暗能量
Dark energy

context: zh_tw: 在物理宇宙學中，暗能量是一種充溢空間的、增加宇宙膨脹速度的難以察覺的能量形式。暗能量假說是當今對宇宙加速膨脹的觀測結果的解釋中最為流行的一種。在宇宙標準模型中，暗能量佔據宇宙 68.3% 的質能。暗能量現有兩種模型：宇宙學常數（即一種均勻充滿空間的恆常能量密度）和純量場（即一個能量密度隨時空變化的動力學場，如第五元素和模空間 (物理學)）。對宇宙有恆定影響的純量場常被包含在宇宙常數中。宇宙常數在物理上等價於真空能量。在空間上變化的純量場很難從宇宙常數中分離出來，因為變化太緩慢了。
en: In physical cosmology and astronomy, dark energy is an unknown form of energy which is hypothesized to permeate all of space, tending to accelerate the expansion of the universe.  Dark energy is the most accepted hypothesis to explain the observations since the 1990s indicating that the universe is expanding at an accelerating rate.Assuming that the standard model of cosmology is correct, the best current measurements indicate that dark energy contributes 68% of the total energy in the present-day observable universe. The mass–energy of dark matter and ordinary (baryonic) matter contribute 27% and 5%, respectively, and other components such as neutrinos and photons contribute a very small amount. The density of dark energy is very low (~ 7 × 10−30 g/cm3) much less than the density of ordinary matter or dark matter within galaxies. However, it dominates the mass–energy of the universe because it is uniform across space.Two proposed forms for dark energy are the cosmological constant, representing a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant can be formulated to be equivalent to the zero-point radiation of space i.e. the vacuum energy. Scalar fields that change in space can be difficult to distinguish from a cosmological constant because the change may be extremely slow.
qas: question: zh_tw: 什麼場提供貢獻？
en: What kind of fields contribute?
answers: zh_tw: text: 純量
en: text: scalar

context: zh_tw: 宇宙大尺度質量密度的傅立葉譜支援暗能量存在的假設。目前通常假設，暗能量在宇宙中各向同性，密度非常小，且不與通常物質發生任何除引力之外的已知的相互作用（即電磁，強，弱相互作用）。暗能量的密度又非常之小，大概 10−29 g/cm3，因此地球上的實驗室應當很難直接發現它。但是因為暗能量應該充滿了所有的宇宙空間，因此它佔宇宙質能總量的 68%，這顯著地影響了宇宙整體的演化。目前的兩類暗物質理論 —— 宇宙常數理論和基本標量場理論，都包含了暗能量的兩種重要性質 —— 均勻和負壓。
en: The nature of dark energy is more hypothetical than that of dark matter, and many things about it remain in the realm of speculation. Dark energy is thought to be very homogeneous and not very dense, and is not known to interact through any of the fundamental forces other than gravity. Since it is quite rarefied and un-massive — roughly 10−27 kg/m3 — it is unlikely to be detectable in laboratory experiments. The reason dark energy can have such a profound effect on the universe, making up 68% of universal density in spite of being so dilute, is that it uniformly fills otherwise empty space.
qas: question: zh_tw: 實驗室試驗中難以發現多少公斤？
en: How many kilograms is undetectable in laboratory experiments?
answers: zh_tw: text: 10−29 g/cm3
en: text: 10−27 kg/m3

question: zh_tw: 暗能量在宇宙中佔據的比例是多少？
en: What percentage of dark energy is in the universe?
answers: zh_tw: text: 68%
en: text: 68%

context: zh_tw: 威爾金森微波各向異性探測器（WMAP）衛星耗時七年，給出的資料是宇宙物質的 72.8% 是暗能量，22.7% 是暗物質，4.5% 是通常物質。2013 年，普朗克衛星給出的資料是，68.3% 的暗能量、26.8% 的暗物質、及 4.9% 的通常物質。
en: The existence of dark energy, in whatever form, is needed to reconcile the measured geometry of space with the total amount of matter in the universe. Measurements of cosmic microwave background (CMB) anisotropies indicate that the universe is close to flat. For the shape of the universe to be flat, the mass-energy density of the universe must be equal to the critical density. The total amount of matter in the universe (including baryons and dark matter), as measured from the CMB spectrum, accounts for only about 30% of the critical density. This implies the existence of an additional form of energy to account for the remaining 70%. The Wilkinson Microwave Anisotropy Probe (WMAP) spacecraft seven-year analysis estimated a universe made up of 72.8% dark energy, 22.7% dark matter, and 4.5% ordinary matter.
qas: question: zh_tw: 根據威爾金森微波各向異性探測器（WMAP）的研究，宇宙物質的多少是暗物質？
en: According to the WMAP study, how much of the universe is dark matter?
answers: zh_tw: text: 22.7%
en: text: 22.7%

context: zh_tw: 某些標量場理論能回答，“為何宇宙加速膨脹恰好能被我們觀測到？” 這個問題。如果宇宙加速膨脹得稍微早一點，那麼在銀河系形成之前，物質就已經由於宇宙的加速膨脹而互相分離，不能再凝聚成任何星系系統，也就不能產生人類了。這些標量場理論具有一種稱為 “tracker” 的性質，“tracker” 的意思類似追蹤，追蹤的物件是宇宙中的輻射。在宇宙早期輻射為主時期，這種標量場並不表現任何效應，也就是跟著 “追蹤著輻射走”，當輻射逐漸被宇宙膨脹稀釋，密度降到物質密度以下，就觸發了這種標量場開始產生效應，推動宇宙逐漸加速膨脹。
en: The coincidence problem asks why the acceleration of the Universe began when it did.  If acceleration began earlier in the universe, structures such as galaxies would never have had time to form, and life, at least as we know it, would never have had a chance to exist. Proponents of the anthropic principle view this as support for their arguments. However, many models of quintessence have a so-called "tracker" behavior, which solves this problem. In these models, the quintessence field has a density which closely tracks (but is less than) the radiation density until matter-radiation equality, which triggers quintessence to start behaving as dark energy, eventually dominating the universe. This naturally sets the low energy scale of the dark energy.In 2004, when scientists fit the evolution of dark energy with the cosmological data, they found that the equation of state had possibly crossed the cosmological constant boundary (w = −1) from above to below. A No-Go theorem has been proved that gives this scenario at least two degrees of freedom as required for dark energy models. This scenario is so-called Quintom scenario.
qas: question: zh_tw: 巧合問題有什麼可顧慮的？
en: What does the coincidence problem concern itself with?
answers: zh_tw: text: 為何宇宙加速膨脹恰好能被我們觀測到？
en: text: why the acceleration of the Universe began when it did