Arsalan's Web

I am broadly interested in topics in cosmology, quantum information, quantum foundations, and at the intersection of these fields. Over the years, I've studied the nature of dark energy, explored solutions (and non-solutions) to cosmological tensions, CMB anomalies, decoherence & emergent classicality, the inflationary universe, and quantum algorithms for foundational questions.

You can read more about some selected works in the categories below. And here's a link to the arXiv which hosts the manuscripts of all the projects that I've been involved in.

Last updated: Sept. 2024

Circumventing Cosmic Variance

Read the paper here: https://arxiv.org/abs/2403.10895

Suppose you take a bag of colored beans and spill them on the floor. You notice that all the yellow ones have ended up in one corner of the room. You may be inclined to hypothesize that this is due to some fundamental law of nature -- after all if the distribution of colored beans were to be random, it would be a freak coincidence for all the yellow ones to have ended up together. But if you want to do science, you better test that hypothesis: take another bag of colored beans and check if the yellow ones end up together. You do this again. And again. Till you've established the statistical significance of your hypothesis.

Cosmology is (almost) unique in the sciences in that we do not get to re-run the cosmic experiment. This limitation of being able to observe just a single universe is called "cosmic variance" and poses a difficulty for quantifying the statistical significance of our theories -- especially those of the earliest epoch (around the epoch of the big bang). Furthermore, while the cosmic microwave background radiation (CMB) provides some of the best data on the largest scales, and earliest epochs, of the Universe, it is mostly sourced by a 2D surface and we observe it only now and from here.

In this long-awaited work with Ted and Reid, we studied a method of getting around cosmic variance by probing the polarization of CMB radiation from galaxy clusters and found some astonishing results: there is a bunch of new information on the early universe that can be gained by these measurements, compared to our current data of the CMB. The information is akin to if I had intergalactic friends spread out over the Universe who could send me letters telling me what their view of the CMB is like, essentially allowing us to probe the CMB at different times and locations in the Universe. The long-and-short of it is that, by making as few as a 100 of these "remote quadrupole" measurements over just 20% of the sky, we can meaningfully constrain theories of the early Universe that depart from the standard cosmological model.

Classical Subsystems from Quantum Worlds

Read the paper here: https://arxiv.org/pdf/2403.10895

We often look back on projects and think about how much we've learnt in doing them. When I think about this one, I feel there was an even bigger element of unlearning.

The question we asked is, in some sense, a very basic one: who decides the subsystems of a physical theory? In fact, it's so basic that we often take the answer for granted -- I can hear some of you saying, "I know what the electron is, and the apparatus that measures it. Why would I divide these two subsystems any other way?". But this begs the question: why do we live a world where subsystems are nice localized objects? And, moreover, who decides how to splice up the Hilbert space in the cosmological setting?

We propose that the answers to these questions are related to decoherence and "einselection". When we let go of a prior bias on what should the subsystems of a physical theory (in technical terms, this means the only relevant input is the eigenspectrum of the Hamiltonian), we find there is a large ambiguity in the classically emergent "realms". Here "realms" are worlds (in the Everettian sense) that support einselection (i.e. they exhibit certain features of classicality) but otherwise may look nothing like the classical world that we are used to.

Entanglement Masquerades in the CMB

Read the paper here: https://arxiv.org/pdf/2211.11079

In the simplest inflationary models, the expansion of the early universe is driven by a single scalar field. The perturbations in this field evolve into large scale structure and imprint themselves on the last scattering surface of the cosmic microwave background (CMB) radiation. But suppose there is another field laying around, a so-called "spectator", during inflation. This field doesn't interact directly with the inflaton, except gravitationally. How should the spectrum of perturbations from inflation change? Will there be observational signatures from it? In fact, we already know of at least one such field: the Higgs.

It turns out that such a field should generically entangle with the inflaton. In theory terms, this entanglement modifies the vacuum state of the inflaton away from the standard so-called Bunch-Davies state. In practical term, the entanglement leads to modifications of the primordial power spectrum and has observational consequences for large-scale structure and the CMB.

So we're not proposing a new inflationary model (of which there are many!), but instead saying that this is something to be expected from having two quantum fields in the presence of gravity. How much of this signal can actually be detected depends on some details but when we constrain it to data, we find that there is alot that can be swept under the Planck errors -- especially on the largest scales where cosmic variance is high.

But if measuring these signatures can give us a new window of understanding the earliest epochs of our universe (see Rose's follow-up work on this theme!), it's worth exploring other methods of probing them. Which brings me to how this inspired my work on remote quadrupole measurements with Ted Bunn and undergraduate student Reid Koutras (see dropdown above).

Quintessential Cosmological Tensions

Read the paper here: https://arxiv.org/abs/2207.10235

To be updated soon!

Page updated

Google Sites

Report abuse