4gravitons has proposed a challenge: To explain scientific papers appearing on arXiv on a given day in a given (sub)field to a general audience. The challenge is directed towards fellow science communicators so that definitely excludes me. But one of my old year thoughts (turned to a new year resolution) for this blog was to discuss papers on arXiv that interested me, on a monthly basis (apart from some physic(ist)s-related rants like this) in an effort to keep this blog (sort of) active. So, despite not being a science communicator, I feel like the above-mentioned challenge is one way to get into discussing papers this month. So here we go…
Warning as given by 4gravitons: I’m looking at papers in the “High Energy Physics – Theory” area, announced 10 Jan, 2022. I haven’t read these papers, just their abstracts, so apologies if I got your paper wrong!
Extra Disclaimer: I am not an expert in any of the (sub)topics covered by the 11 papers appearing below. I am familiar with enough ‘hep-th jargon’ to have a vague idea about what each paper might be about and that’s what is reflected in my explanations. So don’t take what follows too seriously, or to put it another way, take it seriously at your own risk.
[1] arXiv:2201.02200: A 
The author looks at dynamics of an exotic kind of matter and its implications to cosmology. They find by employing Einstein’s theory of gravity that a Universe filled with this kind of matter would be closed (like the surface of a sphere) and would expand exponentially. They also discuss the possibility of this kind of matter as a dark matter candidate. [Our Universe with ordinary + dark matter (+dark energy) is more or less flat and expanding but not exponentially.]
[2] arXiv:2201.02201: Anisotropic Special Relativity
Einstein’s theory of Special Relativity treats all directions of space and time on equal footing. As the title of this paper suggests, the author here considers breaking the isotropy of space and choosing one preferred direction in space. Using such a setup, they go on to discuss the consequences for quantum field theory, which is the framework used to describe our current understanding of fundamental particles and forces known as the Standard Model. [Not sure how far they reach in ‘reworking’ QFT using ASR as I’ve not read the paper.]
[3] arXiv:2201.02206: Precision Bootstrap for the 
This is a note detailing a computational technique used to determine some physical properties of a 3d lattice of ‘spins’ (variables that can take two values like {+1, -1}) in a better way than previously used techniques. Such 3d systems are useful in studying phase transitions (like ice → water → steam), modelling phenomena like ferromagnetism, spin glasses, and even neurons! So having the ability to compute stuff easily and efficiently is always welcome. [I don’t think I can make it any more interesting.]
[4] arXiv:2201.02407: More on Topological Hydrodynamic Modes
The authors discuss fluid/gravity correspondence, a version of holography where fluid dynamics (or hydrodynamics) is described by a gravity theory living in one dimension higher. Specifically, they find a holographic description of a hydrodynamic phenomenon they had discovered earlier. What they had discovered were special kind of very-low-energy (virtually zero) fluid excitations (or modes) by ‘weakly’ breaking energy-momentum conservation in the hydrodynamic system. In this paper, by choosing a special rotating frame of reference (think of a merry-go-round; using Einstein’s equivalence principle, this rotating system can be thought of as a non-rotating system with a gravitational force), they could find existence of the same very-low-energy modes in the holographic setup too. They also extend their holographic construction to include ‘electric’ charges and find similar very-low-energy modes. [Usually, any paper whose title starts with “More on” or “Note on” is more or less an addendum to a previous paper without much of an independent story. I have also been known to succumb to this practice more than once in the past and will do so again later this year.]
[5] arXiv:2201.02412: 
A framework for building algorithms for quantum computing is being explored in this paper. The authors pick a simple quantum mechanical toy model and discuss ideas to extend it to more complex models in various dimensions ranging from 1d to 4d (our world). One of the motivations for considering this seems to be the possible generalization to QCD (quantum chromodynamics; the theoretical framework describing strong nuclear force where exact analytical calculations are quite hard to perform and usually discretized spacetime lattices are used to perform calculations that are expected to give sensible answers in a continuum limit) such that lattice computations for QCD become more ‘robust’. [Don’t know why but this paper reminded me of one of my parody papers that I wrote ages ago, where one simple equation surprisingly described all of physics!]
[6] arXiv:2201.02493: Solving formally the Auxiliary System of 
I don’t think I’ll attempt to explain this. Can someone else give it a try? [What? I’m still only at the half-way mark… Man, this is harder than I expected even after taking into account that explaining things is hard!]
[7] arXiv:2201.02500: Entanglement Entropy in Horndeski Gravity
Entanglement Entropy is a measure of quantum entanglement between two subsystems of a quantum system. Holographic EE is a quantity computed for a certain gravitational system by considering a subregion in a spacetime with boundary, which encodes the EE of a non-gravitational matter system living on that boundary (with the two subsystems required for EE being the chosen subregion on the boundary and the rest of the boundary). The author chooses a general theory of 4d gravity (Horndeski) and computes HEE for an infinite strip subregion of the boundary. Certain information theoretical and thermodynamical aspects of this setup are then discussed. [Isn’t it always a good idea to refrain from citing papers in the abstract? I have been guilty of doing that myself but that one was unavoidable!]
[8] arXiv:2201.02524: On some new types of membrane solutions
The author provides new solutions (of certain extensions of Einstein’s equations of gravity) in M-theory (mother of all string theories) with two time dimensions and discusses their properties, like no black hole formation. [Usually, one deals with only one time dimension.]
[9] arXiv:2201.02572: Topological invariant of 4-manifolds based on a 3-group
This paper claims to be a generalization of another paper from 2007, which has been flagged in the “arXiv admin note”. Anyway, the authors consider an exotic extension of Maxwell’s theory of electromagnetism to study properties of 4d spacetimes. They construct a quantity ‘Z’ which is a topological invariant, by which it is meant that this Z does not depend on the specifics of the 4d spacetimes (for example, how distances are measured on them) but just on the overall shape (topology) of the spacetime. In a certain sense, it means that Z can be used to classify different types of 4d spacetimes (for example, 2d surfaces can be classified by counting the number of holes). [Also note that they give a nice way to avoid using citations in the abstract!]
[10] arXiv:2201.02575: The spatial Functional Renormalization Group and Hadamard states on cosmological spacetimes
The authors study the ‘flow’ of a special state living on cosmologically relevant spatial and temporal scales. They give a concrete mathematical realization of such a state and study how the state flows from high energy (ultraviolet; let’s say near Big Bang) to low energy (infrared; let’s say our humdrum life now). They find that choosing certain states of low energy in pre-inflationary period (inflation is the epoch of exponential expansion in our cosmological past, very close to the Big Bang) and allowing them to flow using their ‘flow equations’ leads to concretely computable property of the state in the kind of spacetime that we are familiar with now! [So many familiar words, yet their exact order renders them challenging to comprehend! Hopefully, I have gotten the gist of the abstract right.]
[11] arXiv:2201.02595: Celestial holography meets twisted holography: 4d amplitudes from chiral correlators
This paper introduces a new program for computing scattering amplitudes (actually, certain parts of it; the whole thing is expected to give the probability for a given scattering event to take place) in 4d field theories (involving photons, W/Z bosons, gravitons, etc.) from certain 6d theories. They recognize these parts of the scattering amplitudes as something originating in different kinds of 4d theories and/or 2d theories. This connection allows them to reproduce well-known results efficiently as well as compute some new results for certain scattering amplitudes. In addition, they also make contact with other programs like “celestial holography” and “twisted holography” that have become quite popular recently. [For the last statement, I refer to my previous post reviewing ISM 2021 and links therein.]
Life in Physics... 