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Tags astronomy , dark matter

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Old 28th March 2018, 10:58 AM   #1
Pixel42
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Galaxy found without any dark matter / Dark matter or no dark matter

https://www.theguardian.com/science/...es-astronomers

Quote:
A distant galaxy that appears completely devoid of dark matter has baffled astronomers and deepened the mystery of the universe’s most elusive substance.

The absence of dark matter from a small patch of sky might appear to be a non-problem, given that astronomers have never directly observed dark matter anywhere. However, most current theories of the universe suggest that everywhere that ordinary matter is found, dark matter ought to be lurking too, making the newly observed galaxy an odd exception.

“Something like this has never been seen,” said Prof Pieter van Dokkum, of Yale University, the study’s senior author. “It challenges the standard ideas of how we think galaxies form.”
My first guess was that the dark matter had got separated out somehow, reading on that's one of the speculations.

This is an interesting point:

Quote:
Paradoxically, the authors said the discovery of a galaxy without dark matter counts as evidence that it probably does exist. A competing explanation for the fast-orbiting stars is that the way gravity drops off with distance has been misunderstood – but if this were the case, all galaxies should follow the same pattern.
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Old 28th March 2018, 11:05 AM   #2
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Dark matter or no dark matter? That is the question

In 2016, astronomers at Mauna Kea's Keck Observatory and the Gemini North telescope, discovered a distant galaxy composed almost entirely of dark matter...

https://phys.org/news/2016-08-scient...laxy.html#nRlv

Now, that same group of astronomers have discovered another distant galaxy, this time, almost entirely devoid of dark matter

https://phys.org/news/2018-03-dark-galaxy.html#nRlv

I mean, WT actual F? This seems to poke a rather large hole in the current theory of how galaxies form, i.e from blobs of dark matter.

When taken together, what are the implications of these two discoveries?
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Old 28th March 2018, 11:18 AM   #3
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Originally Posted by smartcooky View Post
In 2016, astronomers at Mauna Kea's Keck Observatory and the Gemini North telescope, discovered a distant galaxy composed almost entirely of dark matter...

https://phys.org/news/2016-08-scient...laxy.html#nRlv

Now, that same group of astronomers have discovered another distant galaxy, this time, almost entirely devoid of dark matter

https://phys.org/news/2018-03-dark-galaxy.html#nRlv

I mean, WT actual F? This seems to poke a rather large hole in the current theory of how galaxies form, i.e from blobs of dark matter.

When taken together, what are the implications of these two discoveries?
I know how galaxies born.

I just wonder if anybody really want to know and really want to understand how!!!

😀
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Old 28th March 2018, 11:35 AM   #4
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Originally Posted by Pixie of key View Post
I know how galaxies born.
Is it storks?
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Old 28th March 2018, 11:46 AM   #5
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Beat you to it, smartcooky:

http://www.internationalskeptics.com...d.php?t=328166
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Old 28th March 2018, 12:12 PM   #6
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Originally Posted by Pixel42 View Post
Damn... beat me by seven minutes

I'll report my post and get it merged.
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Old 28th March 2018, 12:38 PM   #7
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Originally Posted by smartcooky View Post
Damn... beat me by seven minutes

I'll report my post and get it merged.
Yeah but, obviously, everyone likes yours better...
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Old 28th March 2018, 01:35 PM   #8
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Originally Posted by smartcooky View Post
In 2016, astronomers at Mauna Kea's Keck Observatory and the Gemini North telescope, discovered a distant galaxy composed almost entirely of dark matter...

https://phys.org/news/2016-08-scient...laxy.html#nRlv

Now, that same group of astronomers have discovered another distant galaxy, this time, almost entirely devoid of dark matter

https://phys.org/news/2018-03-dark-galaxy.html#nRlv

I mean, WT actual F? This seems to poke a rather large hole in the current theory of how galaxies form, i.e from blobs of dark matter.

