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 Mar 29 comment Possibility for radiation in dark matter that is not interactive with regular matter? In fact, simulations of galaxy formation leads to much denser cores at the center of galaxies than what is observationally seen. One of the proposals for resolving this conflict is to allow for interactions amongst dark matter particles - precisely what you are looking for. The original (to my knowledge) paper is arxiv.org/pdf/astro-ph/9909386v2.pdf Mar 29 awarded Supporter Mar 18 comment Can negative energy explain black holes? Things aren't as simple with negative masses. I would imagine that violating the positive energy conditions would mean that an event horizon will not necessarily form. If $M \rightarrow -M$, $r=2M$ won't be the event horizon. A brief survey of the literature arxiv.org/abs/gr-qc/9705007 though shows that such solutions are, in principle, possible. Mar 16 comment Can negative energy explain black holes? I don't understand the relationship with inflation. The exponential expansion happens there due to the equation of state parameter being close to -1, which is typically achieved by slow-roll. The energy density is still positive. It is just that the kinetic energy is much smaller and for a scalar field dominated universe, this leads to an exponential expansion. ($\rho + 3p$ is negative, not $\rho$). FRW and Schwarzschild are different solutions :) Feb 27 awarded Commentator Feb 27 awarded Editor Feb 27 comment Why do we need more power to do a job fast? Edited, thanks! Feb 27 revised Why do we need more power to do a job fast? added 1 characters in body Nov 3 accepted Is the Unruh radiation isotropic? Nov 3 comment Is the Unruh radiation isotropic? Yes, by isotropy, I meant rotational invariance. Say, at a given moment, the observes sees thermal radiation. As he is accelerating, his speed is increasing in one direction. I would have naively expected that the radiation he observes should somehow reflect this. But, I guess not. I don't really understand even your heuristic reasoning, but, of course that is because I don't know enough. Thanks anyway. Nov 2 asked Is the Unruh radiation isotropic? Oct 31 comment How do we know the universe is expanding, and not that its contents are shrinking? continued: For instance, the temperature fluctuations in the CMB are one part in $10^5$. So, we can deduce that the distribution of energy on very large scales is almost isotropic. This is the evidence that our observable universe is almost-FRW. Oct 31 comment How do we know the universe is expanding, and not that its contents are shrinking? If the FRW metric is to hold exactly, there would be no perturbations in the energy density - in particular, you and I wouldn't exist. So, the fact that the FRW metric doesn't hold on very small scales is beyond doubt. If you look at how clusters of galaxies are distributed and how the CMB is distributed, you notice that the perturbations in the energy density are very small. Oct 31 comment How do we know the universe is expanding, and not that its contents are shrinking? I think that Hubble's Law cannot be applied to such short length scales. The reason we use an almost-FRW metric is because, empirically, we observe that matter is distributed like that. But, this applies only to large scales. So, Hubble's law cannot be applied to the Earth-Sun region. It is a "coarse-grained" law, not a fundamental one, because there are perturbations in the energy density. Oct 30 answered How do we know the universe is expanding, and not that its contents are shrinking? Oct 28 comment If nothing in the universe can travel faster than light, how come light can't escape a black hole? This is a bit like asking, if a Bugatti is the fastest car in the world why can't it travel across the whole of Europe in one hour? Does this mean that the Bugatti isn't the fastest car in the world? Oct 27 answered Why do we need more power to do a job fast? Oct 27 comment Energy-time uncertainty and pair creation @annv, I understand how particles are created. My question is about $\Delta E$ itself. I have similar-looking relations in the first and the second paragraph. But, the interpretation for $\Delta t$ in the first is very different from that in the second - at least at first glance. I am basically asking how they can be reconciled. Thanks. Oct 27 comment Energy-time uncertainty and pair creation If I understand correctly, the similarity between momentum-position uncertainty and energy-time uncertainty isn't that straightforward. This is because time isn't an operator like momentum or position. It is an external parameter. So, a standard deviation for t doesn't make sense. Which is why one has to interpret what one means by $\Delta t$ in the uncertainty relation. Interpretations similar to the one I have referred to are the only ones I have seen that sound reasonable to me. But, I don't understand how that is related to what I mention in the second paragraph. Thanks. Oct 27 asked Energy-time uncertainty and pair creation