Timeline for Effects of dark matter
Current License: CC BY-SA 4.0
10 events
| when toggle format | what | by | license | comment | |
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| Feb 14, 2019 at 8:56 | answer | added | Peter Erwin | timeline score: 1 | |
| Feb 13, 2019 at 6:27 | comment | added | Samuel Rawlings | Because the gravity which reaches the central bulk from the spiral arms is where the arms were 50,000 years ago, not where they are at that moment. The same applies to the stars in the outer arms, they are reacting to gravity from the core which left the core 50,000 years ago. They are also reacting to gravity from stars half way along the arm 25,000 years ago, and so the stars half way along the arms have already moved further along their orbit around the core by the time the gravity from them reaches the other arms. So are our simulations of galaxies taking this into account? | |
| Feb 9, 2019 at 15:06 | history | edited | Qmechanic♦ |
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| Feb 9, 2019 at 7:52 | comment | added | seVenVo1d | @SamuelRawlings I did not understand how the delayed gravitational effect would change the velocity of the outer spiral ? | |
| Feb 9, 2019 at 2:58 | comment | added | Samuel Rawlings | We have 100k LY across galaxy. It takes 50k years for grav from the bulk to reach outer arms. Grav from stars 25k LY away from the bulk take 25kY to reach the tips of the arms & core. Grav from stars 90k years from the core reach the tips at 10k years, while still 90k years away from the bulk. We have not properly encoded relativistic effects in to galactic grav sims. We could test this. Galaxies with fast orbits of far stars will appear to have more “dark matter” than those with closer/slower orbits. Galaxies with slow/no rotation will have little/no “dark matter” | |
| Feb 9, 2019 at 0:55 | comment | added | Samuel Rawlings | EDIT Shadowing gravity as in causally delayed, in the sense that, for our galaxy, gravity from the central bulk of the bar doesn't reach the tips of the spiral arms for about 40k years. For matter half way along that spiral arm it would take 20k years for gravity to reach the tips, and so on. Are our simulations of galactic rotation taking into consideration the delayed nature of gravity reaching the tips of the spiral arms from the bulk of the core? It strikes me that the very nature of arms that spiral is due to of the speed of causality. The further the arms reach, the more they spiral | |
| Feb 9, 2019 at 0:46 | comment | added | Samuel Rawlings | Shadowing gravity in the sense that, for our galaxy, gravity from the central bulk of the bar doesn't reach the tips of the spiral arms for about 40k years. | |
| Feb 8, 2019 at 23:51 | comment | added | PM 2Ring | People have tried all sorts of schemes to avoid dark matter. They don't work. If they succeed in getting galaxy rotation curves right, they fail in something else, eg gravitational lensing. But what do you mean by "shadowing gravity"? Consider: during a lunar eclipse, the Earth doesn't shadow the Moon from the Sun's gravity, only from its light. If gravitational shadowing happened, it'd have an obvious effect on the Moon's orbit. | |
| Feb 8, 2019 at 23:10 | review | First posts | |||
| Feb 9, 2019 at 2:54 | |||||
| Feb 8, 2019 at 23:07 | history | asked | Samuel Rawlings | CC BY-SA 4.0 |