Exam stress

This week is by far the most stressful week of the teaching semester for lecturing staff, as Friday marks the deadline for submission of exam papers. If you teach four courses, which is fairly typical for Institute of Technology lecturers, that’s eight papers of questions and model answers to be handed in if you include the repeats.

I’m not saying it’s a bad idea in principle to have exams submitted half way through the semester, far from it (besides, the timetable is set by the fact that the papers have to be posted to the external examiners). However, it’s a lot of prep and the stress is twofold. First there’s all those neat questions you thought of as you were teaching the course, but now can’t find. Second, it’s the realisation that, in every course, you are far behind where you should be. If only the students knew!

Enthralled class

Worst of all are the shared papers. This involves inumerable meetings, changes and compromises, until all participants are happy.I often feel it would be easier to write the whole damn paper myself, even if I only teach part of that course!

Still, shouldn’t complain. Next week is midterm and we have a whole week off. I intend to make the most of it, with a weekend in the Comeragh mountains with the Glenwalk Hillwalking Club, followed by some surfing and music in Doolin during the week. I deserve it.

With Glenwalk in the mountains of Mourne

Tunes in Doolin last summer

Current status of the concordance model

This week I’m studying a very nice article on the ArXiv by L.Perivolaropoulos on recent observational challenges to the ΛCDM model (thanks Bee).

The ΛCDM model is the technical name given to the concordance model of Big Bang cosmology (see final post in cosmology 101 series). Essentially, the model is the best attempt to account for the three main strands of observational evidence: the measurements of the cosmic microwave background, the measurements of the large scale structure of the universe by gravitational lensing, and the supernova measurements of the accelerated expansion of the universe. CDM stands for Cold Dark Matter, the postulate that much of the matter holding the galaxies and galaxy clusters together is unseen – i.e. does not couple with the electromagnetic interaction (see previous post on Dark Matter). Λ refers to the so-called cosmological constant -  i.e.  the ‘dark energy’ term thought to be responsible for the current acceleration of the universe expansion (see previous post on dark energy here).

The matter-energy composition of the universe according to ΛCDM

However, cosmologists are well aware that there is an alternative: the ΛCDM model could simply be wrong, and the postulates of dark matter and dark energy completely spurious, if our underlying theory of gravity – general relativity – does not apply at the largest scales. Both postulates arise from the attempt to shoehorn the observational data into gravitational theory, and it is always possible that the underlying theory is incomplete (after all, we know GR breaks down at the smallest scales). There is a very nice discussion of this in Perivolaropoulos’ s paper, in the context of six experimental observations that have emerged in the last few years that don’t seem to fit easily into the ΛCDM model.

Of course, given the spectacular success of general relativity in explaining so many aspects of our universe so far, the betting money is on relativity being correct, while the new observational data may modified as more measurements are made (this has happened countless times before). Either way, it’s a really nice update on the current state of play and shows how good science is done – not to mention the usefulness of the ArXiv database.

Update

Over on the DiscoverScience blog, Sean Carroll also has very nice post on a specific challenge to the concordance model from measurements of the large scale structure of the universe by weak gravitational lensing. Again, both the post and the discussion afterwards are excellent and give a good idea of how this sort of science is done.

It’’s worth mentioning that both dark matter and dark energy are favourite targets of skeptics, philosophers of science and other commentators. To be sure, they both probably seem like an obvious fix to an outsider, particularly given their postulated prevalence relative to ordinary matter (our universe is estimated to comprise 73% dark energy, 23%  dark matter and only 4% ordinary matter!). However, in this sort of debate, it’s important to listen to the experts. While keeping an open mind, most cosmologists seem convinced that dark matter almost certainly exists. The general line is that you can see it – by its gravitational effect, not electromagnetic. This is perfectly feasible if dark matter is made up of WIMPS (weakly interacting massive particles), a not unreasonable proposition. Such particles may even be detected at the LHC, which would be very exciting. It should also be remembered that the existence of dark matter is also invoked to account for the nucleosynthesis of the elements, a seperate plank of the big bang model. Finally, there are now strong experimental hints of the existence of dark matter from studies of galaxy collisions

Evidence for dark matter in the bullet cluster

As for dark energy, it is certainly true that this is a lot more speculative, and could turn out to be one of many different things  (see wiki for a good summary). However, it’s important to note that the postulate does not arise solely from the supernova measurements – there are also indirect evuidence of dark energy from measurements of the cosmic microwave background.