“Silently, one by one, in the infinite meadows of heaven,
Blossomed the lovely stars, the forget-me-nots of the angels.” -Longfellow
The cosmic microwave background is a thing of beauty, as not only does its uniform, cold temperature reveal a hot, dense past that began with the hot Big Bang, but its fluctuations reveal a pattern of overdensities and underdensities in the very early stages of the Universe.
It's fluctuations just like these that give rise to the stars, galaxies, groups and clusters that exist today, as well as the voids in the vast cosmic web. But effects at the surface of last scattering are not the only ones that affect the CMB's temperature; if we want to make sure we've got an accurate map of what the Universe was born with, we have to take everything into account, including the effects of matter as it gravitationally grows and shrinks.
As we do exactly this, we find ourselves discovering the causes behind the biggest anomalies in the sky, and it turns out that the standard cosmological model can explain it all. Find out what it all means on this week's Ask Ethan!
- Log in to post comments
I quite like your blog, and I appreciate the lengths you go to to ensure that the science you present is correct. This is why I am disappointed to see that you are in this instance giving credence to ridiculous hype.
Let me summarise some facts regarding this supervoid "explanation" of the Cold Spot:
1. In November last year, a detailed study of the possible ISW effect due to exactly such a supervoid was published (by me and others) in Physical Review D, explicitly showing that this supervoid fails to account for more than 10% of the total temperature discrepancy at the centre of the Cold Spot unless all of GR is wrong. This reference is here: http://adsabs.harvard.edu/abs/2014PhRvD..90j3510N
2. We also showed that in order to explain all of temperature discrepancy at the centre of the Cold Spot one would require a void that is so large and so empty that the chance of it existing in our Universe would be less than 1 in 1 million. The comparable probability of the Cold Spot just being a random fluctuation that requires no special explanation is roughly 1 in 1000, even according to the most pessimistic estimates (and likely much larger).
3. The Cold Spot is not unusually cold at the centre. If one selects the coldest spot in the CMB map, naturally it is cold compared to the average point in the same map. This is just a selection effect. The question is whether it is cold compared to the coldest spot in CMB maps in alternative versions of the Universe. The answer is it is not: 100% of the coldness of the Cold Spot at the centre can be explained by the selection effect (Figure 6 in the paper above), compared to just 10% from the supervoid.
4. What is unusual about the Cold Spot is that there is a hot ring surrounding the central cold region, which does not happen in random CMB maps. The supervoid in no way accounts for this, because supervoids do not produce hot rings, unless all of GR is wrong.
5. This "supervoid" is not particularly super. It's rare, but not so very rare: theory predicts roughly 20 or so such voids should exist in our local Universe, and simulations agree. Several such voids have already been seen in other parts of the sky, some of which are larger than this "supervoid" and all of which are emptier. Yet we do not see 20 Cold Spots! Can it be that GR fails only in one region of the sky? Or is it more likely that the supervoid and the Cold Spot are unrelated?
To be honest the most disappointing thing about this whole episode is that these results were published months in advance of the current press release, and the authors of this paper and the press release are well aware of them. I'm sorry to say that in my opinion the fact that they went ahead with demonstrably incorrect and over-hyped claims reflects very poorly on them. Publicity in news media who know nothing about physics is one thing, but it is much more worrying that such pseudo-scientific claims receive prominence on your own well-informed blog too.
"Let me summarise some facts "
Sorry, need more than claim to fact for it to be taken a fact.
@Wow #2: The author of #1 was not making unsupported claims. Unfortunately, the link to his as-published PRD abstract was truncated. However, arXiv has the article (as a trivial search on the as-posted name revealed), and not behind a paywall: http://arxiv.org/abs/1408.4720.
You can certainly argue whether the paper is correct or not, but you'll need to hold yourself to a higher standard than we usually do with "cranks." This is a quite properly peer-reviewed article in Physical Review D. If you can find fault with it, then you're going to need to present us with similarly peer-reviewed, citable evidence to support your own claims.
One interesting point, noted in the paper, is that during this author's writing an entirely independent analysis of the Cold Spot was also pre-printed to arXiv, with broadly similar conclusions. To my mind, two independent studies reaching the same conclusion adds weight to the argument.
@Michael Kelsey: Thanks for making my points for me. And yes, Jim Zibin's work reaches the same conclusions as ours.
Actually the web address I provided is correct and leads to the NASA ADS entry, but as it contains a .. the link got incorrectly terminated. Copy and paste should work.
