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• Christiansen, Øyvind; Hassani, Farbod & Mota, David Fonseca (2025). Environmental cosmic acceleration from a phase transition in the dark sector. Journal of Cosmology and Astroparticle Physics (JCAP). 2025. doi: 10.1088/1475-7516/2025/01/043. Full text in Research Archive Show summary

  A new degravitation mechanism within the framework of scalar tensor gravity is postulated and included by prescription. The mechanism eliminates all constant contributions from the potential to the Friedmann equation, leaving only the kinematic and the dynamic terms of the potential to drive cosmic acceleration. We explore a scenario involving a density-triggered phase transition in the late-time universe, and argue that the resulting effective energy density and equation of state parameter can explain late-time cosmology when extrapolated to a region of the parameter space.

• Christiansen, Øyvind; Adamek, J.; Hassani, Farbod & Mota, David (2025). Gravitational waves from dark domain walls. Journal of Cosmology and Astroparticle Physics (JCAP). 2025(1). doi: 10.1088/1475-7516/2025/01/149. Full text in Research Archive Show summary

  For most of cosmic history, the evolution of our Universe has been governed by the physics of a `dark sector', consisting of dark matter and dark energy, whose properties are only understood in a schematic way. The influence of these constituents is mediated exclusively by the force of gravity, meaning that insight into their nature must be gleaned from gravitational phenomena. The advent of gravitational-wave astronomy has revolutionised the field of black hole astrophysics, and opens a new window of discovery for cosmological sources. Relevant examples include topological defects, such as domain walls or cosmic strings, which are remnants of a phase transition. Here we present the first simulations of cosmic structure formation in which the dynamics of the dark sector introduces domain walls as a source of stochastic gravitational waves in the late Universe. We study in detail how the spectrum of gravitational waves is affected by the properties of the model, and extrapolate the results to scales relevant to the recent evidence for a stochastic gravitational wave background. Our relativistic implementation of the field dynamics paves the way for optimal use of the next generation of gravitational experiments to unravel the dark sector.

• Christiansen, Øyvind; Hassani, Farbod & Mota, David (2024). Asimulation: Domain formation and impact on observables in resolved cosmological simulations of the (a)symmetron. Astronomy and Astrophysics (A & A). ISSN 0004-6361. 689. doi: 10.1051/0004-6361/202449188. Full text in Research Archive Show summary

  The symmetron is a dark energy and dark matter candidate that forms topological defects in the late-time universe and holds the promise of resolving some of the cosmological tensions. We performed high-resolution simulations of the dynamical and non-linear (a)symmetron using the recently developed relativistic N-body code asevolution. By extensively testing the temporal and spatial convergence of domain decompositioning and domain wall stability, we determined criteria and physical intuition for the convergence. We applied the resolution criteria to run five high-resolution simulations with 12803 grids and a box size of 500 Mpc h−1 of the (a)symmetron. We considered the behaviour of the scalar field and the domain walls in each scenario. We find the effect on the matter power spectra, the HMFs, and observables computed over the past light cone of an observer, such as the integrated Sachs-Wolfe and non-linear Rees-Sciama effect and the lensing, compared to ΛCDM. We show local oscillations of the fifth force strength and the formation of planar structures in the density field. The dynamics of the field was visualised in animations with high resolution in time. The simulation code is made publicly available.

• Christiansen, Øyvind; Hassani, Farbod; Jalilvand, Mona & Mota, David (2023). Asevolution: a relativistic N-body implementation of the (a)symmetron. Journal of Cosmology and Astroparticle Physics (JCAP). 2023(5). doi: 10.1088/1475-7516/2023/05/009. Full text in Research Archive Show summary

  We present asevolution, a cosmological N-body code developed based on gevolution, which consistently solves for the (a)symmetron scalar field and metric potentials within the weak-field approximation. In asevolution, the scalar field is dynamic and can form non-linear structures. A cubic term is added in the symmetron potential to make the symmetry-broken vacuum expectation values different, which is motivated by observational tensions in the late-time universe. To study the effects of the scalar field dynamics, we also implement a constraint solver making use of the quasi-static approximation, and provide options for evaluating the background evolution, including using the full energy density averaged over the simulation box within the Friedmann equation. The asevolution code is validated by comparison with the Newtonian N-body code ISIS that makes use of the quasi-static approximation. There is found a very small effect of including relativistic and weak-field corrections in our small test simulations; it is seen that for small masses, the field is dynamic and can not be accurately solved for using the quasi-static approximation; and we observe the formation of unstable domain walls and demonstrate a useful way to identify them within the code. A first consideration indicates that the domain walls are more unstable in the asymmetron scenario.

• Christiansen, Øyvind; Jiménez, Jose Beltrán & Mota, David (2021). Charged black hole mergers: Orbit circularisation and chirp mass bias. Classical and Quantum Gravity. ISSN 0264-9381. 38(7). doi: 10.1088/1361-6382/abdaf5. Full text in Research Archive Show summary

  We consider the inspiral of black holes carrying U(1) charge that is not electromagnetic, but corresponds to some dark sector. In the weak-field, low-velocity regime, the components follow Keplerian orbits. We investigate how the orbital parameters evolve for dipole-dominated emission and find that the orbit quickly circularises, though not as efficiently as for a gravitationally dominated emission. We then regard circular orbits, and look for modifications in the gravitational waveform from the components carrying small charges. Taking this into account we populate the waveform with simplified LIGO noise and put it through a matched filtering procedure where the template bank only consists of uncharged templates, focusing on the charges' effect on the chirp mass estimation. We find a consistent overestimation of the 'generalised' chirp mass, and a possible over- and underestimation of the actual chirp mass. Finally, we briefly consider the effect of such charges on hyperbolic encounters, finding again a bias arising from interpreting the generalised chirp mass as the actual chirp mass.

