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Why do astronomers believe in dark matter?


2019-09-20 | Source: Michael J. I. Brown The Conversation

Dark matter is invisible in nature, cannot be monitored by a telescope, and particle physicists have yet to be honored to discover it through experiments. So why do I, and thousands of my colleagues, believe that most of the mass of the universe consists of dark matter, rather than the belief that the traditional matter that forms stars, planets and all other beings visible in our sky is the tyrant in the mass of the universe? To answer this question, we have to appreciate what dark matter can do, what it can do, understand where it is in the universe, and understand that being "dark" is only the beginning of the puzzle. Invisible effect Our story about dark matter begins with speed and gravity. We see, along the universe, objects traveling in orbits under the influence of gravity. As the planet orbits the Sun, the Sun revolves around the center of our galaxy. The speed that any space object needs to reach in order to stay in orbit is an equation that combines mass and distance. For example, the Earth in our solar system moves at 30 kilometers per second, while the more distant planets move at a few kilometers per second. Our galaxy is extremely massive, and the Sun revolves around the center of the galaxy 26,700 light-years away, at 230 km per second. However, while distances are further away from the center of the galaxy, the orbital velocities of stars remain almost constant. Why? Unlike our solar system, whose mass is dominated by the sun, the mass in our galaxy spreads over thousands of light years, and the greater the distances between space objects and the center of the galaxy, the greater the number of stars and the amount of gas trapped within its By distant stars in our galaxy? not exactly. In the 1960s, leading American astronomer Vera Rubin measured the orbital velocities of the galactic galaxy next to the Milky Way, at distances up to 70,000 light-years from the center of the galaxy. Remarkably, although this distance is more than double the stars and gases of the chain, the orbital velocity remains around 250 km per second. This phenomenon does not distinguish specific galaxies from one another. In the 1930s, Swiss-American astronomer Fritz Zweecki discovered that galaxies circling within the Hungarian clusters were moving at much higher speeds than expected. So what's going on? One possibility is that there are vast amounts of invisible mass extending beyond stars and gases, dark matter. In fact, the work of Zweiki and Robin, and subsequent generations of astronomers, suggest that the existence of dark matter in the universe far exceeds the amount of conventional matter (as for dark energy, this is a completely different story). Paradoxically, our inability to see or see dark matter gives us some clue as to how it behaves. Other. Although dark matter often reacts through gravity alone, it has some peculiar properties. A cloud of hot gas in space could lose energy by emitting light, causing it to cool; a gas cloud that collapses large enough and cool, under its gravity, could collapse into stars. In contrast, dark matter does not lose its energy by light emissions. Therefore, while conventional matter can collapse in dense bodies such as stars and planets, dark matter remains more dispersed. This explains a clear contradiction; while dark matter may dominate the mass of the universe, we do not think that much of it exists in our solar system. Successful simulation Because gravity is the only force that influences the motion of dark matter, it is also relatively easy to analyze it in a typical manner and in simulations. Since the 1970s, we have been able to find formulas for several dark matter structures, which also happen to predict the number of giant galaxies and galaxy clusters. Moreover, simulations can model the accumulation of structures through the history of the universe. The dark matter model is not only compatible with data, but also has predictive power. Is there an alternative to dark matter? We extracted its existence using gravity, but what guarantees that the way we understand gravity is correct? What if we deal with it wrong? Gravity may be stronger than we think in large distances. There are several alternative theories of gravity, but the "modified Newtonian dynamics" of physicist Mordechai Milgrum is the most famous example. How do we distinguish dark matter from modified gravity? Well, in most theories, gravity is attracted to mass, so if there is no dark matter, gravity is attracted to conventional matter, whereas if dark matter is dominant, gravity will be drawn primarily toward dark matter. So it must be easy now, knowing the correct theory, right? No, not at least exactly. Dark matter and traditional materials almost follow each other, but there are some useful exceptions. When the clouds of gases hit the dark matter, something remarkable results from this process; the gas collides into one cloud, while the particles of dark matter continue to move under the influence of gravity. This happens when Hungarian clusters collide with each other at tremendous speeds. So how do we measure the gravitational pull in the collision of Hungarian clusters? Well, gravity affects not only mass but also light, so distorted images of galaxies can track gravity. In a clash of galaxy clusters, gravity pulls toward where dark matter should be, rather than conventional matter. Ripples in time We can see the effect of dark matter in the deep past, not just in our time, down to the Big Bang. The cosmic microwave background radiation, the huge auroras caused by the Big Bang, can be seen in all directions; These sound waves resulted from the interaction between gravity, pressure and temperature in the early stages of the universe. Although dark matter affects gravity, it does not react to temperature and pressure like conventional materials, so the power of sound waves depends on the proportion of conventional matter from dark matter. As expected, measurements of these ripples, captured by satellite lenses and ground observatories, reveal that dark matter is more widespread than conventional matter in our universe. So, is the story over? Is dark matter the only answer available to this dilemma? Most astronomers may assume that the existence of dark matter is the simplest and best explanation for many of the phenomena we see in the universe. Although there are potential problems with the simplest models of dark matter, such as the number of small lunar galaxies, they are interesting and not compulsive defects. But the fact remains that we haven't discovered dark matter directly, so far, and that doesn't particularly bother me, because physics has a history with particles that took decades to discover directly; Darkness is the explanation for these phenomena.