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PetTastic
2013-Oct-04, 09:09 AM
Does this sound right?

The local dark matter density is equivalent to less than one 1 km iron asteroid per cubic AU.
It seems too small.

Just doing a final cross check on some facts in a doc, and the NASA website my notes point to is down.

crosscountry
2013-Oct-04, 09:51 AM
I get 1.4e-8 kg/m^3
or
2x10^26 kg in a AU sphere. That would be about one Neptune + one Uranus within the earth's radius.

To get the density I took the entire mass of our solar system (~2e30 kg) divided by the volume of our solar system (using pluto's radius ~6x10^9 km) and multiplied by 6 (rough estimate for mass of dark matter divided by regular matter)

Converting to iron, I get something about 4x bigger than the earth at ~26,000 km radius. But I may have misplaced some decimals.

PetTastic
2013-Oct-04, 10:51 AM
That sounds more like what I was expecting, but when I just googled it myself I am seeing numbers in the range
0.025 solar masses per cubic parsec.
So only 2x10^12 kg per cubic AU.
This seems to agree with the spiral galaxies formula here:
http://en.wikipedia.org/wiki/Dark_matter#Galaxy_rotation_curves
But as I say that looks too small.

crosscountry
2013-Oct-04, 11:20 AM
that gives me 1.4x10^-12 kg/km^3 - far less than I calculated the other way.

Universe today reported on a paper suggesting that dark matter is not uniformly distributed. Perhaps that's part of the problem?
http://www.universetoday.com/15266/dark-matter-is-denser-in-the-solar-system/

antoniseb
2013-Oct-04, 11:48 AM
Your lower numbers look a lot closer. The actual number is something close to equal the normal matter density here in the galactic plane, but unlike the normal matter, it is roughly equally dense in the sphere that surrounds the galaxy at any given radius.

Grey
2013-Oct-04, 02:09 PM
To get the density I took the entire mass of our solar system (~2e30 kg) divided by the volume of our solar system (using pluto's radius ~6x10^9 km) and multiplied by 6 (rough estimate for mass of dark matter divided by regular matter)This significantly overestimates the amount of dark matter. Doing it this way doesn't count in all the relatively empty space between the stars. There's very little normal matter there, but the dark matter should be spread throughout that volume as well, not just localized near the solar system. Indeed, that's largely the point, that dark matter doesn't aggregate into clumps. It may seem to us like the solar system out to Pluto is largely empty, but the average density is much higher than the area outside.

You'll really have to take the total amount of dark matter in the Milky Way, work out a rough distribution profile for the Sun's distance from the center, and use that to determine the density.

crosscountry
2013-Oct-04, 03:11 PM
This significantly overestimates the amount of dark matter. Doing it this way doesn't count in all the relatively empty space between the stars. There's very little normal matter there, but the dark matter should be spread throughout that volume as well, not just localized near the solar system. Indeed, that's largely the point, that dark matter doesn't aggregate into clumps. It may seem to us like the solar system out to Pluto is largely empty, but the average density is much higher than the area outside.

You'll really have to take the total amount of dark matter in the Milky Way, work out a rough distribution profile for the Sun's distance from the center, and use that to determine the density.

That makes sense. I've definitely overestimated the density of regular matter in the Milky way. It just goes to show us why we shouldn't make assumptions.

Thanatos
2013-Oct-09, 01:32 AM
The amount of dark matter in the solar system is limited to less than the mass of the moon - http://arxiv.org/abs/0806.3767