There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

We study the imprints of new ultralight particles on the gravitational-wave signals emitted by binary black holes. Superradiant instabilities may create large clouds of scalar or vector fields around rotating black holes. The presence of a binary companion then induces transitions between different states of the cloud, which become resonantly enhanced when the orbital frequency matches the energy gap between the states. We find that the time dependence of the orbit significantly impacts the cloud's dynamics during a transition. Following an analogy with particle colliders, we introduce an $S$-matrix formalism to describe the evolution through multiple resonances. We show that the state of the cloud, as it approaches the merger, carries vital information about its spectrum via time-dependent finite-size effects. Moreover, due to the transfer of energy and angular momentum between the cloud and the orbit, a dephasing of the gravitational-wave signal can occur, which is correlated with the positions of the resonances. Notably, for intermediate and extreme mass ratio inspirals, long-lived floating orbits are possible, as well as kicks that yield large eccentricities. Observing these effects, through the precise reconstruction of waveforms, has the potential to unravel the internal structure of the boson clouds, ultimately probing the masses and spins of new particles.

Gravitational collider physics

We analyze the charge-to-mass structure of BPS states in general infinite-distance limits of $$ \mathcal{N} $$
 = 2 compactifications of Type IIB string theory on Calabi-Yau three-folds, and use the results to sharpen the formulation of the Swampland Conjectures in the presence of multiple gauge and scalar fields. We show that the BPS bound coincides with the black hole extremality bound in these infinite distance limits, and that the charge-to-mass vectors of the BPS states lie on degenerate ellipsoids with only two non-degenerate directions, regardless of the number of moduli or gauge fields. We provide the numerical value of the principal radii of the ellipsoid in terms of the classification of the singularity that is being approached. We use these findings to inform the Swampland Distance Conjecture, which states that a tower of states becomes exponentially light along geodesic trajectories towards infinite field distance. We place general bounds on the mass decay rate λ of this tower in terms of the black hole extremality bound, which in our setup implies $$ \lambda \ge 1/\sqrt{6} $$
 . We expect this framework to persist beyond $$ \mathcal{N} $$
 = 2 as long as a gauge coupling becomes small in the infinite field distance limit.

/pdf/merging-the-weak-gravity-and-distance-conjectures-using-bps-yt6zhxwcsa.pdf

Merging the Weak Gravity and Distance Conjectures Using BPS Extremal Black Holes

We explore whether the axion which solves the strong CP problem can naturally be much lighter than the canonical QCD axion. The $$ {Z}_{\mathcal{N}} $$
 symmetry proposed by Hook, with $$ \mathcal{N} $$
 mirror and degenerate worlds coexisting in Nature and linked by the axion field, is considered in terms of generic effective axion couplings. We show that the total potential is safely approximated by a single cosine in the large $$ \mathcal{N} $$
 limit, and we determine the analytical formula for the exponentially suppressed axion mass. The resulting universal enhancement of all axion interactions relative to those of the canonical QCD axion has a strong impact on the prospects of axion-like particle experiments such as ALPS II, IAXO and many others: experiments searching for generic axion-like particles have in fact discovery potential to solve the strong CP problem. The finite density axion potential is also analyzed and we show that the $$ {Z}_{\mathcal{N}} $$
 asymmetric background of high-density stellar environments sets already significant model-independent constraints: 3 ≤ $$ \mathcal{N} $$
 ≲ 47 for an axion scale fa ≲ 2.4 × 1015 GeV, with tantalizing discovery prospects for any value of fa and down to $$ \mathcal{N} $$
 ∼ 9 with future neutron star and gravitational wave data, down to the ultra-light mass region. In addition, two specific ultraviolet $$ {Z}_{\mathcal{N}} $$
 completions are developed: a composite axion one and a KSVZ-like model with improved Peccei-Quinn quality.

/pdf/an-even-lighter-qcd-axion-3k9ychliua.pdf

An even lighter QCD axion

We describe a method for finding flux vacua of type IIB string theory in which the Gukov-Vafa-Witten superpotential is exponentially small. We present an example with ${W}_{0}\ensuremath{\approx}2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}8}$ on an orientifold of a Calabi-Yau hypersurface with $({h}^{1,1},{h}^{2,1})=(2,272)$, at large complex structure and weak string coupling.

Vacua with Small Flux Superpotential

We study the topological properties of Calabi-Yau threefold hypersurfaces at large h1,1. We obtain two million threefolds X by triangulating polytopes from the Kreuzer-Skarke list, including all polytopes with 240 ≤ h1,1 ≤ 491. We show that the Kahler cone of X is very narrow at large h1,1, and as a consequence, control of the α′ expansion in string compactifications on X is correlated with the presence of ultralight axions. If every effective curve has volume ≥ 1 in string units, then the typical volumes of irreducible effective curves and divisors, and of X itself, scale as (h1,1)p, with 3 ≲ p ≲ 7 depending on the type of cycle in question. Instantons from branes wrapping these cycles are thus highly suppressed.

/pdf/the-kreuzer-skarke-axiverse-19qp89yfoc.pdf

The Kreuzer-Skarke axiverse

We perform an extensive analysis of the statistics of axion masses and interactions in compactifications of type IIB string theory, and we show that black hole superradiance excludes some regions of Calabi-Yau moduli space. Regardless of the cosmological model, a theory with an axion whose mass falls in a superradiant band can be probed by the measured properties of astrophysical black holes, unless the axion self-interaction is large enough to disrupt formation of a condensate. We study a large ensemble of compactifications on Calabi-Yau hypersurfaces, with $1 \leq h^{1,1} \leq 491$ closed string axions, and determine whether the superradiance conditions on the masses and self-interactions are fulfilled. The axion mass spectrum is largely determined by the K\"ahler parameters, for mild assumptions about the contributing instantons, and takes a nearly-universal form when $h^{1,1} \gg 1$. When the K\"ahler moduli are taken at the tip of the stretched K\"ahler cone, the fraction of geometries excluded initially grows with $h^{1,1}$, to a maximum of $\approx 0.5$ at $h^{1,1} \approx 160$, and then falls for larger $h^{1,1}$. Further inside the K\"ahler cone, the superradiance constraints are far weaker, but for $h^{1,1} \gg 100$ the decay constants are so small that these geometries may be in tension with astrophysical bounds, depending on the realization of the Standard Model.

Superradiance in String Theory

We study the topological properties of Calabi-Yau threefold hypersurfaces at large $h^{1,1}$. We obtain two million threefolds $X$ by triangulating polytopes from the Kreuzer-Skarke list, including all polytopes with $240 \le h^{1,1}\le 491$. We show that the Kahler cone of $X$ is very narrow at large $h^{1,1}$, and as a consequence, control of the $\alpha^{\prime}$ expansion in string compactifications on $X$ is correlated with the presence of ultralight axions. If every effective curve has volume $\ge 1$ in string units, then the typical volumes of irreducible effective curves and divisors, and of $X$ itself, scale as $(h^{1,1})^p$, with $3\lesssim p \lesssim 7$ depending on the type of cycle in question. Instantons from branes wrapping these cycles are thus highly suppressed.

The Kreuzer-Skarke Axiverse

We introduce a method for finding flux vacua of type IIB string theory in which the flux superpotential is exponentially small and at the same time one or more complex structure moduli are stabilized exponentially near to conifold points.

Mehmet Demirtas

Papers

Vacua with Small Flux Superpotential

The Kreuzer-Skarke axiverse

Superradiance in String Theory

The Kreuzer-Skarke Axiverse

Conifold Vacua with Small Flux Superpotential