CEFISES Seminar: Gauvain Leconte-Chevillard, “Major Transitions in the Evolution of the Universe: The Case of the Inflation-Reheating Transition”

Attention, location change. This event will not be held in Louvain-la-Neuve as usual, but in Namur.

Venue: L53 (5ème étage de la Fac de philo et lettres, UNamur, entrée dans le haut du rue Grafé 1).

Series: OLOFOS

Speaker: Gauvain Leconte-Chevillard (UNamur)

Title: “Major Transitions in the Evolution of the Universe: The Case of the Inflation-Reheating Transition.”

Abstract:

Cosmology is often portrayed as a natural historical science (Lambert & Reisse 2008, 163; Butterfield 2014, 58; Pearce 2017). Scientific and popular accounts of our current understanding of the universe frequently include a “chronology” divided into distinct epochs or eras (Planck era, inflation epoch, matter-dominated era, etc.). Yet this periodization is not without puzzles. Some epochs overlap (e.g., the electroweak epoch and the inflationary era); some are defined by physical processes (like nucleosynthesis), others by dominant energy contributions (like radiation or matter domination); some last for mere instants, while others span eons. This raises several issues. Methodological questions include, notably: How are the boundaries of a cosmological epoch determined? Is there a unified criterion to circumscribe a cosmological era, or are they characterized in different ways? Related ontological questions arise as well: What occurs during the transition between epochs? Are these transitions smooth and continuous, or do they involve threshold events marking clear ruptures?

This talk addresses these questions by drawing a comparison between cosmology and another natural historical science: evolutionary biology. In that field, Maynard Smith and Szathmáry (1997) introduced the concept of “Major Transitions” to describe eight pivotal events in the evolution of living beings (e.g. the eukaryogenesis or the apparition of multicellular organisms) – episodes that involve fundamental changes in biological structure, complexity and ecological systems. I begin by stressing important differences in how scientists study the evolution of life and the evolution of the Universe – differences that reflect the fact that cosmology is not a natural science in the same sense as evolutionary biology, paleontology, or geology (Cleland 2002, 2011; Currie 2024). However, I argue that the conceptual distinction between minor and major transitions is one that can be transferred from evolutionary biology to cosmology and that it is useful for identifying events in cosmic evolution that stand out due to the radical novelties they introduce – novelties that require new theoretical constructs and new methodologies to describe the behavior of the universe.

As a case study, I focus on one of the leading hypotheses in primordial cosmology, according to which the very early universe underwent a rapid phase of accelerated expansion known as cosmic inflation. Inflation ended approximately 10⁻32 seconds after the expansion began, through a process known as reheating, during which the inflaton field decayed into the particles of the Standard Model. I argue that this transition exemplifies what some philosophers (Humphreys 2016; Guay & Sartenaer 2016, Fletcher 2021) describe as diachronic emergence: although the particles of the Standard Model emerged on the basis the inflaton field, they nonetheless represent something fundamentally new that dramatically change the behavior of the universe. This makes the transition from inflation to reheating a major transition and justifies the distinction between the inflationary epoch and the cosmic eras that followed.

In conclusion, I suggest that this approach can be fruitfully applied to identify other major transitions in the chronology of the universe, and to clarify the extent to which cosmology can truly be considered a natural historical science.

Bibliography

Butterfield, Jeremy. “On under-determination in cosmology.” Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics 46 (2014): 57-69.

Cleland, Carol E. “Methodological and epistemic differences between historical science and experimental science.” Philosophy of science 69.3 (2002): 474-496.

Cleland, Carol E. “Prediction and explanation in historical natural science.” The British Journal for the Philosophy of Science (2011).

Currie, Adrian. Rock, bone, and ruin: An optimist’s guide to the historical sciences. MIT Press, 2024.

Fletcher, Samuel C. “Similarity structure and diachronic emergence.” Synthese 198.9 (2021): 8873-8900.

Guay, Alexandre, and Olivier Sartenaer. “A new look at emergence. Or when after is different.” European Journal for Philosophy of Science 6 (2016): 297-322.

Humphreys, Paul. Emergence: A philosophical account. Oxford University Press, 2016.

Lambert, Dominique, and Jacques Reisse. Charles Darwin et Georges Lemaître : une improbable mais passionnante rencontre. Académie royale de Belgique, 2008.

Maynard Smith, John, and Eörs Szathmáry. The major transitions in evolution. OUP Oxford, 1997.

Pearce, Jacob. “The unfolding of the historical style in modern cosmology: Emergence, evolution, entrenchment.” Studies in History and Philosophy of Science Part B: Studies in History and Philosophy of Modern Physics 57 (2017): 17-34.

Szathmáry, Eörs. “Toward major evolutionary transitions theory 2.0.” Proceedings of the National Academy of Sciences 112.33 (2015): 10104-10111.