研究称暗能量或隐匿于宇宙空洞(图) 研究称暗能量或隐匿于宇宙空洞 北京时间11月26日消息，新科学家报道，近日研究发现宇宙空洞，也就是几乎是空洞的巨型宇宙空间，能够帮助我们了解暗能量，一种加速宇宙膨胀的神秘物质。如果引力是其中的主要力量，那么宇宙膨胀将会逐渐变慢，因为被吸收的物质也会受到其它物质的牵引。但10多年前，对超新星的研究表明宇宙的膨胀其实是不断加速的。这表明时空的真空区内一定存在某种内在的能量排斥引力。 据称，暗能量组成了宇宙的70%，关于暗能量密度的几个观点也随着时间的变化而发生改变。测试这些观点的方法之一便是测试宇宙随着时间推移的膨胀率，这个过程会在宇宙空洞中留下印记。早期宇宙的量子起伏会导致原始物质密度的变化。在密集区域形成的星系和星系群之间会产生宇宙空洞，从而产生星系调查，诸如斯隆数字巡天（SDSS），中观测到的宇宙大型结构。空洞一般宽达1.5亿光年。 宇宙膨胀将近乎球体的空洞拉伸成鸡蛋形状的气泡。“空洞对暗能量高度敏感。”荷兰格罗宁根大学的里恩·凡·德·威格尔特（Rien van de Weygaert）这样说道。根据威格尔特研究小组进行的电脑模拟显示，将不同时期宇宙空洞的形状进行对比将揭露暗能量密度的变化，从而帮助科学家区分不同理论。 然而这种效应微乎其微，同时宇宙空洞的形状也受到因强烈引力相互作用产生的星系运动的影响。因此加拿大安大略滑铁卢大学的吉尔海姆·拉沃（GuilhemLavaux ）和同事提出了一项解决方案：将某一特定体积的所有空洞的所有形状堆叠起来，从而产生一个平均的拉伸力。 研究小组已经将这项技术应用于最新SDSS数据中的宇宙空洞。然而不够精确的宇宙距离测量使得宇宙空洞边界非常模糊。这项调查也没有触及宇宙深空，因此没有提供宇宙空洞进化的完整历史。 但平方公里射电阵（Square Kilometre Array，简称SKA)无线电望远镜或许能够改变这一现状。到2024年，它将能够监测到数十亿颗星系。
因为觉得这篇文章不错，自己通读了一遍，我虽然是做材料科学的研究的，但是还是很喜欢宇宙学的。所以就把这篇文章转出来，毕竟大家可能不经常去MIT的网页看东西。我是经常跑去看的，虽然我并不是MIT的学生。哈哈！相信看这篇博文的您应该都可以通读这篇文章，所以我就不在进行细枝末节上的翻译了。 这篇文章是科学美国人杂志的记者 Davide Castelvecchi (此处的超链接是连接到他在科学美国人上的个人专题报道)写的，他曾经在2004年的时候采访过Alan Guth. 全文如下： On the night of December 6, 1979–32 years ago today– Alan Guth had the “spectacular realization” that would soon turn cosmology on its head. He imagined a mind-bogglingly brief event, at the very beginning of the big bang, during which the entire universe expanded exponentially, going from microscopic to cosmic size. That night was the birth of the concept of cosmic inflation （此处连接是关于the growth of inflation,也是Davide的手笔）. Such an explosive growth, supposedly fueled by a mysterious repulsive force, could solve in one stroke several of the problems that had plagued the young theory of the big bang. It would explain why space is so close to being spatially flat (the “flatness problem”) and why the energy distribution in the early universe was so uniform even though it would not have had the time to level out uniformly (the “horizon problem”), as well as solve a riddle in particle physics: why there seems to be no magnetic monopoles, or in other words why no one has ever isolated “N” and “S” poles the way we can isolate “+” and “-” electrostatic charges; theory suggested that magnetic monopoles should be pretty common. In fact, as he himself narrates in his highly recommendable book, The Inflationary Universe , at the time Guth was a particle physicist (on a stint at the Stanford Linear Accelerator Center, and struggling to find a permanent job) and his idea came to him while he was trying to solve the monopole problem. Twenty-five years later, in the summer of 2004, I asked Guth–by then a full professor at MIT and a leading figure of cosmology– for his thoughts on his legacy and how it fit with the discovery of dark energy and the most recent ideas coming out of string theory. The interview was part of my reporting for a feature on inflation that appeared in the December 2004 issue of Symmetry magazine . (It was my first feature article, other than the ones I had written as a student, and it’s still one of my favorites.) To celebrate “inflation day,” I am reposting, in a sligthly edited form, the transcript of that interview. Twenty-five Years of Cosmic Inflation: A QA With Alan Guth Davide Castelvecchi: What is cosmology? Alan Guth: Cosmology is the study of the history and large-scale structure of the universe, and my own niche in cosmology is the very early universe—the first small fraction of a second of the history of the universe. DC: How is it possible that people can understand the universe itself, as opposed to studying things the universe contains? AG: We do have a number of pieces of information that we can put together to try use as a basis for constructing theories. Observations about the distributions of galaxies within the visible part of the universe, and the motions of galaxies. Also now very important are observations of the cosmic background radiation—radiation that we believe is the afterglow of the big bang’s explosion itself. And now we have very precise measurements, both of the spectrum of this radiation and also of the small ripples that exist in its intensity pattern. The radiation is almost perfectly uniform. In all different directions in the sky, the intensity we observe is the same to about one part in 100,000. But nonetheless, one does see minute differences from one direction to another. This pattern of ripples is tied directly to two things: theories about how the ripples were formed—which is where inflation comes in—and