Is dark matter real? The decades-long mystery of astronomy

Modern astronomy is a bit in turmoil. Astronomers are understanding how stars form, burn and die, and they are improving their understanding of how planets fit together into planetary systems like ours.

But astronomers have a problem: They don’t understand how galaxies can exist – a problem that hasn’t been solved after decades of research.

The problem is relatively simple. Galaxies are collections of stars held together by gravity. Like our solar system, they rotate, with stars walking majestic paths, orbiting the galactic center. At any fixed distance from the center of the galaxy, faster moving stars require stronger gravity to keep them in that orbit. When astronomers measure the orbital speed of stars in galaxies at different distances from the center, they find that stars are moving so fast that galaxies should be torn apart.

The most common explanation for this observational conundrum is a hitherto unknown form of matter: dark matter. If it exists, dark matter exerts gravity, but it does not emit light or any form of electromagnetic radiation. This means it cannot be seen by telescopes or any other instrumentation that astronomers use to observe the cosmos. However, this invisible dark matter would add to any galaxy’s gravitational pull, explaining why stars orbit the galaxy so quickly.

The problem with the dark matter hypothesis is that no one knows what form dark matter takes. When the term was first proposed in 1933 by Swiss-American astronomer Fritz Zwicky, it was possible that the extra mass was simply clouds of hydrogen gas. Interstellar hydrogen gas is largely invisible to telescopes. However, as technology has improved, astronomers have found ways to measure the amount of hydrogen gas in galaxies, and while there is plenty of it, there isn’t enough to explain. the mystery of the rotation of galaxies.

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Other explanations that have been offered include things like scorched stars, black holes, and other objects that are known to exist in galaxies but don’t emit light. However, astronomers searched for such objects (called MACHO, short for MAssive Compact Halo Objects) in the 1990s and, again, although they found examples of MACHO, there were not enough to explain the motion of stars in galaxies.

WIMP

With some of the simplest explanations ruled out, scientists began to speculate that perhaps dark matter existed as some kind of “gas” or particles never seen before. These particles are generically called “WIMP”, short for “Weakly Interacting Massive Particles”. WIMPs, if they exist, are essentially stable subatomic particles, with a mass somewhere in the range of the mass of a proton up to 10,000 protons, or even more.

Like all candidate dark matter particles, WIMPs interact gravitationally, but the “W” in the name means they also interact via the weak nuclear force. The weak nuclear force is involved in some forms of radioactivity. much stronger than gravity, but unlike gravity’s infinite range, the weak nuclear force only acts over tiny distances – distances much smaller than a proton. If WIMPs exist, they invade galaxies, including our Milky Way, and even our own solar system. Based on the mass of WIMPs, astronomers estimate that if you clench your fist, a dark matter particle could be in there.

Scientists have been looking for direct and convincing evidence for the existence of WIMPs for many decades. They do this in several ways. For example, some WIMP theories suggest that WIMPs can be made in particle accelerators, like the Large Hadron Collider in Europe. Particle physicists look at their data, hoping to see the signature of WIMP production. No evidence has been seen so far.

Another way researchers look for WIMPs is by directly observing dark matter particles floating around the solar system. Scientists build very large detectors and cool them to very cold temperatures so that the atoms in the detectors move slowly. They then placed these detectors half a mile or more underground to shield them from radiation from space. Then they wait, hoping that a particle of dark matter will interact in their detector, disrupting one of the nearly stationary atoms.

gone

But despite decades of effort, no WIMPs have been observed. Predictions from the 1980s suggested that researchers could expect to detect WIMPs at a particular rate. When no WIMPs were detected, the researchers built a series of detectors with much greater sensitivity, all of which failed to find any WIMPs. Current detectors are 100 million times more sensitive than those of the 1980s, and no definitive observations of WIMPs have taken place, including a very recent measurement from the LZ experiment, which uses 10 tons of xenon to achieve sensitivity unmatched at WIMPs.

Looking forward

After decades of failure to detect dark matter, the scientific community is re-examining the situation. What do we know for sure? Among other things, astronomers are certain that galaxies spin faster than can be explained using the known laws of motion and gravity and the amount of observed matter. The dark matter hypothesis is a solution to a matter deficit, but it may not be the answer. Perhaps the real explanation is that the laws of motion and gravity need to be reexamined.

The name of such an approach is called MOND – short for “MOdifications of Newtonian Dynamics”. The first such solution was proposed in the 1980s by Israeli physicist Mordehai Milgrom. He proposed that for the familiar motion we experience day to day, the laws of motion developed by Isaac Newton in the 1600s work very well. But for very small forces and very small accelerations (like on the outskirts of galaxies), these laws had to be adjusted. After making these adjustments, he could correctly predict the rotation of galaxies.

Although such an achievement could be considered a resounding success, he modified the equations to match the observed rotational properties of galaxies. It is not the successful test of a theory. He knew the answer before he created the equations.

In order to test Milgrom’s theory, researchers needed to compare his predictions in other situations, such as applying it to the motion of large clusters of galaxies held together by their mutual gravitational attraction. The MOND theory struggles to make a prediction of this movement that agrees with the theory, and it also disagrees with other observations.

So where are we? We are in this delightful phase of a scientific enigma – a mystery that is still seeking a solution. While the majority of the scientific community sides with dark matter, the failure to prove the existence of dark matter leads some to look much more seriously at theories that alter accepted theories of gravity and motion. movement.

If dark matter exists, it is five times more prevalent than ordinary atomic matter. If the correct answer is that we need to revise our laws of motion and gravity, this will have important consequences for our modeling of the history of the universe. The LZ experiment continues to run, hoping to improve its already impressive performance, and researchers are building new detectors, hoping to find dark matter and solve the mystery for good.

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