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Thomas Colas @cosmocolas.bsky.social · 04/09/2026

8/ On CMB scales Δθₖ* ~ e⁻¹¹⁷. This gives a new inflationary consistency relation: the primordial power spectrum fixes the phase uncertainty🧭 A measurable loss of coherence would point to new primordial physics: non-adiabaticity or decoherence.

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Thomas Colas @cosmocolas.bsky.social · 04/09/2026

7/ In fact, the result is insensitive to essentially any intermediate expansion history: prolonged reheating/preheating, early matter or PBH domination, kination, early dark energy, or a time-dependent equation of state. None changes the final coherence if ζ is conserved.

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Thomas Colas @cosmocolas.bsky.social · 04/09/2026

6/ More generally, if ζ is conserved on super-Hubble scales and evolves unitarily, the phase variance Δθₖ² ∝ 1/Pζ(k) × [late-time background quantities]. No inflationary Hubble scale. No duration of inflation. No reheating history. This is the central result of the paper.

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Thomas Colas @cosmocolas.bsky.social · 04/09/2026

5/ Why? Inflation is only part of the story. The transition out of inflation and the later expansion also squeeze perturbations, compensating the dependence accumulated during inflation⚖️ The final result remembers the power spectrum but forgets the inflationary scale.

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Thomas Colas @cosmocolas.bsky.social · 04/09/2026

4/ With Siméon Vareilles, Julien Grain, Federico Piazza and Vincent Vennin, we follow phase uncertainty through cosmological history, up to recombination. Scenarios with very different inflationary scales end inflation with very different coherence... but later converge.

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Thomas Colas @cosmocolas.bsky.social · 04/09/2026

3/ Inflation provides a beautiful explanation. Beyond the Hubble radius, perturbations become squeezed: one solution survives while the other decays. The longer this lasts, the sharper the phase🌀 One might expect: longer inflation → stronger coherence. But this is wrong.

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Thomas Colas @cosmocolas.bsky.social · 04/09/2026

2/ Think of each primordial perturbation as an oscillator. Sharp acoustic peaks require modes of the same wavelength to start with almost the same phase. Random phases would wash the peaks out. This is phase coherence🎯 arxiv.org/abs/hep-ph/0...

arxiv.org

Coherent Phase Argument for Inflation

Cosmologists have developed a phenomenally successful picture of structure in the universe based on the idea that the universe expanded exponentially in its earliest moments. There are three pieces of...

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Thomas Colas @cosmocolas.bsky.social · 04/09/2026

1/ 📄 New paper’s out: we study phase coherence in primordial #cosmology. The sharp acoustic peaks of the CMB and their late-time imprint in BAO require primordial perturbations to oscillate with extremely coherent phases. What fixes this coherence? 🧵 arxiv.org/abs/2609.03012

arxiv.org

Universal CMB Phase Coherence from Single-Field Inflation

The phase coherence of primordial perturbations shapes the acoustic peaks of the CMB and is regarded as a key signature of inflation. Inflationary scenarios at different energy scales, thus with diffe...

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Thomas Colas @cosmocolas.bsky.social · 13/05/2026

Many thanks to everyone who discussed stochastic inflation, Schwinger–Keldysh EFTs and RG ideas with us over the past months. Discussions and feedback were extremely valuable throughout this project 🙏

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Thomas Colas @cosmocolas.bsky.social · 13/05/2026

This perspective opens many directions for stochastic inflation: 🔄 non-equilibrium RG flows 📉 infrared fixed points ⚛️ power counting and operator hierarchies 🌊 non-equilibrium steady states and more generally a Wilsonian understanding of real-time cosmological evolution.

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Thomas Colas @cosmocolas.bsky.social · 13/05/2026

Overall, the picture is: stochastic inflation is not an independent prescription added on top of QFT in de Sitter, but the leading infrared description of a non-equilibrium Wilsonian renormalization group 🌌⚛️ arxiv.org/abs/2605.11096

arxiv.org

Stochastic inflation from a non-equilibrium renormalization group

Understanding stochastic inflation, and in particular the systematic computation of controlled corrections from first principles, remains an important open problem. In this work, we address this probl...

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Thomas Colas @cosmocolas.bsky.social · 13/05/2026

From this RG flow, we derive a generalized Fokker–Planck equation. This provides a unified picture where Wilsonian RG ↔ open EFT ↔ stochastic inflation are all different perspectives on the same infrared dynamics.

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Thomas Colas @cosmocolas.bsky.social · 13/05/2026

We then reformulate the whole construction as a real-time RG flow for the density matrix. This leads to a Polchinski-type equation directly on the Schwinger–Keldysh contour.

