Comments on: Watch That Space! http://asymptotia.com/2006/08/15/watch-that-space/ Tue, 16 Mar 2010 12:53:37 +0000 http://wordpress.org/?v=2.7.1 hourly 1 By: Less In The Dark Than Before? - Asymptotia http://asymptotia.com/2006/08/15/watch-that-space/comment-page-1/#comment-4595 Less In The Dark Than Before? - Asymptotia Wed, 15 Nov 2006 16:45:25 +0000 http://asymptotia.com/2006/08/15/watch-that-space/#comment-4595 [...] From NASA, tomorrow, at 1:00pm EST: An announcement about Dark Energy. If the pattern of last time is to be followed, there must be new evidence from the space telescope team (represented by Riess and Livio at the press conference) in favour of one interpretation or another. Since they are saying that they will “announce the discovery that dark energy has been an ever-present constituent of space for most of the universe’s history”, I imagine that means that there is stronger evidence than before for Dark Energy being a cosmological constant. [...] [...] From NASA, tomorrow, at 1:00pm EST: An announcement about Dark Energy. If the pattern of last time is to be followed, there must be new evidence from the space telescope team (represented by Riess and Livio at the press conference) in favour of one interpretation or another. Since they are saying that they will “announce the discovery that dark energy has been an ever-present constituent of space for most of the universe’s history”, I imagine that means that there is stronger evidence than before for Dark Energy being a cosmological constant. [...]

]]> By: MOND Laid to Rest? - Asymptotia http://asymptotia.com/2006/08/15/watch-that-space/comment-page-1/#comment-475 MOND Laid to Rest? - Asymptotia Mon, 21 Aug 2006 18:24:30 +0000 http://asymptotia.com/2006/08/15/watch-that-space/#comment-475 [...] Well, the press conference I told you about has happened. There’s new and very direct proof from observations 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. [...] [...] Well, the press conference I told you about has happened. There’s new and very direct proof from observations 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. [...]

]]> By: Jeff Booth http://asymptotia.com/2006/08/15/watch-that-space/comment-page-1/#comment-474 Jeff Booth Mon, 21 Aug 2006 17:13:01 +0000 http://asymptotia.com/2006/08/15/watch-that-space/#comment-474 http://chandra.harvard.edu/photo/2006/1e0657/media/ Listening to press conference online right now. Above has a link to images and the paper and the real audo file. Peir Stefano above got the interpretation spot on (bravo!). http://chandra.harvard.edu/photo/2006/1e0657/media/

Listening to press conference online right now. Above has a link to images and the paper and the real audo file. Peir Stefano above got the interpretation spot on (bravo!).

]]> By: Pier Stefano Corasaniti http://asymptotia.com/2006/08/15/watch-that-space/comment-page-1/#comment-394 Pier Stefano Corasaniti Fri, 18 Aug 2006 00:57:56 +0000 http://asymptotia.com/2006/08/15/watch-that-space/#comment-394 I am not an expert of dark matter and halo model at all as I spend most of my time thinking to the dark side of the energy, but what I understood from the abstract I might speculate that the evidence they are claiming would come from the fact that the center of mass of the X-ray images may not coincides with that inferred from the weak and strong lensing analysis. This is because while the gas has been slowed down by the shock, the dark matter particles (which interact only gravitationally) may have continued their motion causing a displacement of the clustered mass, hence a lensing signal which would not be compatible with the position of the baryons. Presumably it might require a very weird theory of gravity to allow baryons to produce a lensing mirage somewhere else with respect to the center of their mass distribution...but it is just my speculation..very interesting stuff indeed. I am not an expert of dark matter and halo model at all as I spend most of my time thinking to the dark side of the energy, but what I understood from the abstract I might speculate that the evidence they are claiming would come from the fact that the center of mass of the X-ray images may not coincides with that inferred from the weak and strong lensing analysis. This is because while the gas has been slowed down by the shock, the dark matter particles (which interact only gravitationally) may have continued their motion causing a displacement of the clustered mass, hence a lensing signal which would not be compatible with the position of the baryons. Presumably it might require a very weird theory of gravity to allow baryons to produce a lensing mirage somewhere else with respect to the center of their mass distribution…but it is just my speculation..very interesting stuff indeed.

]]> By: Brad Holden http://asymptotia.com/2006/08/15/watch-that-space/comment-page-1/#comment-386 Brad Holden Thu, 17 Aug 2006 18:27:47 +0000 http://asymptotia.com/2006/08/15/watch-that-space/#comment-386 To answer the Count's question, yes the majority of baryons are in the gas. The usual fraction is 85 or so percent with the remainder in stars. Why that changes the conclusions about MOND versus other stuff, I do not understand. To answer the Count’s question, yes the majority of baryons are in the gas. The usual fraction is 85 or so percent with the remainder in stars. Why that changes the conclusions about MOND versus other stuff, I do not understand.

