How much dark matter is there?

[tkdguy]

What kind of math did they use to come up with this prediction?

 

Probably something way over my head, but I'd still like to know.

[RexMundi]

Re: How much dark matter is there?

 

Ow. That Article reads like one of those Math things that Math guys write up as a joke to make the brains of non math people hurt.

 

~Rex

[L. Marcus]

Re: How much dark matter is there?

 

I think I see a printer's math error in there. If one cubic light year of dark matter weighs seven solar masses, how does a space the size of our solar system come in at 14,000 times the mass of the planets, asteroids, and the sun?

[tkdguy]

Re: How much dark matter is there?

 

I'll have to reread the article, after a good night's sleep.

[RexMundi]

Re: How much dark matter is there?

 

I think I see a printer's math error in there. If one cubic light year of dark matter weighs seven solar masses' date=' how does a space the size of our solar system come in at 14,000 times the mass of the planets, asteroids, and the sun?[/quote']

 

Where's Nyrath when you need the guy.....We need SCIENCE!

 

~Rex

[L. Marcus]

Re: How much dark matter is there?

 

Cancer's about soonish.

[Ian Mackinder]

Re: How much dark matter is there?

 

I get 'Page Not Found'. Is the URL okay, or was the article taken down?

 

In any case, I do wonder about the whole Dark Matter matter .

 

As I understand it (badly, I'm certain), DM has been "detected" in that certain maths (mainly related to things like movements of galaxies) don't seem to balance out correctly. Which leads me to question if it is actually the maths itself that is "off". Kind of like a calculator that keeps telling you that 2 + 2 = 5. Rather than theorizing about new elements to explain the anomaly, maybe there is something screwy with the process itself.

 

I'm not a 'Dark Matter Denier', truly. Could be that I am totally abysmally wrong, I know that. Just wonderin'.

[RexMundi]

Re: How much dark matter is there?

 

I'm all for the Dark Matter thing personally. The Mass has to be somewhere. Still, when folks start saying there is a Limit of "THIS MUCH!" and it Masses "THIS MUCH!" and they have still yet to actually find/prove "THIS" is actually There? Sometimes it reads a bit funny.

 

~Rex

[austenandrews]

Re: How much dark matter is there?

 

As I understand it (badly' date=' I'm certain), DM has been "detected" in that certain maths (mainly related to things like movements of galaxies) don't seem to balance out correctly. Which leads me to question if it is actually the maths itself that is "off". Kind of like a calculator that keeps telling you that 2 + 2 = 5. Rather than theorizing about new elements to explain the anomaly, maybe there is something screwy with the process itself.[/quote']

While I'm sure there's plenty of mass we haven't seen out there, I suspect most scientists believe some other factor is missing from the calculations.

[McCoy]

Re: How much dark matter is there?

 

I think I see a printer's math error in there. If one cubic light year of dark matter weighs seven solar masses' date=' how does a space the size of our solar system come in at 14,000 times the mass of the planets, asteroids, and the sun?[/quote']

Let's see. The heliopause (as good as anything else for determining a "boundary") is about 120 AU or 960 light minutes. Round up to a thousand. 565,600 minutes in a year, so 1000 light-minutes roughly .002 light years as the radius of the solar system. 3/4 pi r^3, and the solar system is about 2*10^-8 cubic light years, containing on the average 1.4*10^-7 solar masses of dark matter. Only off by eleven orders of magnitude, close enough for what passes for journalism any more.

 

While I'm sure there's plenty of mass we haven't seen out there' date=' I suspect most scientists believe some other factor is missing from the calculations.[/quote']

I don't believe most do, yet, but a significant minority. One of the real attractions of the MOND hypothesis is that if it turns out to be correct the dark matter drops out of the equation. I'm sure Cancer can explain it better.

[Xavier Onassiss]

Re: How much dark matter is there?

 

I think I see a printer's math error in there. If one cubic light year of dark matter weighs seven solar masses' date=' how does a space the size of our solar system come in at 14,000 times the mass of the planets, asteroids, and the sun?[/quote']

 

That partly depends on where you draw the boundaries of 'our solar system.' The 'old' solar system ended at Pluto's orbit (or Neptune's) but now, Pluto is known to be one of an entire class of dwarf planets which orbit beyond Neptune, and we don't yet know how far out their orbits extend. Beyond that, the Oort Cloud is also considered part of our solar system, and it may extend as far as half-way to the nearest star. If so, it's volume would be a sphere almost 2.2 light years in radius: about 44 cubic light years.

 

Don't look at me,

Xavier Onassiss

[L. Marcus]

Re: How much dark matter is there?

 

Yes, but if the math in the article is correct, our system has the volume of two thousand cubic light years. That sounds a bit exessive, I feel.

[L. Marcus]

Re: How much dark matter is there?

 

... That gives a cube about twelve light years a side.

[Wyrm Ouroboros]

Re: How much dark matter is there?

 

Sphere: V=4/3*pi*r^3 -- not 3/4.

[L. Marcus]

Re: How much dark matter is there?

 

... Yes?

[Xavier Onassiss]

Re: How much dark matter is there?

 

Any way you look at it, the math just doesn't work out as the article is written. I'm getting a solar system volume around 2000 cubic light years using their figures. No, I don't think so....

 

Here's one possible problem: "...it would weigh about 14,000 times more than the mass of the eight planets, the asteroids, and the Sun combined."

