## Evidence of cosmological extra dimensions? February 25, 2009

Posted by keithkchan in Journal club.

Yesterday, we had the journal club. It was my turn to present a paper. I would like to summarize the paper I discuss here.

The paper I chose was a recent paper arXiv:0812.2244 , which claims that there might be evidences of cosmological extra dimension. The authors presented several evidences which seems to be at odd with the standard model LCDM. The evidences are:
1. the cross power spectrum between the galaxy and CMB, the power is larger than the expected value by $2 \sigma$
2. existence of large scale bulk flow which is also larger than expected by $2 \sigma$
3. Cosmic Background Imager measure the power in CMB larger than expected by 35% at l~3000
4. Lyman-alpha forest power spectrum, which probe the structure around z~3 find that the the power is larger by 35%
5. The CMB correlation at angular scale larger than 60 degree is much smaller than expected. This corresponds to the quadrupole in the CMB power spectrum. People have parametrized the unnaturalness of the smallness in many ways. If it is due to statistical fluctuation, the chance of having such a small value is less than 0.03%.

Evidences 1 to 4 require that the structure in the large scales are more evolved than expected from LCDM model, while evidence 5 usually mean that the evolution cannot be too large. The authors fit the data using the a phenomenological generalization of the DGP model. In their model, the number of extra dimensions is more than 1. In this kind of model, our 3-brane embeds in more than one brane. e.g. our 3-brane embeds in some 4-brane, which is again embedded in 5-brane. This kind of construction seems to avoid the ghost instability that kills DGP. In intermediate scales the extra scalar degree of freedom can mediate extra force and enhances the gravitational strength. They tune some free parameter so that the Sachs-Wolf effect cancels the Integrated Sachs-Wolf effect. It improves the fitting to CMB a little bit, so that the problem 5 is alleviated a little bit. Their model also seem to alleviate the tension between the theory and observations for problem 1 to 4.

Although I am not very familiar with most of these observations, my impression is that the potential systematics in these observations are rather large. They may be rather nonlinear, difficult to model, e.g. evidence 3. The differences from LCDM by 2 $2 \sigma$ is not so significant. Among all the evidences, I take No. 5 most seriously. But their model can only improve it a little bit. Also their calculations are rather crude. I don’t really believe that there are already evidences for cosmological extra dimensions.

On the other hand it is interesting to note that this model seems to alleviate the tension between LCDM and data in several different observations. It is useful to keep this in mind to see what happens when the measurements improve.