]]> By: Count Iblis http://asymptotia.com/2006/08/15/watch-that-space/comment-page-1/#comment-374 Count Iblis Thu, 17 Aug 2006 13:05:48 +0000 http://asymptotia.com/2006/08/15/watch-that-space/#comment-374 Does the gas account for most of the baryonic mass? If not then I don't see how this disproves MOND. Does the gas account for most of the baryonic mass? If not then I don’t see how this disproves MOND.

]]> By: Aaron F. http://asymptotia.com/2006/08/15/watch-that-space/comment-page-1/#comment-361 Aaron F. Thu, 17 Aug 2006 03:54:51 +0000 http://asymptotia.com/2006/08/15/watch-that-space/#comment-361 Oh maaaaaan! I totally called it! The moment I read "dark and normal matter have been forced apart in an extraordinarily energetic collision," I thought, "that sounds like the bullet cluster!" If only I had posted a time-stamped comment about it before the spoiler came out... :) Oh maaaaaan! I totally called it! The moment I read “dark and normal matter have been forced apart in an extraordinarily energetic collision,” I thought, “that sounds like the bullet cluster!” If only I had posted a time-stamped comment about it before the spoiler came out… :)

]]> By: Count Iblis http://asymptotia.com/2006/08/15/watch-that-space/comment-page-1/#comment-358 Count Iblis Thu, 17 Aug 2006 00:24:35 +0000 http://asymptotia.com/2006/08/15/watch-that-space/#comment-358 The dark matter density is pretty low in these clusters. So, "collisionless" is still compatible with pretty large cross sections. The dark matter density is pretty low in these clusters. So, “collisionless” is still compatible with pretty large cross sections.

]]> By: Levi http://asymptotia.com/2006/08/15/watch-that-space/comment-page-1/#comment-342 Levi Wed, 16 Aug 2006 12:47:50 +0000 http://asymptotia.com/2006/08/15/watch-that-space/#comment-342 The short paper at the link below seems to be very much on topic: http://www.journals.uchicago.edu/ApJ/journal/issues/ApJL/v567n1/15847/15847.html This would really be the end of MOND, wouldn't it? Does anybody have advance word on whether the dark matter is collisionless or not? The short paper at the link below seems to be very much on topic:

http://www.journals.uchicago.edu/ApJ/journal/issues/ApJL/v567n1/15847/15847.html

This would really be the end of MOND, wouldn’t it?

Does anybody have advance word on whether the dark matter is collisionless or not?

]]> By: anon. http://asymptotia.com/2006/08/15/watch-that-space/comment-page-1/#comment-336 anon. Wed, 16 Aug 2006 05:20:33 +0000 http://asymptotia.com/2006/08/15/watch-that-space/#comment-336 Real information from Slashdot? That's a first. Over at Uncertain Principles, Steinn Sigurdsson linked a paper (Markevitch et.al.) constraining the dark matter self-interaction cross section from this object, and raised an interesting question that I will repeat in the hope that someone here can answer: Does anyone know if it's possible that the new data allow an actual <i>measurement</i> of this cross section? Real information from Slashdot? That’s a first.

Over at Uncertain Principles, Steinn Sigurdsson linked a paper (Markevitch et.al.) constraining the dark matter self-interaction cross section from this object, and raised an interesting question that I will repeat in the hope that someone here can answer:

Does anyone know if it’s possible that the new data allow an actual measurement of this cross section?

]]> By: JoAnne http://asymptotia.com/2006/08/15/watch-that-space/comment-page-1/#comment-335 JoAnne Wed, 16 Aug 2006 05:18:35 +0000 http://asymptotia.com/2006/08/15/watch-that-space/#comment-335 WooHoo!!!! WooHoo!!!!

]]> By: Clifford http://asymptotia.com/2006/08/15/watch-that-space/comment-page-1/#comment-334 Clifford Wed, 16 Aug 2006 05:15:47 +0000 http://asymptotia.com/2006/08/15/watch-that-space/#comment-334 See what I mean? :-) -cvj See what I mean? :-)

-cvj

]]> By: Evil Old Slashdot User http://asymptotia.com/2006/08/15/watch-that-space/comment-page-1/#comment-333 Evil Old Slashdot User Wed, 16 Aug 2006 05:05:48 +0000 http://asymptotia.com/2006/08/15/watch-that-space/#comment-333 Bwahaha! Bwahaha!

