# Tag Info

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(C)harge conjugation + (P)arity transformation + (T)ime reversal - CPT - is observed to be an exact symmetry of nature (as far as anyone can tell so far anyway). Applying the C, P and T operators to the entire Universe would exchange every particle for its antiparticle (and also mirror-image everything, and flip all momenta). Provided CPT is in fact an exact ...

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Almost, but not quite. The answer to the questions you ask Are all the constants (Boltzmann's, gravitational, c) the same? Would elements react differently? Would it look the same? are all "yes", but various processes controlled by the weak nuclear interaction violate charge symmetry (sometimes known a C-symmetry) and would be different. Most of those ...

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Yes, it is a test of General Relativity. The red shifts had been observed before . The history is written in the wiki page on Hubble's law. . Hubble inferred the recession velocity of the objects from their redshifts, many of which were earlier measured and related to velocity by Vesto Slipher in 1917. The predictions from General Relativity came ten ...

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There are two parts to your question. First, why can we see things "46 billion light years away" if the Universe is only about 13.8 billion years old? Because the Universe is expanding. How far does a photon travel in 13.8 billion years in an expanding Universe? It depends on the rate of expansion. I'll give a simplified example to illustrate the point: ...

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When they say the universe was the size of a baseball about a billion billion billion billionth of a second after the big bang, does that means the observable universe was the size of a baseball, or does it mean the entire universe? In the past, the answer would have been the entire universe. Big bang cosmology is all about the expanding universe starting ...

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You could append clusters and superclusters of galaxies to the first list , and perhaps begin it with vacuum and fields that aren't properly waves or energy.

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A black hole is little more than a spatially closed, gravitationally bound quantity of matter with an escape velocity greater than or equal to the speed of light, but for a black hole to have a validated existence, it must be perceived from OUTSIDE, not inside. In a universe-sized black hole, it would not seem like a black hole from within. The mass inside ...

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Because near matter spacetime isn't expanding, and if it isn't expanding it can't be stretching the matter. The expansion of spacetime is a prediction of general relativity for the special case of a matter distribution that is homogenous and isotropic. If we feed in this condition we find that the geometry of spacetime is described by an equation called the ...

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General relativity deals with equivalence classes of maps. If you take one map of space-time, put it through a continuously differential equation, and alter the metric accordingly, you get an equivalent map. You can't talk about one or the other being right. They're equivalent. Either they're both right or they're both wrong. To be fair, that's just a ...

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In my Physics Class we had to look into possible areas where Dark Matter could be "hiding". Where is all the dark matter? Actually, I think it's hiding in plain sight. If you've read up on relativity I think you can work it out. See Einstein's Leyden Address where he described a gravitational field as space which was "neither homogeneous nor isotropic", and ...

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There are two main misconceptions about dark matter. One is that dark matter is a clump of stuff traveling with the matter. The other is that matter does not interact with dark matter. Dark matter fills 'empty' space. 'Empty' space has mass. Spacetime has mass. Dark matter is displaced by matter. The Milky Way moves through and curves spacetime. The ...

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There are several reasons to believe that dark matter is a particle. The most widely accepted alternative explanations for the different phenomena that led us to conjecture dark matter in the first place, can collectively be labeled "we don't understand gravity well enough". But no matter what, the effects of dark matter are sort of "localized". The ...

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The answer is simple - yes. There is a nice picture illustrating the idea of the grand unification.

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The reason it's thought of as expansion of space rather than just things moving farther apart through space is that the math of general relativity describes it that way, and GR has been well-supported by experiments so far. GR is all about curvature of spacetime, and curvature of anything can be determined by how we measure distances. A lot of the math in ...

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The balloon analogy imagines the universe as a 2D surface expanding around a central point as it moves through a 3rd dimension of time. This may be the origin of confusion as in reality there is no 2D surface of expansion, like a wave front, but rather an expansion of 3D spacetime, wherein every point in space quite literally is its own central point from ...

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Your question may presuppose that the Universe had a beginning in its own past. If we apply the logic of our experience and perception that we live with in space/time, this may seem like a reasonable inference. But just as complex systems may need more than knowledge of the sum of their constituents to be understandable, broader knowledge of the Universe ...

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I'll answer your question with an analogy. Imagine a really small balloon, so small that it occupies a point. Now, imagine that the balloon is expanding uniformly outward from that point. Note that that central point is not part of the balloon. It's the same idea as to what happened with the BB. In this analogy, the universe is the surface of the balloon. ...

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it must have started from a single point This is a common misconception popularized buy the media. Imagine this grid: Imagine each square getting larger. If you think about it, you will see that each point on the grid is expanding. The grid is the universe. Each point on the universe is its own "singularity".

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The universe does not "calculate". The universe just exists as it is and evolves. We calculate things like how far a particle with a given speed goes in a given time because we don't actually have the particle or the time. The universe just has everything in a state, has the laws of nature in place, and everything works itself out. In the universe an actual ...

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This is a commonly considered idea, of which one variant is the "Hubble bubble". Anything that happens outside of the visible universe, is, after all, in principle unknowable to us.

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The latest data is from the Planck satellite, and you can find the results in this 15MB PDF. The matter and dark energy densities are; \begin{align} \Omega_M &= 0.315 \pm 0.017 \\ \Omega\Lambda &= 0.686 \pm 0.020 \end{align} So we get a total density of $\Omega = 1.001 \pm 0.026$. So within the 2.6% experimental error spacetime is flat.

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The premise of your question is flawed. It is not possible to know everything with perfect accuracy, even in a non-quantum fully deterministic system. (In a quantum system, even if you do know its state with perfect accuracy, you can't predict it accurately.) Ever hear of chaos? the butterfly effect? the three-body problem? No matter how well you know the ...

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We don't know is probably the right answer to this question. With current knowledge of quantum mechanics it is believed that randomness is inherent in quantum mechanics. Or we may simply do not yet know the mechanics behind. In conclusion if randomness is real than there is no way for a universe to produce exactly the same result when started multiple ...

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To my taste, the most coherent way to think about it is to realize that the whole concept of "randomness" is strongly related do lack of information. It only makes sense to talk about randomness when we have a small "agent", who is a part of a large "universe" -- thus he cannot have a complete knowledge about that universe. But he needs to make some ...

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This is a well known and kenned estimate. The Wikipedia article Observable Universe covers this well; look especially at the sections under headings "Mass of ordinary matter" and "Matter content — number of atoms" for how it is derived. In summary: Cosmological models, especially the Lambda CDM-Model refined from the FLRW Metric, imply a relationship ...

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Firstly we should note that the universe as a whole is not described by the Schwarzschild metric, so the Schwarzschild radius of the universe is a meaningless concept. However if you take the mass of the observable universe you could ask what the Schwarzschild radius of a black hole of this mass is. For a mass $M$ the Schwarzschild radius is:  r_s = ...

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Yes! The center of the universe is the one place where time is "correct". That is, not influenced by extraneous gravitational fields. So by 'correct' I mean where time is running faster (or no slower) than anywhere else. It is left as an exercise to the reader how this location might be found.

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