"The author of #1 was not making unsupported claims"
The claim of fact was a crank call, Michael. The lack of valid support within the text and a link to arxiv did not counter that and the lack of support remains an issue.
It's quite arrogant to claim "let me tell you some facts" as they did to Ethan who isn't a neophyte. It's basically a dick move.
There was NO NEED WHATSOEVER to go and start off like that.
If the post had been truncated at the beginning to exclude that, the assessment of "He's a dick" would be unjustified.
As it is, they are a dick as far as I'm concerned.
I don't understand - all of the 5 points listed are facts. It is a fact that we published a paper last November. It is a fact that this paper contained a proof of each one of the other statements. It is also a fact that the same conclusions were independently reached by another paper at around the same time.
Despite not being a neophyte, Ethan was clearly not aware of these facts, otherwise being a responsible science communicator he would probably have mentioned them. (If he was aware of the fact of the existence of our paper but disagreed with the contents, I'd like to hear why, and so would PRD I'm sure.)
I'm really not sure what you mean by a "lack of valid support within the text." Our PRD paper is more than 13 pages of double-column text, equations and figures, constituting plenty of valid support. Do you expect me to reproduce all of that in a comment on a blog?
"I don’t understand"
"all of the 5 points listed are facts."
And it's a fact you're a dick. HOWEVER, this still makes you think "Well, Wow really shouldn't say that, it's just *rude*!"
What ISN'T fact are:
"I am disappointed" Opinion.
"giving credence to ridiculous hype." Opinion, again.
"What is unusual about the Cold Spot is that there is a hot ring surrounding the central cold region," Nope. Because it's not unusual for the same reasons you placed the other claims there: ANY cold spot, if it appears, will appear to have a hot ring around it, though the variation of the values and the levels you decided (again, opinion) to use will either make those rings appear or disappear.
ANY minima will have higher values around it. If the minima doesn't have much extent, then it will be annular shaped. If the scale at which you produce the same colour and the local variations are roughly concordant, there will appear to be a ring around SOME of these local minima, in most cases, random chance indicates that they would be quite random in shape. But we look at the shapes and make them. We will spot a pattern, even if one doesn't exist, because that's how our visual system works.
"and the authors of this paper and the press release are well aware of them."
Maybe they thought that it was credulous alarmist rubbish, like you did of their paper?
Thanks for your comment. I was only aware of a portion of this, and I'll be happy to highlight (and expound further) on this in the comments of the week coming up on Saturday.
By the way, Syksy Rasanen, coauthor of yours, is someone with whose work I'm intimately familiar. We were coming up and working on related topics in the mid-2000s, so we've crossed paths professionally, but not personally.
Hi Ethan - Cheers, look forward to seeing it. Syksy still works on inhomogeneities and backreaction. He also writes a blog with a similar purpose to yours, but in Finnish only!
Wow: Perhaps part of your annoyance is because you think that I think that the Cold Spot is truly unusual and inexplicable. Actually, I don't - my position on this is very much the same as Zhang and Huterer's from 2009, i.e. that the Cold Spot only looks unusual under a very particular method of analysis. We say precisely this in our paper. This seems to also be what you think, so don't get so annoyed.
However the interesting thing is that, even under this particular method of analysis, the reason the Cold Spot looks unusual is because of the hot ring surrounding it. The coldness at the centre on its own would not make it unusual under any method of analysis. All these details are in our paper - please read it, you might even enjoy it. I'm afraid I can't post figures here, but if you'd like to see them, they're in the paper.
It would be quite funny if Szapudi et al did indeed think our paper were credulous and alarmist, since they're the ones postulating an effect ten times larger than permitted by known physics while we're very much in line with orthodox LCDM. But stranger things have happened!
"Wow: Perhaps part of your annoyance is because you think that I think that the Cold Spot is truly unusual and inexplicable. "
No, it isn't.
It's from this: "This is why I am disappointed to see that you are in this instance giving credence to ridiculous hype." followed by this “Let me summarise some facts ”
But you don't get it.
Because you don't want to.
And now you flail around for some other reason that takes it into my fault so you can ignore what you "don't get".
Perhaps your problem is that you've got a postgraduate qualification and thing that this means you should be allowed to be a dick
Or perhaps I made that up so as to illustrate why your claim there was, at best, ill-advised.
Here is my follow-up.
Let me know if I misinterpreted anything from your paper.