A Spot of Hype
A few weeks ago a paper came out in Monthly Notices of the Royal Astronomical Society (accompanied by a press release from the Royal Astronomical Society) about a possible explanation for the now-famous cold spot in the cosmic microwave background sky that I’ve blogged about on a number of occasions:
If the standard model of cosmology is correct then a spot as cold as this and as large as this is quite a rare event, occurring only about 1% of the time in sky patterns simulated using the model assumptions. One possible explanation of this ( which I’ve discussed before) is that this feature is generated not by density fluctuations in the primordial plasma (which are thought to cause the variation of temperature of the cosmic microwave background across the sky), but by something much more recent in the evolution of the Universe, namely a local large void in the matter distribution which would cause a temperature fluctuation by the Sachs-Wolfe Effect.
The latest paper by Mackenzie et al. (which can be found on the arXiv here) pours enough cold water on that explanation to drown it completely and wash away the corpse. A detailed survey of the galaxy distribution in the direction of the cold spot shows no evidence for an under-density deep enough to affect the CMB. But if the cold spot is not caused by a supervoid, what is it caused by?
Right at the end of the paper the authors discuss a few alternatives, some of them invoking `exotic’ physics early in the Universe’s history. One such possibility arises if we live in an inflationary Universe in which our observable universe is just one of a (perhaps infinite) collection of bubble-like domains which are now causally disconnected. If our bubble collided with another bubble early on then it might distort the cosmic microwave background in our bubble, in much the same way that a collision with another car might damage your car’s bodywork.
For the record I’ve always found this explanation completely implausible. A simple energy argument suggests that if such a collision were to occur between two inflationary bubbles, it is much more likely to involve their mutual destruction than a small dint. In other words, both cars would be written off.
Nevertheless, the press have seized on this possible explanation, got hold of the wrong end of the stick and proceeded to beat about the bush with it. See, for example, the Independent headline: `Mysterious ‘cold spot’ in space could be proof of a parallel universe, scientists say’.
No. Actually, scientists don’t say that. In particular, the authors of the paper don’t say it either. In fact they don’t mention `proof’ at all. It’s pure hype by the journalists. I don’t blame Mackenzie et al, nor the RAS Press team. It’s just silly reporting.
Anyway, I’m sure I can hear you asking what I think is the origin of the cold spot. Well, the simple answer is that I don’t know for sure. The more complicated answer is that I strongly suspect that at least part of the explanation for why this patch of sky looks as cold as it does is tied up with another anomalous feature of the CMB, i.e. the hemispherical power asymmetry.
In the standard cosmological model the CMB fluctuations are statistically isotropic, which means the variance is the same everywhere on the sky. In observed maps of the microwave background, however, there is a slight but statistically significant variation of the variance, in such a way that the half of the sky that includes the cold spot has larger variance than the opposite half.
My suspicion is that the hemispherical power asymmetry is either an instrumental artifact (i.e. a systematic of the measurement) or is generated by improper substraction of foreground signals (from our galaxy or even from within the Solar system). Whatever causes it, this effect could well modulate the CMB temperature in such a way that it makes the cold spot look more impressive than it actually is. It seems to me that the cold spot could be perfectly consistent with the standard model if this hemispherical anomaly is taken into account. This may not be `exotic’ or `exciting’ or feed the current fetish for the multiverse, but I think it’s the simplest and most probable explanation.
Call me old-fashioned.
P.S. You might like to read this article by Alfredo Carpineti which is similarly sceptical!
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In the Dark 
May 19, 2017 at 6:24 pm
Thanks Peter – your account is probably fairer than we deserve. The one thing we are certain about is Cold Spot is not caused by a void. That leaves the 1-in-50 chance that its a statistical fluctuation in standard model or something more exotic. If its the latter then the bubble collision possibility has to be mentioned because the multiverse is now claimed to be inevitable in standard inflation. The other possible explanations mentioned eg by Planck team all involve “non-standard” inflation. So that’s the case for prioritising bubble universe collision of the more exotic possibilities – its more “standard”.
Sabine Hossenfelder also suggested that bubble universes wouldnt survive collision but maybe other theorists could comment.
May 19, 2017 at 11:17 pm
Can’t win – I’ve always liked standard model inflation and standard model inflation predicts the multiverse. So why imply I am biased against standard model in this particular instance? Genuinely don’t understand!
May 20, 2017 at 11:01 am
I suppose the answer to that question depends on how many anomalies we’ve tried to find with a 1-in-50 shot at showing up?
Unlike many areas of interest, the CMB data has been studied to the point of torture. I wouldn’t be surprised if it confessed something like this eventually.
May 22, 2017 at 9:57 am
If you believe in the multiverse, is there any observation that could possibly be made that would make you stop believing in it?
