Brookhaven Forum 2013 Part III

First session after lunch, and we have a cosmology talk from Marc Kamionkowski.


Talk 5: 2pm: Marc Kamionkowski, "Fields, Fossils, Spins and Symmetry: New Physics from Cosmology"

That title is too long.  It messes up the formatting on my blog.

This is a seminar, so it's longer and involves more background.

Review:

• The CMB is a surface map.  One of the basic tools is an expansion in terms of spherical harmonics.
• The power spectrum converts the coefficients of this expansion into rotationally invariant quantities, and so is a true observable.
• Data today is so good that the oscillations in the power spectrum are unambiguous.  Even without a theoretical model, we would see their presence.

 The standard interpretation, followed here, is that the fluctuations in the CMB are the legacy of inflation.  There was a relatively recent paper on the health of inflation in light of Planck, that I want to talk about at a later date.  The most common view is that Planck not only supports inflation, but strongly restricts the possible models.  The paper I have in mind argues that the only models left actually struggle theoretically; they introduce problems at least as severe as the ones they were meant to solve.  But that's the topic of a later post.

Marc is emphasising the spectral index.  This parameter measures how the primordial power spectrum varies with scale; it is close to, but distinct from, strict scale invariance.  The departure has only been confirmed recently, and is expected from inflation.

Polarisation:

• A sign of gravity waves/gravitation perturbations
• A direct probe of the very early universe; t ~ t(Big Bang).  This is because the universe is effectively  transparent to gravity waves.
• Polarisation also sensitive to CP-violation, through E-B cross-correllation.  Arises for some generic evolving field coupling to the SM (photon).
• So it's a pity we haven't found them yet.

Non-Gaussianity

• A number of ways the inflationary (Gaussian) prediction can be modified.
• Gravitational lensing is one possibility, now observed by correlating with galactic surveys; and more recently just from the CMB itself.

Departures from (Statistical) Isotropy:

• Long an assumption, we can now actually test this hypothesis.
• Models exist which predict this; might be worth looking at some of these.
• Even simplest inflation models do this!  Albeit suppressed, of course, from the tensor-scalar-scalar coupling.
• A Power Asymmetry.  This seems to be what used to be called the cold spot.

Summary: this is a reasonable seminar, it's just all been a bit away from the stuff I'm really interested in.