Innocuous title; powerful conclusion (re dark matter)

Nereid · #1 (**permalink**) 09-September-2007, 02:10 AM

"Astrometry of Galactic Star Forming Region Sharpless 269 with VERA : Parallax Measurements and Constraint on Outer Rotation Curve", a very recent astro-ph preprint.

Before I mention the conclusion, just look at an intermediate part: "We have successfully detected a trigonometric parallax of 189+/-8 micro-arcsec, corresponding to the source distance of 5.28 +0.24/-0.22 kpc. This is the smallest parallax ever measured, and the first one detected beyond 5 kpc."

Wow! Direct trig parallax for an object >5 kpc distant!

But wait! There's more!!

Quote:

This [...] provides a direct confirmation on the existence of large amount of dark matter in the Galaxy's outer disk.

Long live the VERA project!

matt.o · #2 (**permalink**) 09-September-2007, 02:59 AM

They really need to work on their paper titles - perhaps something like "Direct empirical proof for the existence of dark matter" a la Clowe et al. might have made me read the abstract!

folkhemmet · #3 (**permalink**) 09-September-2007, 05:47 AM

What an amazing accomplishment in terms of spatial resolution, although the title is kind of dry.

Is this direct evidence for the existence of dark matter in anyway newer or more powerful than previous examples of evidence for dark matter? I always thought that galactic rotation curves were one of the earliest pieces of evidence for dark matter, but MOND also successfully explains galactic rotation curves.

William · #4 (**permalink**) 09-September-2007, 07:03 AM

From what I have read, I believe the MOND hypothesis is not valid. For example:

"Alternatives to Dark Matter (?)" by Anthony Aguirre

http://xxx.lanl.gov/pdf/astro-ph/0310572

Abstract.

Quote:

It has long been known that Newtonian dynamics applied to the visible matter in galaxies and clusters does not correctly describe the dynamics of those systems. While this is generally taken as evidence for dark matter it is in principle possible that instead Newtonian dynamics (and with it General Relativity) breaks down in these systems. Indeed there have been a number of proposals as to how standard gravitational dynamics might be modified so as to correctly explain galactic dynamics without dark matter. I will review this general idea (but focus on “Modified Newtonian Dynamics”, or “MOND”), and discuss a number of ways alternatives to dark matter can be tested and (in many cases) ruled out.

This paper based on observations of the collision of two clusters provides what the authors believe is direct empirical proof of the existence of dark matter.

"A DIRECT EMPIRICAL PROOF OF THE EXISTENCE OF DARK MATTER"

http://arxiv.org/pdf/astro-ph/0608407

Abstract

Quote:

We present new weak lensing observations of 1E0657−558 (z = 0.296), a unique cluster merger, that enable a direct detection of dark matter, independent of assumptions regarding the nature of the gravitational force law. Due to the collision of two clusters, the dissipationless stellar component and the fluid-like X-ray emitting plasma are spatially segregated.

The question as to what “Dark Matter” really is however as far as I know has not been answered. Also from Anthony Aguirre’s above quoted paper.

Quote:

The great majority of astronomers now believe that the universe is dominated by cold, collisionless, non-baryonic dark matter. But despite more than 20 years of intense effort, no non-gravitational evidence for dark matter has ever been found: no direct detection of dark matter, no annihilation radiation from it, no evidence from reactor experiments supporting the physics (beyond the standard model) upon which dark matter candidates are based. We know nothing about dark matter, except for the properties that we have attributed to it, and also that it is not enough: we need to postulate …dark energy to supplement it.

Questions/Comments
1) I have looked at some of the papers concerning exotic or/and baryonic dark matter. There appear to be theoretical issues concerning other observations (i.e. If dark matter exists) related to dark matter. (eg Does dark matter clump at the galactic core and so forth. No, based on observations. Why? Does dark matter merge to form large dark matter clumps. No. Again. Why?

2) I was surprised that there were so few papers that discussed the more practical observational problems associated with dark matter. What I could find was limited to a power point presentation. i.e. No published papers (that I could find any way) that layed out the problems. Was anyone seen any papers that discuss the problems?

Nereid · #5 (**permalink**) 10-September-2007, 01:42 AM

Quote:

Originally Posted by folkhemmet

What an amazing accomplishment in terms of spatial resolution, although the title is kind of dry.

