Twisted Deja Vu: How the Universe Is Exhibited Near Black Holes

Advertisement

In the proximity of black holes, space is so warped that even light rays may bend around them many times. This event may allow us to observe many variants of the same thing. While this has been known for decades, only now do we have an accurate, scientific expression, thanks to Albert Sneppen, a student at the Niels Bohr Institute. The result, which even is more valuable in genuine black holes, has just been issued in the journal Scientific Reports.

You have probably learned about black holes — the astonishing blocks of gravity from which light can not escape. You may also have learned that space itself and even time behave strangely near black holes; space is warped.

A disk of lustrous gas whirls into the black hole “Gargantua” from the movie Interstellar. Source: Interstellar.wiki/CC BY-NC

In the proximity of a black hole, space curves enormously that light rays are diverted, and very nearby light can be diverted so much that it travels many times around the black hole. Hence, when we see a faraway background galaxy (or some other celestial body), we may be fortunate to see the same image of the galaxy many times, albeit more and more distorted.

Galaxies In Multiple Versions

The process is displayed in the figure below: A faraway galaxy shines in all directions — some of its light reaches near to the black hole and are easily deflected; some light comes even more closer and circumvolves the hole a single time before disappearing. By closely observing the black hole, we notice more and more versions of the same galaxy, the more adjacent to the edge of the hole we are looking at.

How much nearer to the black hole do you have to see from one picture to see the next image? The result has been acknowledged for over 40 years and is some 500 times (for the math aficionados, it is more exactly the “exponential function of two pi,” written e2π).

Light from the background galaxy orbits a black hole an expanding number of times, the closer it reaches the hole, and we, therefore, see the same galaxy in different directions. Source: Peter Laursen

Calculating this is so difficult that, until recently, we had not yet formed a mathematical and physical intuition as to why it occurs to be this specific factor. But applying some skillful, numerical methods, master’s scholar Albert Sneppen from the Cosmic Dawn Center — a central study center under both the Niels Bohr Institute and DTU Space — has now succeeded in explaining why.

“There is something fantastically wonderful in now explaining why the images replicate themselves in such an artistic way. On top of that, it gives new possibilities to experiment with our knowledge of gravity and black holes,” Albert Sneppen clarifies.

Explaining something mathematically is not only entertaining in itself; indeed, it takes us closer to a conclusion of this spectacular phenomenon. The factor “500” reflects how black holes and gravity work, so the repetitions of the images now become a means to check and experiment with gravity.

The condition is seen “face-on,” i.e. how we would examine it from Earth. The additional images of the galaxy become more distorted and squeezed, the closer we see at the black hole. Source: Peter Laursen

Spinning Black Holes

As a completely unique trait, Sneppen’s process can also be generalized to use not only to “trivial” black holes but also to black holes that revolve. Which they all do.

“It turns out that when it revolves fast, you no longer have to get more adjacent to the black hole by a factor 500, but significantly less. Each image is now barely 50, or 5, or even down to only 2 times nearer to the edge of the black hole,” explains Albert Sneppen.

Having to see 500 times closer to the black hole for every new image, indicates that the images are immediately “squeezed” into one annular image, as seen in the figure on the right. In practice, several images will be tough to examine. But when black holes revolve, there is more room for the “extra” images, so we can expect to prove the theory observationally in a not-too-distant future. In this way, we can study not just black holes, but also the galaxies behind them:

The time of the light increases, the more times it has to go around the black hole, so the images become increasingly “delayed.” If, for example, a star explodes as a supernova in a background galaxy, one would be able to see this explosion again and again.

Source: “Divergent reflections around the photon sphere of a black hole” by Albert Snepppen, 9 July 2021, Scientific Reports.

DOI: 10.1038/s41598-021-93595-w


Advertisement