An Intro to our recent paper'Quantum Gravity Meets DESI'

This post is a brief plug for my latest paper collaborated with Prof.Yi-fu Cai’s group and now on arXiv (https://arxiv.org/pdf/2504.07791), accepted by JCAP,

I originally came up with the core idea, and shortly after we began, Brandenberger’s introductory article appeared, which confirmed me and I said to my collaborators that we were on the right track.

Working with Prof.Cai’s group is a real pleasure: I’m proud that in just one week we produced a manuscript. The pace at which cosmologists work never ceases to amaze me—we truly accomplished everything possible in that time.

Here’s how the story begins: when I noticed DESI 2025 data favoring evolving dark energy, I felt uneasy. In string phenomenology, the cosmological constant is normally treated as the vacuum expectation value of a quintessence field, and under the Swampland Distance Conjecture we assume we reside on a flat moduli space. But the latest DESI results reveal two striking things. First, dark energy’s negative pressure is weakening—its equation-of-state parameter is rising toward less negative values. While a weakening cosmological constant can fit certain distance-conjecture expectations, strictly speaking a true constant should remain fixed. Second, both the DESI collaboration’s w0wa fit, and the independent reconstruction from NOAC report multiple crossings of the “w = –1” line. This’s called “Phantom Crossing” or “Quintom Scenario”, which challenges conventional cosmological constant theory.

These findings pose serious issues. Ordinary fields can’t drop below w = –1 unless one invokes phantom fields with the wrong-sign kinetic term—objects that are unstable even classically. From a fluid perspective, w < –1 violates the null energy condition in general relativity, undermining the singularity theorems and raising questions about black-hole and wormhole formation. And single-field quintessence models simply cannot realize a Quintom Scenario, as shown in earlier work by Cai and collaborators. Altogether, these observations confront the string-phenomenology community’s earlier LCDM-based assumptions. As a formal-theory student, I felt compelled to respond—and in need of input from cosmologists, I put this preprint together.

As the old saying goes, “Writing carries the Way.” With this paper, we hope to call formal theorists to pay closer attention to experimental results, especially when they pose such fundamental puzzles. While scalar-field models struggle with NEC violation, modifications of gravity offer geometric effects whose effective potentials can violate the NEC. Existing modified-gravity frameworks were the strongest answers we could deliver within just one week.

In the acknowledgments, I thank Qin-xun for passionately publicizing DESI updates; Dr. Ren Xin for his deep expertise in modification of gravity; Prof. Pi Shi for introducing me to Prof. Shao-jiang’s for his outstanding work on dark energy; Dr. Zhi-zhen for his string-cosmology insights; and Yu Hang for his knowledge of f(Q) and f(T) gravity. Chun Yu led our choice of the f(T) model, performed the calculations, and prepared most of the figures. Dr. Dong Dong refined the manuscript’s style, formatting, and even coined our catchy title—I wish him every success in his academic job search. Prof. Emmanuel at the National Observatory of Athens provided incisive feedback that sharpened our arguments. Finally, “The Lotus and Swampland” workshop organized by Prof. Fengjun and Prof. Babak at TSIMF this past spring played a pivotal role in my growth from a landscape learner into someone ready to lead a project.

Above all, my deepest gratitude goes to Professor Yi-Fu Cai—without his support and kindness, this work would never have come to fruition.

July 7th, 2025

Thomas J. Wang

Contact: ThomasJWang.theophys@outlook.com