@非人間的すぎる

3 months ago

Dark matter is a mysterious, invisible substance that makes up about 95% of the mass of our galaxy. It's thought to be the glue that holds together stars, dust, and gas in galaxies, and forms the foundation of the universe's structure. However, dark matter's influence is much weaker in our solar system than in other parts of the galaxy

Dark matter is difficult to detect in the laboratory because it doesn't interact with light or the electromagnetic field, and it's only known to interact with ordinary matter and radiation through gravity. In our solar system, dark matter is thought to make up about 45% of the gravitational force, while normal matter makes up the other 55%. However, some say that the total amount of dark matter in our solar system is probably about the mass of a small asteroid, and would have almost no effect. This is because the values of dark matter in our solar system are tiny compared to other masses, such as the sun, which has a mass of 2 × 1030 kg, while Earth is more like 6 × 1024 kg.
One way to learn more about dark matter is to observe the evolution of galaxies since the earliest stages of the universe. For example, the ESA's Euclid mission, which launched in July 2023, aims to map the geometry of matter in the universe, specifically the distribution of galaxies, to learn more about dark matter.

Pictures of the Milky Way show billions of stars arranged in a spiral pattern radiating out from the center, with illuminated gas in between. But our eyes can only glimpse the surface of what holds our galaxy together. About 95 percent of the mass of our galaxy is invisible and does not interact with light. It is made of a mysterious substance called dark matter, which has never been directly measured.

Now, a new study calculates how dark matter’s gravity affects objects in our solar system, including spacecraft and distant comets. It also proposes a way that dark matter’s influence could be directly observed with a future experiment. The article is published in the Monthly Notices of the Royal Astronomical Society.

“We’re predicting that if you get out far enough in the solar system, you actually have the opportunity to start measuring the dark matter force,” said Jim Green, study co-author and advisor to NASA’s Office of the Chief Scientist. “This is the first idea of how to do it and where we would do it.”

Dark matter in our backyard

Here on Earth, our planet’s gravity keeps us from flying out of our chairs, and the Sun’s gravity keeps our planet orbiting on a 365-day schedule. But the farther from the Sun a spacecraft flies, the less it feels the Sun’s gravity, and the more it feels a different source of gravity: that of the matter from the rest of the galaxy, which is mostly dark matter. The mass of our galaxy’s 100 billion stars is miniscule compared to estimates of the Milky Way’s dark matter content.

To understand the influence of dark matter in the solar system, lead study author Edward Belbruno calculated the “galactic force,” the overall gravitational force of normal matter combined with dark matter from the entire galaxy. He found that in the solar system, about 45 percent of this force is from dark matter and 55 percent is from normal, so-called “baryonic matter.” This suggests a roughly half-and-half split between the mass of dark matter and normal matter in the solar system.

“I was a bit surprised by the relatively small contribution of the galactic force due to dark matter felt in our solar system as compared to the force due to the normal matter,” said Belbruno, mathematician and astrophysicist at Princeton University and Yeshiva University. “This is explained by the fact most of dark matter is in the outer parts of our galaxy, far from our solar system.”

A large region called a “halo” of dark matter encircles the Milky Way and represents the greatest concentration of the dark matter of the galaxy. There is little to no normal matter in the halo. If the solar system were located at a greater distance from the center of the galaxy, it would feel the effects of a larger proportion of dark matter in the galactic force because it would be closer to the dark matter halo, the authors said.

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