暗黒物質のハリケーン、地球に接近　銀河系を逆走中　１００個の恒星が、大量の暗黒物質を伴って太陽系に接近食い止める術はないうまき

暗黒物質のハリケーン、地球に接近　銀河系を逆走中　１００個の恒星が、大量の暗黒物質を伴って太陽系に接近食い止める術はない
http://www.asyura2.com/17/jisin22/msg/630.html
投稿者うまき日時 2018 年 11 月 20 日 22:28:28: ufjzQf6660gRM　gqSC3IKr

暗黒物質のハリケーン、地球に接近　銀河系を逆走中
11/20(火) 16:18配信 CNN.co.jp
暗黒物質のハリケーン、地球に接近　銀河系を逆走中
およそ１００個の恒星が、大量の暗黒物質を伴って太陽系に接近しているという
（ＣＮＮ）　このほど発表された論文によると、地球は宇宙ハリケーンの直撃を受ける見通しだ。およそ１００個の恒星が、大量の暗黒物質を伴って太陽系に接近している。それを食い止める術はない。

写真特集：打ち捨てられたソ連製宇宙船

ただし、このハリケーンに危険はない。むしろ、まだ観測されたことのない暗黒物質を検出する絶好のチャンスになるかもしれない。

欧州宇宙機関（ＥＳＡ）の衛星「ガイア」は今年４月、太陽を取り囲む銀河系の近くにある２０億の恒星の位置と軌道に関する情報を公表した。

このデータを調べた研究者は、一部の恒星が特異な動きをしていることを発見。１年ほど前には、太陽系を通過する「星の流れ」が観測されていた。その１つで、同じような年代や成分の恒星１００個近くで形成される「Ｓ１」は、、普通の恒星とは逆方向の軌道で銀河系を周回していた。

これは高速道路を走行する車の一部が逆走するような状況だが、距離が離れているため衝突の心配はない。逆走する恒星は数千光年の距離に分散した状態で、数百万年以内に太陽系の近くを通過する。

Ｓ１は、１０億年ほど前に銀河系と衝突した矮小（わいしょう）銀河の残骸の一部と思われる。

矮小銀河はとてつもない量の暗黒物質を伴うことがある。暗黒物質はまだ観測されていない仮説上の物質だが、宇宙の謎の多くを説く鍵になるとされ、宇宙全体で普通の恒星やガスや惑星の質量の５倍の量が存在していると考えられている。

矮小銀河は暗黒物質の量がさらに多く、銀河系を周回する矮小銀河の炉座矮小銀河の場合、１０～１００倍と推定される。

もしそれがＳ１に当てはまるとすれば、Ｓ１の暗黒物質は、普通の暗黒物質の約２倍の速度で地球を突き抜ける。Ｓ１の暗黒物質は秒速およそ５５０キロの速度で太陽系を飛行していると推定される。ただし暗黒物質は、もし本当に存在しているとしても、極めて拡散性が高く、太陽系に対して目に見えるような影響は及ぼさない。

こうした数字は、それを裏付ける根拠は豊富にあるものの、今のところ仮説でしかない。しかし暗黒物質が高速で地球を通過すれば、観測する絶好のチャンスとなる。

論文を発表した研究チームは、現存する観測装置と提案されている観測装置を使って暗黒物質を発見できる可能性を計算した。暗黒物質のうち、質量の重い「ＷＩＭＰ」は質量の特定範囲を検出できる可能性があり、質量の軽い「アクシオン」を検出できる可能性はさらに大きいとしている。

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最終更新:11/20(火) 18:57
https://headlines.yahoo.co.jp/hl?a=20181120-35128933-cnn-int 　

1. 2018年11月20日 22:43:59 : jXbiWWJBCA : zikAgAsyVVk[1541] 報告

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Dark matter hurricane: Measuring the S1 stream with dark matter detectors
Ciaran A. J. O’Hare, Christopher McCabe, N. Wyn Evans, GyuChul Myeong, and Vasily Belokurov
Phys. Rev. D 98, 103006 – Published 7 November 2018
https://journals.aps.org/prd/abstract/10.1103/PhysRevD.98.103006

　
Dark Matter Blowing Like a Hurricane
November 7, 2018
The dark matter in our stellar neighborhood may be moving at high speed, which might produce a signature that future dark matter searches could detect.
Synopsis figure
C. O'Hare; NASA/Jon Lomberg
The cosmic weather forecast calls for strong dark matter winds in our region of the Galaxy. The prediction is based on a new study of a nearby collection of stars that are all moving in the same direction. This so-called S1 stream is believed to be the remnant of a dwarf galaxy that was swallowed by the Milky Way billions of years ago. The dark matter from this long-gone galaxy may be blowing past us at roughly 500 km/s, according to the study’s authors. Such a “dark matter hurricane” might leave a detectable imprint in data from dark matter searches.

The S1 stream was identified last year in an ongoing billion-star survey by the Gaia satellite. This is not the first stellar stream—in fact, astronomers have previously detected around 30 of these moving throngs in our Galaxy. The accepted understanding is that each of these streams is the debris of a small galaxy that crashed into the Milky Way.

The unique interest in S1 stems from the fact that its path crosses that of the Sun. Ciaran O'Hare from the University of Zaragoza, Spain, and colleagues calculated the effect of S1 on the dark matter in our region. The team considered different models for the density and distribution of the inflowing dark matter from the S1 progenitor galaxy. They then predicted possible signatures of this fast-moving population in searches for dark matter. Current detectors looking for weakly interacting massive particles (WIMPs)—one widely discussed form of dark matter—probably won’t see any effect from S1, but future WIMP detectors might. The chances are better for axion detectors, as the energy spectrum from axionic dark matter should exhibit a broad bump with an additional narrow peak if the S1 hurricane is truly buffeting our shores.

