Well, the press conference I told you about has happened! This is so exciting! There’s new and very direct proof from observations of the Bullet Cluster with the Chandra X-ray Observatory that Dark Matter really exists.
So the need to make modifications to how gravity works on large scales in order to explain observations seems to be something we can put aside for now.
Your mission: Go to the press release website for more information, and lovely pictures of the Bullet Cluster (such as the composite above, borrowed from the site) and animations of the dynamics. For more explanation, read Sean’s post about it on Cosmic Variance. Sit back and think about it….. Then celebrate!
-cvj
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pah, I’d much rather listen to James Binney on that.
What you say is true. hence the… see my comment above.
It is always hard -impossible maybe- to prove a negative. At this point I could start babbling about white swans and black swans….. etc… etc. But I’ve got to design an entire semester’s course in the next few hours and so I’ll just point to a convenient Wikipedia article.
Cheers,
-cvj
Hey Clifford,
I only mean to say that whatever would cause a ‘MOND-like’ modification to Poisson’s equation may yet account for some/a little of the ~0.3 of dark matter, whereas the majority it seems comes from some kind of particle. As far as I can tell, this recent observation does not exclude the MOND thing as a dark matter candidate on scales of 10s of Megaparsecs.
Indeed, as there never really has been a theory of MOND as such, it would be hard to say one way or the other what to expect!
This scenario would obviously seem to stretch plausibility. However, if (big if!) the same physics that gave a low acceleration modification to Poisson’s equation was the origin of the ~0.7 attributed to dark energy, then this is almost turned on its head; what was originally put forward as an empirical ‘alternative’ to dark matter may well be the signature of what is the dominant constituent of the universe which encroaches somewhat on the 0.3 hitherto attributed to dark matter but not dominantly. The MOND people make a big deal of the acceleration a_{0} being \sim c H_{0}.
That possibility exists….. hence the question mark in the title. And the use of the word “seems”.
Cheers,
-cvj
Clifford,
My point is that MOND “may” have some application to the Dark Energy issue. I am not advocating that as a solution, but this experiment only confirms Dark Matter. I just wanted to be cautious as to the implications of this experiment in general. Certainly it strongly implies that the MOND hypothesis for Dark Matter is greatly weakened.
Elliot
Sandground:- I don’t understand what you mean….I simply said that what was an attempted non-dark-matter explanation of the dark matter problem is no longer viable. You need dark matter after all. I did not say that this meant that we now know what dark matter is.
Cheers,
-cvj
Discussion here. Note the reference to astro-ph/0606216, which actually seems to put the foundation of this “direct proof” in question.
Perhaps not clifford but your words would seem to imply a certainty that what gave ‘MOND’ wasn’t what gave dark energy! MOND is dead, after all.
“We still have that little issue of Dark Energy which is still outstanding.”
Hi.. nowhere in the post does it say that this issue is not outstanding…. I’m confused by what you’re getting at.
-cvj
Clifford,
With all due respect. MOND is not dead per se. We still have that little issue of Dark Energy which is still outstanding. I am not saying this is “good news” for MOND but I don’t think it is completely clear that the the casket is closed, nailed shut, and being lowered into the grave.
It’s a very exciting time for cosmology and cosmologists.
Elliot
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So much for Mond then.:)
So, can we continue to assume “sound in analogy” as to the thinking of gravitational wave detection and possible(?) relations to the “bulk perspective.”
I mean it is important to try to grasp this from a layman standpoint so the ideas here do not seem so “mathematically abstract: in regards to bulk considerations.
If there are so many things wrong with what I am saying then we should correct it while you can, since Wayne Hu and LISA are running with it.
yeah, nice one…
see astro-ph/0212293 . This has been known for a long time to those MOND advocates. MOND is not a theory, it’s an algorithm. They continue only because, to their minds, this algorithm works and keeps on working on smaller scales.