Modern astronomy is in a bit of a turmoil. Astronomers understand how stars form, burn up, and die, and are working to improve their understanding of how planets are grouped into planetary systems like ours.
But astronomers face a problem: they don’t understand how galaxies can exist—a problem that has remained unresolved after decades of research.
The problem is relatively simple. Galaxies are groups of stars bound together by gravity. Like our solar system, it orbits, stars in grandiose paths, and orbits around the galactic center. At any fixed distance from the galactic center, faster-moving stars require stronger gravity to keep them in that orbit. When astronomers measure the orbital speed of stars in galaxies at great distances from the center, they find that the stars are moving so fast that the galaxies must be shredded.
The most common explanation for this observational mystery is a hitherto undiscovered form of matter: dark matter. If it is present, the dark matter exerts gravity, but it does not emit light or any form of electromagnetic radiation. This means that it cannot be seen by telescopes or any instruments that astronomers use to observe the universe. However, this invisible dark matter would add to any gravity of the galaxy, 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, the extra mass could have been just 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 galactic rotation.
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Other explanations that have been proposed include things like burning stars, black holes, and other things that are known to exist within galaxies but do not emit light. However, astronomers looked for such objects (called MACHOs, short for MASSive Compact Halo Objects) in the 1990s, and again, while they found examples of MACHOs, there wasn’t enough to explain the motion of stars in galaxies.
WIMPs
With some simpler explanations ruled out, scientists are beginning to believe that perhaps dark matter exists as a kind of “gas,” or as a particle that has never been seen before. These particles are called “WIMPs”, short for “weakly interacting massive particles”. WIMPs, if they exist, are essentially stable subatomic particles, with a mass somewhere in the proton mass range of 10,000 protons, or even more.
Like all candidate dark matter particles, WIMPs interact gravitationally, but the letter “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 the infinite range of gravity, the weak nuclear force only acts at small distances – distances much smaller than a proton. If WIMPs are found, they spread to galaxies, including our Milky Way, and even our Solar System. Depending on the mass of the WIMPs, astronomers estimate that if you make a fist, a single dark matter particle can be found inside.
Scientists have been searching for direct and compelling evidence for the existence of WIMPs for several decades. They do this in several ways. For example, some WIMP theories suggest that WIMPs can be made in particle accelerators, such as the Large Hadron Collider in Europe. Particle physicists look at their data, hoping to see WIMP’s production signature. No evidence has yet been observed.
Another way researchers are looking for WIMPs is by directly observing dark matter particles traveling through the Solar System. Scientists build very large detectors and cool them to extremely cold temperatures so that the detectors’ atoms move slowly. Then they placed these detectors half a mile or more underground to protect them from radiation from space. Then they wait, hoping that a dark matter particle in their detector will react, disturbing one of the semi-stable atoms.
But despite decades of efforts, no WIMPs have been observed. Predictions in the 1980s indicated that researchers could expect to detect WIMPs at a certain rate. When WIMPs were not detected, the researchers built a series of detectors with much greater sensitivity, all of which failed to find WIMPs. Current detectors are a hundred million times more sensitive than those of the 1980s, and no definitive note has been made for WIMPs, including Ultra-modern size Through the LZ experiment, which uses 10 tons of xenon to achieve unparalleled sensitivity to WIMPs.
looking forward
After decades of failing to detect dark matter, the scientific community is re-examining the situation. What is known for sure? Among other things, astronomers are sure that galaxies are spinning 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 the matter deficit, but it is probably not the solution. Perhaps the actual explanation is that the laws of motion and gravity need to be re-examined.
The name of such an approach is called MOND – an acronym for “MOdifications of Newtonian Dynamics”. The first such solution was proposed in the 1980s by the Israeli physicist Mordihai Milgrom. He suggested that for the familiar motion we experience day in and day out, the laws of motion established by Isaac Newton in the seventeenth century work well. But for very small forces and very small accelerations (as in the outskirts of galaxies), these laws need to be modified. After making these adjustments, he can correctly predict the rotation of galaxies.
Although this achievement might be seen as a resonance success, he changed the equations to match the observed spin properties of galaxies. This is not a successful test of the theory. He knew the answer before he created the equations.
In order to test Milgrom’s theory, the researchers needed to compare its predictions in other situations, such as applying it to the motion of large groups of galaxies held together by their mutual gravity. MOND theory struggles to make a prediction of this motion that is consistent with the theory, nor is it consistent with other observations.
So where are we? We are at that exhilarating stage of a scientific puzzle – a mystery still searching for a solution. While the majority of the scientific community aligns with dark matter, the failure to prove the existence of dark matter leads some to take a more serious look at theories that modify the accepted theories of gravity and motion.
If dark matter is present, it is five times more diffuse than ordinary atomic matter. If the correct answer is that we need to reconsider the laws of motion and gravity, this would have major consequences for modeling the history of the universe. The LZ experiment continues to run, hoping to improve on its already impressive performance, and the researchers do too Building new detectorsHoping to find dark matter and finally solve the mystery.