A Hot Mess: Inflation and Fine-Tuning

Casey McCoy

On the first page of his preface to the English edition of The Logic of Scientific Discovery, Popper writes:

I, however, believe that there is at least one philosophical problem in which all thinking men are interested. It is the problem of cosmology: the problem of understanding the world—including ourselves, and our knowledge, as part of the world. All science is cosmology, I believe, and for me the interest of philosophy, no less than of science, lies solely in the contributions which it has made to it. For me, at any rate, both philosophy and science would lose all attraction if they were to give up that pursuit. (p. 15)

Popper states here the classic, inclusive view of cosmology as the study of the all-inclusive, big ‘U’ Universe. Given the suggested philosophical nature of cosmology, it may seem somewhat surprising that philosophers have paid relatively little attention to the physical study of cosmology, namely, what one might call the science of little ‘u’ physical universes. (For the difference between Universe and universe, have a look at Edward Harrison’s classic textbook on cosmology, Cosmology: The Science of the Universe; it’s the book that first got me interested in the subject.) If philosophy aims at understanding the Universe, then surely an important piece of the complete story is to be found in its physics.

Admittedly, cosmology qua physics only came into its own as a branch of physics with the advent of more powerful telescopes and the theoretical possibilities afforded by the cosmological models of the general theory of relativity in the early-twentieth century, although it remained a research backwater for most of the rest of the century—that is, until once again improved observational capabilities and theoretical developments made it into a high-profile hotbed of research activity, which it continues to be.

One might take the attitude that philosophical study of scientific research may only bear fruit once enough time has passed for the research to become well established. If that attitude is right, then cosmology is too much in flux now for any firm conclusions to be drawn with confidence. Like many contemporary philosophers of physics, however, I am intrigued by the flux. I believe many important lessons in the philosophy of science can be learned by looking at current physics, and I believe that philosophical investigations can have a constructive role to play in contemporary physics (and science more generally) as well.

Before saying a bit about what the philosophy of cosmology can do, I should say a little about what current physical cosmology is up to and how it has got to this point. The full history of cosmology, from ancient times until now, is certainly a fascinating subject. To save some time, though, I will start with what was taken as the standard model of cosmology of the latter half of the twentieth century: the hot big bang model.

According to this model, the universe began in an extremely hot, dense state, after which it expanded and cooled, generally remaining in equilibrium. At certain times, the expansion rate caused it to fall out of equilibrium; during these periods, new structures formed out of the equilibrium soup: from the fusion of nuclei and the formation of hydrogen atoms to the creation of stars and the generation of galaxies. It’s quite a grand story, as befits the subject, but what is perhaps most amazing is that we have very good evidence that most of it happened.

Nevertheless, anomalies began to be discovered in recent decades. Among the most serious were (i) the discovery of discrepancies between the amount of gravitating matter and the amount of visible matter in the universe (and galaxies), and (ii) the discovery that distant galaxies were receding at an accelerated rate (whereas according to the standard model they should be receding at a decelerated rate). The first led cosmologists to introduce dark matter into their models (dark matter interacts gravitationally, but otherwise negligibly), and the second led them to introduce dark energy. The physical nature of these dark components remains the target of considerable amounts of observational and theoretical investigation, and it is quite unclear what they might be.

There is a third component of one such new standard model, the Lambda-CDM model. As well as dark energy and dark matter, it includes inflation. (‘Lambda’ is the symbol for Einstein’s cosmological constant, appropriated for the cosmological constant-like dark energy, and ‘CDM’ stands for ‘cold dark matter’.) Unlike dark matter and energy, there was no empirical problem that ushered inflation into contemporary cosmology; it was primarily explanatory considerations that motivated its proposal and adoption. These considerations centred on the explanation of certain cosmological conditions of the present universe—in particular, the geometrical flatness of space and the uniformity of its contents.

The hot big bang model’s explanations of these conditions depend on the existence of precise initial conditions of the universe. These ‘special’ initial conditions are thought to be problematic in a way that motivates the search for a theoretical solution. Inflationary theory purports to solve these ‘fine-tuning’ problems by introducing a brief epoch in the very early universe where space underwent an exponential, accelerated expansion before relaxing into the decelerated expansion of the hot big bang model. This expansion (intuitively) thins out any inhomogeneities that may have existed and flattens any curvature of the geometry of space, or so the story goes. Thus inflationary theory is taken to solve the hot big bang model’s fine-tuning problems by providing a better explanation of flatness and uniformity than that offered by its predecessor.

What makes the case interesting is that inflationary theory was soon shown to lead naturally to predictions of a spectrum of small, unobserved inhomogeneities in the otherwise apparently uniform contents of the universe. It took a couple of decades to develop the observational capabilities to detect them, but eventually these inflationary theory predictions were observationally confirmed. Although (like dark energy and dark matter) little remains known about the inflationary mechanism itself, this episode is generally seen as a triumph of theoretical reasoning in physics.

Not only do these stories of dark matter, dark energy, and inflation make for fascinating tales from the forefront of physics, they provide philosophers of science with ample opportunity to investigate methodological and foundational questions in a quickly moving field where much remains unknown. Only a handful of philosophers have dived in so far. Although the scope for investigation is large, I will mention here just a couple strands of my own research that attempt to address these kinds of questions.

Until recently I have primarily concentrated on inflationary theory, as I found it puzzling how a theory motivated ostensibly on explanatory grounds alone could lead to empirically sanctioned success. Why should solving fine-tuning problems matter to methodology? The empiricist-minded among us will likely see no puzzle: science is in the business of saving phenomena, and how it does so doesn’t matter to epistemology. I have found inspiration in the problem-solving approaches to progress, however, and believe that some valuable insights could come from carefully analysing fine-tuning problems and what makes them problematic. Cosmologists are, unfortunately, seldom very clear about what they mean by fine-tuning and the problems associated with it, so real interpretive work can be required. The results of this particular strand of my research are found in my papers, ‘Does Inflation Solve the Hot Big Bang Model’s Fine-Tuning Problems?’ and ‘Do Typicality Arguments Dissolve Cosmology’s Flatness Problem?’. My answers to the questions in these two titles (in accord with Betteridge’s law of headlines) were ‘No’—although I emphasize that I do see some interesting possibilities for developing accounts of fine-tuning that could change the answer to the former question to a ‘Yes’.

Currently, I am a researcher on Michela Massimi’s ERC-funded project, ‘Perspectival Realism: Science, Knowledge, and Truth from a Human Vantage Point’. Stage one of the project looks at how experimental and observational physicists in cosmology and high-energy physics deal with the perspectival nature of modelling. The cosmological half of this stage focuses on the dark energy survey, a large collaborative project aiming to probe the cosmos for further observational data relevant to uncovering the nature of dark energy. (The high-energy half focuses on beyond-standard-model searches at the large hadron collider at CERN.) We are interested in the extent to which observation here is ‘model independent’, and how data from different probes are integrated. The relationship between observation and theory is particularly interesting due to the remarkable diversity in possible theoretical explanations of dark energy: a feature of gravity (Einstein’s cosmological constant), a modified theory of gravitation, a new fundamental quantum field… or just an effect of averaging the inhomogeneities of a more fine-grained model. We hope to be sharing our initial results soon!

Casey McCoy
University of Edinburgh