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It may be early to say it is not a cyclic universe. Everything we see around us (in spite of increasing entropy), appears to be cyclic. See atoms, solar systems, galaxies, clusters etc. Even the rate of expansion of universe has gone through cycles of acceleration and slow down. The last switch from slowed down expansion to accelerated expansion is expected ...


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First off, let's start with the more common misunderstanding. The Big Bang was not an explosion of any kind. Popular science likes to depict it as an explosion because of the name "Big Bang" and also because it's more visually appealing than what the Big Bang actually was. The actual definition of the Big Bang is a little complicated, but suffice to say it ...


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Given a few plausible assumptions about the universe its spacetime geometry is described by a solution to the Einstein equations called the FLRW metric. If we know the densities of various types of matter/energy present, e.g. photons/matter/dark energy/anything else, then we can calculate how the expansion of the universe varies with time. Generally ...


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You must not really have looked hard enough. They are the same phenomenon The Big Freeze, which is also known as the Heat Death, is one of the possible scenarios predicted by scientists in which the Universe may end. It is a direct consequence of an ever expanding universe. The most telling evidences, such as those that indicate an increasing rate of ...


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Absolute location, absolute direction, and absolute time are in-explainable with current physics. Therefore absolute speed/velocity does not make sense either. The absolute point where all the expanded matter was concentrated at/before the big bang, may pertain to some location within universe. Even if the point itself expanded, the original reference still ...


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The answer is the same reason why a glass of water left out at room temperature will evaporate. Even though most of the particles will be below the boiling point, the equilibrium one expects is not entirely in the liquid phase. The occasional particularly energetic water molecule will vaporize, just as the occasional neutral hydrogen atom will be struck by a ...


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You are right that the universe formed atoms much earlier (at the temperature when photons can no longer ionize the atoms, i.e. at around $T = 150,000 K$ as you point out with your order of magnitude calculation). However, photons could still scatter off these atoms. Indeed this was quite likely considering the high density of matter in the universe. The ...


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I wonder of you are overthinking this. Wald says: If the universe had always expanded at its present rate that is, $\dot{a}$ is a constant and independent of time. In that case the value of $a$ at time $t$ after the Big Bang is simply: $$ a = \dot{a} t $$ So if you define $T$ by $T = a/\dot{a}$ then $T$ is necessarily the age of the universe.


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For the case $\Omega_M + \Omega_\Lambda = 1 = \Omega_\text{total}$, which is a good approximation for $t \gtrsim 10^8 \; \text{yr}$, an explicit formula for a(t) is $$a(t) = \left[ \frac{\Omega_{M,0}}{\Omega_{\Lambda,0}} \sinh^2 \left( \frac32 \sqrt{\Omega_{\Lambda,0}} \, H_0 \, t \right) \right]^{\frac13}$$ or, plugging in numbers from Pulsar's answer, $$...


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The idea of a "Zero-Energy Universe" is a theory held by a limited number of scientists. There are several stackexchange question that expand on the theory and may help you. Zero energy universe Total energy of the Universe


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We have to be careful what we mean when we say "moving away". Imagine a grid in which we are at the origin and there are light sources located at each of the grid intersections. If the grid stays as it is while the light sources accelerate away from us, their light will appear to be redshifted. If you set up the accelerations such that everything moves ...


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That is not quite that simple. There is theory and measurements, and it places some constraints. There is more. First, if the universe is infinite now it was infinite at the Big Bang. You can have an infinite universe and have it all in the spacetime at the Big Bang. It does not grow to be infinite, it either is or is not. (Ignoring multiple dimensional ...


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Infinity is a mathematical concept, as well as the concept of variables describing dimensions. Physics is about observations, either in the laboratory or of the cosmos, which are fitted with mathematical models. It started with the geocentric system, became the heliocentric system and then the realization that the galaxy is composed out of sun like stars, ...


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The raisin bread analogy can be used to help in understanding this too: Dough is much more expandable than the raisin material. Raisins will expand a bit due to the heat and the pull from the dough stuck on their surface, but it is the dough that is moving. The forces that are holding the raisin together are much stronger than the force expanding the ...


