while it makes up 84 per cent of all matter, and is all around us, it has never been seen as it does not produce or reflect light.
http://www.telegraph.co.uk/science/9383625/After-the-Higgs-boson-scientists-at-Cern-aim-for-super-LHC-turn-their-sights-on-dark-matter.html
이야말로 굉장하지 않은가, 만물의 84%를 구성하지만 보이지 않을 뿐 더러 빛을 생성하지 않을 뿐더러 반사조차 하지 않는다니 !
자료: wikipedia
우주 구성 물질의 비율
암흑 물질(暗黑物質, 영어: dark matter)은 전자기파를 복사하지 않고, 오직 중력적으로만 관찰되는 물질이다. (예를 들어, 암흑 물질은 주변 항성이나 은하의 운동을 교란한다거나, 근처의 전자기파를 굽힌다.) 암흑 물질의 존재는, 은하 따위의 총 질량을 계산할 때 전자기파로 계산한 값이 중력적 효과로 계산한 값보다 현저히 작다는 사실로부터 유추할 수 있다. 암흑 물질의 존재는 현재 정설로 인정되며, 빅뱅 이론 및 우주론의 표준 모형 (ΛCDM model)의 핵심 요소다. 아직 암흑 물질이 어떤 입자로 만들어졌는지는 알려지지 않았다. 이를 암흑 물질 문제(dark matter problem)라 한다. 현재, 학계에서는 아직 발견되지 않은 입자 (초짝입자나 액시온 따위)일 것이라는 이론이 주류다. 암흑 물질은 우주의 물질의 대략 22%를 차지하며, 나머지는 가시광선으로 관측할 수 있는 물질과 암흑 에너지로 이루어진다.
암흑 물질의 존재에 대한 의문은 지구 위에 우리의 존재와는 무관한 듯 보인다. 그러나 암흑 물질이 실제로 존재하느냐 않느냐는 현대 우주론의 최종 운명을 결정지을 수 있다. 우리는 먼 천제들로부터 멀어지는 은하에서 오는 빛의 적색편이를 통해 우주가 현재 팽창하고 있음을 안다. 우리가 빛으로 관찰할 수 있는 일반 물질의 양은 이러한 팽창을 멈출 만한 충분한 중력이 없으며 그래서 그러한 팽창은 암흑 물질이 없다면 영원히 계속될 것이다. 이론적으로 우주에 암흑 물질이 충분히 있다면 우주는 팽창을 멈추거나 역행(최후에 대붕괴로 이끄는)하게 될 수도 있을 것이다. 실제로는 우주의 팽창이나 수축 여부는 암흑 물질과는 다른 암흑에너지에 의해 결정될 것이라는 것이 일반적인 생각이다.
존재의 증거
암흑 물질에 대한 대부분의 증거는 은하 집단들의 연구로부터 온다. 이런 것들의 대부분이 대략적으로 정적이고 상당히 균일하게 나타나므로 그 중요한 이론에 의해 총 운동에너지는 은하들을 묶으려는 총 중력에너지의 반이 되어야 한다. 그러나 실험적으로 그 규모의 몇 배로 훨씬 더 많이 방출되고 있음이 발견되었으며 이는 보이는 물질들은 그 집단에서 극히 일부분일 거라는 추측이 이를 설명하기에 가장 직접적인 방법으로 남는다.
중력이론과 새로운 전산분석들로 천문학자들은 현재 암흑 물질이 어디에 위치할 것인가를 풀 수 있게 되었다. 그 결과는 당신이 암흑 물질과 은하들이 정확하게 같은 방식으로 운집되었을지의 예상했을 그대로다. 또한 은하 자체는 주로 암흑 물질을 이루고 있다는 신호들이 보인다. ㅡ 예를 들어, 은하 내부에서의 회전과 실제 우리 은하 표면의 존재는 은하가 펼쳐진 암흑 물질의 무리를 포함하고 있는지를 가장 쉽게 설명해 준다.
암흑 물질의 위치를 아는 것은 그 물질이 얼마나 존재하는지도 보여준다: 일반물질의 약 7배(우주의 팽창을 멈추게 하기 위해 느리게 하는데 필요한 양의 1/4의 해당한다고 생각됨)
그것은 시각적으로는 탐지될 수 없기 때문에 암흑 물질의 구성은 이론상으로만 남는다. (DAMA연구기관에선 지구를 통과하는 암흑 물질을 직접적으로 탐지를 주장해오고 있지만, 많은 과학자들이 그러한 증거를 기다리기 보단 회의적인 반응이다.) 은하규모의 블랙홀 같은 커다란 질량들은 시각적 자료의 근거에서 배제할 수 있다.
