arXiv subjects

Nemanja Kaloper

Publications and source records attributed to Nemanja Kaloper.

At least 19 recordsLinked to original sources

arXiv2026

Wading the String Bog: CMB Birefringence in the Swampland

We point out that observations of cosmic birefringence may provide direct experimental tests of Swampland conjectures. Ultralight scalar field birefringence mechanisms are constrained by Weak Gravity Conjecture, limiting their parameter space. Conversely, confirming that CMB birefringence is caused by ultralight scalars could spell problems for the Swampland framework. Resolving these difficulties without a revision of Swampland ideology points toward thin axionic domain walls, which can explain birefringence without propagating ultralight degrees of freedom. Importantly these conflicting scenarios for cosmic birefringence could be experimentally distinguished by a search for, or exclusion of, birefringence anisotropies, that could be measured by POLARBEAR, Simons Observatory and LiteBIRD. We also note that cosmic birefringence observables emerge via a Sakharov-like asymmetry, where linear polarization is first generated cosmologically and subsequently twisted by a parity-violating dynamics after last scattering.

hep-th↗

arXiv2026

Localization of Chiral Electromagnetic Waves on Thick Axion Domain Walls

We analyze Maxwell theory coupled to an axion domain wall as a spectral boundary value problem. We find that a finite-width axion domain wall generically supports a localized normalizable chiral electromagnetic mode with linear, gapless dispersion. This mode arises from helicity-dependent coupling sourced by the axion gradient: one polarization experiences an effective attractive potential and forms a bound state, while the opposite polarization is repelled. The existence of this chiral surface photon is robust over a wide regime of wall structures and axion masses. Our result shows that axion domain walls generically support a localized chiral photon that has been missed in previous analyses.

hep-th↗

arXiv2026

Chiral Electromagnetic Surface Waves on Chern-Simons Interfaces

We show that Maxwell theory with a codimension-$1$ Chern-Simons interface supports chiral electromagnetic surface waves on the interface, even when the bulk theory on both sides is conventional vacuum electrodynamics in infinite space. Solving the exact boundary value problem we find that the Chern-Simons interaction acts with opposite sign on the two helicities oriented along the interface, giving rise to one normalizable mode localized on the interface. This mode is a gapless chiral surface photon with linear dispersion and a frequency-independent index of refraction set by the Chern-Simons coefficient. This mode exists despite the absence of ambient material response or geometric confinement.

hep-th↗

arXiv2026

CMB Birefringence from Vacuum Interfaces

Hints of cosmic microwave background polarization rotation ($\Delta\vartheta \sim 10^{-3}$ rad) are commonly attributed to late-time dynamics of ultralight axions. We show that such ultralight degrees of freedom are not required. Polarization rotation naturally arises as a geometric interface phase acquired when photons cross interfaces between topologically distinct dark sector vacua. The effect is a discrete phase shift fixed by the normalization of a wall-supported electromagnetic Chern--Simons interaction and protected by an emergent $1$-form symmetry of the low energy effective theory. This mechanism reproduces the familiar adiabatic rotation induced by light axion domain walls, but persists for arbitrarily thin walls where the axion is heavy or absent. In this regime the rotation manifests as a Pancharatnam phase localized at vacuum interfaces, independent of redshift and photon frequency below a natural ultraviolet cutoff. Cosmic birefringence thus emerges as a probe of vacuum structure in the dark sector, rather than of light-field dynamics.

hep-th↗

arXiv2026

Cloaking Cosmic Light

Light crossing dark domain walls that source a top form coupled to gauge Chern--Simons terms mixing visible and dark $U(1)$ gauge fields generically converts into dark photons. The effect is entirely localized on the wall and requires no additional ingredients. The conversion rate is a sharp function of the photon frequency in the wall rest frame, vanishing above the ultraviolet cutoff of the top form sector. Partial cloaking may also induce a rotation of the polarization of transmitted light of order $\Delta\vartheta \sim 10^{-3}$ radians, modify the cosmic microwave background power spectrum, and violate Etherington's reciprocity relation at low frequencies. These effects can impact cosmological determinations of the Hubble rate.