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You are correct, the recession velocity predicted by the hubble law is negligible at the local group, even if gravity among them could be absent. Their gravitational attraction though, is hard enough to keep them bound together.


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That is the case. The conformal transformation of the metric $g_{ab}~\rightarrow~\Omega^2g_{ab}$ transforms the line element as $$ ds^2~\rightarrow~ds^2~=~\Omega^2(du^2~-~dx^2~-~dy^2~-~dz^2). $$ Then for the case that $du~=~\Omega^{-1}dt$ we have the metric $$ ds^2~=~dt^2~-~\Omega^2(dx^2~+~dy^2~+~dz^2), $$ that is the de Sitter spacetime metric for $\Omega^2~...


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Bob Bee's answer already covers a lot of extra detail, so I just want to give the very concise answer to your specific question. One form of one of the Friedmann equations is: $$H(t) = \sqrt{\frac{8\pi G}{3}\rho(t) - \frac{kc^2}{(a(t))^2}}$$ In a universe with zero global spatial curvature ($k=0$), like ours is thought to be, then the expansion rate $H(t)$ ...


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There is no centre of mass for the universe because it is isotropic and homogeneous. That is every point in the universe is like every other point so there is no one point you can point to and say this is the centre. For more on this see Did the Big Bang happen at a point?. However we can certainly pick any point in the universe as the centre for our ...


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Universe is expanding because the space itself is expanding. The expansion is not due to acceleration of the bodies with respect to space. Therefore, the bodies are getting away from others with the expanding space, not due to their own acceleration. So, their speed with respect to space is not changing and so, the mass approaching infinity does not apply. ...


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The universe, by definition, is all there is. So there is nothing outside it, not even space, since if you extrapolate the present expansion of the universe back far enough in time, you end up with a tiny space, just after the big bang occurred. So, from a personal perspective, I ignore the ordinary definition of the word space, as in the "space between New ...


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The comment by Walter is on the right track: The "acceleration" does not refer to the fact that recession speed increases with distance, because this is just a consequence of space expanding everywhere. This is why we measure the expansion in km/s per megaparsec. Today, the expansion rate (the Hubble constant) is $H_0 \simeq 70\,\mathrm{km}\,\mathrm{s}^{-1}\,...


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The FLRW energy equation for the motion of test masses in the universe is $$ \left(\frac{\dot a}{a}\right)^2 = \frac{8\pi G\rho}{3}. $$ the scale factor for space is $a$ and its time derivative is $\dot a$. I derived this from Newtonian dynamics. The density of mass $\rho$ for the case of a quantum vacuum energy level is constant. I now replace this with ...


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I would argue that the expanding of space cannot and should not be understood adding space into space, nothingness into nothingness. We have no way of observing the space itself as a reason like the one you presented likes. Things seem to get away from us through and the observed mechanism is called redshift, which, in close distances(inside let's say the ...


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The Standard cosmology model, the $\Lambda$CDM model, rearranged from the Friedmann equations, looks like, $H = H_0 {\sqrt{L_m a^{-3} + L_r a^{-4} + L_{de}}}$ This assumes zero curvature space, pretty well measured now (note, not zero spacetime curvature, just the spatial slices). H is the Hubble parameter as function of time, from the Big Bang. $H_0$...


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This is right with respect to the FLRW metric being a conformal coodinate system. The metric $$ ds^2 = \Omega^2(u)(du^2 - d\Sigma^{(3)}) $$ is with the change of variables $du^2 = \Omega(u)^{-2}dt^2$ lets us identify the above metric as the conformal time dependent metric $$ ds^2 = dt^2 - \Omega^2(u)d\Sigma^{(3)} $$ with the expansion factor $a(t) = \Omega^...


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This is a very good question. But before I attempt to give you some of the details, the increase in the entropy is not what causes the universe to expand but is rather a consequence of the expanding universe. In fact, to understand why the entropy of the universe was so low before the inflationary epoch is an open question. Based on this question, I am ...



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