May 19, 2017 at 9:14 pm
Something important is missing in the standard model of cosmology. The organisational structure of what we observe, including all scales, can only fit the standard model by fixing arbitrarily many parameters, themselves fitted by observations. How these parameters have been fixed? Fixed forever in a timeless universe? And now that we have discovered Dark Energy and Dark Matter, isn’t it too strange and unwelcome to keep the standard model? I am very skeptical and unsatisfied. I think parallel universes are ridiculous but I think matter with different sets of parameters and all having gravity in common (or a few common parameters) is likely to be the solution. There should be some signs of it, besides dark energy and dark matter. Forgetting Inflation brings more substance to the table. Flatness, horizon, etc…
But I know I should take my shower colder, just too get use too it…
Skeptical of skeptics and believers at the same time, even though I’m one of them!
May 20, 2017 at 1:54 pm
“If the standard model of cosmology is correct then a spot as cold as this and as large as this is quite a rare event, occurring only about 1% of the time in sky patterns simulated using the model assumptions”
This is not correct. In fact spots as cold and as large as this are very common, occurring in roughly 2 out of 3 simulated universes. My colleagues and I pointed this out three years ago, in https://arxiv.org/pdf/1408.4720.pdf, see Figure 6. You can see the same plot reproduced in Mackenzie et al’s paper, Figure 8. It was also recently reproduced in a paper by Ofer Lahav and his student.
Tom Shanks then says there is “a 1-in-50 chance that it is a statistical fluctuation”. This number doesn’t refer to the size or coldness of the spot, but to a measure of how closely its temperature profile happens to resemble the shape of a special filter. The shape of the filter is basically chosen to match the shape of the observed cold spot profile, so this is obviously circular reasoning: in other words, the 1-in-50 number suffers from a terrible a posteriori bias and is thus essentially meaningless.
This point was made back in 2009 (8 years ago!) in a paper by Zhang and Huterer. Accounting for the a posteriori bias by using different filters reduces the 1-in-50 chances to 1-in-2-or-3. (Shanks must know this, since his paper mentions it explicitly in the introduction.)
Given these numbers I have to say I am puzzled that people think the cold spot is interesting at all, much less that it is “evidence” for exotic multiverses.
May 20, 2017 at 2:00 pm
I do agree with Shanks’ statement that we are certain the cold spot is not caused by a void. But I don’t think this is a particularly new insight, as we made most of the same arguments in our paper three years ago – including pointing out that there are several equally large or larger voids elsewhere in the sky, but only one supposed cold spot!
May 20, 2017 at 2:21 pm
I agree that I could have been more precise in my wording, but to quote from your own paper “..in agreement with the Planck analysis [7], we find that the real Cold Spot is still unusual at the ∼ 3σ level in ΛCDM, in that fewer than 1% of the coldest spots in random maps have as cold a total filtered temperature.”
May 20, 2017 at 2:36 pm
Yes, given the choice of filter we reproduce the Planck numbers. The sentence immediately following the one you quoted explains exactly why this depends on the choice of filter.
We also say:
“we identify the importance of a posteriori selection effects in the identification of the Cold Spot” (in the abstract), and
“at its centre our Cold Spot is well within the expected range of temperatures of the coldest spots on random CMB maps” (in the conclusions)
May 20, 2017 at 2:27 pm
One final thing: you are very generous when you say you don’t blame the authors or the RAS. Because when I look at the RAS press release, I see the headline which mentions “ancient origin for the Cold Spot” and I see a quote from Tom Shanks, which says the the Cold Spot could have been “caused by a collision between our universe and another bubble universe” and that it “might be taken as the first evidence for the multiverse”.
I don’t blame journalists for this at all.
May 20, 2017 at 3:00 pm
The “look elsewhere” effect is always tricky to quantify and, as you say, we go into this issue in the paper. But I agree the 1-in-50 is based on the cold spot profile as well as the minimum temperature reached. The Planck team’s 1-in-50 does take some account of “look elsewhere” but as I said its not easy to quantify, View we took was that given the large numbers of previous papers on the void explanation, having ruled it out, it would be wrong not to mention the other interpretations, assuming it isnt a statistical fluctuation. Basically the 1-in-50 statistical chance has to be balanced against the importance of the possible evidence for the multiverse. And we always said that this interpretation would have to be confirmed by CMB polarisation results before it could be taken seriously.
Note that the “ancient” in the press release title covers the 1-in-50 chance too.
Finally, I also dont blame the journalists nor the RAS press office. I/we take full responsibility. To some extent we “benefitted” from the Guardian connecting our paper to the argument over inflation in Scientific American. And at the very least it then opened up the issue of the testability of things like inflation and the multiverse to the general public. So actually I believe that the journalists did a good job, e.g. Stuart Clark in the Guardian, to whom we express our thanks.
May 20, 2017 at 3:09 pm
I thought your paper handled the issues well and fairly. I thought the wording of the press release was practically inviting the kind of silly speculation that we have in fact seen. My personal opinion is also that numbers that are hard to correctly quantify should then not be misleadingly quantified in press releases.
May 20, 2017 at 3:23 pm
In a perfect world i would agree but there is a tension that the press officers have to square between accuracy and accessibility. I think in our case the RAS press office just about got the balance right.
May 22, 2017 at 1:29 pm
They seek it here, they seek it there,
some cosmologists seek it everywhere.
Is it a bless, is it a curse?,
that damned elusive multiverse.
May 22, 2017 at 2:24 pm
In the dark?
Not for Chance. He would say “I like to watch”.