Thomas Colas

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]

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

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

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

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]

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

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

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]

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

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]

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

Key features — like non-Gaussianities in scalar perturbations, primordial gravitational wave statistics, or momentum correlations — are either weakly constrained or not measured at all [9/20]

Unfortunately, our window into the early Universe is still pretty narrow [8/20]

It sets the ultimate limit on how precisely we could measure a parameter — if we had ideal access to the early Universe’s quantum state [7/20]

So how do we approach this? We bring in quantum metrology — specifically, the quantum Fisher information — and apply it to the state of cosmological fluctuations right after inflation [6/20]

This raises two deep questions:
· How well can we infer a given parameter?
· What’s the best way to do it? [5/20]

I've always found it wild that we just collect photons with telescopes — and somehow extract things like the dark matter abundance or the Universe’s expansion rate [4/20]

In cosmology, this process shapes nearly everything we know about the Universe: its composition, its history, its fate [3/20]

Estimating the values of a model’s parameters — what we call parameter inference — lies at the heart of scientific discovery [2/20]

Quantum estimation of cosmological parameters Understanding how well future cosmological experiments can reconstruct the mechanism that generated primordial inhomogeneities is key to assessing the extent to which cosmology can inform fundamental ...

What’s the best way to extract information about the early universe? In our new paper, we tackle this classic cosmology problem using tools from quantum information theory🧵[1/20] #Physics #Cosmology #Quantum
arxiv.org/abs/2507.12228

After more than a year of questions, setbacks, and slow progress, this work finally took shape. Huge thanks to the physicists who joined the journey, especially my past and present collaborators [25/25]

We recomputed the primordial tensor spectrum for this model and derived the tensor-to-scalar ratio. Using BICEP’s current bound, r < 0.036, we placed an upper limit on noise sourcing tensors during inflation [24/25]

Interestingly, dissipative birefringence appears at lower derivative order than conservative birefringence. Exploring explicit models that realize this would be an exciting direction [23/25]

The main novelty is that we can now study gravitational waves propagating through a medium — featuring non-trivial speed, dissipation, noise, and even birefringence [22/25]

An Open Effective Field Theory for light in a medium In many scenarios of interest, a quantum system interacts with an unknown environment, necessitating the use of open quantum system methods to capture dissipative effects and environmental noise. With...

This mirrors exactly the structure we see in electromagnetism within a medium [21/25] arxiv.org/abs/2412.12299

This effect appears clearly in the modified continuity equation [20/25]

We derived the modified Einstein equations in this framework. Assuming homogeneity and isotropy, we get the modified Friedmann equations below — describing cosmic evolution influenced by a dissipative matter sector [19/25]

The Open Effective Field Theory of Inflation In our quest to understand the generation of cosmological perturbations, we face two serious obstacles: we do not have direct information about the environment experienced by primordial perturbations ...

In the decoupling limit, the scalar sector separates from gravity — recovering the results of arxiv.org/abs/2404.15416 [18/25]

The scalar clock emerges explicitly after performing two Stückelberg tricks — one for each branch of the path integral [17/25]

Once the symmetry breaking pattern is set, we can build the most general effective action compatible with these symmetries — organized order by order in a derivative expansion. At lowest order, the functional looks like this: [16/25]