When you look through a telescope and see stars and galaxies across space, there is a reason why that happens: gravitational lensing. It shows us many things, but perhaps the most important is that it can show us the things we can’t see, like dark matter. Gravitational lensing and dark matter, when combined with Einstein’s general theory of relativity, can give us the answer to what dark matter is (check out my cool post about what is dark matter – at least to the extent we know).

Check this article out, too: How How are Neutron Stars: The Density Machines

Gravitational lensing makes distant objects seem closer and clearer when massive things like galaxy clusters bend space. This bending makes light from far-off galaxies brighter and clearer. This mainly happens due to the gravity. For example, when The Hubble Space Telescope looked at galaxy cluster Abell 370, 4 billion light-years away, it saw nearly 100 galaxies quite easily. That was because the gravities of these galaxies bend and shape their light by gravitational lensing. Depending on the shape and the location of galaxies, stars, or other objects in space, they may be easier or harder to spot. The galaxy that we call “the Dragon” has a unique stretched shape because of this.

Now, since gravitational lensing allows us to do things that gravity bends, that’s where the connection between gravitational lensing and dark matter comes in. Dark matter only interacts with gravity, so if dark matter bends the light via gravity, we can use gravitational lensing to understand where it is and what it is.

Let’s see this in much more detail and how it can answer a lot of questions, including some about dark matter.

What is Gravitational Lensing?

In its essence, gravitational lensing was predicted by Einstein in his General Theory of Relativity – one of the core theories of modern astrophysics -, and it’s proven in our present day. Astronomers study objects billions of light-years away using gravitational lensing.

Gravitational lensing happens when a massive object’s gravitational field bends light. This is like a lens in optics but on a cosmic scale. Imagine looking at a faraway streetlamp through a glass of water. The glass bends and distorts the light from the lamp, making it appear in a slightly different position, stretched, or even doubled. That’s how gravitational lensing works in space but at different sizes and levels. It makes distant galaxies appear magnified and sometimes multiplied.

This method helps scientists see things they couldn’t before. It also helps in gravitational wave astronomy. By studying bent light, astronomers learn about distant cosmic events. The biggest example of gravitational lensing is Hubble’s telescope. Hubble’s space telescope uses gravitational lensing to explore the universe. It can see the density of matter and the growth of cosmic structures.

The Mechanics of Gravitational Lensing

Gravitational lensing process begins when we detect a light source behind a massive object, like a quasar or galaxy. That’s when the gravity can affect that light and how we see it from Earth. Whatever the object is, it’s gravity bends this light towards Earth. The bigger the object, the better and easier to distinguish this. This can create multiple images of the same object with different brightness levels. These images can form spectacular rings, known as Einstein rings. Named after Albert Einstein, these rings are rare. They occur when everything is perfectly aligned, creating a stunning ring of light.

Exploring Dark Matter Through Cosmic Lenses

We’ve been studying (or trying to) dark matter for some time now. Even though we’ve made great progress, I’d say there is still a lot to go. Gravitational lensing is one of the primary methods of studying dark matter. Dark matter is a big part of our universe but can’t be seen because it doesn’t reflect light. The method of gravitational lensing helps here. It lets astronomers find dark matter by seeing how it bends light from far-off galaxies.

Think of a huge galaxy cluster as a cosmic lens. It bends and magnifies light from objects behind it. This bending, or gravitational lensing, helps us see where dark matter is in these clusters. El Gordo, a galaxy cluster, is a great example. It’s mostly dark matter, even though we can only see a small part of it. Scientists learn about dark matter’s amount and where it is by studying how light bends around it.

The Various Levels of Gravitational Lensing

I’ve explained gravitational lensing, but there is not one type. There are three. Strong, weak, and micro. These gravitational lensing methods affect how we see stars and planets far away and have different uses. We call these gravitational spectrum effects, and as I said, they vary from strong gravitational lensing to weak gravitational lensing. The third one is microlensing. I’ll try to explain all three in a more detailed way below, but here is a very simple brief of the three effects:

• Strong Lensing: Produces highly visible effects like arcs, rings (Einstein rings), or multiple images of the same distant object.
• Weak Lensing: This causes slight distortions in the shapes of background galaxies, which are detectable only through statistical analysis of large samples.
• Microlensing: Occurs on smaller scales, like stars bending the light of background stars, often used to detect exoplanets.

Gravitational Lensing Effects: Strong, Weak, Micro

Strong gravitational lensing happens near massive objects like galaxy clusters. The light gets bent, creating amazing sights like multiple images or Einstein rings. These visuals are essentially amazing for scientists and hobbyists. VLBI observations have given us detailed views of these massive objects. They show us the structure of galaxies and where potentially dark matter may be. Strong lensing is used to study the mass distribution of the lensing galaxy or cluster. It also helps to magnify the view of extremely distant galaxies that would otherwise be too faint to observe.

Weak gravitational lensing is more subtle. It looks at small changes in background galaxies’ shapes. Weak lensing occurs when the gravitational influence of the lensing object is less pronounced or the alignment isn’t as close. Instead of dramatic arcs or multiple images, weak lensing causes subtle distortions in the shapes of background galaxies. These distortions are too small to see in individual galaxies, but we can detect them statistically across a large sample.