Is this direct evidence for the existence of dark matter in anyway newer or more powerful than previous examples of evidence for dark matter? I always thought that galactic rotation curves were one of the earliest pieces of evidence for dark matter, but MOND also successfully explains galactic rotation curves.

Section 4.2 ("Constraint on Galactic Rotation") addresses this.

Some excepts:

Quote:

In previous works, the Galactic rotation curve has an uncertainty up to 100 km s−1 in the outer region if one includes the strong dependence on Galactic constants [...]

The coincidence of rotation velocities at the Sun and S269 simply indicates that the rotation curve there is basically flat, as was known for rotation curves of other spiral galaxies (Rubin et al. 1980, Rubin 1983, Sofue & Rubin 2001).

[...]

The values of Vexp and Vobs obtained above give (Vexp/Vobs)2 = 0.70, and thus within the position of S269 at least ~30% of the enclosed mass must be composed of dark matter.

So what's new isn't so much that spiral galaxies have flat rotation curves, but that the amount of DM 'close to' the solar system has been constrained.

Given that we can study what's 'close to' the solar system in far greater detail than we can even nearby spiral galaxies, this is good news indeed.

William · #6 (**permalink**) 10-September-2007, 03:55 AM

I have noticed some people referring to hot dark matter in the forum. I believe the current standard cosmological model is strictly cold dark matter. Attached is a paper by Joel Primack that outlines why hot dark matter has been abandoned.

In addition to the issues Primack refers to in the attached, I thought there was a fundamental theory problem from the perspective of hot dark matter, to explain small galactic black holes. i.e. The hot dark matter would continually fall into the galactic black holes (and other smaller black holes in the respective galaxies), which should therefore all grow without limit, and become super massive.

I believe cold dark matter is hypothesized to revolve about the galactic centres and as it cannot dissipate energy, it cannot loss angular momentum and fall into the central black hole. The black hole evolution papers I looked at hypothesized that black holes formed primarily from baryonic matter.

Has anyone else in forum looked into Black Hole evolution? Issues associated the Hot and Dark Matter hypothesis?

What ever happened to Hot Dark Matter? By Joel R. Primack

http://arxiv.org/pdf/astro-ph/0112336.pdf

Quote:

Here’s the HDM story in a nutshell: For a few years in the late 1970s and early 1980s, hot dark matter looked like the best bet dark matter candidate. Such HDM models of cosmological structure formation led to a top-down formation scenario, in which superclusters of galaxies are the first objects to form, with galaxies and clusters forming through a process of fragmentation. Such models were abandoned by the mid-1980s when it was realized that if galaxies form sufficiently early to agree with observations, their distribution would be much more inhomogeneous than it is observed to be. Since 1984, the successful
structure formation models have been those in which most of the mass in the universe is in the form of cold dark matter (CDM). But the HDM stock rose again a few years later, …And like all m = 1 theories, CHDM predicted that clusters and galaxies would mostly form at low redshift. This turned out to disagree with observations, as clusters were discovered at higher and higher redshifts. Now increasing observational evidence favors _CDM — i.e., CDM with m ≈ 1/3 and a cosmological constant _ or some other form of “dark energy” making up _ ≈ 2/3 so that tot = m + _ = 1 as implied by recent observations of the cosmic background radiation. The question now is how much room there is for HDM. At present, cosmology provides the best available upper limit on the neutrino masses.

P.S.
I should caution that Joel Primack is one of the founders of the cold dark matter hypothesis. The warm dark matter founders pointed out some problems with the cosmological model in the early Universe, which they believe warm dark matter would resolve. (The warm dark matter founders do not discuss the issue with black hole evolution.) The warm dark matter founders have just recently, hypothesized a warm dark matter that evolves to resolve the problem in the early universe.

antoniseb · #7 (**permalink**) 10-September-2007, 11:56 AM

Quote:

Originally Posted by William

Has anyone else in forum looked into Black Hole evolution? Issues associated the Hot and Dark Matter hypothesis?

Hot Dark Matter probably makes up 1 to 4% of the mass of the universe, and could possibly all be in the form of neutrinos. It is my understanding that Hot Dark Matter is no more, and probably less likely to be absorbed by black holes than Cold Dark Matter, which more often orbits SMBHs in galactic scale orbits. The difference is that Hot Dark Matter particles move so fast that they cannot be bound by the gravity of galaxies (or anything except black holes if they happen to cross the event horizon).

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