[편집]암흑 물질의 발견
최근에 암흑 물질로 만들어진 '보이지 않는 은하'가 발견되었다고 한다. 이것은 역사상 처음이다. 이것은 얼핏 보면 상당한 질량을 가지고 있고 자전하는 은하처럼 보이지만, 그 내부는 암흑 물질로 이루어져 있다. 이 은하는 지구로부터 5,000만 광년 떨어져 있고, 육안이나 일반망원경은 물론, 적외선이나 자외선 탐지기로도 관측되지 않는다. 영국·이탈리아·프랑스·호주 등 4개국 과학자들로 구성된 연구진은 우주에 떠도는 수소를 연구하던 중, 처녀자리에서 태양의 1억 배 질량을 가진 이 '수소 원자 덩어리'(암흑 물질)을 발견했다. 이 암흑 물질은 방사선을 내뿜고 있어 영국 체셔주와 푸에르토리코에 설치된 전파망원경을 통해 그 존재가 드러날 수 있었다. 연구진의 한 과학자는 "만약 보통의 은하였다면 매우 밝아서 아마추어 망원경으로도 관측되었을 것"이라고 말했다. 천문학자들은 현재 우주이론상 암흑 물질은 일반 물질보다 5배 이상 많기에, 이번 발견은 우주 연구에 상당히 중요한 계기가 될 것이라고 하였다.
[편집]암흑 물질의 후보 물질
암흑 물질을 구성하는 입자는 거의 전자기적으로 상호작용하지 않으므로, 일상적인 양성자나 전자 따위의 중입자로 구성되기 힘들다. 암흑 물질을 구성하는 가설적인 중입자 물질을 마초(MACHO, massive compact halo object)[1]라고 한다. 예를 들어 블랙홀, 중성자별, 아주 어두운 백색왜성이나 갈색왜성, 떠돌이 행성 따위다. 현재 학계의 정설에 따르면, 설사 마초가 존재하더라도 이들은 우주 전체 암흑 물질 양 가운데 소량만을 이룬다.
일부 중입자나 중성미자는 전자기적으로 상호작용하지 않으므로 암흑 물질을 구성할 수 있으나, 학계의 정설에 따르면 이들 입자는 우주의 전체 암흑 물질 양 가운데 소량만을 이루고, 나머지는 현재 발견되지 않은 입자로 이루어진다. 현재 주로 거론되는 암흑 물질 후보는 최경 (最輕) 초짝입자 (LSP), 액시온, 비활성 (sterile) 중성미자 따위다. 이들을 통틀어 윔프(WIMP, weakly interacting massive particle)[2]라고 부른다. 초대칭 이론은 수많은 초짝입자(superpartner)의 존재를 예측한다. 그 중 가장 가벼운 입자는 (대부분의 모형에서는) 안정하다. 정확하게 어느 입자가 가장 가벼운지는 모형에 따라 다르지만, 대개 초중성입자(neutralino)나 초액시온 (axino) 따위다. 액시온은 페체이 퀸 이론에서 CP 문제를 풀기 위하여 도입하는 입자다. 이 입자 역시 전자기적으로 상호작용하지 않기 때문에 암흑 물질을 이룰 수 있다. 비활성 중성미자는 일반적 중성미자의 미세한 질량을 설명하기 위하여 시소 메커니즘(seesaw mechanism)에서 도입하는 입자다. 만약 비활성 중성미자가 매우 무겁다면 일반적 중성미자는 그만큼 가벼워진다.[3]
역사적으로 암흑 물질은 세 범주로 나누어 왔다. 그 세 범주는 입자가 우주팽창으로 인해서 느려지기 전에 초기우주에서 무작위 운동으로 움직일 수 있었던 거리에 따라 분류되었다. 그 거리를 free-streaming length라고 하며 그에 따라 암흑 물질은 다음과 같이 나뉜다.
- 고온 암흑 물질 (HDM, hot dark matter) – free-streaming length가 원시 은하보다 훨신 큰 물질
- 중온 암흑 물질 (WDM, warm dark matter) – free-streaming length가 원시 은하와 비슷한 물질
- 저온 암흑 물질 (CDM, cold dark matter) – free-streaming length가 원시 은하보다 훨신 작은 물질
우주론의 표준 모형(standard model of cosmology)은 저온 암흑 물질을 채택한다. 우주론의 표준 모형은 일명 ΛCDM 모형이라고도 부르는데, 여가서 "Λ"는 우주상수 (암흑 에너지), "CDM"은 저온 암흑 물질을 뜻한다. 그러나 저온 암흑 물질은 은하의 생성을 잘 설명하지 못하기 때문에, 아직 학계에서 정설이 없는 상태다.
