The universe as seen by Planck - Days Three and Four II

[Continued from yesterday...]

In the first piece of this post I covered the implications of Planck for the paradigm of inflation. This piece covers the rest.

The anomalies

This is what the CMB would look like in an unphysical Bianchi universe. A worry for our physicality is that this unphysical Bianchi universe seems to fit the data better than a physical \(\Lambda\)CDM universe.

It would be impossible to provide an overview of this conference without mentioning the features and anomalies that Planck has chosen to draw significant attention to. I have a bunch of notes that I've written down that I might one day turn into a new blog post, but I'm not going to delve into them now.

These features and anomalies are clearly going to become a contentious issue in cosmology for the next few years. In fact, the words believer, atheist and agnostic were even being used by speakers during talks regarding whether the anomalies are real or statistical effects. Each time someone declared themselves an anomaly atheist or anomaly agnostic, someone in the audience inevitably spoke up and passionately defended the significance of the questioned anomaly.

The list of potential anomalies is long. There is the cold spot, the anomalously low quadrupole, the hemispherical asymmetry, the statistical difference between the odd and even multipoles at large scales, there is the dipole modulation, there is the general lack of power at large scales, there is the feature in the temperature power spectrum at small scales, the fact that the universe seems to be in an unphysical Bianchi model and there is the "axis of evil" (to name a few).

Pick your side. Atheist, believer or agnostic. The great anomaly wars of cosmology are about to begin (another inevitable consequence of an observational, rather than experimental science, I suppose - i.e. there is only a finite quantity of information available to us, so for some observables we can't just do the experiment again to check who is right).

What should we make of Planck vs SPT and Planck vs the local universe?

Why do galaxies rotate?

[Note from Shaun: The following is a guest post from cosmologist Bjoern Malte Schaefer, who works at the University of Heidelberg. He also writes at the blog, Cosmology Question of the Week, which, although aimed at undergraduate and postgraduate students of cosmology, is worth a look for everyone.]

Image illustrating why there is a relationship between angular momentum direction and inclination angle (and hence  the apparent shape of a galaxy)

Why do galaxies rotate?

Galaxies rotate, every child knows that. Looking at the images of grand spiral galaxies it is quite suggestive to think how all the stars and gas that make up a galaxy all move in a more or less orderly fashion about the galaxy's centre. However, when we think about mechanisms through which galaxies can acquire angular momentum the matter seems very obscure: how do they start rotating in the first place? 

The formation of cosmic structure, including galaxies and the larger clusters and superclusters in which they are embedded, is a fluid mechanical phenomenon, where gravity is the only force acting on the  distribution of matter on large scales. It is in fact gravity that amplified the tiny fluctuations present in the primordial distribution of matter that filled the early Universe (matter here means mostly cold dark matter) and caused them to evolve into the large-scale structure that we observe in the present Universe. These tiny fluctuations grew by self-gravity: a region in the matter distribution that is slightly denser than its surroundings generates a gravitational pull and accumulates more matter, hence its density increases with time.

At first sight it is very difficult to imagine how gravity could introduce rotation. After all, on the scales of cosmic structures, gravity is well approximated by the scalar Newtonian potential, which is parity invariant and does not possess any chirality: At which point would a galaxy in the forming decide whether it should rotate clockwise or counterclockwise? The answer to this lies in a process called tidal shearing, which consists in a misalignment between the tidal forces (the second derivatives of the gravitational field) and the moment of inertia of the protogalaxy (the second moments of the matter distribution). The meaning of these technical terms can be explained quite easily: Imagine a curling stone sliding along the sheet, where the ice surface on its left side is a bit smoother compared to the right side. The stone will be set into clockwise rotation by this difference in force. For a protogalaxy, it is the variation in gravitational force moving the galaxy along that introduces rotation, and is responsible for the galaxy's angular momentum.