astro-ph.CO↗

arXiv2026

Electromagnetic Couplings of Dark Domain Walls

We extend Maxwell electrodynamics with a Chern--Simons coupling to a dark sector top form sourced by domain walls. Cosmic birefringence can arise from a distinct mechanism in which photon polarization is rotated when crossing vacuum interfaces, rather than through adiabatic propagation in a background field. Ultrathin walls induce a finite, frequency-independent polarization rotation generated by an electromagnetic Chern-Simons term localized at the interface. The effect persists even in the absence of ultralight axions or other propagating scalar degrees of freedom. For phenomenologically viable parameters, such walls can generate cosmic microwave background polarization rotation at the level $\Delta\vartheta \sim 10^{-3}$ rad, providing a signature of the topological structure of the dark-sector vacuum.

hep-th↗

arXiv2026

Very-High-Frequency Gravitational Waves from Multi-Monodromy Inflation

We show that in multi-stage axion monodromy inflation an interruption near the end of the penultimate stage can lead to a spike in the gravitational wave background. These gravitational waves are in the frequency range and with an amplitude accessible to proposed terrestrial detectors such as the Einstein Telescope, Cosmic Explorer, and future Levitated Sensor Detector experiments.

hep-ph↗

arXiv2025

Discretely Evanescent Dark Energy

We propose a new UV-complete dark energy model which is \underbar{\it neither} a cosmological constant nor a slowly rolling scalar field. Our dark energy is the flux of a top form in a hidden sector gauge theory similar to QCD. The top form controls the vacuum energy generated by dark sector CP violation. Its flux discharges by the nucleation of membranes that source it. The tension and charge of the membranes are set by the chiral symmetry breaking scale $\sim 10^{-3} eV$, and the dark energy is a transient. It decays on the order of the current age of the universe. The decays decrease dark energy discretely and randomly, instead of gradually like rolling scalars. Since the decay rate is close to the present Hubble scale, $\Gamma \ga H_0^4$, in a time $\sim {\cal O}(1/H_0)$ the cosmic acceleration may even cease altogether.

hep-th↗

arXiv2025

A Quantal Theory of Restoration of Strong CP Symmetry

We propose a mechanism for relaxing a gauge theory CP violating phase in discrete steps to very small values. The idea is that the CP violating phase includes the magnetic dual of a $4$-form flux which can discharge by the nucleation of membranes. Inside the bubbles surrounded by the membranes, the total CP violating phase is reduced. When the bubbles are produced rapidly during radiation domination in the early universe, near the chiral symmetry breaking scale, they will collide and percolate, melting away into gauge theory radiation and dramatically relaxing CP violation.

hep-ph↗

arXiv2025

Alternative to Axions

The $U(1)$ CP-violating phase which arises below chiral symmetry breaking in a non-Abelian gauge theory is ``secretly" the magnetic dual of the flux of a $U(1)$ $4$-form gauge field. Thus the discharge of this $4$-form flux by Schwinger production of charged membranes reduces the total CP-violating phase toward zero. This can safely restore CP symmetry to within the observational limits, $\theta_{\cancel{\tt CP}} \la 10^{-10}$, while also satisfying all limits from cosmology, when the charge and the tension of the gauge field sources are in the $\la keV$ range.

hep-ph↗

arXiv2024

Reviving QFT in $2+1$ de Sitter Spacetime

We consider a conformally coupled scalar QFT on $2+1$ dimensional static Einstein universe $R \times S^2$, and write down the free theory Hilbert space. We explain that this theory is secretly a QFT in $2+1$ de Sitter space because all the quantum observables experience {\it quantum revivals}, which naturally restricts the timelike $R$ to the appropriate de Sitter time range. Our construction circumvents the causal obstruction to formulating QFT in de Sitter due to event horizons. There aren't any in static Einstein. The `unitary gauge' description of the theory is realized by the zonal harmonics $P_\ell(\hat n \cdot \hat n')$. We verify that interactions with conformally invariant external sources are mediated only by these modes. Hence these modes comprise the complete basis of the "bulk" theory. When the theory is cut off in the UV the basis dimension scales as the Bekenstein-Hawking formula.

hep-th↗

arXiv2024

Falsifying Anthropics

We propose using fuzzy axion dark matter to test the anthropic principle. A very light axion can be directly detectable, at least by black hole superradiance effects. The idea then is that gravitational and astrophysical observations can discover a light axion in the regime where it must be all of dark matter with abundance which must be set up by the anthropic principle, due to excessive primordial misalignment induced by inflation-induced Brownian drift of fluctuations. Yet it may turn out that dark matter is something else instead of this axion. Since the de Sitter-induced axion misalignment controlled only by the de Sitter curvature cannot be evaded, this would invalidate the anthropic prediction of the dark matter abundance.