Estimated distribution of matter and energy in the universe, today (top) and when the CMB was released (bottom).
자료: NASA
http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/
Dark Energy, Dark Matter
In the early 1990's, one thing was fairly certain about the expansion of the Universe. It might have enough energy density to stop its expansion and recollapse, it might have so little energy density that it would never stop expanding, but gravity was certain to slow the expansion as time went on. Granted, the slowing had not been observed, but, theoretically, the Universe had to slow. The Universe is full of matter and the attractive force of gravity pulls all matter together. Then came 1998 and the Hubble Space Telescope (HST) observations of very distant supernovae that showed that, a long time ago, the Universe was actually expanding more slowly than it is today. So the expansion of the Universe has not been slowing due to gravity, as everyone thought, it has been accelerating. No one expected this, no one knew how to explain it. But something was causing it.
Eventually theorists came up with three sorts of explanations. Maybe it was a result of a long-discarded version of Einstein's theory of gravity, one that contained what was called a "cosmological constant." Maybe there was some strange kind of energy-fluid that filled space. Maybe there is something wrong with Einstein's theory of gravity and a new theory could include some kind of field that creates this cosmic acceleration. Theorists still don't know what the correct explanation is, but they have given the solution a name. It is called dark energy.
What Is Dark Energy?
More is unknown than is known. We know how much dark energy there is because we know how it affects the Universe's expansion. Other than that, it is a complete mystery. But it is an important mystery. It turns out thatroughly 70% of the Universe is dark energy. Dark matter makes up about 25%. The rest - everything on Earth, everything ever observed with all of our instruments, all normal matter - adds up to less than 5% of the Universe. Come to think of it, maybe it shouldn't be called "normal" matter at all, since it is such a small fraction of the Universe.
One explanation for dark energy is that it is a property of space. Albert Einstein was the first person to realize that empty space is not nothing. Space has amazing properties, many of which are just beginning to be understood. The first property that Einstein discovered is that it is possible for more space to come into existence. Then one version of Einstein's gravity theory, the version that contains a cosmological constant, makes a second prediction: "empty space" can possess its own energy. Because this energy is a property of space itself, it would not be diluted as space expands. As more space comes into existence, more of this energy-of-space would appear. As a result, this form of energy would cause the Universe to expand faster and faster. Unfortunately, no one understands why the cosmological constant should even be there, much less why it would have exactly the right value to cause the observed acceleration of the Universe.
Another explanation for how space acquires energy comes from the quantum theory of matter. In this theory, "empty space" is actually full of temporary ("virtual") particles that continually form and then disappear. But when physicists tried to calculate how much energy this would give empty space, the answer came out wrong - wrong by a lot. The number came out 10120 times too big. That's a 1 with 120 zeros after it. It's hard to get an answer that bad. So the mystery continues.
Another explanation for dark energy is that it is a new kind of dynamical energy fluid or field, something that fills all of space but something whose effect on the expansion of the Universe is the opposite of that of matter and normal energy. Some theorists have named this "quintessence," after the fifth element of the Greek philosophers. But, if quintessence is the answer, we still don't know what it is like, what it interacts with, or why it exists. So the mystery continues.
A last possibility is that Einstein's theory of gravity is not correct. That would not only affect the expansion of the Universe, but it would also affect the way that normal matter in galaxies and clusters of galaxies behaved. This fact would provide a way to decide if the solution to the dark energy problem is a new gravity theory or not: we could observe how galaxies come together in clusters. But if it does turn out that a new theory of gravity is needed, what kind of theory would it be? How could it correctly describe the motion of the bodies in the Solar System, as Einstein's theory is known to do, and still give us the different prediction for the Universe that we need? There are candidate theories, but none are compelling. So the mystery continues.
The thing that is needed to decide between dark energy possibilities - a property of space, a new dynamic fluid, or a new theory of gravity - is more data, better data.
What Is Dark Matter?
By fitting a theoretical model of the composition of the Universe to the combined set of cosmological observations, scientists have come up with the composition that we described above, ~70% dark energy, ~25% dark matter, ~5% normal matter. What is dark matter?
We are much more certain what dark matter is not than we are what it is. First, it is dark, meaning that it is not in the form of stars and planets that we see. Observations show that there is far too little visible matter in the Universe to make up the 25% required by the observations. Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons. We know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them. Third, dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with matter. Finally, we can rule out large galaxy-sized black holes on the basis of how many gravitational lenses we see. High concentrations of matter bend light passing near them from objects further away, but we do not see enough lensing events to suggest that such objects to make up the required 25% dark matter contribution.
