The Dark Energy Survey (DES) concluded its biannual Collaboration meeting at University of Chicago in mid-June. DES is one of the largest surveys in cosmology searching for evidence of dark energy, the elusive entity that according to the so-called “concordance model” in cosmology should constitute 73% of the whole mass-energy of the universe. After years of observations at the Blanco Telescope in Chile, spanning the southern sky and mapping 200 million galaxies, DES Year 1 data will soon be publicly released; and there is a lot of anticipation as to whether the data will prove consistent with the current concordance model or not.

DES uses four different probes — baryonic acoustic oscillations (BAO), weak gravitational lensing, Supernova of type Ia, and galaxy clusters — to measure both how fast the universe is accelerating in its expansion and how clumpy the universe was at different epochs after the Big Bang. Precise measurements of both quantities are crucial for establishing whether dark energy is indeed a non-zero vacuum energy responsible for the accelerated expansion of the universe; or, whether instead Einstein’s general relativity needs be modified to account for the observed accelerated expansion.

This received view finds no home in the more complex landscape of contemporary particle physics and cosmology

I am a philosopher of science and I have taken an active interest in DES over the past few years because DES — like most of the high-energy physics currently going on at CERN, which is also part of my research interests — raise important and surprisingly analogous methodological questions about how evidence, model-building, and ultimately theory choice are deeply inter-related.

There are some surprising similarities in some of the methodological challenges that cosmologists and particle physicists face today in the light of the wealth of data coming out of large cosmological surveys like DES, no less than from run 2 at LHC (at higher energy of 13 TeV). How to make an effective use of the complex and bewildering amount of data? What kind of evidence can ultimately answer the pressing questions that cosmologists, and particle physicists alike, are asking: namely, is there really dark energy? And if so, what is it exactly? Or, are there really particles whose physics goes beyond the Standard Model? And if so, what are they like?

Largest ever map of the universe points to mysterious ‘dark energy’ Read more

A typical measure of scientific success is the ability of a scientific theory to deliver novel predictions, which — if experimentally proved — might constitute an important advance for our scientific knowledge. Philosophy of science has built an industry around confirmation theory. But unprecedented methodological challenges are facing contemporary cosmology and particle physics today. These challenges force philosophers to go back to the drawing-board and re-think some of the traditional ways of thinking about scientific progress in cutting-edge areas, where fast-growing technologies are delivering an unprecedented wealth of experimental data, and model-building is crucial in the interface between experimentalists and theoreticians.

Two major methodological challenges arise equally in contemporary cosmology and particle physics. First, when it comes to mapping the still largely unknown theoretical landscape of both Beyond Standard Model physics and the Dark Energy-Dark Matter paradigm in cosmology, there is a considerable variety of theoretical options on the table. In particle physics, the search for Beyond Standard Model physics takes the form of searching for possible supersymmetric particles, or more exotic kinds of particles for which scientists do not necessarily have well-understood theoretical models. And even within the family of supersymmetric particles, there is a proliferation of possible fully-fledged theoretical models, which cannot be tested one-by-one with the experimental data from LHC Run 2. Similarly, in cosmology, a plurality of models is on offer regarding the nature of dark energy (ranging from the standard view that dark energy is a non-zero energy density of the vacuum, to more exotic proposals that modify gravity and go under the name of quintessence, among others).

But there is more. The goal of experimental physicists at the LHC is to offer good quality data (i.e. data that have been robustly selected and statistically analysed) to point us in the right direction. Having then model-independent methods — i.e. methods that bracket as much as possible assumptions from the Standard Model and are more data-driven, so to speak — becomes very important in the search for Beyond Standard Model physics. Interestingly enough, cosmologists are dealing with very similar methodological problems. DES data coming from the four aforementioned probes (Supernova, baryonic acoustic oscillations, galaxy clusters and gravitational lensing) will be integrated to discern which one among the rival models currently available about dark energy might be on the right path. Here too, as in high energy physics, the need for a model-independent and more data-driven approach becomes very important in the search for dark energy.

Unsurprisingly, some cosmologists have been exploring model-independent frameworks such as effective field theory of cosmological perturbations, which focus on a number of parameters at the interface between the plurality of fully-fledged theoretical models and the experimental data coming from DES and other large-scale surveys. In high-energy physics, simplified models are designed to deliver on the same methodological goal. Simplified models focus on a handful of parameters for hypothetical Beyond Standard Model particles (mass values, cross-sections, branching ratios) so as to provide an effective interface between the wealth of data coming from LHC and the very many theoretical models available for Beyond Standard Model physics.

There was once a received wisdom in philosophy of science that portrayed scientific inquiry as the activity of coming up with theoretical models, deducing empirical consequences and test them to either confirm or reject the model. This received view finds no home in the substantially more complex landscape of contemporary particle physics and cosmology. The increasing appeal to model-independent searches is redefining the terms of how experimentalists and theoreticians interact as research communities engaged in the same task of finding an answer to fundamental questions. A variety of model-independent practices is currently being designed in both cosmology and particle physics to facilitate this interaction and to handle the unprecedented challenges that the wealth of data poses in both fields.

Jon Butterworth (@jonmbutterworth) David Brooks @uclmaps optics engineer, & £1M+ worth of lens for Dark Energy Spectroscopic Instrument https://t.co/6rDgJ5QHli in our basement pic.twitter.com/YRQcHbwqvP

At a cocktail party at the end of a conference in philosophy of cosmology at the Rotman Institute (Canada), I spoke to Oxford cosmologist Tessa Baker about how much her talk on effective field theory of cosmological perturbations reminded me of similar methodological strategies adopted by the ATLAS collaboration in their phenomenological Minimal Supersymmetric Model (pMSSM). And the thought dawned on me that maybe our role —as philosophers of science — is also that of creating bridges between research communities that might (consciously or not) be adopting similar methodologies to answer very similar challenges. And with this thought, the philosophical urge to re-think altogether the landscape of how experimental data, model-building, and theory-choice are related to each other.

Michela Massimi is the Principal Investigator of a philosophy of science project funded by the European Research Council (ERC-Cog-647272) entitled Perspectival Realism. Science, Knowledge and Truth from a human vantage point, which is looking both at LHC physics and DES as case studies (www.perspectivalrealism.org). Michela is the recipient of the 2017 Royal Society Wilkins–Bernal–Medawar Prize for her interdisciplinary work in the area of history and philosophy of modern physics.