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# Tag Info

4

Short answer: Yes, mostly, with some exceptions. Long answer: Nuclei can only be changed by nuclear processes. So the everyday chemical transformations that go on in our bodies and in the world around us do not affect the nuclei of the atoms involved. However, some nuclei are naturally unstable and will spontaneously decay or transform themselves into ...

3

What is referred to as the "Hubble constant", $H_0$, is in fact the value of the Hubble parameter now. The Hubble parameter, defined as $\dot{a}/a$ where $a(t)$ is the scale factor, varies with cosmic epoch. It was larger in the past and it seems to be decreasing now.

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I think it is a common misconception. Hubble constant is only a constant a fixed time (of course). In fact the whole point of Friedmann equation is to find the evolution of Hubble "constant".

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$h$ is a dimensionless version of the Hubble parameter $H_0$: $$H_0=h\times 100\text{ km s}^{-1}\text{ Mpc}^{-1}$$ (See Wikipedia.) For example, $h=0.7$ means that the Hubble parameter is 70 kilometers per second per megaparsec.

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This is a question that entails the ability to manage QCD at low energies and this is an active field of research yet as we are not able to do it, unless for lattice computations. It can be considered as part of the more general problem of the determination of the phase diagram of QCD (see my answer here). The idea of chiral symmetry breaking in strong ...

2

The second law essentially just says the most likely thing that can happen will happen. If this law is violated for the universe then there would need to be some sort of external influence that makes this not the case. In other words, treating the universe as an entire system, it couldn't be a closed system so that something external could lower the entropy ...

1

The Big Bang was a singularity. By analogy, the function $f(x)=1/\sqrt{x}$ has a domain $(0,\infty)$, which doesn't include $x=0$ because the function misbehaves there. The Big Bang singularity is technically not considered an "event" because it's not a point or set of points that we include on the spacetime manifold. So time since the big bang means the ...

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Yes, that's how it's done. You can see that it furnishes an estimate rather than a precise answer; that estimate can be improved by other techniques that astrophysicists have in their toolbox.

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Inflation ? It doesn't have to be massive structures. As long you have a time dependent energy momentum tensor $T_{\mu \nu}$ that has non zero quadrupole moment, it will generate gravitational waves.

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