The universe’s most mysterious and powerful objects – black holes – have recently gained a new identity as cosmic lenses. This theory of gravitational lensing, which is actually a direct consequence of Einstein’s theory of general relativity, tells us how extremely massive objects distort the fabric of space and time around them so much that they begin to act as natural lenses for light. Black holes, due to their extremely dense mass, prove to be the most effective in this field, capable of not only bending light’s path but twisting it into a powerful magnifying glass. These cosmic lenses are providing us access to corners of the universe that were considered completely beyond the reach of traditional telescopes – whether they are the first galaxies formed near the beginning of time or the extremely close regions of black holes about which we had virtually no information. This paper will delve deep into this amazing physical phenomenon while clarifying how today’s astronomers are using black holes as nature’s powerful lenses to uncover countless secrets of the universe.

The fundamental principle of gravitational lensing lies in Einstein’s theory of general relativity, which tells us that mass creates curvature in the fabric of space and time. This curvature is so fundamental that without it, the concept of the universe’s structure would be impossible. When a light ray passes through a region where space-time curvature exists, it leaves its straight path and bends, exactly like a ball changes direction when passing over a slope. However, the case of black holes is much more complex and interesting. A black hole, due to its extremely concentrated mass, creates such intense curvature in the surrounding space-time that not only light but even the speed of time itself is affected. This is why black holes act as the most powerful lenses for light. When a distant light source (such as a galaxy or quasar), a black hole, and the observer (telescope on our Earth) all align in a straight line, the black hole in the middle acts as a gigantic lens. This lens not only twists the light coming from the background source but can also create multiple images by dividing it into different parts. In this process, the intensity of light also increases extraordinarily, allowing us to see celestial objects that would normally be beyond the observational limit of our telescopes.

Black holes have the honor of being the universe’s most powerful lenses, and the reason is their extremely dense mass. Compared to an ordinary star, the mass of black holes is so concentrated that they create a gravitational field around them from which nothing, not even light, can escape. This characteristic makes them unparalleled for gravitational lensing compared to other celestial bodies. The lensing capability of black holes can be estimated from the fact that they can not only bend the light of distant galaxies but also capture light passing very close to their own event horizon. A magnificent example of this is a supermassive black hole named PKS 1830-211, which bent gamma rays from a supernova in such a way that they appeared much brighter to us than normal. This observation gave scientists the first opportunity to directly observe the extremely close regions of the black hole’s jets – an area that, despite being billions of light-years away from us, covered only about 100 astronomical units (100 times the distance between the Sun and Earth). This observation is actually living proof of the power of gravitational lensing, which told us that black holes are not only terrifying objects of the universe but also an invaluable source for understanding the universe.

In a special type of gravitational lensing called microlensing, a star or another massive object temporarily comes in front of a distant star’s light and makes it temporarily brighter for a few days or weeks. This process occurs when two stars, as seen from our Earth, pass in front of each other. If the intervening star has sufficient mass, it bends the background star’s light in such a way that it becomes temporarily brighter. The same principle is used for the discovery of black holes, but in this case, the situation is somewhat more complex. When a black hole passes between a distant star and Earth, it not only bends the star’s light but also creates a specific type of temporary increase in its brightness. This increase is particularly important because its observation not only detects the presence of the black hole but also makes it possible to accurately determine its mass. It is through such observations that nearby black holes like Gaia BH1 have been discovered, which are located only 1,560 light-years from Earth. This discovery is also particularly important because it tells us how many black holes might exist in our own galaxy that we didn’t know about until now.

When the background source (such as a distant galaxy), the lensing body (such as a black hole), and the observer (our Earth) are perfectly aligned, an extremely interesting phenomenon occurs. In this case, light bends equally from all sides and forms a complete circle, called an “Einstein ring.” This circle is actually a completely distorted image of the background source, which appears to us in the shape of a ring. If there is a slight misalignment in the straight line, four separate paths of light are formed, creating four separate images of the same source, called an “Einstein cross.” These beautiful phenomena not only magnificently confirm Einstein’s theory but also make it possible to accurately determine the mass of the lensing body. The most famous example of Einstein crosses is Q2237+030 or “Einstein Cross,” in which four separate images of a distant quasar are formed due to the gravitational lens effect of a nearby galaxy. In another similar observation, in a system named SDSS J0946+1006, two galaxies at different distances have together created two separate Einstein rings of the same background galaxy, called a “double ring.” These observations are not only beautiful but also a valuable source for scientists to understand the structure of the universe.

