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Can a physics minded HNer confirm something for me (or set me straight)? One piece of evidence for dark matter is observed when galaxies collide. Two galaxies

by wpasc 8y ago

Can a physics minded HNer confirm something for me (or set me straight)? One piece of evidence for dark matter is observed when galaxies collide. Two galaxies collide, but by observation of gravitational lensing on the light from stars "behind" the colliding galaxies show that much of the matter from each continues moving. This matter (dark matter) is seemingly unobstructed by the collision (because of dark matter's lack of electromagnetic interaction). I heard this explained to me once, wondering if it is true.

joshvm 8y ago
> because of dark matter's lack of electromagnetic interaction This is still an assumption, because we don't know for sure that dark matter is only weakly interacting, but this is what was observed in the Bullet Cluster collision: https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster There's a good explanation here: https://astrobites.org/2016/11/04/the-bullet-cluster-a-smoking-gun-for-dark-matter/ https://astrobites.org/2016/11/04/the-bullet-cluster-a-smoki... The lensing was found to be strongest near the visible matter (stars), and the amount of observed lensing couldn't be accounted for with the total stellar mass alone. So the assumption is that the dark matter, like the stars, wasn't slowed much by the collision. The gas, on the other hand, interacts electromagnetically and slows down in the middle. That's not to say that the stuff that stars are made up of isn't affected by electromagnetism, but it's a short-range effect and even in a collision, interstellar separation is large (i.e. contrary to what you might think, a galaxy collision doesn't involve all the stars smashing into each other, most of them will just sail right on through). Gravity will dominate in that case.
- wpasc 8y ago
  Fascinating, thank you for the specific example. With a memorable name like "Bullet Cluster" I won't forget it.
- ncmncm 8y ago
  "Gas", here, as usual, means "plasma". Gas does not interact electromagnetically. Use of the term "plasma" invites banishment from the community of astronomers, and loss of access to telescopes.
  - joshvm 8y ago
    It's referred to as plasma in the primary literature: https://arxiv.org/abs/astro-ph/0608407 https://arxiv.org/abs/astro-ph/0608407 Many people use hot/ionised gas and plasma interchangeably though. For example: https://academic.oup.com/mnras/article/389/2/967/974617 https://academic.oup.com/mnras/article/389/2/967/974617
acqq 8y ago
The most famous example: the Bullet Cluster https://www.forbes.com/sites/startswithabang/2017/11/09/the-bullet-cluster-proves-dark-matter-exists-but-not-for-the-reason-most-physicists-think/#2769ca511738 https://www.forbes.com/sites/startswithabang/2017/11/09/the-... “This argument has been made for over a decade, now, with no satisfactory counterargument coming from detractors of dark matter. It isn’t the displacement of gravitation from normal matter that “proves” dark matter exists, but rather the fact that the displacement only occurs in environments where dark matter and normal matter would be separated by astrophysical processes.”
layoutIfNeeded 8y ago
When galaxies "collide" stars barely ever get into contact. The interaction is completely dominated by gravity.

by wpasc 8y ago

joshvm 8y ago

> because of dark matter's lack of electromagnetic interaction This is still an assumption, because we don't know for sure that dark matter is only weakly interacting, but this is what was observed in the Bullet Cluster collision: https://en.wikipedia.org/wiki/Bullet_Cluster https://en.wikipedia.org/wiki/Bullet_Cluster There's a good explanation here: https://astrobites.org/2016/11/04/the-bullet-cluster-a-smoking-gun-for-dark-matter/ https://astrobites.org/2016/11/04/the-bullet-cluster-a-smoki... The lensing was found to be strongest near the visible matter (stars), and the amount of observed lensing couldn't be accounted for with the total stellar mass alone. So the assumption is that the dark matter, like the stars, wasn't slowed much by the collision. The gas, on the other hand, interacts electromagnetically and slows down in the middle. That's not to say that the stuff that stars are made up of isn't affected by electromagnetism, but it's a short-range effect and even in a collision, interstellar separation is large (i.e. contrary to what you might think, a galaxy collision doesn't involve all the stars smashing into each other, most of them will just sail right on through). Gravity will dominate in that case.

wpasc 8y ago
Fascinating, thank you for the specific example. With a memorable name like "Bullet Cluster" I won't forget it.
ncmncm 8y ago
"Gas", here, as usual, means "plasma". Gas does not interact electromagnetically. Use of the term "plasma" invites banishment from the community of astronomers, and loss of access to telescopes.
- joshvm 8y ago
  It's referred to as plasma in the primary literature: https://arxiv.org/abs/astro-ph/0608407 https://arxiv.org/abs/astro-ph/0608407 Many people use hot/ionised gas and plasma interchangeably though. For example: https://academic.oup.com/mnras/article/389/2/967/974617 https://academic.oup.com/mnras/article/389/2/967/974617

acqq 8y ago

The most famous example: the Bullet Cluster https://www.forbes.com/sites/startswithabang/2017/11/09/the-bullet-cluster-proves-dark-matter-exists-but-not-for-the-reason-most-physicists-think/#2769ca511738 https://www.forbes.com/sites/startswithabang/2017/11/09/the-... “This argument has been made for over a decade, now, with no satisfactory counterargument coming from detractors of dark matter. It isn’t the displacement of gravitation from normal matter that “proves” dark matter exists, but rather the fact that the displacement only occurs in environments where dark matter and normal matter would be separated by astrophysical processes.”

layoutIfNeeded 8y ago

When galaxies "collide" stars barely ever get into contact. The interaction is completely dominated by gravity.