Tuesday, May 5, 2015

Mysterious Cold Spot in the CMB: Still a mystery

Summary: A research group has recently suggested that a supervoid can explain the Cold Spot in the CMB. The problem is that a supervoid (via the ISW effect) can't explain the actual Planck TT data.

There has been a lot of attention over the last decade to a particularly large Cold Spot in the CMB, as seen both by WMAP and Planck (Image from this article.) Though, the Cold Spot is somewhat hard to see in the Planck data without a circle around it because there are so many "large-scale cold spots." The mystery behind the famed Cold Spot in the CMB is that the cold region is surrounded by a relatively hot region, and there is a difference of ~ 70 µK between the core of the cold spot and the surround region. Typical variations between locations these small is only 18 µK.

The two images directly above are from Planck 2015 results. The top of these two figures is the poliarization data, and the bottom of the top is the temperature data. Note that the scale goes from -300 µK to +300 µK.

While this new finding of a massive, supervoid of galaxies in the region near the Cold Spot is interesting, it should and has already been be pointed out that such a supervoid can't explain the ∼ -100 µK cold spot in the CMB via the standard ISW effect. As stated in the article "Can a supervoid explaint he Cold Spot?" by Nadathur et al., a supervoid is always disfavoured as an explanation compared with a random statistical fluctuation on the last scattering surface. There's just not enough of a void to explain the Cold Spot because the temperature would only be ∼  -20µK below the average temperature due to the late-time integrated Sachs-Wolfe effect (ISW.) Nadathur et al. state, "We have further shown that in order to produce ∆T ∼ −150 µK as seen at the Cold Spot location a void would need to be so large and so empty that within the standard ΛCDM framework the probability of its existence is essentially zero." The main argument against the supervoid-only explanation can be seen in the Figure by Seth Nadathur on his blog post regarding the paper he first-authored on this topic.

Here's the main problem with this supervoid-only explanation:   there's no lack of dark matter in the vicinity of the supervoid. So, while there is likely a supervoid of galaxies, there's no supervoid of dark matter. This means that there's no way to obtain even a - 20 µK cold spot in the CMB. As seen in the figure below (Fig.2.) from the paper by Mazotti  and Dodelson, there's no lack of gravitational lensing, and hence dark matter, where the cold spot (CS) is located. Mazotti  and Dodelson conclude that "There is no evidence, from the reconstructed ISW signal in the Cold Spot region, for an entirely ISW origin of this large scale anomaly in the CMB."

When Mazotti  and Dodelson take the data from Planck (lensing) and NVSS (galaxies), they can only "reconstruct" a cold spot in the CMB of ~ −8 µK. (See Fig.8. from their paper below)

A similar value of ~6-10 µK in the region of the CMB Cold Spot was obtained even more recently by Planck researchers in their paper Planck 2015 results. XXI. The integrated Sachs-Wolfe effect. One of the main figures from their paper is presented below. Note that they estimate a lot of cold spots of up to ~ −27 µK due to the integrated ISW effect; however, the famed Cold Spot is not one of these locations. There's a ISW effect in the entire area of the Cold Spot on the order of  −10 µK. This means that a supervoid (via the integrated ISW effect) can't be the explanation for the fame Cold Spot.

So, where does this leave us?
The authors of the new analysis of the supervoid put it this way, "While the existence of the supervoid and its expected effect on the CMB do not fully explain the Cold Spot, it is very unlikely that the supervoid and the Cold Spot at the same location are a coincidence."  Meaning, this study doesn't really explain the famed Cold Spot.

So, now, I'd like to speculate on one possible cause of the Famed Cold Spot.

In the traditional ISW effect, it is assumed that dark energy is constant. Therefore, there will be regions of the CMB that are colder than other regions because the photons first enter a region with relatively little dark matter (compared with other photons that enter regions of more dark matter.) As the photons descend into the dark matter region, those photons that enter dark-matter-scarce regions gain less energy than those that enter dark-matter-rich regions. After awhile and after dark energy stretches these regions, the photons leave the regions give back some of the energy they obtained while falling into the dark matter regions. However, the amount they give back is small compared with how much they obtained. Hence, those photons that first entered the dark-matter-rich regions are more energetic than those that first entered the dark-matter-rich regions.  (This is the traditional late-time ISW effect.)
But what if dark energy is not constant for all locations in space?
What if dark energy is really just the quantum degeneracy pressure of the light, active neutrinos? In particular, what if these neutrinos are made in the center of black holes when the black holes consume dark matter (i.e. sterile neutrinos)? What's interesting about black holes is that they could be locations where dark matter is converted into cold, light neutrinos. The dark matter falls into the center where the temperature can reach ~TeV, and the sterile neutrinos convert into active neutrinos. The light, active neutrinos can escape because the surface of the black hole radiates meV photons and neutrinos. [Note: there's a large increase in entropy when black holes consume keV (or greater) dark matter and turn it into meV neutrinos. And also note that black holes are well known to consume to lots of matter. The question really is whether black holes can consume dark matter.]

If black holes can convert dark matter into dark energy, you could have locations in the universe in which there is lots of dark matter (but if there's no galaxies and no black holes), then the dark matter can't convert into dark energy.
If there's a lack of dark energy in locations with lots of dark matter, then the photons would have to give back a lot of their energy, and be much colder locally than the photons of the surrounding regions that have black holes and hence dark energy.
This explanation takes into account both the lack of galaxies as well as the lack of any appreciable deficit of dark matter in the vicinity of the famed Cold Spot. However, it should be noted that I have not calculated or estimated how much of a temperature decrease would be expected from a region with a typical amount of dark matter, but with a lack of dark energy. If you know how to calculate this, please leave a comment in the comment section below.
I would be interested in knowing where the Mysterious Cold Spot could be explain by a lack of dark energy (due possibly to a lack of black holes that could turn the dark matter into dark energy.)

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