Microlensing is used to find exoplanets. Microlensing occurs on much smaller scales when a single massive object (like a star or planet) passes in front of a background light source. The gravitational field of the closer object acts like a lens, temporarily magnifying the background star’s light. It’s like a cosmic magnifying glass. Microlensing

Here’s a brief table:

Type of Lensing	What Happens	Effects Observed	Key Applications
Strong Lensing	Large gravitational field, close alignment	Arcs, Einstein rings, multiple images	Mapping dark matter in galaxy clusters, magnifying distant galaxies
Weak Lensing	Small gravitational field, less alignment	Slight distortion of galaxy shapes	Mapping large-scale dark matter distribution, studying cosmic structure formation
Microlensing	Small-scale lensing by stars/planets	Temporary brightening of a star	Detecting exoplanets, rogue planets, black holes

Gravitational Lensing and Dark Matter: How Are They Connected

Gravitational lensing and dark matter are deeply connected because lensing lets us see the effects of dark matter, even though it’s invisible. Imagine a massive galaxy cluster sitting between us and a distant galaxy. The cluster’s gravity bends the light from the galaxy, distorting or magnifying its image, like looking through a warped glass. This bending of light is gravitational lensing. It shows not just the cluster’s visible mass but also the dark matter hidden within it.

Dark matter makes up most of the universe’s mass, but we can’t see it directly because it doesn’t emit or reflect light. However, its gravity shapes the universe, and gravitational lensing shows us where it is. Scientists create maps of dark matter by studying how light bends, showing how it spreads through space.

Lensing also shows that visible matter, like stars and gas, accounts for only a small part of the total mass in these clusters. The rest is dark matter. The Bullet Cluster that prove this is a magnificent famous example. In this and other examples, gravitational lensing reveals that dark matter moves differently than ordinary matter, confirming the existence of a matter that we don’t account for.

Reconstructing the Mass of Clusters: The Case of the Bullet Cluster

The Bullet Cluster plays a key role in linking gravitational lensing and dark matter. It’s a pair of galaxy clusters colliding, creating one of the clearest pieces of evidence for dark matter. When these clusters crashed, the visible parts—like stars and hot gas—behaved differently. The stars passed through almost unaffected, but the gas clouds collided, slowed down, and heated up.

Gravitational lensing mapped the cluster’s mass and revealed that most of it wasn’t in the gas but in regions following the stars. The lensing effect proves that dark matter didn’t slow down during the collision, unlike the gas. This behavior shows that dark matter interacts very weakly, if at all, with itself or ordinary matter, except through gravity. Without lensing, we wouldn’t know where the dark matter was, but the distorted light patterns make it visible. The Bullet Cluster is a cosmic “smoking gun.” It shows that dark matter isn’t just a theory—it’s real, and it behaves differently than ordinary matter.

Conclusion

Gravitational lensing and dark matter open up a hidden side of the universe, and they show how much lies beyond what we can see. By bending light, gravitational lensing acts as a cosmic magnifying glass. Apart from uncovering distant galaxies, it helps us to research the invisible mass that holds the universe together: dark matter. Dark matter doesn’t shine or interact with light, but it interacts with gravity. The famous Bullet Cluster is an amazing real-life proof of how using gravitational lensing can show us dark matter and help us finally detect dark matter.

As a conclusion, gravitational lensing doesn’t just help us see what’s far away. It also helps us to see what is visible to the naked eye. This connection between lensing and dark matter offers a clearer picture of the universe’s formation and evolution. Every distorted galaxy and a bent beam of light helps answer big questions about what our universe is made of and how it works.

FAQ

What is Gravitational Lensing?

Gravitational lensing is when a massive object, like a galaxy, bends and magnifies light from behind it. It works like a magnifying glass, letting us see distant objects. This effect comes from Einstein’s theory of relativity, showing how massive objects warp space.

How Does Gravitational Lensing Relate to Dark Matter?

Gravitational lensing is linked to dark matter because it lets us find and study dark matter. Dark matter doesn’t reflect light, but its gravity bends light, which we can see. This helps scientists understand where and how much dark matter is in the universe.

Can Gravitational Lensing Help in the Study of the Early Universe?

Yes! Gravitational lensing makes it possible to see very distant galaxies that are too faint to see normally. This lets scientists study how galaxies formed and changed over time. Telescopes like Hubble have used this to explore the universe, and the James Webb Space Telescope will continue this work.

What Are the Types of Gravitational Lensing?

There are two main types: strong and weak. Strong lensing creates clear effects like multiple images and rings. Weak lensing causes small distortions that can be used to find the mass of objects. There is also a third one, microlensing. It occurs on smaller scales, like stars bending the light of background stars, often used to detect exoplanets.

What is an Einstein Ring?

An Einstein ring is a rare sight in strong lensing. It happens when everything lines up perfectly, bending light into a ring. It’s named after Albert Einstein, who predicted it in his theory of general relativity.