Supermassive black holes often emit powerful jets of matter traveling near the speed of light. These jets sometimes extend for thousands of light-years and emit extreme energy. However, understanding the origin and structure of these jets was a difficult task because these areas are very small and distant. Here, gravitational lensing comes to our aid once again. In an active galaxy named PKS 1413+135, scientists observed that clumps of hot matter from the jet emerging from the black hole passed behind an intermediate-mass lens (called a “milli-lens”) and as a result, their brightness significantly increased. This “milli-lens” has a mass about 10,000 times greater than the Sun. Such observations have given scientists the opportunity to see the extremely close regions of these jets, which span only a few light-days, with unprecedented detail. These are the areas where jets form and their initial dynamics are determined – areas that would normally be beyond the resolution of our telescopes.

In recent years, a new type of “milli-lenses” has been discovered, which is the link between traditional microlenses (small stars) and full-scale gravitational lenses (entire galaxies). These lenses consist of a few thousand to thousands of solar masses. In the case of PKS 1413+135, this milli-lens could consist of a star cluster or perhaps a collection of dark matter. These small lenses are also special because they don’t completely cover the background source but magnify its small parts separately, giving us the opportunity to closely watch the movements occurring in the black hole’s jets. The discovery of milli-lenses has also raised several new questions for scientists. Are these intermediate-mass black holes? Are they collections of dark matter? Or are they unusual star clusters? Finding answers to these questions will be one of the most important research fields in astronomy in the coming years.

The concept of gravitational lensing actually predates Einstein. In 1804, Johann Georg von Soldner first proposed the idea that gravity could affect light. However, Einstein calculated the correct effect in his general theory of relativity in 1916. Interestingly, Einstein himself did not believe that observing this effect would ever be possible. In 1919, astronomer Arthur Eddington confirmed the theory during a total solar eclipse. He and his team saw that the apparent position of stars near the Sun had the same difference that Einstein had predicted. This success brought Einstein and his theory worldwide fame. Over time, the theory of gravitational lensing continued to develop. In 1936, Einstein mentioned in an article that stars could act as lenses for each other. In 1937, Fritz Zwicky proposed that entire galaxies could also act as lenses. However, the first gravitational lens system was discovered in 1979, when a double quasar named Q0957+561 was discovered. Since then, thousands of gravitational lens systems have been discovered, each teaching us something new about the universe.

Another revolution is about to occur in the field of gravitational lensing in the coming years, due to the emergence of new technologies. The European Space Agency’s Euclid space telescope, launched in 2023, and the Vera Rubin Observatory in Chile, which will be fully operational in 2024, will take extremely high-quality images of large portions of the sky. It is estimated that these missions together will discover more than 100,000 new gravitational lenses, which is a hundred times more than the currently known number. This data will completely change our understanding of dark matter, dark energy, and the evolution of the universe. The Euclid mission is specifically designed for the study of dark energy and dark matter, and its primary tool is gravitational lensing. The Vera Rubin Observatory will collect more than 20 TB of data every night, a large portion of which will consist of gravitational lens observations. In addition to these projects, the Nancy Grace Roman Space Telescope, to be launched in 2027, will be another major advancement in the field of gravitational lensing. This telescope will offer a field of view 100 times wider than Hubble, enabling it to discover thousands of new gravitational lenses.

The data obtained from projects like Euclid, Vera Rubin, and Roman will be so vast that scientists will not be able to process it using traditional methods. To deal with this challenge, help is being sought from ordinary people through “citizen science” programs. Volunteers classify images on online platforms and train artificial intelligence models, which can then automatically identify potential gravitational lenses in thousands of images. In this regard, a project called “Space Warps” on the Zooniverse platform has been particularly successful, where thousands of volunteers have identified new gravitational lenses in Hubble Space Telescope images. Artificial intelligence, especially deep learning networks, has brought revolutionary changes to this field. Modern AI models have now received so much training that they can identify gravitational lenses better than the human eye. These models not only identify lenses but can also analyze their characteristics, such as lens strength, orientation, and background source shape. This automated analysis is providing scientists with the ability to work on data at an unprecedented scale.

