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.
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.