line then light coming from source is uniformly bended by the lens which results a formation of bright light ring which we call Einstein Ring.
Weak Gravitational Lensing: We observe weak gravitational lensing when gravitational lens is not much powerful to propose multiple images of source or Einstein Ring. In this case source appears to be extra streched or magnified. If we know the size and shape of source then we can easily discover the properties of gravitational lens by calculating how much the light bend from it's original path.
Generally we don't know about shape and size of source but we know about their average properties which can be used. for example if we have images of 1000 galaxies and all are of spiral shape then we can say that far present this galaxy is of spiral shape. by using weak Gravitational lensing mainly we discover the properties of those galaxies and clusters which are working as weak gravitational lens.
Micro Lensing: This is totally different from strong and weak gravitational lensing because in this case gravitational lens is much smaller. In strong and weak gravitational lensing we observed that gravitational lens are either galaxy clusters or galaxy. but in micro lensing the work of gravitational lens is done by stars or exoplanets, by this type of lensing we can discover those far away planets or stars whose emitted light turns dim while reaching to us but with help of micro lensing this dim light turns out to be shining for some time let us know how?
As we know that gravity bends the light and lead to the formation of gravitational lens. micro lensing occurs when gravity of a star or planet focuses the light coming from the far away planet or star such that those far away star or planets appears to be shining for some time.
NOTE: About 23% of our Universe is composed of Dark Matter, It is not alike our ordinary Matter and we can't see it but it exists.(I will make a separate blog about Dark Matter and Dark Energy )
We know that Gravitational Lensing takes pace when a mass comes between us and a Light source. for example a Galaxy cluster; it acts as a lens which results distorted magnified or multiple images of that Light source. Imagine the light of hidden galaxy needs to pass through galaxy cluster before reaching us which works as a gravitational lens. The amount of Light we have received can be used to calculate the mass of that galaxy cluster (whiz acting as lens) with the help of computer simulation.
As we know that how Gravity works,we prepare the same gravitational lensing effect on computer and enter the data of received Light, Now we enter the observed mass of galaxy cluster(causing gravitational lensing) and see if we get the amount of light on computer same as we get in real life but surprisingly we obtain that original mass of Galaxy Cluster is not sufficient to bend the Light to an extent same as an extent Light has reached us. In fact the original mass of galaxy cluster is only 10% of that mass which actually done the work of Gravitational lens. It means in computer simulation to make the same gravitational lensing effect we require the mass of 9 other galaxy clusters, this missing mass can be of faint baryonic objects like- Black Holes, Interstellar objects, dust and planets.but after observing many galaxies closely we discover that the masses of these objects too are not same as missing masses. which means it is true that In this Universe something is there which we can't see but it has it's mass and is responsible for Gravitational Lensing but question is what is this new entity and where it exists. After further studies we concluded that it is present everywhere in this Universe and as we can't see it we call it as Dark Matter, it do not interact with Light and therefore is not visible.
It is not completely true to say that only we(Humans) have to make efforts and build powerful machines for solving the mysteries of this Universe, sometimes nature also help us to do so. Gravitational Lensing is one amongst the natural phenomenons which stands for the above given fact.
The concept of Gravitational Lensing completely depend upon Properties of Space. It help us to see those Galaxies, Quasars and Stars which are billion of Light years away from us but we can't observe them directly. It is Gravitational Lensing which reveal the concept of Dark Energy.
This Blog will reveal the basic understanding behind Gravitational Lensing along with How Dark Matter was discovered with the help of Gravitational Lensing.
"what is Gravitational Lensing? and how does it work?" before answering these questions I would like to tell you the concept of lenses in normal Life:
Atually Lenses have the tendency to bend the path of Light.In case we have a Convex lens on which a beam of Light is focused it will converge the beam of light at a particular point.but if we have a Concave lens it will diverge the beam of light.
However Gravitational Lensing works similar to a Magnifying Glass. Let us understand how a simple Magnifying glass works:
We could see anything around us only if it's Light is reaching our EYE.When we see an object with our eyes it appears of same size as it is actually but if we see it through a magnifying glass it appears bigger in size.It is because beam of Light coming from the object got converged by the convex lens present in the magnifying glass and if we extend this converged path of light in it's backward direction we would be able to see that object apparently Bigger than it's original size; this is because our human brain can only perceive Light as a straight line.
