Archive for Hemispherical Power Asymmetry

A Spot of Hype

Posted in Astrohype, The Universe and Stuff with tags , , on May 19, 2017 by telescoper

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!