New answers tagged

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EDIT: My first answer seemed to imply that radiation is at rest in the Cosmic Rest Frame. Radiation is not in rest in any frame. See below. The sentence shouldn't be read as "[velocity of energy] forms", but "velocity of [energy forms]"$^\dagger$. The sentence refers to "energy forms", i.e. the different forms in which energy can manifest itself. These ...


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An infinite "universe full of water" is actually very close to how the real universe is typically modeled, except that instead of water it's the right mix of ordinary matter, dark matter, radiation, curvature and cosmological constant. On large scales it's reasonable to assume that the density of each component is everywhere equal at a given time. And ...


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No, we will always be able to see farther than the Hubble sphere (in theory). This spacetime diagram — taken from Pulsar's rendering of Davis & Lineweaver (2003)'s Figure 1, in this excellent answer — can help visualize it: Coordinates In this figure, time increases upward, we're the vertical line in the middle, Big Bang is the bottom line, and our ...


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It's impossible to draw an accurate picture of a 2D hyperbolic surface, because such a surface cannot be embedded into a 3D euclidean space; this is known as Hilbert's Theorem. The saddle surface in the figure is just an approximation, and serves as an illustration that every point on a hyperbolic surface is a saddle point.


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The statement that at the beginning of the universe energy/mass was concentrated in a single region under conditions of extreme temperature and density is usually extrapolated from experimental data on the energy/mass content of the universe and its expansion, which is then analyzed through the classical (i.e. non-quantum) theories of general relativity and ...


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In a static bounded universe, what happens to particles that hit the edge? I venture to say nobody knows for sure. But it's an interesting question, because we have no evidence whatsoever that our universe is infinite or some kind of hypersphere. See this answer to a related subject. The story goes that in the old days, people could not conceive of a ...


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Let's be careful here. The contested statement seems to be this: Someone on-line, who actually seems fairly intelligent, but troll-ish, claims that there is good reasons for it, not from any philosopher, but physics. And this is indeed true, while not proven there are good reasons for it. Here "good" means that it's good enough to write articles about ...


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Just because all things are possible, does not mean that all possibilities actually exist. And then, to say they simultaneously exist goes even further. They merely exist as "possibilities", or possible outcomes but last I checked, possibilities do not fit into the same category of physical realty. I never understood why something is assumed to automatically ...


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At present, mainstream physics posits that the underlying level of all natural phenomena is quantum mechanical. There exists the standard model of particle physics, a mathematical model, where it is posited that the elementary particles and their interactions build up matter as we know it, classical theories emerging smoothly from the underlying quantum ...


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In short: No. Based on the Many-Worlds-Interpretation of QM, you can make statements of this kind, but as the name says, it is just an interpretation, whose ontological statements are highly controversial. There are other interpretations which provide the same physical results, which do not state the existence of multiple worlds, which at least shows that ...


1

What is a simple explanation to the fact that the universe is expanding? Show them a star map and point out the galaxies and the fact that it is a projection of three dimensional space to an image. Astronomers and astrophysicists have spent a lot of effort measuring the behavior of the galaxies, their motions relative to each other and to the solar ...


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Yes, it can. Curvature (whatever measure for it you use, Riemann tensor, Ricci tensor, Ricci scalar, you name it) is a function of spacetime, and hence of time.


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It would still be here, it would just be turned inside out.


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It's possible that I do not understand this theory correctly, but it seems to me that it was disproved by experiment. Indeed, it is possible to strip an atom of its electron cloud. If the nucleus was purely an effect of the electron field, nothing would be left once the electrons are removed. This is not the case. See e.g. this post. Note that you can never ...


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Well, the observations of the acceleration of the universe's expansion is perhaps the strongest argument against the Big Crunch argument. Nonetheless though, it is by no means a disproven theory- we cannot very accurately predict the future until we understand the 95% of the universe that isn't baryonic matter, the 95% that current theories have as the ...


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Until dark energy (and dark matter) are properly understood, it is impossible to be certain of the future fate of the universe. The concordance $\Lambda$CDM model, deduced from observations of distant supernovae, from the cosmic microwave background and from baryon acoustic oscillations suggest that the expansion of the universe is accelerating and that ...


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I am not an expert to give the most proper answer but according to Hubble's law , the most distant galaxies are moving away from us faster than the speed of light, due to space expansion. This fact makes me think that also the gravitational effect of these galaxies is lost forever, and this looks to be the future of our universe. Even if a 'big crunch' ...


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This is a very tough question, since you do not make any further assumptions about the force - most importantly its strength, sign, variation with distance and objects it acts upon. In general, yes, the universe would probably look absolutely different and we would not be here to ask this question. Somebody or something absolutely else might. Our ...


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If we think little more complex and consider every particle in the universe, each particle should be moving with some initial velocity caused by something happened before, due to some other particle You do not state the level of your physics knowledge. I will assume you are a highschool student. What you are describing is the way classical mechanics ...


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Let's analyse the evolution of the curvature in the $\Lambda\text{CDM}$ model. If $\rho_R$, $\rho_M$, and $\rho_\Lambda$ are the densities of radiation, matter and dark energy, and $$ \rho_c = \frac{3H^2}{8\pi G} $$ is the critical density, then we can define $$ \Omega_{R} = \frac{\rho_{R}}{\rho_{c}},\quad \Omega_{M} = \frac{\rho_{M}}{\rho_{c}},\quad ...


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One way to send energy off is to shine a flashlight upwards. The beam will go until it hits something. That may be years for the portion of the beam that happens to be aimed exactly toward a star or other object. It turns out that a lot of the beam will go between stars. Stars and such are really spread out. That part of the beam will keep going. Nothing ...


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A rotating reference frame is not an inertial reference frame: In the rotating frame, objects accelerate even though there are no forces acting on them. In your example, you can in fact determine easily whether you are rotating or the universe is rotating around you. In the first case there is artificial gravity on the ship, and in the second case there is ...


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Yes it's falsifiable and indeed has been falsified. The geometry of spacetime is described by an equation called the metric that we get by solving Einstein's equation. We can get some information about the metric that describes the universe by studying the motion of objects like galaxies, galaxy clusters etc. When we do this we find the observations are ...



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