Timeline for Can the Sun / Earth have a dark matter core?
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28 events
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| Apr 27, 2018 at 6:41 | comment | added | ProfRob | @Allure You can't just use the density of the core -its size is required too. $p \propto \rho r$. | |
| Apr 27, 2018 at 6:38 | comment | added | Allure | I just used the density at the cores, but now that I think about it, a DM particle could presumably interact in the exterior and then fall to the core, so my calculation's wrong too. Still, since the results are pretty close, I think a full calculation is not necessary. Thanks. | |
| Apr 27, 2018 at 6:27 | comment | added | ProfRob | @Allure Correct, you have to do a proper numerical integration of a density profile. The density of the earth is fairly uniform, but the Sun and neutron stars aren't. Do you mean it's 11 orders of magnitude more likely at core densities, or have you calculated the interaction p integrated through the star over a density profile? | |
| Apr 26, 2018 at 23:15 | comment | added | Allure | Repeating your calculations for $p$, I think you have assumed that the Sun / Earth is uniformly dense, which is incorrect? Since $p$ is inversely proportional to the density, we should be able to just use the density of the solar core / Earth's core / neutron star and compare that - I'm getting neutron stars are 11 orders of magnitude more likely to capture dark matter particles. | |
| Apr 16, 2018 at 23:34 | comment | added | Allure | @RobJeffries thanks for the answer. Using one of your links I found this short review, arxiv.org/pdf/1701.03926.pdf, which directly deals with the question in the OP. I still have to digest it, but thought you might be interested. | |
| Apr 16, 2018 at 22:11 | comment | added | John Duffield | @Rob Jeffries : no, because I'd have to answer the question to explain it, and despite all the rock-solid references I might give, I'll just get a pile of downvotes. | |
| Apr 16, 2018 at 21:59 | comment | added | ProfRob | @BertBarrois who said it did? You misunderstood. I said it interacts rarely, not that it was non-interacting. Non-interacting DM cannot be trapped. | |
| Apr 16, 2018 at 21:10 | comment | added | John Duffield | @RobJeffries : So what candidates are there? Inhomogeneous and interacting vacuum energy. The energy of the gravitational field shall act gravitatively in the same way as any other kind of energy. And it isn't made of WIMPs. The FLRW assumption of the homogeneity and isotropy of space is wrong. | |
| Apr 16, 2018 at 20:27 | comment | added | Bert Barrois | @RobJeffries Simpler example: Consider a mixture of two non-interacting gases in a box in a uniform gravitational field. If they are at different temperatures, nothing would force them to equilibrate. They might well have different scale heights, depending on their values of mg/T, but would remain mixed. | |
| Apr 16, 2018 at 20:07 | comment | added | Bert Barrois | @Rob If a non-interacting mass has any angular momentum at all, its orbit will never go all the way to the center. | |
| Apr 16, 2018 at 19:56 | vote | accept | Allure | ||
| Apr 16, 2018 at 16:51 | history | edited | ProfRob | CC BY-SA 3.0 |
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| Apr 16, 2018 at 16:47 | comment | added | luk32 | @RobJeffries I guess it boils down how much and how precisely can we attribute Sun's mass to regular matter. I've decided to ask a separate question to discontinue discussion in comments. I think it's valid and interesting physics.stackexchange.com/questions/400223/… | |
| Apr 16, 2018 at 16:20 | history | edited | ProfRob | CC BY-SA 3.0 |
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| Apr 16, 2018 at 16:08 | comment | added | ProfRob | @JohnDuffied - as opposed to...? The questioner makes the distinction that they are talking about non-baryonic dark matter. So what candidates are there? | |
| Apr 16, 2018 at 15:44 | comment | added | John Duffield | This answer presumes that dark matter consists of particles, -1. I see luk32 also mentioned this. | |
| Apr 16, 2018 at 15:35 | comment | added | ProfRob | @luk32 What do you mean by "visible mass"? We can only see the first ~100km of the Sun. The gravitational effects of dark matter are identical to those of normal matter. There is no debate about what the mass of the Sun is; only what it consists of. | |
| Apr 16, 2018 at 15:35 | comment | added | JimmyB | @luk32 While we may know the Sun's total mass pretty accurately from the masses and velocities of orbiting objects, I think we cannot know how much of that mass may be dark matter. (We may be able to give an upper bound above which the Sun wouldn't work as it does now. But apart from that, I'm confident that a couple of percent of DM inside the Sun wouldn't be observable.) | |
| Apr 16, 2018 at 14:04 | comment | added | luk32 | @RobJeffries Sorry for double commenting, but I thought I can ask another question whether it would be possible to devise such an experiment. I am just not sure if it's interesting enough. I always thought that dark matter must be very low density i.e. as dense as a galaxy or lower as opposed to let's say a star. Otherwise, it's lensing effect would be much more prominent and it would be a well known fact. | |
| Apr 16, 2018 at 13:49 | comment | added | luk32 | @RobJeffries Caused by dark matter? I need to catch up. What I meant specifically in this case is that if Sun had a core (i.e. within it's visible radius) of DM in any significant mass, then we could probably see that it's light-bending effect is stronger just by Sun's visible mass. I am not an astrophysicist so it's just my guess, but I think we can measure it with pretty good confidence. I don't think that it is known that sun bends light more than it should based on it's visible mass, so if there is any additional mass at it's core it's lower than current certainty of the measurment. | |
| Apr 16, 2018 at 13:38 | comment | added | ProfRob | @BertBarrois Trapped dark matter orbits in the gravitational potential. As it (rarely) interacts, it loses kinetic energy and sinks. This has nothing to do with densities, pressures or miscibility (dark matter is the ultimate miscible fluid). | |
| Apr 16, 2018 at 13:36 | comment | added | ProfRob | @luk32 Its "gravitational effects like lensing" are observed! | |
| Apr 16, 2018 at 13:22 | comment | added | luk32 | I think it's important to mention that you assume a particular model of dark matter (I think WIMP fits the bill here), while no dark matter so far was confirmed. It's a theoretical concept that is needed to make star paths/momenta fit the equations of motion we know. I.e. there is some mass missing compared to the mass we can observe and estimate. This is important, IMO, to understand what DM is expected to be. That said, I don't think it can be anything as concentrated as visible matter, because it's gravitational effects like lensing (I think) would be measurable with modern equipment. | |
| Apr 16, 2018 at 13:04 | comment | added | JimmyB | @BertBarrois I strongly doubt your statements. What is keeping all sorts of normal things from sinking to the planet's core? It's definitely non-gravitational interaction, in fact, it's forces in the opposite direction of gravity. If all dark matter does is gravtitationally attracting other matter, there's nothing to hinder it passing through a planet to it's core almost like matter wasn't even there. Hence, dark matter cannot float atop of anything with a mass; until we find negative gravitation that is. | |
| Apr 16, 2018 at 11:28 | comment | added | Bert Barrois | Quite right to emphasize interaction. Non-interacting particles will not reach any sort of hydrostatic or thermodynamic equilibrium. I'd add two fine points: (1) to sink to the center, the dark matter would have to be denser than normal stuff under pressure, and (2) dark and normal matter would have to be * immiscible*, like the molten iron and mantle rock in the Earth. | |
| Apr 16, 2018 at 9:50 | history | edited | ProfRob | CC BY-SA 3.0 |
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| Apr 16, 2018 at 8:10 | history | edited | ProfRob | CC BY-SA 3.0 |
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| Apr 16, 2018 at 7:07 | history | answered | ProfRob | CC BY-SA 3.0 |