Reply to thread

Message

<blockquote data-quote="freyar" data-source="post: 6753200" data-attributes="member: 40227"><p>The answers to these questions are kind of related.&nbsp; While all matter, dark or baryonic/normal, follows the same law of gravity (in any usual type of theory), which is just Newtonian gravity (since relativistic effects are tiny).&nbsp; The issue is that baryonic matter, as you say, has electromagnetic interactions and therefore pressure.&nbsp; Essentially, dark matter interacts so rarely that it can just pass through everything, but normal matter acts like a fluid.&nbsp; That means it's possible for normal matter to collapse into small objects.&nbsp; Think of it this way: the same amount of matter in a smaller space has a lower energy (that's what you get from falling, after all), so you have to lose energy somehow to go from being spread out to being compact.&nbsp; Dark matter doesn't have an effective way to lose energy.&nbsp; In the end, what this means is that, even though there is much more dark matter over all, there's a lot more normal matter in the center of a galaxy like ours.&nbsp; Also, outside the center, the normal matter forms a disk structure (that's where we are), but dark matter mostly stays in a large spherical halo (at least that's what a typical model would say; it's interesting to think about alternatives).&nbsp; In any event, the overall behavior of normal matter is a lot more complex than that of dark matter, even though they obey the same law of gravitation.&nbsp; And those complex processes can feed back into the dark matter via gravity.&nbsp; It's not clear exactly what the feedback will do since there are competing contradictory effects possible, but one possibility is breaking up or smoothing out the subhalos.</p></blockquote><p></p>

[QUOTE="freyar, post: 6753200, member: 40227"] The answers to these questions are kind of related. While all matter, dark or baryonic/normal, follows the same law of gravity (in any usual type of theory), which is just Newtonian gravity (since relativistic effects are tiny). The issue is that baryonic matter, as you say, has electromagnetic interactions and therefore pressure. Essentially, dark matter interacts so rarely that it can just pass through everything, but normal matter acts like a fluid. That means it's possible for normal matter to collapse into small objects. Think of it this way: the same amount of matter in a smaller space has a lower energy (that's what you get from falling, after all), so you have to lose energy somehow to go from being spread out to being compact. Dark matter doesn't have an effective way to lose energy. In the end, what this means is that, even though there is much more dark matter over all, there's a lot more normal matter in the center of a galaxy like ours. Also, outside the center, the normal matter forms a disk structure (that's where we are), but dark matter mostly stays in a large spherical halo (at least that's what a typical model would say; it's interesting to think about alternatives). In any event, the overall behavior of normal matter is a lot more complex than that of dark matter, even though they obey the same law of gravitation. And those complex processes can feed back into the dark matter via gravity. It's not clear exactly what the feedback will do since there are competing contradictory effects possible, but one possibility is breaking up or smoothing out the subhalos. [/QUOTE]

Verification