 

The sun's mass (2e30 kg) is almost a thousand times that of the planets and asteroids put together (2.6E27 kg), so maybe that's where they went wrong. Use the mass of just the planets, and the numbers are (I think) in the ball park, at least. After that, you're back to "how big is the solar system, really?" If you put it around two cubic light years (which personally, I don't) then their numbers might work.

 

Off-topic rant:

 

Popular Science has always done a crappy job editing their astronomy articles, especially when it comes to checking their math. I don't have link, but I pretty much quit reading them when I saw an article quoting the distance to a supernova in miles instead of light-years. Not only was the figure off by 12 orders of magnitude, if the *&^%$#@! supernova was that close, it would kill us all. And anyone qualified to work at a 'science' magazine should know that.

 

Bloody stupid.

 

 

Don't look at me,

Xavier Onassiss

[spaceprincessecho]

Re: How much dark matter is there?

 

Obviously we've annexed more of the universe as our own. It's the American way!

[Cancer]

Re: How much dark matter is there?

 

The if you chase the link back to Science Now, it references smething to come out in the MNRAS, which I can't get to from where I am at the moment.

 

It looks like the number is an attempt to get an upper limit for the dark matter in the Universe based on understanding of galactic evolution involving the supermassive black holes that seem to exist in the core of every galaxy. If the dark matter density is above some limit, then the black holes grow too fast and too large for you to expect to ever get anything that looks like a normal galaxy. As such, the limit depends upon galactic dynamics and is more or less independent of cosmological models etc. Those models made definite predictions about what the dark matter density has to be, but there are precious few ways to measure that number.

 

Other galactic dynamics concerns I've seen present minimum numbers for the dark matter density. That I haven't seen an upper limit published before reflects on the old mindset, that people weren't all that certain that dark matter existed at all. More or less everyone in the biz accepts dark matter now, and the question is whether there's the right amount of it relative to standard cosmological models.

 

It's an upper limit, and worth putting out there in the literature, but it's not a terribly shocking number. In fact, I expect there's stronger upper limits (that is, smaller dark matter densities) that one could find, though perhaps not from the approach the referenced paper uses.

[tkdguy]

Re: How much dark matter is there?

 

I get 'Page Not Found'. Is the URL okay' date=' or was the article taken down?[/quote']

 

It's working for me. Maybe the site was busy at the time?

 

It's on the Popular Science webpage, if you're still having trouble finding it.

[tkdguy]

Re: How much dark matter is there?

 

Any way you look at it' date=' the math just doesn't work out as the article is written. I'm getting a solar system volume around 2000 cubic light years using their figures. [i']No, I don't think so....[/i]

 

Here's one possible problem: "...it would weigh about 14,000 times more than the mass of the eight planets, the asteroids, and the Sun combined."

 

The sun's mass (2e30 kg) is almost a thousand times that of the planets and asteroids put together (2.6E27 kg), so maybe that's where they went wrong. Use the mass of just the planets, and the numbers are (I think) in the ball park, at least. After that, you're back to "how big is the solar system, really?" If you put it around two cubic light years (which personally, I don't) then their numbers might work.

 

Off-topic rant:

 

Popular Science has always done a crappy job editing their astronomy articles, especially when it comes to checking their math. I don't have link, but I pretty much quit reading them when I saw an article quoting the distance to a supernova in miles instead of light-years. Not only was the figure off by 12 orders of magnitude, if the *&^%$#@! supernova was that close, it would kill us all. And anyone qualified to work at a 'science' magazine should know that.

 

Bloody stupid.

 

 

Don't look at me,

Xavier Onassiss

 

Re: rant

 

That's a shame. I notice that's not their focus, but still. I'll look at Sky and Telescope later to see if this report is there too.

[RexMundi]

Re: How much dark matter is there?

 

Hey, it's POPULAR Science. Not Accurate Science. If you want to be Popular, you need to be Pretty (Great looking Magazine).....Not accurate.

 

~Rex

[McCoy]

Re: How much dark matter is there?

 

Computer ate my post! AARGH!

Let's see. The heliopause (as good as anything else for determining a "boundary") is about 120 AU or 960 light minutes. Round up to a thousand. 565,600 minutes in a year, so 1000 light-minutes roughly .002 light years as the radius of the solar system. 3/4 pi r^3, and the solar system is about 2*10^-8 cubic light years, containing on the average 1.4*10^-7 solar masses of dark matter. Only off by eleven orders of magnitude, close enough for what passes for journalism any more.

 

 

I don't believe most do, yet, but a significant minority. One of the real attractions of the MOND hypothesis is that if it turns out to be correct the dark matter drops out of the equation. I'm sure Cancer can explain it better.

 

Yes' date=' but if the math in the article is correct, our system has the volume of [i']two thousand cubic light years[/i]. That sounds a bit exessive, I feel.

 

Sphere: V=4/3*pi*r^3 -- not 3/4.

My bad, I even looked it up and got it wrong!

 

So if we define the edge of the solar system as the heliopause, it's 3.34*10^-8 cubic light years. If we go with a solar system 1 light year in radius (upper limit I'm finding for estimated edge of the Oort Cloud), it's 4.2 cubic light years. The 2000 cubic light year solar system would have (someone please check my math) a radius of 7.8 light years, tad large IMHO.