When taken together, what are the implications of these two discoveries?
that is a cool story I saw earlier, I think the question is why doesn't that galaxy have a dark matter halo?
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Old 29th March 2018, 10:54 PM   #9
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If there is one galaxy without dark matter maybe there are others?

Then the question that is asked in the article how did this happen?
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Old 30th March 2018, 01:10 AM   #10
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I would like to point something else, though. When doing the difference between visible stars and mass, the difference isn't just non-baryonic matter. This new galaxy basically seems to also not leave much room for black holes.

SOMETHING mighty strange happened there, but I'll leave it to astrophysicists to figure out what, 'cause I have no idea.
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Old 30th March 2018, 01:27 AM   #11
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Originally Posted by Samson View Post
I have no specific knowledge to add, but regard dark matter as a disaster for the simple mind to understand.
We send a space craft to Pluto which arrives exactly on time, and the equations pay no heed to dark matter, yet this pervades the galaxy with its 80% of all matter. I think the simple mind can be excused for not getting quickly to grips with dark matter. I wish it could be shown to be a hoax.
Actually, it's not that hard to understand why it doesn't affect the probe much, if you understand the Shell Theorem.

When a star system forms, normal matter forms an accretion disk BECAUSE it interacts with other normal matter and is braked. Dark matter doesn't get braked and doesn't accrete, because it just passes through normal matter.

I.e., it has no real reason to end up in the Sun or very near it, which is the mass that dominates the gravity equations from here to Pluto.

What it ends up being is roughly a blob of dark matter gas filling the whole galaxy.

Essentially in the Pluto travel equation you have somewhere between 99.8 and 99.9 of the mass being the Sun, and buggerall visible mass between it and say, Alpha Centauri. Whereas in the dark matter's case there's buggerall in or around the Sun, and the vast majority of it spread over the vast distances in all directions. It's spread out over all those massive spaces between the stars.

If you do the maths of what affects the gravity that affects the probe when it passes Pluto, all the dark matter that matters is in a sphere with the radius of Pluto's orbit. Cf the shell theorem. Well, there's not that much there. The local density in our solar system is a mere 0.3GeV/cc. By way of comparison that 0.3GeV is less than a third of a proton's mass. The vast majority will be outside the shell, so it doesn't really matter for gravity calculations.

Dark matter makes up by being spread over all those cubic lightyears inside the galaxy, but in any given point the density will be very very small.

And the other reason I mention that shell theorem is also that such galaxy sized blob will mostly affect our movement around the galactic centre. And assuming a more or less radial symmetry in its distribution (which isn't 100% true, but close enough for our purposes here), what matters is the mass contained in a sphere with the radius being the distance between us and the galactic centre.

Well, between us and Pluto that radius doesn't change much. Essentially Pluto just moves around the galactic centre together with the Sun, and the correction you'd need to make because of dark matter is really insignificant.

That's the short and skinny of it, basically.
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Last edited by HansMustermann; 30th March 2018 at 01:40 AM.
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Old 30th March 2018, 01:36 AM   #12
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Originally Posted by Samson View Post
I have no specific knowledge to add, but regard dark matter as a disaster for the simple mind to understand.
We send a space craft to Pluto which arrives exactly on time, and the equations pay no heed to dark matter, yet this pervades the galaxy with its 80% of all matter. I think the simple mind can be excused for not getting quickly to grips with dark matter. I wish it could be shown to be a hoax.
Why on earth would anyone create such a hoax?

It may turn out to be an incorrect interpretation of the data, though this discovery makes that less likely as the quote from the article I linked in the OP points out.

As to the lack of effect on the orbits of probes in the solar system, this article should help:

http://scienceblogs.com/startswithab...-solar-system/
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Old 31st March 2018, 06:34 AM   #13
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Originally Posted by Pixie of key View Post
I know how galaxies born.

I just wonder if anybody really want to know and really want to understand how!!!

😀
Thanks, but no thanks!!!!!!!!!!!!!!!
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