This research is published in Physical Review D.

–Michael Schirber

Michael Schirber is a Corresponding Editor for Physics based in Lyon, France.

https://physics.aps.org/synopsis-for/10.1103/PhysRevD.98.103006

A dark matter hurricane is headed our way
By Don Lincoln

Updated 0544 GMT (1344 HKT) November 20, 2018
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Don Lincoln is a senior scientist at Fermi National Accelerator Laboratory. He is the author of "The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Stuff That Will Blow Your Mind" and produces a series of science education videos. Follow him on Facebook. The opinions expressed in this commentary are his. View more opinion articles on CNN.

(CNN)According to a recent paper, the Earth is caught directly in the crosshairs of a cosmic hurricane. A swarm of nearly 100 stars, accompanied by an even greater amount of dark matter, is aimed directly at our stellar neighborhood and there's nothing we can do to stop it; in fact, the vanguard is already upon us. This sounds like a perfect summer blockbuster movie, starring The Rock and Chris Pratt, or maybe Scarlett Johansson and Charlize Theron.

Don Lincoln
Don Lincoln
Except this is for real. But is it a danger? Well, actually, no. Not at all. But it's potentially incredibly fascinating, with lots of interesting scientific interconnections. So, what is really going on?
The story starts last April, when the Gaia satellite announced the locations and trajectories of 2 billion stars in the vicinity of the Milky Way surrounding our sun. They released the data to the public.
Scientists were then able to look at the data set to see if they could spot anything peculiar. In galaxies like the Milky Way, the most common behavior is that the stars orbit the center of the galaxy in a manner broadly similar to the planets orbiting our sun. However, there are some stars that exhibit unusual motion. About a year ago, astronomers identified some "stellar streams" passing through our celestial neighborhood.
One of them, called S1 (for stream 1), consists of nearly 100 stars of similar age and composition, orbiting the Milky Way in a direction exactly opposite that of normal stars. It's kind of like a handful of cars driving the wrong way down the highway, except with a much greater distance between them and with no likelihood of a collision. These stars are spread out over a few thousand light years and they will pass through the solar system's neighborhood over the course of a few million years.
Something is wrong with dark matter
Something is wrong with dark matter
Astronomers identified S1 as being part of the remnants of a dwarf galaxy that collided with the Milky Way and was consumed in an epic episode of cosmic cannibalism. Dwarf galaxies are very small, typically about 1% the mass of the Milky Way. They can orbit larger galaxies and collide with the bigger galaxy, adding their mass to the parent. This is what appears to have happened in the case of S1, although the process has taken probably a billion years.
Dwarf galaxies often have a disproportionately large fraction of dark matter. Dark matter is a hypothetical and still-undiscovered form of matter that interacts only gravitationally. Scientists have proposed its existence to explain many astronomical mysteries, for example the observation that most galaxies rotate faster than can be explained by the known laws of physics and the stars and gas of which they are composed.
While dark matter has not yet been observed, hypothesizing its existence is the simplest and most economical explanation for myriad astronomical mysteries. Averaged over the entire universe, dark matter is thought to be five times more prevalent than the ordinary mass of stars and gas and planets.
In dwarf galaxies, the fraction of dark matter is often higher. In Fornax, a well-studied dwarf galaxy orbiting the Milky Way, researchers estimate that the dark matter is between 10 and 100 times greater than the mass found in its stars.
If that number holds for S1, the dark matter of the S1 stream is passing through the Earth at a much higher velocity than the more ordinary dark matter that orbits the Milky Way -- about twice as fast. It is thought that S1 dark matter is flying through the solar system at a speed of about 550 km/s, or about 1.2 million mph. While these numbers are impressive, they are misleading. Dark matter, if it exists, is extremely diffuse and it will have no discernible effect on the solar system.
The mystery of the early universe's enormous black hole
The mystery of the early universe's enormous black hole
Because dark matter hasn't been observed yet, these velocity numbers are speculative, although they are strongly supported by a very large body of evidence. However, the prospect of high velocity dark matter flying through the Earth has suggested an opportunity to detect it.
In a paper in the prestigious journal Physical Review D, researcher Ciaran O'Hare and his collaborators calculated the possibilities of discovering dark matter using both existing and proposed dark matter detectors. They considered two varieties of dark matter particles: a very heavy kind called a WIMP (weakly interacting massive particle) and a very light kind called an axion. Because the ultimate nature of dark matter is not known, it is important to be open to all possibilities.
They found that the detectors they evaluated could find WIMPs for certain ranges of the particle mass. However, when they looked at the axion possibility, it appeared the prospects were even better. Because of its light mass and the manner in which an axion would interact with the detector, the apparatus simply has a better chance of seeing the axion. (If axions exist, of course.)
Experiments with names like ADMX, MADMAX and ABRACADABRA are able or will be able to search for the signatures of dark matter proposed in the recent paper. They consist of technologies that are designed to interact with axions in a strong magnetic field and convert them to ordinary microwaves or radio waves that can be easily detected.
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It's important to remember that the S1 stream poses no credible threat to the Earth and humanity. There is no need for an action hero to save us. However, the synergy of science is staggering. A careful catalog of nearby stars has opened the prospect of a better possibility of finding and identifying dark matter, which is one of the great unanswered mysteries of modern physics. It's an amazing time we live in, in which we can study such things.
I'm excited.
https://edition.cnn.com/2018/11/19/opinions/dark-matter-hurricane-headed-our-way-lincoln-opinion/index.html