『What in the worlds is dark matter?』のカバーアート

What in the worlds is dark matter?

What in the worlds is dark matter?

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10月19日まで。※適用条件あり
Do you or someone you know need professional tech guidance for your author, coaching, or independent publishing business? If so, check out our sponsor Cloudessy, providing fractional Chief Technology Officer services. Simplify your tech stack, prevent launch failures, and build systems that actually support growth.https://marketing.cloudessy.com/30-minute-tech-stack-update/Five-sixths of the matter in the universe is a mystery to us. Now, we might have taken a step closer to figuring out what it is. Maybe.On September 1, the team running the LUX-ZEPLIN (LZ) experiment searching for dark matter reported a development at a conference in Tendo, Japan. Nearly a mile underground in an old South Dakota gold mine, the team watches a tank containing ten tons of ultra-pure liquid xenon. Going through 220 days of data collected in 2023 and 2024, they found a flash of light—the signature of a xenon nucleus recoiling with 248 kiloelectronvolts of energy it picked up from a source no known background phenomenon explains. They calculated the significance of the event at 2.6 sigma—meaning the background would throw off an event like this roughly once in two hundred tries. That isn't enough to call the detection a "discovery," which typically requires five sigma or higher. They didn’t claim to have seen direct evidence of dark matter. But they saw something interesting.1If LZ, or another experiment, repeats this finding, it could have major ramifications for astrophysics and cosmology. Dark matter out-masses all the rest of the matter in our universe at a ratio of about five to one,2 and yet we don’t know what it is.In 1933, Swiss astronomer Fritz Zwicky determined the galaxies in the Coma cluster were moving too fast for the cluster’s visible mass to hold them together. Something else—he called it dunkle Materie3—had to be supplying the necessary gravity. The astronomy community basically ignored his finding for 40 years.4 Then in 1970, Vera Rubin and Kent Ford measured how fast the outer regions of the Andromeda galaxy rotate and found the same problem—the edges were turning nearly as fast as the middle, which Newtonian gravity flatly forbids unless the galaxy’s mass is distributed in ways that don’t match what our instruments detect.5Fast-forward to 2006, when a team led by Douglas Clowe mapped a pair of galaxy clusters caught mid-collision. The gas in the clusters—which is where nearly all the ordinary matter lives—had piled up in the middle, slowed by the impact. But gravitational lensing showed the center-of-mass to be separated from the center of the gas by a significance of eight sigma. They called their paper: “A Direct Empirical Proof of the Existence of Dark Matter.”6The cosmic microwave background provides additional confirmation. The Planck satellite’s measurements fix cold dark matter at roughly five times the density of ordinary matter. Everything we can identify—gas, planets, nebulae, stars, galaxies, quasars, etc.—amounts to about a sixth of the material universe.7 Everything else is invisible and intangible, detectable to us only by its gravity.So, what is it?Maybe it’s not invisible, just really hard to see…Before we dive into speculative explanations, let’s address the simplest possibility. Perhaps dark matter isn’t really exotic; it’s just faint—burnt-out stars, drifting planets, stray black holes. These are often called massive compact halo objects (MACHOs). Anything dense enough passing in front of a background star will briefly brighten it by gravitational lensing, so the idea is directly testable.8The EROS-2 survey watched millions of stars in the Magellanic Clouds for 6.7 years. If the Milky Way’s halo were made of objects around four-tenths of a solar mass, roughly 39 lensing events should have shown up in the bright-star sample—only one did.9 The cosmic microwave background and Big Bang nucleosynthesis independently cap the total amount of ordinary matter far below what the galaxies' gravity demands—no matter how faint that matter might be.10Whatever holds the galaxies together, it isn’t made of the same stuff we are.Science fiction is on the case. Three novels in particular—Mike Brotherton’s Spider Star, John Gribbin’s The Alice Encounter, and Stephen Baxter’s Ring—each take a different approach to dramatizing the problem.WIMPs to the rescue?If dark matter isn’t just ordinary matter that’s hard to see, maybe it’s a particle no one has found yet.The favorite candidates for decades have been “weakly interacting massive particles” (WIMPs)—hypothesized to be heavy, stable particles produced in abundance in the early universe and left over when the cosmos thinned out.11 But we’ve hunted for a long time and nothing suitable has turned up.LZ published its low-mass results last December, based on 417 live days—the largest dataset any dark matter detector has ever assembled—and found nothing between three and nine ...
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