Comments on: Seeing the Sky with Different Eyes http://blogs.discovermagazine.com/cosmicvariance/2008/08/28/seeing-the-sky-with-different-eyes/ Random samplings from a universe of ideas. Sun, 05 Jul 2009 06:33:10 +0000 http://wordpress.org/?v=2.3.1 By: Something’s in the air | Cosmic Variance | Discover Magazine http://blogs.discovermagazine.com/cosmicvariance/2008/08/28/seeing-the-sky-with-different-eyes/#comment-49325 Something’s in the air | Cosmic Variance | Discover Magazine Tue, 25 Nov 2008 08:21:48 +0000 http://blogs.discovermagazine.com/cosmicvariance/2008/08/28/seeing-the-sky-with-different-eyes/#comment-49325 [...] There’s been a rash of slightly odd and suggestive results as of late. There was the observation last month by the PAMELA satellite of an anomalous positron excess at ~50 GeV. This week the balloon-borne ATIC experiment reports seeing a bump in electrons and positrons (they can’t tell the difference) at 500 GeV. MILAGRO has recently seen some weird gamma ray hotspots at 10 TeV. As if all this isn’t enough, Doug Finkbeiner has been warning us that there is an unexplained CMB “haze”. [...] […] There’s been a rash of slightly odd and suggestive results as of late. There was the observation last month by the PAMELA satellite of an anomalous positron excess at ~50 GeV. This week the balloon-borne ATIC experiment reports seeing a bump in electrons and positrons (they can’t tell the difference) at 500 GeV. MILAGRO has recently seen some weird gamma ray hotspots at 10 TeV. As if all this isn’t enough, Doug Finkbeiner has been warning us that there is an unexplained CMB “haze”. […]

]]> By: Shantanu http://blogs.discovermagazine.com/cosmicvariance/2008/08/28/seeing-the-sky-with-different-eyes/#comment-42375 Shantanu Sun, 19 Oct 2008 04:47:03 +0000 http://blogs.discovermagazine.com/cosmicvariance/2008/08/28/seeing-the-sky-with-different-eyes/#comment-42375 Sean, I too haven't read all of Blanchet's papers on this subject and my information may be outdated. But his first paper posited that of dark matter consists of a gravitational dipole, then you can easily explain Milgrom's law on galactic scales (without invoking MOND). The downside is that dark matter now consists of particle with negative mass. Does the Bullet cluster rule out these models? Sean, I too haven’t read all of Blanchet’s papers on this subject and my information may be outdated.
But his first paper posited that of dark matter consists of a gravitational dipole,
then you can easily explain Milgrom’s law on galactic scales (without
invoking MOND). The downside is that dark matter now consists of particle
with negative mass. Does the Bullet cluster rule out these models?

]]> By: Fermi-Walker Public Transport http://blogs.discovermagazine.com/cosmicvariance/2008/08/28/seeing-the-sky-with-different-eyes/#comment-42374 Fermi-Walker Public Transport Mon, 01 Sep 2008 10:36:16 +0000 http://blogs.discovermagazine.com/cosmicvariance/2008/08/28/seeing-the-sky-with-different-eyes/#comment-42374 Synchrotron emission is also polarized, sometimes in the tens of percent, so that would be another way of verifying if that is the emission mechanism. However polarization can also be masked by beam size effects (resolution) and by an ionized medium along the line of sight. Still, it may be worth checking and if found, would be good support. Synchrotron emission is also polarized, sometimes in the tens of percent, so that would be another way of verifying if that is the emission mechanism. However
polarization can also be masked by beam size effects (resolution) and by an ionized medium along the line of sight. Still, it may be worth checking and if found, would be good support.

]]> By: Sean http://blogs.discovermagazine.com/cosmicvariance/2008/08/28/seeing-the-sky-with-different-eyes/#comment-42371 Sean Mon, 01 Sep 2008 06:07:22 +0000 http://blogs.discovermagazine.com/cosmicvariance/2008/08/28/seeing-the-sky-with-different-eyes/#comment-42371 Shatanu, I haven't read the paper, so I can't answer in detail. Bullet-Cluster-type measurements imply that there is some "dark" form of energy that acts essentially like dark matter. So the explanation can't be <em>only</em> modified gravity. But some particular theory of "modified gravity" might include extra forms of energy-momentum that mimic dark matter, and therefore be consistent with the data. Shatanu, I haven’t read the paper, so I can’t answer in detail. Bullet-Cluster-type measurements imply that there is some “dark” form of energy that acts essentially like dark matter. So the explanation can’t be only modified gravity. But some particular theory of “modified gravity” might include extra forms of energy-momentum that mimic dark matter, and therefore be consistent with the data.

