Only around 5 percent of the universe contains ordinary matter such as protons and electrons, with the remainder is packed with unknown substances such as invisible matter and dark energy. So far, physicists have struggled to find these mysterious compounds after spending decades looking for them. Yet, recent research could change this around since they have narrowed down the quest considerably.
Dark matter was first suggested more than 70 years ago to clarify why the force of gravity in galaxy clusters is so much greater than predicted. If the clusters held only the stars and gas we see, their gravity could be even lower, causing scientists to believe there is some form of matter buried there that we can't see.
Such dark matter will give extra mass to these massive objects, enhancing their gravitational influence. The material's key candidate is a form of hypothetical particle known as a "weakly interacting massive particle" (WIMP).
To probe the existence of dark matter, scientists search for proof of its correlations outside gravity. If the WIMP theory is right, dark matter particles could be observed by their scattering off atomic nuclei or electrons on Earth. In such "direct detection experiments, a WIMP collision would cause these charged particles to recoil, producing light that we can observe.
Electrons vs. Protons: The Difference
There will be ample heft for WIMPs to bowl over a whole atom sometimes. Yet some experimentalists are putting up smaller bowling pins in situations where dark matter is brighter.
Occasionally, a gentler rain of dark matter particles weighing less than protons might shake electrons out of their host atoms. The Sub-Electron-Noise Skipper CCD Experimental Device (Sensei), which utilizes hardware close to that of digital cameras to amplify signals within materials from spontaneously emancipated electrons, is the first laboratory-developed explicitly to capture this dark matter.
When a Sensei prototype turned on just one-tenth of a gram of silicon, it did not find dark matter. Even then the findings of the team, released in 2018, ruled out those templates immediately.
A team lead by University of Oregon physicist Tien-Tien Yu is up to deploy a 10-gram version, free from intrusive cosmic rays, in an underground laboratory in Canada. Other organizations are planning low-cost substitute experiments targeting the same low-hanging fruit.
Going Lighter
If dark matter is still lighter or blind to electric charge, an electron can fail to be unleashed. By manipulating groups of particles' actions, Kathryn Zurek, a theoretical physicist at the California Institute of Technology, has brainstormed ways that even these pipsqueaks might betray their existence.
For starters, picture a block of silicon as a mattress with springs that reflect atomic nuclei. Bounce a quarter off the mattress, says Zurek, and while no one spring can travel far a ripple that moves over several springs could be set off by the coin. In 2017, she suggested that an equivalent disruption from an encounter with dark matter could produce sound waves that could somewhat warm the system.
At the University of California, Berkeley, one experiment following this path, Tesseract, is currently operating in a basement searching for ripples from dark particles close in heft to those targeted by Sensei. However, technically, responsive potential updates might discover particles up to a thousand times lighter.
Yet there are still more options for Lilliputian particles. The axion may contain dark matter and concurrently solve a mystery regarding the powerful nuclear force, an object so weak that it is more wave than atom. The Axion Dark Matter Experiment (ADMX) recently started searching within a mighty magnetic field for axions decaying into pairs of photons, and other related quests are starting.
Other experiments try to make it much brighter. The lightest dark matter might potentially be is around one-thousandth of a trillionth of a trillionth of the mass of the neutron, resulting in a photon with a wavelength the size of a tiny galaxy that is like an incredibly low-energy wave. To understand why universes stay together, lighter (and therefore longer) entities will be too diffuse.
Clues From Above
Although the next wave of apparatus finding overt communication with dark matter is being planned by experimentalists, others intend to scour the heavens for indirect signposts.
By gravitationally pulling-in visible matter, immense dark matter clouds are assumed to contain galaxies and stars. But this will not be achieved by any smaller dark matter clusters which may occur. Such humble blobs will be black, but they could still bend moving starlight gravitationally. In the data from the ongoing GAIA study, one community of researchers is looking for this "lensing" of starlight by dark matter blobs.
Any such systems heavier than around 100 million suns were not located by the preliminary findings released in September. The researchers aim to discern the potential shapes of wispier dark clouds with wider future data sets. Scientists may infer from these imaginary systems' shapes and sizes whether dark matter particles interfere with themselves and in what manner.
Most scientists expect that dark matter is as universal as it is aloof. If they can come up with sufficient ways to feel the unseen, such as testing if it tickles various kinds of detectors, or if it nudges starlight, warms celestial centers, or even lodges in rocks, it might turn up wherever its ghostly impact.
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