It may sound ironic, but one the most significant drivers of new and exciting discoveries in cosmology (or science in general) has been simple (but often boring) accounting.
Trying to think of the best way of explaining why accounting matters in Science, I remembered Feynman’s introduction to “conservation of energy” in the first volume of his celebrated “Lectures of Physics”, which I read a long long time ago. There, Feynman has an intriguing account of how the mother of Dennis the Menace finds ingenious ways of accounting for Dennis’s toy blocks, simply based on that blocks cannot appear from/or disappear into thin air. While this may sound like a trivial statement, its implications can reveal to Dennis’s mother interesting facts about the properties of various systems in Dennis’s room, such as his toy box or bathtub!
One of the most famous examples of the power of bookkeeping was the discovery (or proposal) of neutrinos by Pauli in 1930, to account for the lost energy/momentum in the beta decay of neutrons. In beta decay, a free neutron decays apparently only into a proton and an electron. Since these are both charged particles, they are easy to track in bubble chambers and so their energy and momentum can be measured accurately. However, the sum of the energies and momenta of these particle was not consistent with energy/momentum of a single massive particle (the original neutron), suggesting a breakdown of energy/momentum conservation. Of course, since the energy/momentum conservation is sanctioned by god (part of the ten commandments … check it ;)), Pauli suggested that there must be another neutral particle that carries the rest of the energy/momentum of the original neutron. Sure enough, neutrinos were detected 25 years later, and are now an integral part of the standard model of particle physics.
As it turns out, the job of cosmologists is very similar to that of Dennis’s mother in Feynman’s example. God (like Dennis; no theological overtone intended!) has hidden the matter in various random places in the Universe, and it is our job as cosmologists to account for this missing matter. As Dennis’s mother did, in this process, we will understand interesting properties of our cosmic bathtubs and toy boxes :)
In cosmology, the power of bookkeeping has famously led to the discoveries (or should I say proposals) of dark matter and dark energy. Dark matter has been proposed as a way to account for factors of 10s to 100s mismatch between the gravitational mass (deduced from star/galaxy velocities) and stellar mass of galaxies and galaxy clusters. Dark energy (or cosmological constant, its less exotic predecessor) was first proposed as way to reconcile the mismatch between the flat cosmic geometry (measured via the cosmic microwave background anisotropies) with the cosmic clumpy matter content (as measured in galaxies and clusters). Of course, the situation is not quite as good as the case for neutrinos, as we are yet to detect dark matter or dark energy. While the former is an active industry, most people would bet we’ll never be able to detect dark energy in a non-gravitational way.
Now, I come to the main point of my post, which is a similar discrepancy in accounting, even for the ordinary matter in the Universe. In fact, I decided to write this post (the first scientific post of this blog), after I read Julianne’s nice post on Cosmic Variance about the missing baryons problem, where she explains how we fail to account for most of the baryons (read ordinary matter) in the Universe.
Let me finish by answering three questions about the “Missing Baryon Problem”:
1- How do we know how much ordinary matter there must be in the Universe?
Well, it turns out that Big Bang theory has been very successful in describing the cosmic evolution in the early Universe. This is evidenced by its successful prediction of light element abundances (produced within the first 3 minutes of big bang), as well as the spectrum of anisotropies in the cosmic microwave background. However, Big Bang theory requires 4% of the mass/energy of the Universe to be baryons.
2- So where are most of the cosmic baryons?
As it turns out, we can only see 10-15% of the baryons that the Big Bang theory predicts (see Peebles and Fukugita’s cosmic inventory). The rest is supposedly in a warm-hot intergalactic medium (dubbed WHIM by my colleagues), with very little emission/evidence in any wavelength. However, with advancement of observational technology, we should start to see WHIM’s signature in X-rays or cosmic microwave background, within a decade or so.
3- Is there something wrong with this picture?
Even where we are supposed to see the hot gas, which, as Julianne mentions is in galaxy clusters, there still seems to be some discrepancy, as the baryonic budget is 20-30% less than the big bang prediction (e.g. see here and here). Now, the solution to this may not be as exciting as the last three accounting problems, but I bet it’ll teach us something interesting about our Cosmology.