also to theories that calculate how the structures in the universe have formed from the ripples. Another important ingredient in terms of the observational basis for cosmology is the chemical abundances that we observe in the universe, Those are measured from the spectral characteristics of gas clouds and stars, and can be compared with theories about how the chemical elements were formed in the first few minutes of the history of the universe. And wonderfully, the calculations agree very, very well with the observed abundances of the lightest elements. DC: When you first had the idea of inflation, did you anticipate that it would turn out to be so influential? AG: I guess the answer is no. But by the time I realized that it was a plausible solution to the monopole problem and to the flatness problem, I became very excited about the fact that, if it was correct, it would be a very important change in cosmology. But at that point, it was still a big if in my mind. Then there was a gradual process of coming to actually believe that it was right. DC: What’s the situation 25 years later? AG: I would say that inflation is the conventional working model of cosmology. There’s still more data to be obtained, and it’s very hard to really confirm inflation in detail. For one thing, it’s not really a detailed theory, it’s a class of theories. Certainly the details of inflation we don’t know yet. I think that it’s very convincing that the basic mechanism of inflation is correct. But I don’t think people necessarily regard it as proven. DC: You recently wrote that “the case for inflation is compelling,” which sounds like a cautious statement. AG: It’s certainly not as well confirmed as the big bang theory itself. But I guess I’d find it hard to believe that there could be any alternatives for solving the basic problems inflation solves, like the horizon and flatness problems. DC: Do you have your favorite version of inflation among the many that have been proposed? AG: Not really, except that I could say that I think cosmology is moving toward describing things in terms of string theory. And there have been a number of attempts to describe inflation in that context. I think that is the future. DC: So you think that string theory will ultimately prove to be right? AG: Yes, I do. I think it may evolve a fair amount from the way people think of it now, but I do think string theory definitely has a lot going for it. DC: Is string theory physics or is it just fancy mathematics so far? AG: I consider it physics. It’s certainly speculative physics so far — unfortunately, it’s working in a regime where there’s no direct experimental test. But there are nonetheless consistency tests. If the goal of string theory is to build a quantum theory that’s consistent with general relativity, that’s a very strong constraint, and so far string theory is the only theory that seems to have convinced a lot of people that it satisfies that criterion. Just from a sociological point of view, theoretical physicists have been looking for a consistent quantum theory of gravity for at least 50 years now, and so far there’s really only one theory that has reached the mainstream — string theory. DC: Has string theory really reached the physics mainstream? AG: Yes. I would say that nowadays, a theoretical particle physicist cannot ignore string theory. DC: Speaking of sociology, in your book you describe your first attempts as a young particle theorist to describe your idea of inflation to cosmologists, and how communication would break down because people used different lexicons. Is the situation any different now? AG: I think the situation has improved tremendously between particle physics and cosmology. Now I think that almost everybody in cosmology is reasonably fluent in the vocabulary of both fields, and I think everybody recognizes that there is a strong interface between these two fields. At the same time, now there are also important implications going the other way, with the discovery of dark energy. DC: Is dark energy more relevant to particle physics than dark matter? AG: I would say yes. I am not sure if everybody will agree — it depends on what your perspective is. I think dark matter is more relevant to the next age of particle physics experiments — hopefully supersymmetry and perhaps other interesting things that we may discover. On the other hand, there’s at least a good chance that dark energy is energy of the vacuum, so it