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Thomas Colas @cosmocolas.bsky.social · 13/05/2026

The framework also systematically generates corrections beyond the standard stochastic picture, including: • field-dependent noise • dissipative terms • higher-order stochastic operators

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Thomas Colas @cosmocolas.bsky.social · 13/05/2026

In this regime, the EFT reduces to stochastic inflation. The usual Fokker–Planck equation appears as the leading infrared/local limit of the Schwinger–Keldysh EFT

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Thomas Colas @cosmocolas.bsky.social · 13/05/2026

A key point is that the EFT becomes local and diffusion-dominated in the super-Hubble regime. Three small parameters control the EFT expansion: • thin-shell coarse graining • gradient expansion • super-Hubble hierarchy

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Thomas Colas @cosmocolas.bsky.social · 13/05/2026

In our paper, we start from the reduced density matrix of long modes. Integrating out short modes in the Schwinger–Keldysh path integral naturally produces an open EFT with: 🌊 diffusion/noise 🌀 dissipation all generated dynamically by modes crossing the horizon.

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Thomas Colas @cosmocolas.bsky.social · 13/05/2026

The problem is well known: light fields in dS develop large IR effects and perturbation theory eventually breaks down. Stochastic inflation successfully resums these effects, but its precise EFT origin is still under investigation, see eg arxiv.org/abs/2007.03693

arxiv.org

Soft de Sitter Effective Theory

Calculating the quantum evolution of a de Sitter universe on superhorizon scales is notoriously difficult. To address this challenge, we introduce the Soft de Sitter Effective Theory (SdSET). This fra...

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Thomas Colas @cosmocolas.bsky.social · 13/05/2026

📄 New paper with Sebastián Céspedes🌌🌊 We revisit stochastic inflation from the viewpoint of Schwinger–Keldysh EFTs and Wilsonian RG. Main message: stochastic inflation emerges as the IR limit of a real-time Wilsonian RG flow arxiv.org/abs/2605.11096 #cosmology #qft #physics🧵

arxiv.org

Stochastic inflation from a non-equilibrium renormalization group

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Thomas Colas @cosmocolas.bsky.social · 16/03/2026

Open gravitational dynamics offers an alternative perspective on the dark sector. This model fits current data and predicts correlated signatures in GW, galaxy clustering and weak lensing, offering multiple observational tests in upcoming surveys. Exciting prospects ahead!🌜

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Thomas Colas @cosmocolas.bsky.social · 16/03/2026

The model predicts enhanced structures at low z compared to Lambda-CDM. This enhancement places this minimal scenario outside the observationally allowed range, though precise bounds would require scanning over cosmological parameters and a precise inclusion of baryons

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Thomas Colas @cosmocolas.bsky.social · 16/03/2026

Scalar perturbations also change. We predict a modification of the Bardeen potentials, producing a non-trivial gravitational slip, η = Φ / Ψ. This deviates from Lambda-CDM model where η = 1, and is rather constrained by DESI results

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Thomas Colas @cosmocolas.bsky.social · 16/03/2026

In our model, dissipative effects modify the propagation of GWs. The GW luminosity distance hence differs from the electromagnetic one. The predicted effect is consistent with current constraints from LVK and could be tested with future observations

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Thomas Colas @cosmocolas.bsky.social · 16/03/2026

The model is made of a single open dark fluid whose effective EOS can be chosen to reproduces BAO + CMB best-fit expansion history, similar to a CPL dark-energy parametrization. But importantly, the acceleration arises without violating the null energy condition

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Thomas Colas @cosmocolas.bsky.social · 16/03/2026

If gravity interacts with an environment, its dynamics can be treated as an open system. Using Schwinger-Keldysh formalism, we developed a general framework for open gravitational dynamics. In this paper we study a minimal model that account for late-time cosmic acceleration

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Thomas Colas @cosmocolas.bsky.social · 16/03/2026

Phenomenology of an Open EFT of Dark Energy 🌌 📄https://arxiv.org/abs/2603.12321 All current evidence for dark matter and dark energy is purely gravitational. Hence, we treat the dark sector as the medium through which the spacetime metric propagates #cosmology #darkenergy Thread 👇

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Thomas Colas @cosmocolas.bsky.social · 16/03/2026

📢Registration open for Contours 2026 — EFTs Meet the Schwinger–Keldysh Formalism! 📅 29 June – 3 July 2026 📍 DAMTP, University of Cambridge Join us for a conference on non-equilibrium EFTs across #hydrodynamics, #blackholes, #holography & #cosmology Submit abstracts: indico.global/event/16432/...

indico.global

Contours 2026 - Effective Field Theories Meet the Schwinger–Keldysh Formalism

Effective field theories in particle physics are usually designed for experiments where the initial state — the vacuum before a scattering event — is clean and isolated. Yet many systems, from condens...

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Thomas Colas @cosmocolas.bsky.social · 14/12/2025

Huge thanks to Xi and Zhehan for this collaboration — it’s been a real pleasure. I’ve learned a ton, from deep conceptual discussions to the nitty-gritty of the computation. Stay tuned: the open cosmological collider signal could be just around the corner! 💥🌌 [10/10]

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Thomas Colas @cosmocolas.bsky.social · 14/12/2025

This case study clarified several subtleties in integrating out heavy fields in the early universe. I hope it will also be useful to others, and help guide which EFTs we should write in cosmology when the UV physics is unknown [9/10]

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Thomas Colas @cosmocolas.bsky.social · 14/12/2025