hep-th↗

arXiv2023

Implications of Weak Gravity Conjecture for de Sitter Decay by Flux Discharge

We examine implications of the weak gravity conjecture for the mechanisms for discharging cosmological constant via membrane nucleations. Once screening fluxes and membranes which source them enter, and weak gravity bounds are enforced, a generic de Sitter space \underline{must} be unstable. We show that when all the flux terms which screen and discharge the cosmological constant are dominated by quadratic and higher order terms, the bounds from weak gravity conjecture and naturalness lead toward anthropic outcomes. In contrast, when the flux sectors are dominated by linear flux terms, anthropics may be avoided, and the cosmological constant may naturally decay toward smallest possible values.

hep-th↗

arXiv2023

Axion Flux Monodromy Discharges Relax the Cosmological Constant

Linear axion monodromy models modulated with higher powers of fields naturally realize the quantum-mechanical flux discharge mechanism for relaxing the cosmological constant toward zero. Working with multiple copies of superposed linear and quadratic flux monodromies, each copy spanned by a pair of fluxes, we show that when the axion is very massive and so effectively decoupled, the membrane discharges relax the cosmological constant toward an attractor $0 < \Lambda/\mpl^4 \ll 1$. If we restrict the flux variations and the intermediate flux values to never venture beyond a finite flux range, the terminal value of the cosmological constant will be tiny but finite. We show how it can reproduce the observed scale of dark energy, and explain how to incorporate matter sector phase transitions.

hep-th↗

arXiv2023

Origin of the Arrow of Time in Quantum Mechanics

We point out that time's arrow is naturally induced by quantum mechanical evolution, whenever the systems have a very large number ${\cal N}$ of non-degenerate states and a Hamiltonian bounded from below. When ${\cal N}$ is finite, the arrow is imperfect, since evolution can resurrect past states. In the limit ${\cal N} \rightarrow \infty$ the arrow is fixed by the "tooth of time": the decay of excited states induced by {\it spontaneous emission} to the ground state, mediated by interactions and a large number of decay products which carry energy and information to infinity. This applies to individual isolated atoms, and does not require a coupling to a separate large heath bath.

hep-th↗

arXiv2023

de Sitter Space Decay and Cosmological Constant Relaxation in Unimodular Gravity with Charged Membranes

General covariant unimodular gravity frameworks, based on the Henneaux-Teitelboim formulation, are, in disguise, precisely $4$-form field theories corrected with higher dimension operators. In the presence of charged tensional membranes, any de Sitter space in all such theories is unstable and decays. If the fluxes sourced by membranes are mutually incommensurate, de Sitter geometries comprise a very refined discretuum of states. Whenever the $4$-form sector is dominated by terms linear in flux the almost-Minkowski space is the unique long-time attractor. As a result, a tiny cosmological constant is natural in all such frameworks, without appealing to anthropic reasoning.

hep-th↗

arXiv2022

Power-law Inflation Satisfies Penrose's Weyl Curvature Hypothesis

Based on entropy considerations and the arrow of time Penrose argued that the universe must have started in a special initial singularity with vanishing Weyl curvature. This is often interpreted to be at odds with inflation. Here we argue just the opposite, that Penrose's persuasions are in fact consistent with inflation. Using the example of power law inflation, we show that inflation begins with a past null singularity, where Weyl tensor vanishes when the metric is initially exactly conformally flat. This initial state precisely obeys Penrose's conditions. The initial null singularity breaks $T$-reversal spontaneously and picks the arrow of time. It can be regulated and interpreted as a creation of a universe from nothing, initially fitting in a bubble of Planckian size when it materializes. Penrose's initial conditions are favored by the initial $O(4)$ symmetry of the bubble, selected by extremality of the regulated Euclidean action. The predicted observables are marginally in tension with the data, but they can fit if small corrections to power law inflation kick in during the last 60 efolds.

gr-qc↗

arXiv2022

New Horizons for Fundamental Physics with LISA

The Laser Interferometer Space Antenna (LISA) has the potential to reveal wonders about the fundamental theory of nature at play in the extreme gravity regime, where the gravitational interaction is both strong and dynamical. In this white paper, the Fundamental Physics Working Group of the LISA Consortium summarizes the current topics in fundamental physics where LISA observations of GWs can be expected to provide key input. We provide the briefest of reviews to then delineate avenues for future research directions and to discuss connections between this working group, other working groups and the consortium work package teams. These connections must be developed for LISA to live up to its science potential in these areas.

gr-qc↗