Welcome back. So in the last lecture, we saw how cosmology has established itself as a science in its own right in just over a century. In this class, we look at the current cosmological model, the so-called concordance model, or lambda CDM. This model builds on Einstein's general relativity and the so-called Friedmann-Lemaitre-Robertson-Walker model, and asserts that our universe is infinite and consists of 5% ordinary matter, 25% cold dark matter and 70% dark energy. So according to this picture, the vast majority of our universe consists of two exotic entities: dark energy and dark matter. While the search for those two entities is at the very forefront of contemporary research in cosmology, from a philosophical point of view, we should ask, what are dark energy and dark matter, and how justified are we in believing in them? The philosophical debate behind dark energy and dark matter concerns the rationality of theory choice. What reasons do scientists have for choosing one theory over another? How do we go about making those rational decisions in the light of the available evidence? So, in this class we look at the history of dark energy and dark matter and some of the experimental evidence for them, as well as examining some of the possible rival theories that have been proposed within the broader philosophical debate about the rationality of theory choice. To get us started with the debate about the rationality of theory choice in science, lets consider for example, the discovery of the planet Neptune back in 1846. The anomalous perihelion of the planet Uranus was known for some time, and two astronomers, Urbain Le Verrier and John Couch Adams, independently of each other tried to reconcile the anomaly with Newtonian theory by postulating the existence of a new planet called Neptune, whose orbital mass was supposed to interfere with and explain the anomalous orbit of Uranus. The new planet was indeed observed on the 23rd September, 1846. A very similar phenomenon was also observed for the planet Mercury, and also in this case, Le Verrier postulated the existence of a new planet, Vulcan, to explain the observed anomaly. But this time, the predicted planet was not found, and a final explanation of the phenomenon came only with the advent of general relativity. This historical example illustrates a phenomenon that we have already encountered in the introductory lecture, namely, the problem of underdetermination of theory by evidence. Whenever we have more than one scientific theory or hypothesis, the available evidence may not be sufficient to determine the choice for one theory over a rival one. For example, in the case of Uranus, the anomalous perihelion was evidence that there was something wrong with the set of assumptions, including both main theoretical hypotheses about Newtonian mechanics, as well as auxiliary hypothesis about the number of planets in the solar system, their masses, and orbits. But the anomalous perihelion by itself didn't tell scientists whether the culprit for the anomaly was one of the main theoretical assumptions, as opposed to one of the auxiliary hypothesis about number of planets, masses and so forth. As is seen in our case of the anomalous perihelion of Mercury shows, finding the right answer to these questions may well be far from obvious. The physicist and philosopher Pierre Duhem at the beginning of the 20th century concluded that scientists often followed their good sense in making decisions in such situations. But Duhem's solution in terms of good sense is not satisfactory. For one thing, it's not clear what good sense is. Second, it's not clear why scientist X's good sense should agree with scientist Y's good sense. And worse, Duhem's solution delegates the rationality of theory choice to whatever a scientific community deems as the most sensible choice to make, even if that choice may well be the wrong one. Let's then take a closer look at the argument from underdetermination and how it challenges the rationality of theory choice. The argument proceeds from three premises to a conclusion. So, premise one of the argument says that scientists' belief in theory T1 is justified. They have good reasons for believing that theory T1 is true, or corresponds the way things are in nature. Premise two says that scientific theory T1 has to be read literally. In other words, if the theory talks about planetary motion, we must take what the theory says about planetary motions at face value. Premise three says that theory T1 is empirically equivalent to another theory T2, whenever T1 and T2 have exactly the same empirical consequences. And given those three premises, one, two, three, we draw the conclusion that premises two and three jointly imply that premise one must be false. In other words, it is not the case that scientists have good reasons for believing that theory T1 is true. In other words, the scientists are not justified in believing that a theory T1 is true, or corresponds to the way things indeed are in nature, if there is another rival theory, T2, which is empirically equivalent to T1. These considerations resonate in the work of the influential historian and philosopher of science, Thomas Kuhn. In his 1977 book, The Essential Tension, Kuhn argued that theory choice seems to be governed by five seemingly objective criteria. The five seemingly objective criteria for theory choice, according to Kuhn, are the following ones. Number one, accuracy. The theory we go for has to be accurate, has to agree with the available experimental evidence. Number two, consistency. The theory has to be consistent with other theories accepted at the time. Number three, broad scope. The theory has to be able to go beyond the original realm of phenomena it was designed to explain. Number four, simplicity. We want our theory to be simple. Number five, fruitfulness. Our theory should also be able to predict novel, undreamt of phenomena. However, Kuhn continued, those five criteria are either imprecise, for example, we don't know how to define simplicity, or they conflict with one another. Take the example of Copernican astronomy. Copernican astronomy seems better than Ptolemaic astronomy on the basis of accuracy, but it fared worse than Ptolemaic astronomy on the basis of consistency with other well accepted theories at the time such as Aristotle's physics. Therefore Kuhn concluded that the five criteria are not sufficient to determine theory choice. And external sociological considerations are decisive in gathering scientists' consensus around one theory. Going back to our topic, we should ask what the evidence is for the concordance model in cosmology, and whether in this case too there might be empirical equivalent rivals. These questions are the more pressing if we consider that the search for dark energy and dark matter is still ongoing, with large galaxy surveys currently underway. In the next section we review the evidence for dark energy, and dark matter. And we return to the underdetermination problem and the rationality of theory choice at the very end of today's class, when we assess the prospects and promises of the concordance model in cosmology.
