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Gnargenox
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09 Oct 2017 11:57

I can't understand how rotation curves of all observed galaxies are all pretty much equally flat, except near the center, implying they all have just the right amount of Dark Matter, when they each create normal matter in star formation at different rates.
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Watsisname
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09 Oct 2017 17:16

Okay, let's first consider the distribution of matter and how it changes. :)  Is star formation "creating normal matter"?  No, it just recycles the matter that was already there from the interstellar medium.  (In fact, because star death does not return 100% of that material back, we would say the ISM is being depleted over time, as more mass is converted from gas to stars to stellar remnants, at least for as long as star formation continues).  But the point is, the total amount of normal matter is the same.

Another problem is that because there are more stars per unit volume of space near the galactic center than the outer regions, the hypothesis that star formation helps explain dark matter by creating new matter does not work.  What we would need is more matter being created at larger distances, to even things out to produce a flat rotation curve.  Furthermore, we still need to explain what that matter is and why we don't see it.  We know that regular matter is not distributed in such a way as to be consistent with the rotation curves.

How do we know that?  Consider what we are able to measure.  Stars are easy.  They are luminous and we know how to measure their masses, and thus we can get a pretty good measure of a galaxy's stellar mass.  Gas is also luminous, but at different wavelengths.  We can survey ionized gas, which occurs in star formation regions, by their emission lines.  We can also observe neutral molecular gas by 21cm wavelength emission.  And we can observe hot gas (like the galactic corona) in X-rays.  In short, we have methods of surveying all the types of regular matter, and we know pretty well how much of it there is and how it is distributed.  When we do this, and plot how this matter is distributed as a function of distance, and then apply the laws of gravity (basically Kepler's Third Law) to predict how quickly things should orbit as a function of distance, we get a specific predicted curve, and that curve drops off with distance, much like how the orbital velocity of planets in a solar system drops off with distance.

But what we actually observe that the velocity vs. radius curve doesn't drop off as expected.  It is essentially flat out to large radii -- even out to the very edge of the galactic disk.  What this means is that either our understanding of gravity is wrong, or that there really is more matter there.  In fact, we can also extend this beyond the galactic disk -- by measuring the velocities (velocity dispersion) of the gas in the galactic corona.  We get the same result -- there must be more matter even out there than what we are able to detect by electromagnetic observations.

There are only two options -- either our models of gravitation must be adjusted at large distances (or slow accelerations)... or there really is more matter there than what we can see: "dark matter".  Both of these are ad hoc hypotheses, but they make further predictions which we can test.  It turns out that the dark matter hypothesis has much greater predictive success than the modified gravitation.

(There is a third option which is that there is more regular matter but in the form of "dark" components that we already know about -- like rogue planets and black holes, but when we go through the maths and the constraints from observations of how many of these things there can be out there, there are not enough.  Dark matter as an exotic new type of particle is still necessary.)

Dark matter naturally predicts that there should be places in the universe where it gets separated from normal matter (such as during galactic collisions, like the Bullet Cluster), and indeed this is what we observe.  Another check is that because we measure the spatial curvature of the universe to be flat (mass density ~= critical density), yet what we observe for the density of regular matter is only about 5% of that, then adding together the amount of dark matter and dark energy needed to explain the formation of structure in models of cosmological simulations ought to bring us to 100% of the critical density.  And indeed it does.  This isn't number fudging -- it is different observational tests yielding consistent results when they easily could have worked out differently.

Astronomers use the LCDM (Dark Energy and Cold Dark Matter) model with Ωm ~0.3 and ΩΛ ~0.7, meaning the universe is 30% matter, of which only ~5% is normal matter.  25% is dark matter, and the remaining 70% is dark energy.  These numbers make specific predictions for a variety of cosmological observations, and they all happen to agree very well.  That is why this model is considered the concordance -- or "most accepted and used" model of cosmology.
 