seems to be telling us something about the fundamental structure of physical law, which is a big surprise. The vacuum energy has been a haunting question for particle theorists since the advent of quantum field theory in the 1930’s. As soon as we had quantum field theory we knew that the vacuum was not a simple state: It was a very complicated state with all kinds of quantum fluctuations going on. And there was no reason at all why the energy of the vacuum should turn out to be zero or small. In fact, nobody knows how to calculate the energy of the vacuum, but if particle physicists were to try to estimate it, the natural answer would be something like 120 orders of magnitude larger than the experimental bound. So it was always a big mystery, but until the advent of dark energy, the belief was that the real number was zero, because of some kind of symmetry that we didn’t understand yet — an exact cancellation between the positive and negative contributions. If dark energy is the energy of the vacuum, now you need that symmetry to make it almost zero, and then some small breaking of that symmetry to make it a small number that’s not zero. And it all gets very complicated and baroque. Nobody has the faintest idea of how it might actually work. There is also the possibility that the vacuum energy is not determined at all by the fundamental laws of physics, but instead it’s determined anthropically, using the idea of a multiverse. It’s quite possible in the context of string theory that there are many vacuum-like states, and all of them are stable enough that they could provide the underpinnings of a universe. And the one that we happen to find ourselves in is determined by random choice. One would imagine that the universe would inflate eternally through all the different possible vacua of string theory, with infinite amounts of space of every type of vacuum produced — eventually. DC: Is this the so-called string theory landscape idea ? AG: Yes, that’s the catchword. If this is right, it would mean that in most regions of space the cosmological constant is enormous, and there are some rare regions of space where the cosmological constant happens to be very small. But life can only form if the cosmological constant is very small. So it’s not a surprise that we find ourselves living in one of those regions. An idea like this five years ago would have been completely anathema to particle physicists. It is still anathema to many, but people pay much more attention to this kind of idea now. DC: Does this connect to the idea of eternal inflation, with multiple universes bubbling off from a primodial vacuum? AG: Yes, there are two ideas coming together here. One is the idea from string theory, that there’s a huge number of possible vacuum states. And the other is the idea of eternal inflation, that once inflation starts, it never ends, and it explores all possible vacua. DC: Recently Stanford University cosmologist Andrei Linde, who also made seminal contributions to inflation theory, teamed up with string theorists to try to reconcile the two fields. AG: Yes. I regard that as probably the most interesting approach. I’m a big fan of that work, though I’m not one of the authors. I think it’s the starting point towards what will become a solid embedding of inflation within the context of string theory. Before them, nobody had any good idea for describing within string theory a state that would have a positive cosmological constant. DC: Does the existence of dark energy suggest a possible connection between the “false vacuum” state that produces inflation and the “true vacuum” state of the cosmological constant? AG: In principle, yes, although the vacuum states in string theory are really quite complicated states, with a number of degrees of freedom that describe them. Certainly, the state which drove inflation in the early part of our universe had a large, positive cosmological constant. In the end, they would all be described in the same language of string theory, and they would have many similarities. But there also are many significant differences. They are very different energy scales. So I think it’s somewhat a question in the mind of the beholder to decide whether or not there is a close relationship or a distant relationship. DC: Could there be two different kinds of “repulsive gravity” then, one which acted during inflation, the other one which is acting now? AG: What I believe, and what is the conventional