Gravitational Lensing-I
“I think and think for months and years. Ninety-nine times, the conclusion is false. The hundredth time I am right.” — Albert Einstein
Ever thought about how we can perceive objects in space whose location seems to be behind any other massive object, but we can see it? Or ever wondered why sometimes we can see photographic images of galaxies and objects forming bluish rings or simply how we can identify and detect dark matter or exoplanets or galaxies in this vastness? If such questions ever crowded your head, you clicked on the right link, honey!
Today, we are going to touch upon Gravitational lensing. As the name suggests, we are observing lensing effects on an object due to the gravitational influence of a massive lens, an AGN(Active Galactic Nuclei), or a cluster of AGN or stars.
Gravitational lensing is a fascinating phenomenon in astrophysics that occurs due to the bending of light by massive objects in the universe.It was first predicted by Albert Einstein’s theory of general relativity in 1915 and has since been observed and studied extensively by astronomers.
What is Gravitational Lensing?
Gravitational lensing is a visual effect caused by the distortion of space-time caused by large masses. The paths that light follows no longer remain straight.
This distortion creates a lens-like effect, hence the term “gravitational lensing.”
Gravitational lensing is a relatively new field, with strong, weak, and microlensing observations only a few decades old.
The three types of gravitational lensing are: Strong lensing, Weak lensing, and Microlensing.
Strong lensing
Strong lensing occurs when the light from a distant object is significantly distorted, resulting in multiple images or even an Einstein ring. It is the most dramatic and easily observable type of gravitational lensing. It occurs when the gravitational field of a massive object is strong enough to create multiple, highly distorted images of the background source.
These images can appear as arcs, rings, or even complete Einstein rings (if the alignment between the source, the lens, and the observer is perfect). It provides valuable insights into the mass distribution and properties of the lensing object.
If our observer, lens, and object are in perfect alignment, we can witness strong lensing, but if this isn't the case?
The case of multiple imaging:
If the lensing object is not perfectly aligned with the source and the observer, it forms multiple images of the source. These images can appear as separate, distinct objects or as elongated arcs, depending on the exact alignment and the shape of the lensing object. By studying the properties of these multiple images, astronomers can learn about the distribution of matter within the lensing object and the nature of the distant source.
Weak lensing
Weak Lensing causes a subtle distortion of the shapes of distant galaxies, providing valuable information about their mass distribution and the universe's evolution.It occurs when the gravitational field of a massive object is not strong enough to create multiple images but still bends the light passing through it. Due to bending appears slightly stretched or sheared.
Weak lensing is particularly useful for studying dark matter and dark energy.
Microlensing
Microlensing occurs when a massive object, such as a star, passes before a background star, causing a temporary brightening or amplification of its light. Due to weak lensing effects temporary amplification or dimming of a background source due to the gravitational field of a compact object, such as a star or a planet, passing between the source and the observer.
It is caused by the gravitational lensing of individual stars within the lensing object. It can be used to study the distribution of compact objects in the galaxy. It is rare and fleeting, requiring careful observation and analysis to detect and characterize. It involves the gravitational influence of individual objects rather than massive clusters or galaxies.
The lensing effect is temporary and lasts only as long as the foreground and background objects are aligned. It helps detect exoplanets, particularly useful for detecting planets in distant parts of the galaxy, where other detection methods may not be as effective.
The Einstein cross
This phenomenon occurs when a distant quasar, an extremely luminous galactic nucleus, is gravitationally lensed by a massive galaxy in the foreground. The light from the quasar is bent to create four distinct images, forming a cross-like pattern.
Albert Einstein first predicted this particular example of gravitational lensing.
Rings & Einstein’s ring
This occurs when a massive object, such as a galaxy, perfectly aligns with a distant light source, such as a quasar or a galaxy. The massive object's gravitational field acts as a lens, causing the light from the background source to be bent in a circular shape around the foreground object. The result is a beautiful ring-like structure that can be observed by astronomers.
Importance of gravitational lensing
•Gravitational lensing has significant implications for our understanding of the universe.
It is an essential tool in modern astrophysics that enables us to probe the mysteries of the cosmos and gain a deeper understanding of its fundamental properties.
This effect can be used to study and analyze various astronomical phenomena, such as the distribution of dark matter, the formation of galaxies, and the properties of black holes.
Applications
•Gravitational lensing allows astronomers to study the mass distribution of distant galaxies and clusters.
•Another important application of gravitational lensing is studying distant galaxies and the early universe.
•can magnify the light from distant objects, making them appear brighter and allowing astronomers to study them in greater detail.
•Gravitational lensing can provide crucial information about these distant worlds' size, mass, and orbital parameters, expanding our understanding of planetary systems beyond our own.
- study of the early universe(Will discuss on this in the later parts)
Observing the gravitational lensing
•Can magnify the light from distant objects, making them appear brighter and allowing astronomers to study them in greater detail.
•Astronomers study specific patterns in the light to understand to position and nature of the object.
- Gravitational lensing provides a way to detect and map the distribution of dark matter indirectly (I will discuss this later), as it can affect the bending of light.
This part was a simple outline of gravitational lensing in its truest sense. In the next blog, I'll dig deeper into its history and how we detect the lensing effects and derive the lensing equations. I have taken help from “Dynamics and Astrophysics of Galaxies ”by Jo Bovy, and “Introduction to Cosmology” by Barbara Ryden.
Keypoints(Edited)
- Dark Matter is a hypothetical matter present in the cosmos that doesn’t absorb, emit, or reflect electromagnetic radiation.
- Dark Energy is basically the driving factor accelerating the universe's expansion (or contraction). (The specifics of it weren’t part of this blog, but I would try to cover it’s mathematical aspects in the near future as they need an elaborate discussion).
- We consider space-time as we evaluate over the changing time, not a static parameter. Here, space-time(3+1 formulation) is considered the fabric on which all the bodies lie, and light bends accordingly, elaborating the pull effects.(Will be discussed elaborately on the later blogs)
