I’d categorize as ‘more mathematically inclined’ but not a ‘mathematical masochist’, so I’ll go for the middle way. Thanks so much,

B.

Usually when I hear negative density used, people really mean below the cosmic mean density not an absolute density that is less than 0 (which means negative mass particles).

The mass-sheet degeneracy is a huge problem when we look at clusters of galaxies, as even with a 30′ field of view camera you’re not getting far enough away from the cluster to get back to a cosmic mean, so the mass-sheet degeneracy is one of the larger sources of errors for measuring cluster masses. For a large random field like COSMOS, this should be less of an issue since the average should be close to the cosmic mean (whether we know what that value is or not is another question).

For additional reading on weak lensing, there was a ARAA article written by Yannick Mellier in 1999 which has the basics in a fairly user friendly way (astro-ph/9812172). For the more mathematically inclined you can work through Peter Schneider’s 100 page discussion in one of the volumes of the Saas-Fee lectures (astro-ph/0509252, or you can probably buy the book online). For the mathematical masochists, I would suggest the 200 page behemoth by Bartelmann and Schneider in Physics reports (astro-ph/9912508).

Thanks, this is really helpful! It’s not too technical - in fact, if you have a reference about the data reconstruction and subtraction of the mean value I’d like to have it more technical (like, I’m not afraid of equations, except when I have to use them on a blog ) Do I interpret your answer correctly with saying, if we don’t assume the mean value is the cosmic mean, it could in principle be that we have a void surrounded by negative density? Best,

B.

Might it give dark Matter a “Lagrangian view” in cosmology/WMAP, considering such gravitational relationships?

Similar computerize views have been developed in supernova’s. QGP plasma.

I guess what we can be thankful for in the 3-D model presented, is it confirms the fact that Dark Matter does not reflect or emit light, surely if this was not so, then the whole of visible Cosmology would never get off the ground. Our Light leaving our Galaxy would pollute and inhibit our ability to observe Galaxies at far away distance’s.

The night sky is truley dark, for a specific reason, I have heard of this argument that was formulated in the early 20th century, but cannot remember who brought it to the foreground, thanks again.

Cheers,

-cvj

Is there any correlation to the fact that the further one goes back in Time, the less acceleration is imposed upon ordinary visable matter?..and would not this co-incide with the fact that the Universe’s matter (locally) is experiencing a vast increase in its acceleration/expansion?

I suppose what I am inquiring about is, is the expansion responsible for what appears to be an extraction process?..ie visible matter being “extracted” out of a smooth highly strong gravitational energy?

Starting tomorrow, you’ll be able to sign up with MI5 to receive an email notice when the “Threat Level” changes. Right now it’s “severe”, but they have the fine-grained menu of low/moderate/substantial/severe/critic…

-cvj

Weak lensing doesn’t actually measure the mass, it measures the change in mass. This means you take the edge of your area you are reconstructing and set it to some fixed value, then integrate inward to build up a picture of the peaks and valleys of the mass distribution. You’re left over with a free constant, which is the mean value of the mass at the edges (or averaged across any other region you choose).

For a field as large a the one Massey et al reconstructed, the mean mass in the field should be very close to the cosmic mean, which we think we’ve measured reasonably well using WMAP, supernova surveys, etc. So you can get rid of that free parameter by assuming that the mean mass averaged over your field is that measured cosmic mean (it is an assumption though).

As usual, the press coverage in what is new and interesting about this result is slightly misleading. Larger areas than this one have been covered by weak lensing before (the largest survey was about 100 square degrees, which is about 400 full moons). People have also done 3-D reconstructions before (most weak lensing results project all of the mass onto a 2-D screen, the 3-D method requires you to be able to get a rough redshift for each of the background galaxies which then lets you get a third dimension, although with much larger errors in that third dimension than in the 2 on the sky). This is the first time, however, that someone has done both. It’s also the largest field reconstructed from HST imaging (which can get a better signal-to-noise reconstruction than ground based because you can measure shapes for more galaxies with the smaller point spread function).

In answer to Charles T’s question -
unless they did something truely novel (and a quick read of the Nature article didn’t indicate so), then the transformation of the shear (shapes of the galaxies) into the mass distribution assumed that gravity was Newtonian.

yeah, that would be nice. The thing is that someone told me they actually subtract some background value to normalize the minimum value to zero, but I couldn’t find any reference on that. Have a nice day,

B.

Okay, here is the rest:

Also, a void in a non-zero density ( Dark Matter less than zero) acts as a diffraction lens, no? Do you know if anything like this is observed? Best,

B.

PS: Sorry, some self-ad - see also my post about Dark Matter

Cheers,

-cvj

this is a very interesting post! It really is amazing what can be extracted from such tiny distortions. I have a question about the analysis though that might sound a bit weird, but it’s been puzzling me. How do they set a ‘zero’ to the distortions? I mean, they can find out , but how do they set ? Do they just define the minimum of to be zero? I’ll give you an analogy: take a plane slice of glass. It doesn’t cause any lensing, but the density isn’t zero. How do they get rid of this normalization? Also, a void in a non-zero density ( Dark Matter

Basically, you use a computer to runa a simulation of the evolution of the universe, putting in dark matter of various sorts. The type of dark matter that these new observations are suggesting is really there fits very well with the simulation results. The simulations already do a good job of showing that you get the right structures we observe to day for the visible matter (see image links etc from that post), as a result of evolving the universe with cold dark matter:

…and this is fitting reasonably well with the dark matter late-time distributions too (although not on the scales and resolutions of the above image…. I don’t think that the weak lensing can give you that level of detail).

See also a remark on that post by Risa Wechsler, who is an expert on that topic (simulations). She also points to her own lectures and other sources.

-cvj

A layman’s question: Do the data unambiguously point to dark matter causing the distortion rather than something rather more fanciful - gravitational influence leaking though bulk dimensions from another brane (or another part of ours) for example.

Of course, the phrase “work with” implies that my software works. Which it currently doesn’t…