belief, is that the repulsive gravity is really a feature of general relativity itself — and in fact Einstein made use of it himself in 1917 when he introduced the cosmological constant and tried to use it to describe how the universe could be static, with ordinary gravity pulling everything together and repulsive gravity — the cosmological constant — pushing everything apart. So from the very beginning general relativity incorporated the possibility of repulsive gravity. What creates repulsive gravity is negative pressures. That’s the feature of the cosmological constant and also of states of scalar fields dominated by their potential energy, which is the way conventional inflation works. Certainly the most plausible explanation for acceleration today, and for inflation early in the universe, was that the universe contains materials that have negative pressures. So at that level of description it’s the same mechanism — because it’s the only mechanism we know. But what the material is that creates the negative pressure is a more detailed question. Whether or not we believe that the KKLMT papers are on the right track, I think we don’t really know how closely related the actual state that drove inflation in the early universe was to the state the universe is in now, with this slow inflation that we attribute to dark energy. DC: Could there ever be a particle physics experiment to probe dark energy? AG: I guess I do not see the dark energy influencing or being influenced by particle physics experiments in the foreseeable future. It certainly is highly relevant for astrophysical observations. One important thing we’d love to know about dark energy is whether or not the energy density is constant over time, as it would be if it were a cosmological constant. Or, it could vary with time — in which case, our best explanation would be that it’s energy of a slowly evolving scalar field that fills all of space. That’s usually called the quintessence. There is some hope of answering that question by more detailed astronomical observations. And the best handle of that is probably still the distant supernovae, with experiments such as SNAP . DC: So is dark energy relevant to particle physics not so much on the experimental side, but because it points to an open problem in its theoretical foundations, i.e., the prediction that the vacuum of quantum field theory should create a much stronger repulsive force? AG: Yes, in terms of trying to understand the foundations of theoretical particle physics, I think it’s very important. In particular, it seems to be suggesting that there may be no physical principles that determine what the vacuum of string theory is. Maybe it is just all possible vacua happening in all different places. Now, I really hope that that turns out not to be the case, because I like to think that physics is more predictive than that. But that is certainly the direction that the dark energy is pointing towards — and it may turn out to be the right direction. DC: In either case, will a better understanding of dark energy shed light on inflationary cosmology? AG: Yes, I think so. If it turns out that the only explanation for the dark energy is this landscape idea, that says that if we want to understand how inflation really works, we have to understand it in the context of the landscape of string theory. DC: Inflation predicts that the universe is spatially flat, a fact which is in accordance with our best cosmological observations, in particular of the cosmic microwave background. Does inflation rule out the possibility that the universe might be spatially closed — what mathematicians call topologically compact? Before inflation and dark energy were talked about, the idea was that a universe that’s spatially flat would expand forever, whereas one that curves onto itself would recollapse. AG: Not completely. The statement that the universe is flat is only an approximation. Inflation drives the universe towards flatness — in fact, if enough inflation happens, it drives it incredibly close to being flat. But you could still imagine a universe that started out closed, and at the end it would be very large, but still closed. It would look flat, because the radius of curvature would be huge. On the other hand, it does all become much more complicated, because remember that we’re talking about spacetime, and not just space. And inflation tends to make the spacetime structure of the universe very