Abstract:
Scientists recently discovered a new clue that may help humans "see" the dark matter of the Milky Way. Although dark matter itself does not emit light and cannot be directly observed by ordinary telescopes, researchers can infer its existence through the gravitational effects it exerts on surrounding celestial bodies. The latest research further suggests that the Earth may be passing through a huge "wind" of dark matter as it orbits the center of the Milky Way, and this relative motion may be able to leave observable visual features around the Milky Way.
Dark matter is one of the most important and mysterious components of modern cosmology. According to the current mainstream cosmological model, ordinary matter only accounts for a small part of the total mass and energy of the universe, while the amount of dark matter is approximately several times that of ordinary matter. The stars, planets, gas and dust we see are all ordinary matter, but the Milky Way's ability to maintain its current rotational speed and overall structure requires a large amount of invisible mass to provide additional gravity.
The biggest characteristic of dark matter is that it hardly interacts with electromagnetic waves, so it does not emit light like stars, and cannot be directly seen by absorbing or reflecting light. Scientists currently determine the existence of dark matter indirectly through phenomena such as the rotation speed of the Milky Way and other galaxies, the motion of galaxy clusters, and gravitational lensing.
The Milky Way itself is surrounded by a huge halo of dark matter. The scale of this dark matter halo far exceeds the Milky Way star disk that can be seen with the naked eye. The solar system is not located outside this dark matter halo, but has always been within it.
The solar system moves around the center of the Milky Way at a speed of about hundreds of kilometers per second, so from a dark matter perspective, the Earth is actually moving through the dark matter environment in the Milky Way. Because the direction of motion of the solar system and the Earth's revolution around the sun are constantly changing, theoretically this relative motion may cause the Earth to experience a directional dark matter "wind."
Researchers believe that if dark matter is composed of some kind of particle that can interact extremely weakly with ordinary matter, then the direction changes caused by the movement of the Earth may leave regular signals in some observation data. What's more, this signal could not only help scientists search for dark matter particles, but may also provide information about how dark matter is distributed within the Milky Way.
One prediction that deserves particular attention is that the Earth's orbit around the sun will cause periodic changes in the direction of incident dark matter. As the Earth moves toward the solar system, the numbers and relative velocities of dark matter particles it theoretically encounters may be different than when the Earth moves to the other side. This annual change is called "annual modulation" and is one of the important signals long sought by many direct detection experiments of dark matter.
If this change can be reliably measured, it could become a special "dark matter map." Scientists can gradually reconstruct the structure of the Milky Way's dark matter halo based on signal changes in different directions and at different times, without having to rely on the light emitted by the dark matter itself.
Another important factor that researchers are concerned about is that the Milky Way is not static in the universe. The solar system, the Milky Way, and the entire local group of galaxies are in continuous motion, so there are complex relative motions between dark matter and ordinary matter. Dark matter halos are not simple, completely uniform spheres, but may contain density changes, substructures, and local features formed by the gravitational influence of the Milky Way.
These structures may theoretically change the speed and direction distribution of dark matter particles near the Earth. If future detectors can measure weak enough differences, it may be possible to infer the spatial structure of dark matter within the Milky Way through these changes.
However, this type of research still faces a very big challenge: scientists currently don’t even know what dark matter is composed of.
In the past few decades, researchers have proposed a variety of dark matter candidates, including weakly interacting massive particles, axions, and other ultralight particles. Different candidate particles differ greatly in their mass, speed and how they interact with ordinary matter, so they can leave completely different signals in detectors.
This is why multiple laboratories around the world have been looking for weak recoil signals produced by dark matter particles directly hitting ordinary matter. Once they can determine that this signal is indeed coming from galactic dark matter, scientists can further analyze the direction, energy and time changes of the particles.
One of the important directions is the so-called "directional dark matter detection". Traditional dark matter detectors can usually only record how much energy a collision occurred, but cannot determine from which direction the particles entered the detector. If sufficiently sensitive directional detectors can be built in the future, it may be possible to directly measure the so-called dark matter wind.
The significance of this method is that the particles from the Milky Way's dark matter halo do not move completely randomly. The solar system itself is passing through a dark matter halo, so from the perspective of an observer on Earth, there should be an overall preferred direction of incidence for dark matter particles. If the detector can identify this direction, it could provide more direct evidence that dark matter does exist in the Milky Way.
At the same time, there are also some dark matter structures in the central region of the Milky Way that are particularly worthy of study. Because the density of stars in the center of the Milky Way is very high and the gravitational environment is extremely complex, dark matter may form a higher density there than in the outer reaches of the Milky Way. Scientists are also investigating whether dark matter might form more concentrated structures near the center of the Milky Way.
It needs to be emphasized that the so-called "visual clues" currently are not that scientists have directly photographed dark matter with ordinary telescopes. Dark matter still cannot be imaged directly like stars, gas clouds or dust. The “visualization” mentioned here refers more to the use of the gravitational effect produced by dark matter and the relative motion between the earth and dark matter to establish a dark matter distribution map from observation data.
In fact, scientists have been able to create dark matter distribution maps through methods such as gravitational lensing. In large-scale structures such as galaxy clusters, the gravitational pull generated by large amounts of dark matter can bend the light emitted by distant galaxies. By measuring this faint optical distortion, researchers can infer the distribution of invisible mass that generates gravity.
The situation inside the Milky Way is even more special, because we are inside the Milky Way and cannot directly observe the entire Milky Way from the outside like we can observe distant galaxies. Therefore, studying dark matter in the Milky Way requires a combination of stellar motion, gas motion, gravitational lensing, and future particle detection data.
If these different types of observations can be combined in the future, scientists may be able to build a more accurate map of the dark matter of the Milky Way and understand the shape, density and local structure of the dark matter halo.
Such research can also help scientists test different dark matter theories. If the dark matter velocity distribution predicted by a certain candidate particle is obviously inconsistent with the actual observations of the Milky Way, then the corresponding theory may be ruled out. In turn, if a directional signal that is highly consistent with a model is detected, it can provide important support for that model.
Therefore, the real significance of this research is not that "scientists have seen dark matter", but that dark matter research is gradually developing from simply looking for an unknown particle to trying to map the spatial distribution of dark matter in the galaxy. Our solar system itself is in this huge dark matter halo, and the Earth has been passing through it.
If future high-sensitivity detectors can capture this weak directional change, humans may eventually be able to use the movement of the Earth in the Milky Way to gradually map a material world that is completely invisible to the naked eye.
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