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10 Oct 2017 08:32

A most excellent answer as always!
Perhaps this missing matter is baryonic?
https://arxiv.org/abs/1709.10378v1

https://arxiv.org/abs/1709.05024
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10 Oct 2017 18:43

Gnargenox wrote:
Source of the post Perhaps this missing matter is baryonic?
https://arxiv.org/abs/1709.10378v1

There is a common misconception about these studies.  The researchers were not looking for a component of dark matter.  They were looking for a component of normal baryonic matter -- specifically warm gas in the filaments of the cosmic web -- which we expect to be there from models but had so far had been coming up short in observations.  

So the dark matter is still there, both in the filaments and in the galactic haloes.
 
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11 Oct 2017 12:41

Darn.

In other news, you'll need to update Haumea to have rings. Stellar occultation observations of the elliptically-shaped Kuiper Belt object (and dwarf planet) Haumea reveal that in addition to the two known moons, a narrow ring orbits the object near the 3:1 resonance with Haumea's rotation period.

This is now the third outer-solar system small body discovered to have a ring in the past few years, joining Chariklo and Chiron. Apparently rings around high-mass asteroids is not a rare phenomenon in the distant outer areas of the Solar System (and planetary systems in general?).

The size, shape, density and ring of the dwarf planet Haumea from a stellar occultation
http://www.nature.com/nature/journal/v5 ... lback=true

http://www.sciencemag.org/news/2017/10/ ... umea-15729
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11 Oct 2017 16:49

That's odd, because I normally think of rings consisting of asteroids, not being held in sway by one. Also, is Haumea really as streached as shown in that diagram?

Edit: I just looked it up in this handy but obscure astronomy program called Space Engine, and yes, it is that stretched out. Wow.
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11 Oct 2017 17:36

Watsisname wrote:
<dark matter>

Would a plethora of tiny, moon-massed black holes effect our view of other galaxies? I heard that, since the universe was dense enough to form a black hole right after the big bang, the universe formed lots of mini black holes when it began. This could explain dark energy matter, if it happened just right that the black holes aren't light enough to be exploding from hawking radiation and that they're small enough that we're not seeing distorted galaxies all over the place. These little black holes could apparently even pass through planets at high enough speeds without sucking them up.

Edit: correction.
Last edited by Mr. Missed Her on 12 Oct 2017 05:01, edited 1 time in total.
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11 Oct 2017 21:56

Press Conference at ESO HQAnnouncing Unprecedented Discovery

The Europeans are learning from NASA how to create media hypes?
NIL DIFFICILE VOLENTI
 
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12 Oct 2017 00:46

Mr. Missed Her wrote:
Source of the post Would a plethora of tiny, moon-massed black holes effect our view of other galaxies?

Perhaps surprisingly, no!

To give an idea, the average mass density of dark matter in the universe equates to about 1.4 protons per cubic meter.  If it is all in the form of black holes with the mass of the Moon (7.3x1022kg), then the average distance between them would be about 1 light year.  Furthermore, each black hole would have an event horizon only about 0.1 millimeters across!  And just as the Moon doesn't noticeably deflect light at its surface, one of these black holes would not significantly deflect light unless it happened to pass much closer than a lunar radius of it.  So our view of distant galaxies will not be affected.

However, for certain mass ranges, primordial black holes would betray their existence through observations of gravitational microlensing.  This is where the black hole (or some other compact object) by chance passes in front of a more distant point source of light, such as a star, and bends the light around it by a very small angle.  It is such a small effect that we don't really see the light being bent, but rather it appears as a momentary brightening of the star, and specific way in which the brightness changes over time reveals it as a microlensing event.

Observations indicate we can rule out the existence of primordial black holes, at least for being all of the dark matter, for mass ranges above about 1018kg.  Models of dark matter as primordial black holes are forced to peak just below that, at around 1017kg, which is about 10 times more than the mass of Phobos.

Mr. Missed Her wrote:
Source of the post This could explain dark energy


It could explain dark matter, but not dark energy.  (This might just be a slip of terminology, and the two do often get confused).  Dark energy must be a sort of repulsive force which is uniform through the entire universe (does not collapse into structures the size of galaxies like dark matter does).  It also must not be diluted as the universe expands.  This points to dark energy being a property of the vacuum itself, which is why you may sometimes hear it called "vacuum energy".

