arXiv subjects

Christian F. Schmidt

Publications and source records attributed to Christian F. Schmidt.

5 recordsLinked to original sources

arXiv2026

Superluminal modes in a quantum field simulator for cosmology from analog trans-Planckian physics

The quantum-field-theoretic description for the U(1)-Goldstone boson of a scalar Bose-Einstein condensate with time-dependent contact interactions is developed beyond the acoustic approximation in accordance with Bogoliubov theory. The resulting effective action is mapped to a relativistic quantum field theory on a dispersive (or rainbow) cosmological spacetime which has a superluminal Corley-Jacobson dispersion relation. Time-dependent changes of the s-wave scattering length to quantum-simulate cosmological particle production are accompanied by a time-dependent healing length that can be interpreted as an analog Planck length in the comoving frame. Non-adiabatic transitions acquire a dispersive character, which is thoroughly discussed. The framework is applied to exponentially expanding or power-law contracting $(2+1)$-dimensional spacetimes which are known to produce scale-invariant cosmological power spectra. The sensitivity of these scenarios to the time-dependence of the Bogoliubov dispersion is investigated: We find a violation of scale-invariance via analytically trackable Transplanckian damping effects if the cut-off scale is not well separated from the horizon-crossing scale. In case of the exponential expansion, these damping effects remarkably settle and converge to another scale-invariant plateau in the far ultraviolet regime where non-adiabatic transitions are suppressed by the high dispersion. The developed framework enables quantitative access to more drastic analog cosmological scenarios with improved predictability in the ultraviolet regime that ultimately may lead to the observation of a scale-invariant cosmological power spectrum in the laboratory.

gr-qc↗

arXiv2025

Emergent quantum field theories on curved spacetimes in spinor Bose-Einstein condensates: from scalar to Proca fields

We consider excitations of a spin-1 Bose-Einstein-condensate (BEC) in the vicinity of different mean-field configurations and derive mappings to emergent relativistic quantum field theories minimally coupled to curved acoustic spacetimes. The quantum fields are typically identified with Nambu-Goldstone bosons, such that the structure of the analogue quantum field theories on curved spacetimes depends on the (spontaneous) symmetry breaking pattern of the respective ground-state. The emergent spacetime geometries are independent of each other and exhibit bi-metricity in the polar and antiferromagnetic phase, whereas one has tri-metricity in the ferromagnetic phase. Compared to scalar BECs, the spinor degrees of freedom allow to investigate massive vector and scalar fields where the former is a spin-nematic rotation mode in the polar phase which can be cast into a Proca field that is minimally coupled to a curved spacetime that emerges on length scales larger than the spin-healing length. Finally, we specify the Zeeman couplings and the condensate trap to be spacetime-dependent such that a cosmological FLRW-metric can be achieved. This work enables a pathway towards quantum-simulating cosmological particle production of Proca quanta via quenching the quadratic Zeeman-coefficient or via magnetic field ramps, which both result in the creation of spin-nematic squeezed states.

cond-mat.quant-gas↗

arXiv2024

Experimental particle production in time-dependent spacetimes: a one-dimensional scattering problem

We experimentally study cosmological particle production in a two-dimensional Bose-Einstein condensate, whose density excitations map to an analog cosmology. The expansion of spacetime is realized with tunable interactions. The particle spectrum can be understood through an analogy to quantum mechanical scattering, in which the dynamics of the spacetime metric determine the shape of the scattering potential. Hallmark scattering phenomena such as resonant forward scattering and Bragg reflection are connected to their cosmological counterparts, namely linearly expanding space and bouncing universes. We compare our findings to a theoretical description that extends beyond the acoustic approximation, which enables us to apply the model to high-momentum excitations.

cond-mat.quant-gas↗

arXiv2024

Cosmological particle production in a quantum field simulator as a quantum mechanical scattering problem

The production of quantum field excitations or particles in cosmological spacetimes is a hallmark prediction of curved quantum field theory. The generation of cosmological perturbations from quantum fluctuations in the early universe constitutes an important application. The problem can be quantum-simulated in terms of structure formation in an interacting Bose-Einstein condensate (BEC) with time-dependent s-wave scattering length. Here, we explore a mapping between cosmological particle production in general (D+1)-dimensional spacetimes and scattering problems described by the non-relativistic stationary Schrödinger equation in one dimension. Through this mapping, intuitive explanations for emergent spatial structures in both the BEC and the cosmological system can be obtained for a large class of analogue cosmological scenarios, ranging from power-law expansions to periodic modulations. The investigated cosmologies and their scattering analogues are tuned to be implemented in a (2+1)-dimensional quantum field simulator.

gr-qc↗

arXiv2023

On the sensitivity of weak gravitational lensing to the cosmic expansion function

We analyse the functional derivative of the cosmic-shear power spectrum $C_\ell^γ$ with respect to the cosmic expansion function. Our interest in doing so is two-fold: (i) In view of attempts to detect minor changes of the cosmic expansion function which may be due to a possibly time-dependent dark-energy density, we wish to know how sensitive the weak-lensing power spectrum is to changes in the expansion function. (ii) In view of recent empirical determinations of the cosmic expansion function from distance measurements, independent of specific cosmological models, we wish to find out how uncertainties in the expansion function translate to uncertainties in the cosmic-shear power spectrum. We find the following answers: Relative changes of the expansion function are amplified by the cosmic-shear power spectrum by a factor $\approx 2-6$, weakly depending on the scale factor where the change is applied, and the current uncertainty of one example for an empirically determined expansion function translates to a relative uncertainty of the cosmic-shear power spectrum of $\approx10\,\%$.

astro-ph.CO↗