Now we should move on to Gravitational Lensing Phenomenon:
We know that Light always travel in a straight in vacuum, but if Einstein's general theory of Relativity was true then it is obvious that higher mass objects like: Stars, galaxies, clusters, etc. would also warp the space-time according to their masses.which means when light passes near to these objects,it also changes it's original path because of shape of space-time at that position indicating to a fact that Gravity will act as a Lens similar to a Magnifying Glass, if this theory is true then we should get a natural Magnifying Glass witch would magnify the far present Galaxies, clusters, etc. along with we would be able to observe those stars, quasars, etc. which are hidden behind another big galaxies.but these are only theoretical considerations therefore there was need for experimental verification.
It was the year of 1919, when Eddington decided to prove this theory. He observed sun in Total Solar Eclipse through Telescope at that time a star was present behind the sun which we can't observe as sun was present just before it resisting it's vision. But if Einstein was right and sun warps space-time due to it's mass then due to Gravitational Lensing phenomenon we can observe that star, but you won't believe that Predictions of EINSTEIN'S theory were true and Eddington observed that star.
Originally light from that star shouldn't get to Earth by any means but as sun has bend the space-time by some extent so when light from that star passes through the sun it also gets bended by some extent and due to this virtual position of star changes and we can observe the star (hereby we can't see the original position of star but its apparent position).
TYPES OF GRAVITATIONAL LENSING:
Strong Lensing reveals one more wonderful phenomenon called Einstein Ring, when source of Light, observer and lens are present in a same line then light coming from source is uniformly bended by the lens which results a formation of bright light ring which we call Einstein Ring.
Weak Gravitational Lensing: We observe weak gravitational lensing when gravitational lens is not much powerful to propose multiple images of source or Einstein Ring. In this case source appears to be extra streched or magnified. If we know the size and shape of source then we can easily discover the properties of gravitational lens by calculating how much the light bend from it's original path.
Generally we don't know about shape and size of source but we know about their average properties which can be used. for example if we have images of 1000 galaxies and all are of spiral shape then we can say that far present this galaxy is of spiral shape. by using weak Gravitational lensing mainly we discover the properties of those galaxies and clusters which are working as weak gravitational lens.
Micro Lensing: This is totally different from strong and weak gravitational lensing because in this case gravitational lens is much smaller. In strong and weak gravitational lensing we observed that gravitational lens are either galaxy clusters or galaxy. but in micro lensing the work of gravitational lens is done by stars or exoplanets, by this type of lensing we can discover those far away planets or stars whose emitted light turns dim while reaching to us but with help of micro lensing this dim light turns out to be shining for some time let us know how?
As we know that gravity bends the light and lead to the formation of gravitational lens. micro lensing occurs when gravity of a star or planet focuses the light coming from the far away planet or star such that those far away star or planets appears to be shining for some time.
Let us now understand how Dark Matter was Discovered with the help of Gravitational Lensing:
NOTE: About 23% of our Universe is composed of Dark Matter, It is not alike our ordinary Matter and we can't see it but it exists.(I will make a separate blog about Dark Matter and Dark Energy )
We know that Gravitational Lensing takes pace when a mass comes between us and a Light source. for example a Galaxy cluster; it acts as a lens which results distorted magnified or multiple images of that Light source. Imagine the light of hidden galaxy needs to pass through galaxy cluster before reaching us which works as a gravitational lens. The amount of Light we have received can be used to calculate the mass of that galaxy cluster (whiz acting as lens) with the help of computer simulation.
As we know that how Gravity works,we prepare the same gravitational lensing effect on computer and enter the data of received Light, Now we enter the observed mass of galaxy cluster(causing gravitational lensing) and see if we get the amount of light on computer same as we get in real life but surprisingly we obtain that original mass of Galaxy Cluster is not sufficient to bend the Light to an extent same as an extent Light has reached us. In fact the original mass of galaxy cluster is only 10% of that mass which actually done the work of Gravitational lens. It means in computer simulation to make the same gravitational lensing effect we require the mass of 9 other galaxy clusters, this missing mass can be of faint baryonic objects like- Black Holes, Interstellar objects, dust and planets.but after observing many galaxies closely we discover that the masses of these objects too are not same as missing masses. which means it is true that In this Universe something is there which we can't see but it has it's mass and is responsible for Gravitational Lensing but question is what is this new entity and where it exists. After further studies we concluded that it is present everywhere in this Universe and as we can't see it we call it as Dark Matter, it do not interact with Light and therefore is not visible.
"WE HAVE QUESTIONS BUT DARKNESS HAS ANSWERS TO THEM"
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