However, at this point, there are still a few dark matter possibilities that are viable. Baryonic matter could still make up the dark matter if it were all tied up in brown dwarfs or in small, dense chunks of heavy elements. These possibilities are known as massive compact halo objects, or "MACHOs". But the most common view is that dark matter is not baryonic at all, but that it is made up of other, more exotic particles like axions orWIMPS (Weakly Interacting Massive Particles).
Recent Discoveries
자료: http://math.ucr.edu/home/baez/physics/Relativity/GR/dark_matter.html
Updated 1993 by SIC.
Original by Scott I. Chase.
What is Dark Matter?
The story of dark matter is best divided into two parts. First we have the reasons that we know that it exists. Second is the collection of possible explanations as to what it is.
Why the Universe Needs Dark Matter
We believe that that the Universe is critically balanced between being open and closed. We derive this fact from the observation of the large scale structure of the Universe. It requires a certain amount of matter to accomplish this result. Call it M.
We can estimate the total baryonic matter of the universe by studying Big Bang nucleosynthesis. This is done by connecting the observed He/H ratio of the Universe today to the amount of baryonic matter present during the early hot phase when most of the helium was produced. Once the temperature of the Universe dropped below the neutron-proton mass difference, neutrons began decaying into protons. If the early baryon density was low, then it was hard for a proton to find a neutron with which to make helium before too many of the neutrons decayed away to account for the amount of helium we see today. So by measuring the He/H ratio today, we can estimate the necessary baryon density shortly after the Big Bang, and, consequently, the total number of baryons today. It turns out that you need about 0.05 M total baryonic matter to account for the known ratio of light isotopes. So only 1/20 of the total mass of the Universe is baryonic matter.
Unfortunately, the best estimates of the total mass of everything that we can see with our telescopes is roughly 0.01 M. Where is the other 99% of the stuff of the Universe? Dark Matter!
So there are two conclusions. We only see 0.01 M out of 0.05 M baryonic matter in the Universe. The rest must be in baryonic dark matter halos surrounding galaxies. And there must be some non-baryonic dark matter to account for the remaining 95% of the matter required to give Ω, the mass of the Universe, in units of critical mass, equal to unity.
For those who distrust the conventional Big Bang models, and don't want to rely upon fancy cosmology to derive the presence of dark matter, there are other more direct means. It has been observed in clusters of galaxies that the motion of galaxies within a cluster suggests that they are bound by a total gravitational force due to about 5-10 times as much matter as can be accounted for from luminous matter in said galaxies. And within an individual galaxy, you can measure the rate of rotation of the stars about the galactic center of rotation. The resultant "rotation curve" is simply related to the distribution of matter in the galaxy. The outer stars in galaxies seem to rotate too fast for the amount of matter that we see in the galaxy. Again, we need about 5 times more matter than we can see via electromagnetic radiation. These results can be explained by assuming that there is a "dark matter halo" surrounding every galaxy.
What is Dark Matter?
This is the open question. There are many possibilities, and nobody really knows much about this yet. Here are a few of the many published suggestions, which are being currently hunted for by experimentalists all over the world. Remember, you need at least one baryonic candidate and one non-baryonic candidate to make everything work out, so there there may be more than one correct choice among the possibilities given here.
- Normal matter which has so far eluded our gaze, such as:
- dark galaxies
- brown dwarfs
- planetary material (rock, dust, etc.)
- Massive Standard Model neutrinos. If any of the neutrinos are massive, then this could be the missing mass. On the other hand, if they are too heavy, as the purported 17 keV neutrino would have been, massive neutrinos create almost as many problems as they solve in this regard.
- Exotica (See the Particle Zoo FAQ entry for some details.)
Massive exotica would provide the missing mass. For our purposes, these fall into two classes: those which have been proposed for other reasons but happen to solve the dark matter problem, and those which have been proposed specifically to provide the missing dark matter.
Examples of objects in the first class are axions, additional neutrinos, supersymmetric particles, and a host of others. Their properties are constrained by the theory which predicts them, but by virtue of their mass, they solve the dark matter problem if they exist in the correct abundance.
Particles in the second class are generally classed in loose groups. Their properties are not specified, but they are merely required to be massive and have other properties such that they would so far have eluded discovery in the many experiments which have looked for new particles. These include WIMPS (Weakly Interacting Massive Particles), CHAMPS, and a host of others.
References: Dark Matter in the Universe (Jerusalem Winter School for Theoretical Physics, 1986-7), J.N. Bahcall, T. Piran, & S. Weinberg editors.
Dark Matter (Proceedings of the XXIIIrd Recontre de Moriond), J. Audouze and J. Tran Thanh Van. editors.
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