As in flat space, the EFT coefficients controlling this open EFT are generally non-analytic, exhibiting branch cuts. Expanding in the heavy mass recovers the asymptotic nature of de Sitter EFTs (arxiv.org/abs/2505.17820) [8/10]

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Thomas Colas @cosmocolas.bsky.social · 14/12/2025

Dissipation is always subleading, suppressed exponentially by the heavy mass. This provides a clear open-system perspective on the cosmological collider signal [7/10]

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Thomas Colas @cosmocolas.bsky.social · 14/12/2025

In de Sitter, cosmological dynamics bring additional complications. Interestingly, different sectors of the EFT produce distinct physical signals: in parity-preserving theories, the non-local signal comes from stochastic operators, while the local signal arises from the unitary sector [6/10]

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Thomas Colas @cosmocolas.bsky.social · 14/12/2025

The relevant EFT coefficients are generically non-analytic in momenta. In flat space, this non-analyticity appears as a series of folded poles. Imposing a mass hierarchy smooths these features and allows one to recover a local open EFT [5/10]

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Thomas Colas @cosmocolas.bsky.social · 14/12/2025

Working in the Schwinger–Keldysh formalism already brings a few surprises. Reproducing the flat-space signal requires more than a unitary EFT: dissipative and stochastic operators for the massless field must also be included [4/10]

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Thomas Colas @cosmocolas.bsky.social · 14/12/2025

We proceed in three steps. We decompose the signal and study its physical origin, integrate out the massive scalar to obtain a non-local EFT, and finally recast this non-locality as non-analytic EFT coefficients — which smooth out once a scale hierarchy is imposed [3/10]

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Thomas Colas @cosmocolas.bsky.social · 14/12/2025

In this work, we study the exchange of a massive scalar, probed through the trispectrum — the in-in analogue of 2→2 scattering — of a massless scalar [2/10]

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Thomas Colas @cosmocolas.bsky.social · 02/10/2025

More lecture notes from the Disordered Universe Summer School are on the way! Topics include: 🌌 Λ > 0 universes (D. Anninos) 📊 Out-of-equilibrium systems (T. Anous) 🔍 Amplitudes & correlators (A. McLeod) Stay tuned! Details: indico.sns.it/event/91/ove...

indico.sns.it

Summer School on The Disordered Universe

The school aims to attract graduate students as well as young postdocs interested in the areas of de Sitter physics, theoretical cosmology, holography, scattering amplitudes, open systems, and disorde...

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Thomas Colas @cosmocolas.bsky.social · 02/10/2025

Last summer I taught at the Disordered Universe Summer School — an interdisciplinary program on de Sitter physics, cosmology, holography, scattering amplitudes, and open/disordered systems. I prepared lecture notes on Open Effective Field Theories, now on arXiv📜 arxiv.org/abs/2510.00140

arxiv.org

Lectures on Open Effective Field Theories

Effective field theories offer a powerful method to unify diverse models under a small set of control parameters, allowing systematic expansions around well-established theories. These techniques, dev...

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Thomas Colas @cosmocolas.bsky.social · 17/07/2025

It’s been a real pleasure collaborating with physicists outside cosmology on this project. Huge thanks to Alessio, Ana, and Michał for the countless hours of discussions and the effort to bridge quantum information and cosmology 🌌⚛️[20/20]

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Thomas Colas @cosmocolas.bsky.social · 17/07/2025

...we’re effectively stuck probing a classical probability distribution — one that hides clear quantum signatures [19/20]

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Thomas Colas @cosmocolas.bsky.social · 17/07/2025

Even though the quantum states of cosmological inhomogeneities may show strong non-classical traits, our limited observational access during inflation means... [18/20]

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Thomas Colas @cosmocolas.bsky.social · 17/07/2025

This tension highlights a key puzzle about quantum perturbations in the early universe [17/20]

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Thomas Colas @cosmocolas.bsky.social · 17/07/2025

It would require accessing the decaying mode of inflationary perturbations — something that’s extremely challenging and may still fall short of true optimality [16/20]

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Thomas Colas @cosmocolas.bsky.social · 17/07/2025

But — and it’s a big but — this measurement is far beyond the reach of current observations [15/20]

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Thomas Colas @cosmocolas.bsky.social · 17/07/2025

The conclusions are twofold: first, there is an optimal measurement strategy that can constrain the tensor-to-scalar ratio extremely well [14/20]

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Thomas Colas @cosmocolas.bsky.social · 17/07/2025

Our main result: there is a huge gap between the classical and quantum Fisher information — one that grows exponentially with the duration of inflation [13/20]

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Thomas Colas @cosmocolas.bsky.social · 17/07/2025

For concreteness, we focus on the tensor-to-scalar ratio — a key observational target in upcoming experiments [12/20]

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Thomas Colas @cosmocolas.bsky.social · 17/07/2025

To understand how much information we lose due to limited access to early-universe data, we contrast the quantum Fisher information — the best-case scenario — with the classical Fisher information extracted from measurements of the Gaussian curvature perturbations alone [11/20]

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Thomas Colas @cosmocolas.bsky.social · 17/07/2025

This limited observational access makes it much harder to reconstruct the quantum state produced by inflation [10/20]

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