Guillermo Franco Abellán
  • Member for 5 years, 2 months
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Expansion of space or contracting of matter?
4 votes

This alternative that you mention is very imaginative, but It poses several problems. For example, if all matter is contracting, galaxies and clusters or any large scale structure would eventually ...

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Divergence of gravitational field
3 votes

I think the most intuitive way of understanding it is using $\textbf{Gauss Law}$ for the physical part and $\textbf{Gauss Theorem}$ for the mathematical part. If you integrate the divergence of the ...

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Do post-big-bang quantum fluctuations determine star and planet formation?
2 votes

I would say that the only quantum fluctuations that are relevant for the formation of structures (such as stars or galaxies) are those of the very early universe. In particular, they play a key role ...

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How is Hubble constant measured by the method of observing BAO?
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1 votes

BAO experiments essentially measure two quantities, one parallel to the line-of-sight \begin{equation} \beta_{\parallel} = H(z_{\rm eff}) r_s (z_{\ast}) \end{equation} and the other perpendicular to ...

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Explain how scaling of the inverse square law breaks down at a stars surface
1 votes

For the answer to the question a), just use a Taylor expansion in the parameter $x= R/d \ll 1$, so that \begin{equation} (1-x^2)^{3/2} \simeq 1- \frac{3}{2} x^2 \end{equation} and then you obtain a ...

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Newtonian Gravitational Waves
1 votes

I would say no, since the solution that you found is stationary, it cannot be propagating. For example, in one dimension, a propagating wave needs to have the functional form $\Psi = \Psi (x-ct)$, ...

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Scale factor for flat universe filled with radiation and cosmological constant
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1 votes

I see some mistakes: First of all, according to your definition of $H_{\Lambda}$, in the right-hand side of your equation you should have $H_{\Lambda}^{2}$ and not $H_{\Lambda}$. When you solve by ...

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Diagrammatic Representation of non-Gaussian perturbation expansions
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1 votes

Note that the reason why you have bubbles is that you have many repeated (i.e. dummy) indices. In the first diagram for example, i and j label the two ($k=2$) lines joined by $-V_{ij}$, but they also ...

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What's the difference between "dark energy" and "vacuum energy"?
1 votes

I would say it is partially correct. Dark energy is, as you said, anything that accelerates the expansion of the universe. One can show that any component with an Equation of state parameter ...

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If global phase doesn't matter why is relative phase important?
1 votes

Remember that for a 1/2 spin particle the only states we have are spin up $|\uparrow >$ and spin down $|\downarrow >$, or a general linear combination of both: $$ \chi = \cos{(\beta/2)} e^{-i\...

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How do we know that the universe is really fine-tuned?
0 votes

I'm not going to enter in why fundamental constants such as the electric charge or the mass of the proton are fine-tuned to favour life in the universe, since it requires a long explanation and I'm ...

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How do you work out the coefficients of the metric tensor?
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0 votes

What we tipically call the metric is $g_{\mu \nu}$, whereas the inverse metric is called $g^{\mu \nu}$ and it is defined through $ g^{\mu \alpha} g_{\alpha \nu} = \delta^{\mu}_{\nu}$. The metric ...

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Free and bound current- and charge density in Maxwell equations
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0 votes

The electric field $E$ is the field we apply, what we express with the first Maxwell equation is that its sources must come from the total density charge $\rho$. In a material, there will be some ...

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Dynamics High School Question
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0 votes

Remember frictional force goes always in the opposite direction of the movement. This is expressed by changing the sign respect the force of movement, in this case the force that pull the blocks is ...

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