# Tag Info

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We are continually told that the Universe will eventually be a void and everything will have burned up, no stars, no nothing. In my school days we were told that energy can neither be created or destroyed. So, if the universe does become "nothing" what has happened to the energy?

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On your first question the answer should be here http://en.m.wikipedia.org/wiki/Metric_expansion_of_space, after the big bang only the distance between space is expanding but you should be able to understand on the link i have included, yes the speed of light remains constant but the expansion is causing it to have longer time than expected to arrive it is ...

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Space is indeed expanding everywhere, and not only between galaxies. The reason we don't grow with it, is that the attraction between the electrons and the protons is strong enough to keep them bounded. You can look at it as if they always re-adjust their position to counter the expansion of space. This also applies to our solar system, our galaxy, and even ...

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I think this question is primarily opinion based. Different people keep different systems, but I'll share my system. To start, version control is absolutely essential. I recommend git. Here are some nice resources: Git for Scientists - an overview aimed at scientists Atlassian Git tutorials - a nice overview and discussion of different strategies for ...

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There are (at least) two things going on. Perhaps the easiest place to start is with the temperature as estimated from the radiation in the universe - possibly what you are referring to when you say the temperature is approaching 0K? The radiation in the universe takes the form of thermal blackbody radiation. It is emitted by material in thermal equilibrium ...

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I believe that you are safe in assuming it is cooler because it is spread out more. Wikipedia, on the subject

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If you are using Python then I would recommend Sphinx and git. Check your code into git so you have a history of your work and use Sphinx to generate the documentation off your code. This is a common combination and should meet most of your documentation needs.

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The Big Bang was originally defined as the zero time limit of the FLRW metric, so it's a mathematical construct and not primarily something physical. We have chosen to apply it to the zero time limit of the universe because we thought the FLRW metric was a good description of the universe, but then inflation gatecrashed the party and spoiled the fun. So if ...

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In my opinion it all hinges on whether one includes quantization of gravity or not. The classical Big Bang just uses General Relativity and solutions of its equations. A singularity has a well defined meaning in the classical approach. As physicists are convinced that the underlying framework of nature is quantum mechanical it is expected that gravity ...