The light-amplifying capability of gravitational lenses provides us with a unique opportunity to look back in time. Advanced machinery like the James Webb Space Telescope (JWST) is using these natural lenses to see the first galaxies that came into existence just 300 million years after the Big Bang. These observations are helping us understand how our own galaxy, the Milky Way, formed and how it will change in the future. Without gravitational lenses, these early galaxies are so faint that they cannot be seen even with our most powerful telescopes. But when these galaxies are behind a black hole or a galaxy cluster, their light bends and reaches us, providing us with valuable information about them. In this regard, a galaxy cluster named Abell 1689 has gained considerable fame, which gave us the opportunity to see thousands of galaxies from the early period of the universe. JWST has opened new paths in this field, which for the first time has observed the detailed structure of galaxies that existed in the early stages of the universe’s formation.

Perhaps the most important application of gravitational lenses is the study of dark matter. Dark matter, which makes up 85% of cosmic matter, neither emits nor absorbs light, so it is impossible to see directly. However, it has mass, therefore it twists space and time through gravity. Scientists can observe the gravitational lensing caused by galaxy clusters and calculate how much dark matter is there and how it is distributed. In this way, gravitational lenses are proving to be a powerful source for bringing the mysterious world of dark matter to light. The observation of the Bullet Cluster (1E 0657-558) proved to be an important milestone in this regard. In this cluster, two galaxies collided with each other, and observations revealed that dark matter separated from ordinary matter. This observation provides direct evidence of the existence of dark matter. The technique of mapping dark matter through gravitational lensing is continuously improving. Through weak lensing methods, scientists are creating a detailed map of the distribution of dark matter throughout the universe, which is providing us with valuable information about dark energy as well.

In modern research, gravitational lenses are being used in several new ways. Through interferometry, scientists are combining images obtained from different telescopes to achieve extremely high resolution. The Event Horizon Telescope (EHT) used this same technique to obtain the first direct image of the M87* black hole. Now, by further improving the network of EHT and other radio telescopes, we will be able to observe more complex effects of gravitational lensing around black holes. In addition, through spectroscopy, the spectrum of lensed light is being analyzed to study the composition and dynamics of the source. Through time-delay measurements, scientists are determining the accurate value of the Hubble constant, which indicates the rate of expansion of the universe. The H0LiCOW project has achieved notable successes in this regard. These new methods are taking research in the field of gravitational lensing to new heights and providing us with deep insight into the fundamental principles of the universe.

The role of data analytics in the study of gravitational lenses is becoming increasingly important day by day. The data obtained from modern telescopes is so vast and complex that the latest algorithms and supercomputers are needed to process it. Through machine learning and artificial intelligence, scientists are succeeding in recognizing and analyzing patterns hidden in this data. Modern methods of Bayesian inference are helping scientists create complex models of gravitational lens systems. These models not only help understand the geometry of the lens but also provide valuable information about the background source. Through statistical methods like MCMC (Markov chain Monte Carlo), scientists analyze millions of possible configurations and arrive at the most probable model. These complex analytical methods would be impossible without modern computational power. Without data analytics, today’s astronomy would be unimaginable, and gravitational lensing is its best example.

Gravitational lenses, especially those associated with black holes, are providing us with a completely new perspective for seeing and understanding the universe. These natural “cosmic magnifying glasses” not only allow us to see extremely distant and faint objects but also give us the opportunity to go back in time and observe the early history of the universe, unravel the mysteries of black holes, and examine hidden aspects like dark matter. With new technologies and observational projects, this field will bring even more amazing discoveries in the coming years, which will guide us in finding answers to fundamental questions about our universe. Gravitational lensing has indeed begun a new era in astronomy – an era in which we are learning to use gravity not only as a force but as a tool, which is providing us access to parts of the universe that were previously beyond our reach.