]]> By: Clifford http://asymptotia.com/2006/08/15/watch-that-space/comment-page-1/#comment-332 Clifford Wed, 16 Aug 2006 03:48:04 +0000 http://asymptotia.com/2006/08/15/watch-that-space/#comment-332 Anonymous:- Thanks! That <em>is</em> very interesting.... I also must remark that slashdot is a really really scary place. This is the first time I've ever gone there. Anyway, to save people the need to wade through a lot of the scary chatter, here is what the spolier said: <blockquote> They're referring to the Bullet Cluster. It's a merging system where a small cluster is passing through a large cluster leaving a shockwave that looks like a bullet's wake, hence the name. Dark Matter is collionless, i.e. the DM from the smaller system hasn't been slowed down by the collion and just zooms through. The gas is slowed down. So, the DM and gas are no longer in the same place. We can see the gas in an X-ray telescope (Chandra) and detect the mass by the gravitational lensing effect on the background galaxies. This is the first time that this has been shown, and it basically disproves the entire category of theories that DM is an illusional caused by us not understanding the action of gravity at long ranges (MOND). <a href="http://www.cosis.net/abstracts/COSPAR2006/02655/COSPAR2006-A-02655.pdf?PHPSESSID=c6403a60cabe182bacf64c055e0609d2" rel="nofollow">Abstract from a conference talk about this.</a> [cosis.net] (PDF) </blockquote> Interesting stuff. The abstract says: <blockquote> Dark matter and the bullet cluster M. Markevitch (1), S. Randall (1), D. Clowe (2), A. Gonzalez (3), M. Bradac (4) (1) Harvard-Smithsonian Center for Astrophysics, (2) University of Arizona, (3) University of Florida, (4) KIPAC, Stanford University 1E0657-56, the "bullet cluster", is a merger with a uniquely simple geometry. From the long Chandra X-ray observation which revealed a classic bow shock in front of a small subcluster, we can derive the velocity of the subcluster and its direction of motion. Recent accurate weak and strong lensing total mass maps clearly show two merging subclusters, including the host of the gas bullet seen in X-rays. This cluster provided the first direct, model-independent proof of the dark matter existence (as opposed to any modified gravity theory) and a direct constraint on the self-interaction cross-section of the dark matter particles. I will review these and other related results. </blockquote> This is great stuff! -cvj Anonymous:- Thanks! That is very interesting…. I also must remark that slashdot is a really really scary place. This is the first time I’ve ever gone there.

Anyway, to save people the need to wade through a lot of the scary chatter, here is what the spolier said:

They’re referring to the Bullet Cluster. It’s a merging system where a small cluster is passing through a large cluster leaving a shockwave that looks like a bullet’s wake, hence the name.
Dark Matter is collionless, i.e. the DM from the smaller system hasn’t been slowed down by the collion and just zooms through. The gas is slowed down. So, the DM and gas are no longer in the same place. We can see the gas in an X-ray telescope (Chandra) and detect the mass by the gravitational lensing effect on the background galaxies.
This is the first time that this has been shown, and it basically disproves the entire category of theories that DM is an illusional caused by us not understanding the action of gravity at long ranges (MOND).

Abstract from a conference talk about this. [cosis.net] (PDF)

Interesting stuff. The abstract says:

Dark matter and the bullet cluster

M. Markevitch (1), S. Randall (1), D. Clowe (2), A. Gonzalez (3), M. Bradac (4)

(1) Harvard-Smithsonian Center for Astrophysics, (2) University of Arizona, (3) University of Florida, (4) KIPAC, Stanford University

1E0657-56, the “bullet cluster”, is a merger with a uniquely simple geometry. From the long Chandra X-ray observation which revealed a classic bow shock in front of a small subcluster, we can derive the velocity of the subcluster and its direction of motion. Recent accurate weak and strong lensing total mass maps clearly show two merging subclusters, including the host of the gas bullet seen in X-rays. This cluster provided the first direct, model-independent proof of the dark matter existence (as opposed to any modified gravity theory) and a direct constraint on the self-interaction cross-section of the dark matter particles. I will review these and other related results.

This is great stuff!

-cvj

]]> By: Anonymous http://asymptotia.com/2006/08/15/watch-that-space/comment-page-1/#comment-330 Anonymous Wed, 16 Aug 2006 02:40:23 +0000 http://asymptotia.com/2006/08/15/watch-that-space/#comment-330 Somebody posted <a href="http://science.slashdot.org/comments.pl?sid=194095&cid=15911401" rel="nofollow">spoilers</a> at Slashdot. (It also came up during a meeting today, and a team member confirmed the spoilers are broadly correct.) Somebody posted spoilers at Slashdot. (It also came up during a meeting today, and a team member confirmed the spoilers are broadly correct.)

]]>