complicated, with inflation continuing in some regions and stopping in others. Imagining the kind of complicated things that can evolve, I think the right conclusion is that the words open and closed don’t really apply anymore. On a very large scale, the universe is really neither of those. DC: Correct me if I’m wrong: The onset of inflation being a very local phenomenon, the universe to which our physical laws apply isn’t likely to have interesting topology, because it arose from a local fluctuation. AG: That’s right. On scales much larger than we can observe there might be an interesting topology. But inflation would suggest that in the scales that we can observe, the topology would be locally R^3 . But this has not stopped cosmologists from exploring other possibilities. One of the anomalies that people are concerned about currently is the observation by WMAP of the very low values of L — the low multiples. Those fluctuations are significantly smaller than what was expected from inflationary models. It could just be a fluke, but people have suggested other possibilities, such as a universe that is periodic in space, with periodicity of the order of the current horizon distance. But so far people have not found anything along those lines that’s consistent with the data that’s observed. DC: A mathematician called Jeffrey Weeks, together with a group of physicists, have published a controversial paper in Nature last fall. They searched the WMAP data and claimed it revealed a “house of mirrors” pattern, and thus that the universe was spatially finite and with the topology of a Poincaré dodecahedral space. If that evidence were to be confirmed, would it pose a problem for inflation? AG: Yes, I think it would be very hard to reconcile with inflation. DC: Virtually all the cosmologists and astronomers I have talked to seem to think that the next big thing in inflation studies will be to look for traces of primordial gravitational waves in the polarization of the cosmic microwave background. In particular, a pattern called the B-mode, if found, would carry information about the first instants of the universe, and thus about the mechanism of inflation. AG: Yes, that is very exciting. The B-mode, if present, would be the sign that we have found the effect of gravity waves, and not just of density perturbations. Gravity waves would give us a handle on the energy scale at which inflation occurred. One of the big uncertainties in the wide class of inflation theories is that inflation may have at happened at any of a tremendously broad range of possible energies. The kind of physics that you want to think about, to understand how it happened, depend very much on that. So it would be very important to get some observational information. DC: Is this going to be an exciting time for you, to see how things evolve? AG: Certainly, yes. It’s been incredibly exciting, ever since COBE . In the early days of inflation, when I and a number of other people tried to calculate the density perturbations that would arise from inflationary models, I really never thought that anybody would ever actually measure these things. I thought we were just calculating for the fun of it. So I was kind of shocked when the COBE people made the first measurements of the non-uniformities of the CMB. And now they’re measuring them with such high precision — it really is just fantastic. DC: And that could happen again — experiments that were considered beyond the realm of possibility will become reality? AG: Yes, that seems to happen almost every year now. 这应该是2004年的Alan
首先，对一个可能是无限的体系来说，则无论怎样倒推和倒推何时，它依旧是无限的，而不能说它过去一定是趋向越来越小的。 其次，即便对一个有限体系持续膨胀的模式作倒推，也得看它的膨胀是线性膨胀还是非线性膨胀。如果是线性膨胀，那么它的过去是趋向越来越小的，在有限的时间内可直至小到了极点，即 奇点 ，这是宇宙大爆炸理论创建者和拥护者采用的简单思维方式。但是，若宇宙膨胀是非线性膨胀，如 r = r o e Ho t 膨胀模式，则在有限的时间内永远倒推不出所谓 奇点 。所以，宇宙膨胀并不一定意味着 奇点 和大爆炸起源。 所谓的宇宙大爆炸，仅是一种假想引发膨胀的第一推动的代名词而已，这在引力的作用下，应呈减速膨胀。而新发现的加速膨胀，对于宇宙大爆炸一说是根本上的否定。为了保住大爆炸说法，唯一的办法是假设存在大量的斥力性的暗能量，这样，再加上原来假设的存在大量的引力性的暗物质，等等，大爆炸论更加乱了套，已无理性和逻辑可言。对于人为拼凑、问题百出的大爆炸之说，实际观测已经做出根本性的回答：不！ 类似地，宇宙过去因密度较高使得温度较高，逐渐膨胀至今温度下降 2.7K 背景辐射 , 以及氦元素丰度等也都不能作为大爆炸的直接的逻辑上必然的证据，仅仅是对应性猜想性测算而已，甚至还有锂元素问题、热平衡问题等反正据，以及为何以临界速度且各向同性膨胀等的解释问题。其实所有这些倒都可以作为 r = r o e Ho t 非奇点非大爆炸膨胀模式的证据，且不引起各种无法解决的疑难问题。 为什么宇宙学的研究经常处于混乱无序状态？其根本原因在于作为主导宇宙运行的万有引力其机制尚不明了！ 对 r = r o e Ho t 膨胀模式取时间导数，恰是哈勃定律 V = H o r ，其中隐含着自然的加速膨胀。 这些均可由物质粒子的本底引力子辐射引发的质量时变关系 dM = － Ho M dt 所导出，并能说明万有引力的机制和物质系统的自然演化，而无需引入大爆炸第一推动和暗物质、暗能量等虚妄概念。
标签: 宇宙膨胀
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