]]> By: Shantanu http://blogs.discovermagazine.com/cosmicvariance/2008/08/28/seeing-the-sky-with-different-eyes/#comment-42370 Shantanu Mon, 01 Sep 2008 02:53:13 +0000 http://blogs.discovermagazine.com/cosmicvariance/2008/08/28/seeing-the-sky-with-different-eyes/#comment-42370 Sean, does the bullet cluster rule out Luc Blanchet's explanation of MOND as advocated in <a href="http://arxiv.org/abs/0807.1200" rel="nofollow">this and references in</a> Sean, does the bullet cluster rule out Luc Blanchet’s explanation of MOND
as advocated in this and references
in

]]> By: Rich http://blogs.discovermagazine.com/cosmicvariance/2008/08/28/seeing-the-sky-with-different-eyes/#comment-42363 Rich Sun, 31 Aug 2008 11:26:35 +0000 http://blogs.discovermagazine.com/cosmicvariance/2008/08/28/seeing-the-sky-with-different-eyes/#comment-42363 Galactic foregrounds are becoming more and more important to understanding how to interpret various astronomical data. It's really striking how areas of CMB astrophysics, such as polarisation and the search for primordial non-Gaussianity, are becoming limited by our understanding of the effects of the foregrounds. Fermi might well be a strong driver for trying to develop better models of the "known" physics that gives us gamma ray emissions in the galactic place, so we can distinguish between that and new effects (eg. dark matter decays). Galactic foregrounds are becoming more and more important to understanding how to interpret various astronomical data. It’s really striking how areas of CMB astrophysics, such as polarisation and the search for primordial non-Gaussianity, are becoming limited by our understanding of the effects of the foregrounds.

Fermi might well be a strong driver for trying to develop better models of the “known” physics that gives us gamma ray emissions in the galactic place, so we can distinguish between that and new effects (eg. dark matter decays).

]]> By: Lawrence B. Crowell http://blogs.discovermagazine.com/cosmicvariance/2008/08/28/seeing-the-sky-with-different-eyes/#comment-42369 Lawrence B. Crowell Sat, 30 Aug 2008 13:23:08 +0000 http://blogs.discovermagazine.com/cosmicvariance/2008/08/28/seeing-the-sky-with-different-eyes/#comment-42369 Most of the gamma rays in the universe, and the galactic center, are from e-e^+ annihilations and related processes. If dark matter is associated with neutrolinos, the gamma rays might be far upscale in energy from there in the multi TeV range. Another candidate for dark matter is strangelet matter, but that might be completely stable. It is not clear whether this is something we should hold our breath over. Lawrence B. Crowell Most of the gamma rays in the universe, and the galactic center, are from e-e^+ annihilations and related processes. If dark matter is associated with neutrolinos, the gamma rays might be far upscale in energy from there in the multi TeV range. Another candidate for dark matter is strangelet matter, but that might be completely stable.

It is not clear whether this is something we should hold our breath over.

Lawrence B. Crowell

]]> By: Gabe http://blogs.discovermagazine.com/cosmicvariance/2008/08/28/seeing-the-sky-with-different-eyes/#comment-42373 Gabe Sat, 30 Aug 2008 00:56:57 +0000 http://blogs.discovermagazine.com/cosmicvariance/2008/08/28/seeing-the-sky-with-different-eyes/#comment-42373 Lawrence, In fact it is expected that the DM density near the center of the galaxy is much higher than in the rest of the halo. In the Hooper, Finkbeiner and Dobler paper that Sean linked to, one of the first things they do is constrain the halo profile that the Haze would require. The "cusped" profile necessary falls roughly between the Navarro-Frenk-White and the Moore et al. N-body simulation profiles. So the Haze doesn't require a shocking distribution of DM. The higher density (and thus higher annihilation rate) is also why there are other experiments trying to detect gamma rays and other particles produced by DM annihilations in the galactic center. Lawrence,

In fact it is expected that the DM density near the center of the galaxy is much higher than in the rest of the halo. In the Hooper, Finkbeiner and Dobler paper that Sean linked to, one of the first things they do is constrain the halo profile that the Haze would require. The “cusped” profile necessary falls roughly between the Navarro-Frenk-White and the Moore et al. N-body simulation profiles. So the Haze doesn’t require a shocking distribution of DM.
The higher density (and thus higher annihilation rate) is also why there are other experiments trying to detect gamma rays and other particles produced by DM annihilations in the galactic center.

]]> By: Lawrence B. Crowell http://blogs.discovermagazine.com/cosmicvariance/2008/08/28/seeing-the-sky-with-different-eyes/#comment-42368 Lawrence B. Crowell Fri, 29 Aug 2008 22:55:36 +0000 http://blogs.discovermagazine.com/cosmicvariance/2008/08/28/seeing-the-sky-with-different-eyes/#comment-42368 As much dark matter exists in galactic halos we might question why dark matter would annihilate near the galactic centers and not in the halo. I suspect that this galactic central glow is not related to dark matter. Lawrence B. Crowell As much dark matter exists in galactic halos we might question why dark matter would annihilate near the galactic centers and not in the halo. I suspect that this galactic central glow is not related to dark matter.

Lawrence B. Crowell

]]> By: Eric Habegger http://blogs.discovermagazine.com/cosmicvariance/2008/08/28/seeing-the-sky-with-different-eyes/#comment-42372 Eric Habegger Fri, 29 Aug 2008 04:05:22 +0000 http://blogs.discovermagazine.com/cosmicvariance/2008/08/28/seeing-the-sky-with-different-eyes/#comment-42372 I think this new data is very interesting but it is better not to superimpose one's own thought processes on that data. You said "Dark Matter anhillilates - so this story goes." That is a very big jump and depends on the usual quantum mechanical machinations that are not really rigorous in bright sunlight. It is only one interpretation that many people, including myself, don't agree with. Let people enjoy the data unfiltered. I think this new data is very interesting but it is better not to superimpose one’s own thought processes on that data. You said “Dark Matter anhillilates - so this story goes.”

That is a very big jump and depends on the usual quantum mechanical machinations that are not really rigorous in bright sunlight. It is only one interpretation that many people, including myself, don’t agree with. Let people enjoy the data unfiltered.

]]>