[astro-ph/0610298] A New Force in the Dark Sector?

Authors:  Glennys R. Farrar, Rachel A. Rosen
Abstract:  We study the kinematics of dark matter using the massive cluster of galaxies 1E0657-56. The velocity of the "bullet" subcluster has been measured by X-ray emission from the shock front, and the masses and separation of the main and sub-clusters have been measured by gravitational lensing. The velocity with gravity alone is calculated in a variety of models of the initial conditions, mass distribution and accretion history; it is much higher than expected, by at least 2.4 sigma. The probability of so large a subcluster velocity in cosmological simulations is <~ 10^{-7}. A long range force with strength ~ 0.4 - 0.8 times that of gravity would provide the needed additional acceleration.
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Simon DeDeo
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[astro-ph/0610298] A New Force in the Dark Sector?

Post by Simon DeDeo » October 12 2006

What do folks think of this?

Is the bullet of the bullet cluster moving too fast?

The idea is that there is some superlight (but not massless) particle that mediates an attractive interaction between dark matter particles and gives an extra oomph to dark matter interactions on scales smaller than tens of Mpc.

I wonder if the expansion history of the universe can be a constraint -- I guess when the universe is small enough for the Yukawa potential to become important, things are *just about* becoming matter dominated. Would be interesting to check if it all squares out.

Tommy Anderberg
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Re: [astro-ph/0610298] A New Force in the Dark Sector?

Post by Tommy Anderberg » October 12 2006

Simon DeDeo wrote:The idea is that there is some superlight (but not massless) particle that mediates an attractive interaction between dark matter particles and gives an extra oomph to dark matter interactions on scales smaller than tens of Mpc.
Oh, so now it's either modified gravity that requires dark matter (TeVeS + 2 eV neutrinos) or dark matter that requires modified gravity?

Got to love that bullet cluster.

Seriously though, if the MOG people could fit the bullet's observed velocity with a lower initial one, that would look better (to me at least) than having to add a fifth force to dark matter. Or it could turn out that they get an even more absurd initial velocity (but intuitively, MOND-style modifications should work in their favor here). So this is an interesting new take for sure.

Pier Stefano Corasaniti
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[astro-ph/0610298] A New Force in the Dark Sector?

Post by Pier Stefano Corasaniti » October 12 2006

Well people has been looking at models of interacting "scalar field dark energy"-"dark matter " since a long while now. On my side I have worked in collaboration with Justin Khoury and Subinoy Das (astro-ph/0510628) on one such a model which can mimic a phantom cosmology, hence reproduce w<-1 without violating the WEC. This is achieved by having a coupling \beta~O(1) between a scalar field dark energy and dark matter. We have found a solution which does not spoil the structure formation at early times, so no sign on the large scale of the CMB, however the field modifies the clustering on cluster and subcluster scales at late time, since DM particles besides feeling the standard tensor gravity feel also a scalar force and therefore the Newton constant for them is G(1+2\beta^2) , hence bigger than standard newtonian gravity. On the other hand the baryons are uncoupled and follow standard gravity...so I am very interested by the findings of this paper..I ll read it very carefully in the next few days..

Su Meng
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[astro-ph/0610298] A New Force in the Dark Sector?

Post by Su Meng » October 12 2006

This is quite an interesting paper! The authors gave a comprehensive study on many astrophyiscal possibilities and rule out them at least about two sigma. Before we could draw any conclusion by using this method to constrain the strength of fifth force, I think we should pay more attension to more "realistic" explanations. We need to check through simulations and more possible candidate clusters.

It is interesting to combine constraints on beta from methods of different scales. Kesden and Kamionkowski have systematically studied the sensitive method to constrain beta through baryon content in leading and trailing tidel tail. If we could find different need of beta in galaxy scale and cluster scale, that would be quite interesting! By reading the anterior part of this paper, it sounds like that we would have a more consistent cosmology by introducing fifth force:)

Maybe we should combine the ideas of new matter component and new force of nature.

Tommy Anderberg
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Re: [astro-ph/0610298] A New Force in the Dark Sector?

Post by Tommy Anderberg » October 13 2006

Su Meng wrote:By reading the anterior part of this paper, it sounds like that we would have a more consistent cosmology by introducing fifth force:)

Maybe we should combine the ideas of new matter component and new force of nature.
You know all this, so just a synthetic cautionary reminder: Epicycles. Add more degrees of freedom, get better fits. The risk: start out like a Ptolemy, end up like a Rube Goldberg.

Simon DeDeo
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[astro-ph/0610298] A New Force in the Dark Sector?

Post by Simon DeDeo » October 13 2006

We talked a little about this at lunch today, and some perceptive points were made by others -- in particular, that it is dangerous to think of this term as simply a modification of G when it comes to thinking on cosmological scales.

In particular, the two forces, while they may obey the 1/r^2 or e^{-r}/r^2 law in the flat space case (i.e., on scales much smaller than the Hubble radius), things get very tricky when it comes to trying to integrate this into cosmology.

The basic problem is that you are essentially filling the universe with a Yukawa charge. This is something Brisudova, Kinney and Woodard looked at in the isotropic case, for a slightly different system (where the charge is associated with a [vector] *gauge* boson and is thus repulsive.) hep-ph/0110174 The whole thing is not as simple as it seems!

I would still be interested to hear from people who have more familiarity with the *astrophysics* behind all of this stuff -- how reliable, e.g., is the determination of the velocity from the shock front properties? Is there a way to get an anomalously high estimate out, perhaps in the presence of magnetic fields?

Niayesh Afshordi
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[astro-ph/0610298] A New Force in the Dark Sector?

Post by Niayesh Afshordi » October 13 2006

We also talked about it at tea today. Apparently the mass of the bullet cluster is not as well-determined as it appears from the paper, and even strong and weak lensing masses have a large discrepancy.

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