Comments on: Avignon Day 1: Calculating Non-Gaussianities

By: Sean

Sean — Tue, 26 Apr 2011 01:16:08 +0000

Oops, you’re certainly right. Fixed it now. Can I blame jet lag?

By: A

Mon, 25 Apr 2011 23:59:43 +0000

Sean, don’t you mean that *scale* invariance implies that F only depends on the ratios k2/k1 and k3/k1? In other words: k2/k1 and k3/k1 are the only quantities that matter for F if and only if the shape of the triangles — and not the overall scale of the triangles — is what matters. Rotational invariance won’t get you that; but you do need rotational invariance to be able to say that only the magnitudes of k1, k2, and k3 matter.

I could be wrong here, but I’ve done some work in this area and I think this is true.

By: Avignon Day 4: Dark Matter | Cosmic Variance | Discover Magazine

Avignon Day 4: Dark Matter | Cosmic Variance | Discover Magazine — Fri, 22 Apr 2011 09:28:16 +0000

[...] few people have strong feelings about non-gaussianities or neutrinos, but many people have strong feelings about reductionism. Quelle surprise! [...]

By: Eugene

Eugene — Wed, 20 Apr 2011 17:59:23 +0000

(I should add that thinking about things in terms of an EFT is an awesome organizing principle, but it is not the only organizing principle out there.)

By: Eugene

Eugene — Wed, 20 Apr 2011 17:36:30 +0000

Hi Sean,

I suppose I should wade in here and say a little bit more about calculating non-Gaussianities. This is not meant as a criticism of the EFT method (which I think is great), but just to address some fairly important but rather technical issues.

Technically, there are some limitations on the EFT method — the original formalism works for a single adiabatic d.o.f., and for strictly slow-roll super smooth type inflation. The former means that you only have a single scalar field (Leonardo + friends have an extension to multifields, but there are extra rules on the theory space you need to impose to make those work in the EFT, which weakens its generality), the latter mean that you have drop all the H”, H”’, H”” etc terms (these are not mere nuisance, as large classes of popular models generates observable nonG use those terms — disclosure is that I wrote them but I think it’s fair to it is one of the four general main mechanisms usually known as “breaking slow roll”). Relaxing these conditions open up the full spectrum of possible nonGaussianities beyond those few “shapes” that EFT has access to. (Daniel probably can say more about additional shapes ala EFT!)

Philosophically, the EFT “gives up”on knowing what the background dynamics are (it is an expansion around some fixed background dynamic). I don’t think there is anything wrong with this approach, however, but I think it’s fair to say that sometimes you do want to know what your background field theory is…

Eugene

By: Albert Zweistein

Albert Zweistein — Wed, 20 Apr 2011 02:10:07 +0000

Ah yes, those pesky over-idealizations of cosmology. They were introduced with such good intentions and led to such good initial progress in the early 20th century, only to come back and bite us in the hinder now that we, well most of us, have entered the 21st century.

Exact homogeneity – I don’t think so.

Exact isotropy – I don’t think so.

Absolute acausal beginning of space-time – I don’t think so.

Strict reductionism – I don’t think so.

We need to move beyond the over-simplifications that made the math easier, and seriously explore models more like Szekeres’ inhomogeneous models that do not assume the idealized symmetries.

Moreover, unbiased observational testing should guide the theoretical loose cannons who have run so amuck in recent decades.

de Vaucouleurs was right on and quite prescient when he wrote “The Case For A Hierarchical Cosmology” way back in the 70s.

Albert Z

By: AnotherSean

AnotherSean — Tue, 19 Apr 2011 16:04:35 +0000

Sounds like a great conference. Put in a good word to the Pope for us.

By: cecil kirksey

cecil kirksey — Tue, 19 Apr 2011 12:37:13 +0000

Sean:
Yes I believe I understand the assumption of the “infinite ” harmonic oscillator. But is that physically realistic? Think of the predicted value of the cosmological constant based on this idea.

By: Sean

Sean — Tue, 19 Apr 2011 12:33:50 +0000

Daniel– Arrgh, thanks. I got it right one out of two times, so it was clearly a quantum fluctuation. I blame jet lag.

cecil– The fluctuations originate as quantum fluctuations in the vacuum, which are Gaussian in the limit of free field theory. That’s just the lowest-energy state; each mode is like a harmonic oscillator.

By: cecil kirksey

cecil kirksey — Tue, 19 Apr 2011 12:17:40 +0000

Can someone explain how exactly does a physical theory predict that a measured variable will have exactly a Gaussian distribution other than the central limit theorem which is a mathematical result not a physical result?

By: Daniel Baumann

Daniel Baumann — Tue, 19 Apr 2011 11:17:09 +0000

Hi Sean, I think you mean Leonardo Senatore, although Salvatore sounds very Italian too Unfortunately, I couldn’t make it to the meeting, but I look forward to hearing more about it.

By: Bee

Bee — Tue, 19 Apr 2011 11:12:47 +0000

Yes, the non-gaussianity is getting a lot of attention recently. I wrote a (quite general) post on that some months back.