On average, a primordial black hole flies past the solar system every ten years.

Written by Gou Lijun

Black holes are commonplace in movies and TV shows. But there is a black hole you may not have heard of: the primordial black hole. And this type of black hole may "visit" our solar system every ten years on average. What is a primordial black hole? Will the solar system be sucked in by it? Today we will talk to you about black holes.

Recently, a paper was published in Volume D of Physical Review D. If dark matter is really made up of micro primordial black holes, then theoretically, on average, one primordial black hole will fly past the solar system every ten years. When passing through the solar system, we can detect their existence by detecting the wobble of Mars' orbit.

We are generally familiar with the stellar black holes, which are the most common type of black hole family. They are formed by the collapse of stars and are usually several to dozens of times more massive than the sun. In our Milky Way, there are estimated to be hundreds of millions of such black holes. Stellar black holes are formed when massive stars explode in a supernova at the end of their lives and their cores collapse.

I had the privilege of studying the famous stellar black hole Cygnus X-1. Cygnus X-1 is the earliest confirmed black hole we know of, and physicists Kip Thorne and Stephen Hawking once made a bet on whether it was a black hole. We now know that it is a black hole, and it forms a binary system with a massive star. The gravitational field of this black hole is so strong that it can strip matter from the companion star, forming an accretion disk around the black hole.

Throughout the study, we made precise measurements of the distance and mass of Cygnus X-1 and the spin of the black hole. Spin is an important property of a black hole, which reflects the speed of its rotation. By analyzing the X-ray radiation emitted by the accretion disk around the black hole, we were able to measure the spin rate of Cygnus X-1 and reveal the mystery of the "rotation" of this black hole. These studies not only deepen our understanding of stellar black holes, but also provide a reference for further exploration of black holes with larger masses.

Unlike stellar black holes, primordial black holes are not formed by stellar evolution. Their origin can be traced back to the beginning of the universe. In the split second after the Big Bang, quantum fluctuations caused local gravitational collapse, giving rise to primordial black holes.

The mass of primordial black holes varies greatly, ranging from as light as a hydrogen atom (about 10^-24 grams) to as heavy as hundreds of millions of times the mass of the sun.

So far, the existence of primordial black holes remains a hypothesis, but more and more scientists are attracted to the study of smaller primordial black holes because they may be candidates for dark matter. Dark matter accounts for most of the mass in the universe, but has never been directly detected. Compared with normal matter, primordial black holes are relatively small in size and rarely collide with ordinary matter to cause interaction, so they are difficult to detect by traditional methods, but their gravitational effects may reveal their traces.

In recent years, scientists have proposed several new methods for detecting primordial black holes. For example, researchers have proposed that they can look for traces of primordial black holes by precisely measuring the orbits of Mars and other planets. When a primordial black hole approaches a planet, its strong gravitational pull will cause tiny wobbles in the planet's orbit. Although these wobbles are extremely small, they can be detected by modern precision astronomical instruments.

In addition, scientists are also trying to detect the existence of primordial black holes through global navigation satellite systems, such as the GPS system. If a primordial black hole of asteroid mass passes by the Earth within a few thousand kilometers, its impact on the satellite orbit will be minimal, but these changes can be recorded by long-term accumulated data. Similar research ideas provide new possibilities for the detection of primordial black holes.

Speaking of primordial black holes, this reminds me that I once participated in a science short film "Panda Legend - Black Hole Kiss" produced by the China Science and Technology Museum. The film incorporates primordial black holes into the plot. The film describes a clever little panda accidentally discovering a primordial black hole, which is approaching the earth at a very fast speed. Fortunately, this primordial black hole did not pose an actual threat to the earth, but disappeared silently in the universe like a "ghost" that visited briefly.

"Panda Legend" is not only an artistic creation, but also my exploration and thinking on the scientific concept of primordial black holes. I hope that through this short film, more people will understand this mysterious celestial body, and I also hope to inspire the audience's curiosity about the unknown areas of the universe.

The study of primordial black holes is still evolving. With the improvement of astronomical observation technology in the future, we may be able to better reveal their existence. Whether through gravitational wave detection, precise measurement of planetary orbits, or analysis of cosmic microwave background radiation, scientists are working hard to find traces of primordial black holes. They may be the key to unlocking the mystery of dark matter, or they may be important clues for our understanding of the origin and evolution of the universe.

On the road to exploring the universe, the study of primordial black holes has undoubtedly opened a new window for us. Just as Cygnus X-1 has brought us new insights into stellar black holes, the study of primordial black holes will continue to expand our understanding of the universe. In the endless frontier of science, every discovery is like a corner of the mysterious veil, waiting for us to reveal the mystery behind it.