Mr. Missed Her wrote:
Source of the post These little black holes could apparently even pass through planets at high enough speeds without sucking them up.

Yeah, a small black hole coming from interstellar space would easily pass through a planet and keep right on going, and the planet would remain intact.  Serious damage would occur locally along its path and around the points of entry/exit however, mostly due to the intensity of light and heat coming off of material that falls into it.  Such event would definitely be noticeable -- fatal to those nearby -- and leave some evidence behind.  Some proposed the Tunguska event was such a microscopic black hole either evaporating or passing through the Earth, though I think that's pretty farfetched compared to an airbursting meteor or comet.

midtskogen, a never before observed astronomical phenomenon, eh?  Boy, that could be anything from aliens to a binary neutron star merger to a sock that floated out the ISS airlock.
 
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12 Oct 2017 04:15

Watsisname wrote:
Mr. Missed Her wrote:
Source of the post This could explain dark energy


It could explain dark matter, but not dark energy.  (This might just be a slip of terminology, and the two do often get confused).  Dark energy must be a sort of repulsive force which is uniform through the entire universe (does not collapse into structures the size of galaxies like dark matter does).  It also must not be diluted as the universe expands.  This points to dark energy being a property of the vacuum itself, which is why you may sometimes hear it called "vacuum energy".

Whoops. I meant dark matter.
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12 Oct 2017 05:17

Watsisname wrote:
Source of the post This points to dark energy being a property of the vacuum itself, which is why you may sometimes hear it called "vacuum energy".

I was actually of the opinion that vacuum energy and dark energy are not quite the same.
 
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12 Oct 2017 12:15

JackDole wrote:
Source of the post are not quite the same

They aren't, however there are some hypotheses that suggest dark energy as it's own thing may not actually be a thing, it may simply be an aspect of vacuum energy.
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12 Oct 2017 13:22

Since we're talking about dark energy, I've got to ask: What if we're just in a less dense region of space bigger than the observable universe? The more dense universe around our patch would be pulling everything away from our observable region, and it looks like the universe is expanding. Of course, this would only account for the acceleration of expansion, and not the expansion which itself came from the Big Bang.
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12 Oct 2017 15:06

Mr. Missed Her wrote:
Since we're talking about dark energy, I've got to ask: What if we're just in a less dense region of space bigger than the observable universe? The more dense universe around our patch would be pulling everything away from our observable region, and it looks like the universe is expanding. Of course, this would only account for the acceleration of expansion, and not the expansion which itself came from the Big Bang.

The universe is most definitely larger than what we can observe- mainly due to inflation.  However it is thought to be homogenous (for the most part.)  Dark flow, which is what you were alluding to, is something that could be accounted for something outside our observable universe pulling the universe outward (perhaps another universe?)

By the way what was that new discovery at the ESA?
 
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12 Oct 2017 17:30

Mr. Missed Her wrote:
Source of the post Since we're talking about dark energy, I've got to ask: What if we're just in a less dense region of space bigger than the observable universe? The more dense universe around our patch would be pulling everything away from our observable region, and it looks like the universe is expanding.

A-L-E-X wrote:
Source of the post Dark flow, which is what you were alluding to, is something that could be accounted for something outside our observable universe pulling the universe outward (perhaps another universe?

Counter-intuitively this would have zero effect on our volume of space.  This is because the gravitational field everywhere inside a hollow sphere, or spherical shell, is equal to zero. The only way to get a uniform accelerating expansion of the observable volume is for the volume to be uniformly filled with something that has this property of dark energy.

JackDole wrote:
Source of the post I was actually of the opinion that vacuum energy and dark energy are not quite the same.

Yeah, they're not quite the same, but share similar features as an energy density associated with the vacuum.  So there is this tantalizing idea that they should be related, but applying concepts of quantum field theory directly to try to predict dark energy's magnitude leads to the infamously wrong prediction by orders of magnitude.

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