Gravitational Lensing

Gravitational Lensing is a phenomenon proposed by greatest physicist Albert Einstein. It actually works like a magnifying lens between observer and the light source in space. This lens is caused by space-time curvature, hence the name.

It is kind of distribution of matters in space which warp it's nearest space-time and hence it bends the incoming light from the distant galaxies. Albert Einstein predicted the amount of bending of space-time with his great theory of relativity.

Principle behind gravitational lensing:

According to general theory of relativity, any matters which contain any mass or energy density can warp the nearest space-time causing a curvature, which create a gravitational field around it. Then, the light ray coming from behind it will get deflected which is called gravitational lensing. It can also be experienced in optical lenses in where light is bent due to its convex or concave properties.

For observing such event, there should be two special celestial things; one is the source and the other is the matter which can warp its space-time. Here, the source could be anything, e.g clusters, supernova, galaxy etc which can act as light sources. The matters will deflect the light coming from the light sources due to its curvature of space-time. The observer will not see the distant light source clearly, rather he will see an apparent image of the light source. The visibility of the image will depend on the classes of gravitational lensing.

Classes of gravitational lensing:

There are mainly three classes of gravitational lensing. They are-

1. Strong Lensing:

In strong lensing, Einstein-ring (also known as Einstein-Chwolson ring) can be seen. When the lens (the distribution of matters which are working as a lens) is too massive and the source of light is too close to the lens, then strong lensing occurs. Also, the source, lens and observer should lie in a straight line.

As light takes different paths in strong lensing, that is why more than one images can be observed by an observer. The first double image quasar was found in 1979 due to this phenomenon. Hubble's constant can be calculated by studying the strong lensing time delay. Due to bending of space, light does not travel in a straight line and hence it covers different distances in different time causing time delay.

2. Weak lensing:

As the name suggests, if the gravitational lens is not powerful enough to show the phenomenon of Einstein ring or multiple images of light sources, then it is called weak lensing. However, it can also stretch and magnify the distant sources like a magnifying glass. If the shape and size of the source is known then, by observing the amount of bending of light, astronomers can calculate the properties of the gravitational lens.

However, scientists only know the average properties instead of all intrinsic properties of distant galaxies. So, by studying the weak lensing phenomena, astronomers can calculate the average properties of other galaxies.

3. Microlensing:

Microlensing is very different from the other two lensing techniques. Because unlike strong and weak lensing techniques, the microlensing does not use large galaxies or clusters. Rather, it uses small objects like stars or exoplanets for lensing process. Due to microlensing, a distant star brighten suddenly for a few days. By using this technique, astronomers can detect those stars or planets which are too far from earth to observe.

When a star or any exoplanet bends the light coming from a distant star, it focuses this light to a certain point. Hence the distant source star appears more brighter than usual. This brightness level remains maximum for a few days and it again gets dimmed with time. If such phenomenon occurs then, an astronomer can detect any distant stars or planets around it.

Concept of dark matter:

First we have to know what dark matter is. According to cosmologists, roughly 21% of the universe is made of dark matter and 75% of the universe is made of dark energy. But only 5% of the universe is only visible. For more information on dark matter and dark energy, visit this link Dark Matter and Dark Energy

From the above explanation we have seen that gravitational lensing occurs, when there is something massive in between observer and the light source. By observing the amount of bending light, an astronomer can calculate the mass of the matter which is acting as a gravitational lens. Using computer simulation model, the astronomers calculated the mass of the lens by putting the amount of observed light which did not synchronize with the real observation. That means, to observe such bending of light, the lens should be too massive. But in practical, there were not seen any matters accept the observable galaxies.

After many studies it was found that, such kind of invisible matters contain roughly 21% of the universe. As such kind of matters cannot be seen in real life, that is why scientists named it as Dark Matter.

Now using these techniques, the scientists are now searching for another galaxies and stars. More research results are still to come.