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The argument is sound given a few oft-omitted (but not too unreasonable) assumptions. Here is one way it can be formulated. Consider a volume $V$. Suppose it has a (possibly infinite) set of possible configurations; call this set of states $S$. Suppose we are interested in a particular configuration, $c \in S$, to within a certain tolerance. Let $C ... 1 The argument rests on the assumed validity of Ergodic theory (see http://en.wikipedia.org/wiki/Ergodic_theory). Quoting it "A central concern of ergodic theory is the behavior of a dynamical system when it is allowed to run for a long time. The first result in this direction is the Poincaré recurrence theorem, which claims that almost all points in any ... 2 This is a statement about a congruence of null geodesics. We are looking for a conjugate point, which is just a place where the null geodesics cross each other. The theorem is putting a bound on how far you can advance the affine parameter$\nu$along the geodesics before the conjugate point occurs (this is what is meant by affine parameter distance). ... 3 How a unit is chosen to be defined depends in large part on how precisely the unit can be reproduced based on that definition. Two different atomic clocks built using the best currently possible methods will produce almost exactly the same answer for how long a second is, to within about 1 part in$10^{14}$. The second is defined in terms of a property of ... 0 In General Relativity, energy momentum flows from one region of spacetime to another. But there isn't necessarily a natural "total energy of the universe." It might help to contrast General Relativity with other theories. In Newtonian mechanics, a particle might gain kinetic energy while a corresponding gravitational potential energy decreases, thus you ... 0 In Newtonian mechanics, a particle might gain kinetic energy while a corresponding gravitational potential energy decreases, thus you get that kind of conservation of energy. The total energy is the same before and after any event. However, the amount of energy depends on who's looking. In Special Relativity a transfer of energy has to happen at an event ... 2 Galaxies would appear stretched along the line of sight, not jumbled. Let's say a galaxy is ten million light years away and, as you proposed, is 100,000 light years across and we see it nearly edge on. The front of the galaxy will appear to us as it did ten million years ago and the back of the galaxy as it did 10,100,000 years ago. Thus, if the galaxy ... 1 I think the very short answer is: galaxies are very small, indeed tiny, compared to how far away they are, and secondly, compared to the size of the universe. The answer to the spirit of your question, is that simple! "Galaxies are tiny." You're used to hearing "galaxies are 100,000 light years across!" but that's a piffle compared to either the size of ... 0 I suspect that the nice rotational formations of galaxies we see come from galaxies that are oriented in parallel with our solar plane so that the light arrives almost synchronously. Considering we are talking of gravitational forces the "almost" could cover a large window, the distortions not being too great to lose track of the shape. A galaxy whose ... 3 Galaxy rotation happens at a very slow rate (compared to the speed of light). Let's suppose you are observing a galaxy edge-on that the delay from the farthest point is$\Delta t = d/c$, where$d$is the galaxy diameter. If we take the lag from one extreme point to the other as D:$D = vt = \frac{v}{c}t$(where$v$is the rotational speed). You can see ... 1 For a stationary massive object like Earth or the Sun, the gravity well does not change from the expansion of space. The gravity well at one time is the same as the gravity well at future times for the same mass. Any minute force imposed by the expansion of space does not constitute a change in the gravity well itself. The well is not stretched or ... 0 If we assume that general relativity is the correct theory for this case, and there are currently no indications, that I am aware of, that it isn't, then the expansion of the universe adds a small modifying term to gravity wells. I doubt that it is measurable at the scale of the solar system. The current best estimate for the Hubble constant is ... 0 Time is that which is measured by clocks. How clocks behave when they are being moved relative to each other is simply a collection of experimental facts. At no time do we need any light to do those experiments, and it totally doesn't matter to moved clocks if we are moving them during the day or the night. So what, exactly is your question? Is it why ... 0 Planes of simultaneity in special relativity don't really mean much of anything. The real physical structure of spacetime is in the light cones. The takeaway from "relativity of simultaneity" is not that there are "different time orderings for different observers", but rather that there is no meaningful time ordering for spacelike separated events. They ... 3 I think that "observable universe" is not defined precisely enough to make such statements about it. The spacetime events that we can see are the events on our past light cone. That light cone intersects the last-scattering surface (about 400,000 years after the big bang) in an approximate sphere. By convention the light cone is cut off there (because we ... 1 Well, i would say no. Why? Because an absolute center of mass would require a uniform covering (coordinate system) over the whole manifold, which, even if it exists, will probably not be on the manifold itself. An analogy would be the center of mass of a spherical surface/manifold. It would be exactly on the center of the sphere (i.e not on the sphere ... 11 What you're asking about is the existence of surfaces of simultaneity. In SR, surfaces of simultaneity can be defined by measurement procedures such as Einstein synchronization, and they turn out to depend on one's frame of reference. In GR it gets a lot tougher to do this. We don't even have global frames of reference. It turns out that what you need in ... 0 Let's assume that the anti-matter galaxy is well isolated from galaxies consisting of ordinary matter (you could assume that at the boundary the annihilation reactions would have proceeded very fast and matter and anti-matter don't come into contact at the time we see the anti-mater galaxy). Then the telltale sign would come from supernova neutrinos, ... -2 Because we can only speculate and guess (although they are very educated speculations and guesses) we don't know anything for certain. In the Universe, anything can happen. Anything. Which is why you never rule something out until you've actually ruled it out- if that makes sense. I'm going to go with Tobias' response that Anti-Matter Galaxies could possibly ... 10 It sounds like you're interested in when a spacetime admits a Cauchy surface. The answer is that every spacetime that is globally hyperbolic has this property. This was proved by Geroch in 1970 (article here, see Section 5). This includes most of the textbook relativistic spacetimes --- Schwarzschild, Kerr, FLRW, and many others. But there are some ... 2 The question is what do we need the matter content of the universe for. As I understand it, in the usual case we want to find the conserved quantity associated with a certain conserved current gained by the projection of the energy-momentum tensor into a Killing vector, as for example in the paper by Abott and Deser. The requirement of asymptotical ... 0 Special relativity is often introduced to students using light clocks because this is a reasonably accessible way to understand that phenomena like time dilation and length contraction must occur. However you should not be mislead into thinking that we use light clocks to define special relativity. The fundamental principle of special relativity (and in fact ... -1 The expansion of the universe is not a force. Forces don't pull things apart at a particular speed; they change the speed by a particular acceleration. The speed itself is just inertia. It's no different in cosmology: there is nothing actively pulling things apart at the speed given by Hubble's law; that speed is just leftover momentum from the big bang, as ... -2 Observer, if you assume as someone watching the experiment or activity, is plainly wrong. Anything that can be detected and measured and thus, in principle, from infinitely hard calculation can tell us about the past or previous states, can be said to be information, and thus, entropy increased while the process was being carried out. Take for example, a box ... 0 The expansion does lead to a kinetic energy term that can be (at least partially) extracted. For objects that are bound to each other, it would lead to a classic acceleration term, which, of course, is equivalent to a classic pseudo-force. In an expanding universe, any two objects that are bound by a potential, are therefor experiencing an additional (albeit ... 1 The notion of kinetic energy is ill-defined in the spacetimes where you have a time-dependent cosmological expansion. If you somehow attached two galaxies to each other with a spring, however, the expansion of the universe would do "work" against that spring, as there would be a force requied to keep the proper distance of the two galaxies fixed. At ... 3 If you take an isolated spherically symmetric object then the spacetime curvature around it is described by the Schwarzschild metric. The bending of the rubber sheet is meant to be an analogy for this curvature, but bear in mind it's just an analogy and is in many ways a poor representation of what actually happens. Anyhow, the Schwarzschild metric only ... 0 Entropy and 2nd law as disorder is a massive misunderstanding (propagated by prestigious physicists non-the less). In fact on this issue there are various physics schools of thought (and especially those of the thermodynamic flavor) which set this whole discussion on a new footing. It is shown that (for example in the work of Nobel-laurate I Prigogine) how ... 2 The current entropy in the Universe is all stored in photons. The first reference by Qmechanic gives you the precise value. Since the photons of the CMBR do not at present interact with anything, the entropy of the Universe is very close to being a constant. What evolution there is, is all due to non-reversible processes in baryonic matter, but it amounts to ... 1 If we assume our universe as an isolated system, then its entropy can only increase. It cannot decrease because of the second law of thermodynamics. It cannot stay unchanged because the universe is undergoing all kinds of irreversible processes. 1 Today, stars don't emit the CMB – they emit much more energetic (hotter, higher-frequency, shorter-wavelength) radiation in general. CMB was emitted by "everything" that could emit radiation at the relevant time, around 400,000 years after the Big Bang. At that time, the radiation was in thermal equilibrium with everything else, so its distribution to ... 0 Theoretically, if a galaxy is far enough away and the universe ends or we become extinct, then it would never reach "us". Unless there is a distance that, once traveled, light dissipates to nothing, which seems unlikely, given ample time it will all reach "us" eventually. 4 The short answer is yes, the presence of dark matter would act to counter the expansion of the universe. And in fact it does--but not enough to stop the expansion. Dark matter has gravity just like normal matter. In fact, that's pretty much the only reason we know dark mater exists at all: we can observe dark matter's gravitation effects in the rotation ... 5 The acceleration of the expansion is currently observed to be happening. This observation is one of the pieces of data we use to infer the amount of dark matter. It tells us that there can't be more than a certain amount of dark matter, because that would be incompatible with the observed acceleration. 1 It's certainly possible, though on current evidence it looks unlikely. The past bound isn't really a bound in the usual sense of the word, but instead it's a singularity. If we solve Einstein's equations for the universe with a few apparently plausible assumptions we find that the universe is described by a scale factor, normally written as$a(t)\$, and as ...

