Searching for the Galaxy Guardians: Why aren't galaxies breaking apart?

Galaxies are the most spectacular and fundamental building blocks of the universe, containing vast quantities of stars, gas, and dust. However, beneath their gorgeous appearance, they actually hide the universe's darkest secrets.

Decades ago, when astronomers were observing spiral galaxies, they discovered a very unreasonable phenomenon on the outer side of the galaxy: the objects there were moving too fast!

Newton and Einstein's theories of gravity tell us that gravity is inversely proportional to the square of the distance. This means that objects far away from the center of a galaxy, where the mass is most concentrated, feel a weaker gravitational pull than those close to the center of the galaxy, so they move slower. However, observations show that the speed of celestial bodies magically begins to level off after exceeding a certain distance, rather than decreasing as expected by theory.

This is a very surprising discovery. If the objects outside the galaxy were moving so fast, they should have already flown out of the galaxy's control, causing the galaxy to fall apart. But this did not happen. Why? What is silently guarding the galaxy? This aroused everyone's curiosity.

Let us consider the first possibility first.

Let's first go back more than 100 years. At that time, astronomer Le Verrier analyzed the orbit of Uranus in the solar system and found that its orbit did not match the prediction of Newton's theory of gravity. Since Newton's theory had been a huge success at the time, no one would question this basic theory. Instead, Le Verrier predicted after detailed mathematical calculations that there was another undiscovered planet outside Uranus that affected Uranus. In 1846, astronomer Galle discovered the missing mass, Neptune, less than 1 degree from the position predicted by Le Verrier.

The same thing may happen outside the galaxy. Although the sum of all visible matter, including stars, cannot explain the high-speed moving objects, what if there are a large number of objects that are much brighter than ordinary stars, such as brown dwarfs, red dwarfs, white dwarfs, neutron stars and black holes, exerting additional gravity? However, after extensive searches, astronomers found that the total amount of these objects is not enough to make up for the missing mass.

Next, let’s look at the second possibility.

Following the success of Neptune, astronomers discovered that the motion of Mercury was also abnormal. A natural inference is that there may be an undiscovered planet near Mercury. However, after searching for a while, no one found another planet near Mercury. Of course, they will never find it. Because this time, what we need is a completely new theory of gravity that goes beyond Newton. In 1915, when Einstein proposed the general theory of relativity, the problem of Mercury was perfectly solved.

So does this mean that, on galactic or larger scales, Einstein's theory actually needs to be revised? It's possible, and many theorists are trying, but no theory has yet emerged as the answer to all the problems.

Schematic diagram of galaxies (picture from TuChong.com)

Now, let's look at the third possibility. In addition to the ordinary matter of twinkling stars and glowing gas and dust in galaxies, is it possible that there is a large amount of invisible matter that interacts with ordinary matter only through gravity? This is a very attractive idea, and this invisible matter is the so-called "dark matter". In other words, galaxies may be surrounded by a huge and diffuse "dark matter halo", which provides additional gravity to keep the galaxy stable.

After that, all other observations also pointed to the fact that the universe should be filled with dark matter that does not emit light, absorb light, or reflect light. Calculations show that about 85% of the matter in the universe is dark matter!

Astronomers use a method called gravitational lensing to map the distribution of dark matter in space. We know that all matter has mass, which means it has gravity, and it affects the surrounding space-time. According to Einstein's theory, when light from a distant background galaxy passes through a foreground galaxy that is closer to the Earth, the gravity of the foreground galaxy will distort the surrounding space-time structure, so the path of the light will bend, just like being bent by an optical lens. From the degree of bending of the light, scientists can infer the mass contained in the foreground galaxy. The results show that the visible matter in the galaxy is not enough to explain the bending of the light, so there must be a large amount of dark matter.

OK, assuming dark matter is real, what is it? Theorists believe that dark matter is most likely one or more completely new particles that have never been discovered. To this end, they have proposed many hypothetical candidate particles, but the masses and properties of these particles are subject to great uncertainty. One class of theories predicts that dark matter consists of a "weakly interacting massive particle" (WIMP), which is favored because it can naturally explain the dark matter density observed in the universe today.

Currently, there are three ways to search for such dark matter particles. The first is direct detection, where scientists build detectors deep underground to directly capture evidence of dark matter particles colliding with atomic nuclei; the second is indirect detection, where scientists look for the annihilation or decay products of dark matter particles in data such as gamma rays and neutrinos; the third is collider detection, where high-energy particle beams collide to directly produce dark matter particles.

At present, my country not only has Jinping Underground Laboratory to directly search for collisions between dark matter particles and atomic nuclei, but also has the Wukong satellite to search for dark matter indirectly. In addition to WIMPs, scientists are of course also actively searching for other candidate particles, such as axions and inert neutrinos. But so far, the nature of dark matter remains a huge mystery.

So, what exactly is guarding the galaxy? We don't know yet. All we can do is stay curious, be patient, and work hard to explore various possibilities. In the end, we may get something unexpected.

This article is a work supported by Science Popularization China Starry Sky Project

Author: Shen Wen

Audit: Korean standard

Produced by: China Association for Science and Technology Department of Science Popularization

Producer: China Science and Technology Press Co., Ltd., Beijing Zhongke Xinghe Culture Media Co., Ltd.