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?

Planck rumours will soon become Planck results

On Thursday, the Planck satellite will be revealing its first cosmological results. In terms of fundamental physics, this will be the biggest event since the Higgs discovery last year. In the cosmology community it is the biggest event for the best part of a decade (possibly in both directions of time). If you don't follow cosmology too closely, you might wonder why this particular experiment might generate so much excitement. After all, aren't there all sorts of experiments, all of the time?

If so, I hope you've come to the right place.

The sky as seen by Planck in 2010. Only, they hadn't removed the foregrounds yet. There's a whole Milky Way galaxy in the way. Why must they make us wait so long?

If you're unaware, Planck is a satellite put in space by the European Space Agency to measure the cosmic microwave background (CMB). The CMB is an incredibly useful source of cosmological information. The impending release of Planck's results on Thursday is big news because Planck has measured the CMB with better resolution than any other experiment that can see the whole sky. Planck might have discovered evidence of interesting new physics, such as extra neutrinos or additional types of dark matter. It might even reveal some effects relating to how physics works at energies we could never probe on Earth. But even if it hasn't discovered anything dramatically new, the precision with which Planck has measured the parameters of the standard cosmological model will immediately make it the new benchmark.

There have been surprisingly few rumours leaked to the rest of the cosmology community about what to expect on Thursday. This has resulted in the most pervasive rumour being that they have simply not found anything worth leaking. Whatever the reality, on Thursday rumours will become results.

What has Planck actually done that is so interesting?

What does the sound of the Big Bang look like?

Six weeks ago I wrote a post where I tried to explain how we know that the Big Bang definitely happened. There are of course other reasons why we know the Big Bang happened, but I decided to focus on one, relatively easily explained piece of evidence, which is the existence and frequency spectrum of the Cosmic Microwave Background (CMB).

Quickly summarising: The CMB is very cold radiation that permeates the entire universe. It was created when the expanding and cooling universe cooled to a point where it was cold enough for hydrogen atoms to form. Before this point, the electrons and protons in hydrogen had enough energy to be free from each other to form an opaque plasma. Once neutral hydrogen formed the universe became transparent and the CMB was formed and travelled (almost) freely forever after. We have detected and measured this CMB and its intensity as a function of its frequency (effectively, the brightness of each colour) is exactly what the Big Bang predicted. If there was no Big Bang there would be no reason to expect a CMB to exist, let alone for it to have this particular property. For more details please read my previous post and the links within.

When writing that post I had intended to say quite a bit more about the CMB and the Big Bang than I ended up having space for. It is not quite true that the mere existence (and spectrum) of the CMB is enough to conclusively determine that the Big Bang must have happened. However the existence of the CMB did build the metaphorical equivalent of a thousand big, bold and bright neon signs that all pointed aggressively towards the Big Bang being true.

When I began writing that earlier post and claimed that the CMB does conclusively prove that the Big Bang happened I had in my mind what I actually discuss in this post. This is the fact that we can see in the CMB the effects of sound waves that existed in the primordial hydrogen plasma. It is these sound waves and our measurements of them that puts the final nail in the coffin of all things not the Big Bang. They also represent what I claimed in that earlier post to be “jaw-droppingly stunning pieces of detective work”.

The glorious Planck satellite, measurer of all things CMB