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As for the straight line, yes. All objects will continue moving along geodesics (a straight line in curved-space but sometimes a curved line in straight-space) if there are no external forces acting on them. Unless, by different velocity you mean the direction is not entirely radial to us. In that case, the expansion will cause the object's path to appear to ...

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I will assume you are talking about the center of mass. If there's no external forces, the center of mass would conserve it's momentum. So, it would stay in constant speed, whatever what that speed is, with respect to whatever inertial frame of reference. This happens because Newton's third law. In the summation of all forces, the internal forces will ...

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I will reply to Why isn't the CMB at the edges of the universe? Why is it flying around in the middle? The occurrence of space time and matter after the Big Bang happened to all points in our universe. The expansion of space happened at the same rate outwards for all points of the universe. All points of the universe 380.000 years ago had ...

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At a basic level: The universe, in the beginning was very hot. So hot in fact that there were no atoms, only electrons and protons and neutrons and photons flying around. The photons were scatting off of the electrons and protons, as they interacted strongly because the electrons and protons are charged. The universe was much like the plasma you find in ...

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Your question and this answer are really better suited to the Meta, and I suspect a moderator will be along some time soon to migrate them. But while your question is still here ... An intuitive understanding of GR is extraordinarily difficult to attain. I've been studying GR (as an interested amateur not a pro) for a decade and I still make naive errors ...

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There are two immediate problems with this idea. First, the acceleration due to the dark energy appears to be the same in all directions. In general relativity (and Newtonian gravity for that matter) the influence of distant matter can only cause a tidal acceleration that expands matter in some directions and compresses it in others. A uniform expansion has ...

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