• Liu, Lang; Christiansen, Øyvind; Ruan, Wen-Hong; Guo, Zong-Kuan; Cai, Rong-Gen & Kim, Sang Pyo (2021). Gravitational and electromagnetic radiation from binary black holes with electric and magnetic charges: elliptical orbits on a cone. European Physical Journal C. ISSN 1434-6044. 81(11). doi: 10.1140/epjc/s10052-021-09849-4. Full text in Research Archive Show summary

  Abstract Extending the electromagnetic and gravitational radiations from binary black holes with electric and magnetic charges in circular orbits in Liu et al. (Phys. Rev. D 102:103520, 2020), we calculate the total emission rates of energy and angular momentum due to gravitational and electromagnetic radiations from dyonic binary black holes in precessing elliptical orbits. It is shown that the emission rates of energy and angular momentum due to gravitational and electromagnetic radiations have the same dependence on the conic angle for different orbits. Moreover, we obtain the evolutions of orbits and find that a circular orbit remains circular while an elliptic orbit becomes quasi-circular due to electromagnetic and gravitational radiations. Using the evolution of orbits, we derive the waveform models for dyonic binary black hole inspirals and show the amplitudes of the gravitational waves for dyonic binary black hole inspirals differ from those for Schwarzschild binary black hole inspirals, which can be used to test electric and magnetic charges of black holes.

• Liu, Lang; Christiansen, Øyvind; Guo, Zong-Kuan; Cai, Rong-Gen & Kim, Sang Pyo (2020). Gravitational and electromagnetic radiation from binary black holes with electric and magnetic charges: Circular orbits on a cone. Physical Review D. ISSN 2470-0010. 102(10). doi: 10.1103/PhysRevD.102.103520. Full text in Research Archive Show summary

  We derive the equations of motion of dyonic black hole binaries with electric and magnetic charges and explore features of static orbits. By using a Newtonian method with the inclusion of radiation reaction, we calculate the total emission rate of energy and angular momentum due to gravitational radiation and electromagnetic radiation for circular orbits. Moreover, we obtain the evolutions of orbits and calculate merger times of dyonic binaries. We show that electric and magnetic charges significantly suppress the merger times of dyonic binaries. Our results provide rich information about dyonic binaries and can be used to test black holes with magnetic charges.

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• Christiansen, Øyvind; Mota, David & Winther, Hans Arnold (2024). Cosmological Simulations of Phase Transitions in Screened Scalar-Tensor Gravity: Cosmic acceleration, topological defects, gravitational waves and relativistic observables. Universitetet i Oslo. Full text in Research Archive Show summary

  Christiansen presents cosmological simulations of the (a)symmetron, a scalar-tensor theory of gravity undergoing a phase transition in the late-time universe. He demonstrates novel signatures in gravitational waves and other relativistic observables, and explores in detail the formation of domain walls and their imprint on cosmic structure formation, including dark matter halos and the cosmic web. Christiansen uncovers an interesting part of the parameter space where phase transitions occur inside the nonlinear underdensities, and proposes an application of the nonlinear and dynamical theory to explain late-time cosmology as well as to unify the dark sector.

• Christiansen, Øyvind & Mota, David (2020). Charged Black Hole Mergers: Emissions, Orbital Evolution, Gravitational Waveform and Observer Bias. Universitetet i Oslo. Full text in Research Archive Show summary

  Gravitational waveforms from mergers or close-encounters of astrophysical compact bodies provide a new and separate window into high energy physics and dark objects. Although there is a zoo of theoretical compact objects that the current detectors of this channel might be sensitive to, we still have not found anything we did not already expect, apart from slightly surprising high stellar mass black hole merger rates and small final spins, both of which are at slight tension with common binary system formation mechanisms. The reason why no such object has appeared in our data might be because we are not looking for them, detections typically being done using matched filtering -- a statistical method requiring a highly precise theoretical prediction of the signal to dig it out of the noise. In this work we extend some of the analysis already done at lowest order to compact objects carrying some U(1) charge. We consider how the extra interaction and emission modifies the dynamics in the slow-inspiral regime and evolve the orbit until its merger phase. The circularisation is shown to be slightly weaker for orbits where the gauge-field dominates the back-reaction, though both here and in the standard gravitationally dominated emission do we expect to have mergers circularise by the time they enter detector sensitivity, given a large enough initial separation. We find the first correction from U(1) emission to the gravitational emission driven orbit, this representing the small charge limit, and find again the orbital evolution, which in turn gives us the gravitational waveform. Having the waveform, we consider how it would be interpreted in gravitational wave detectors where neutral signals are expected, or whether it would be detected at all, using the least squares method and matched filtering. We find, under some assumptions, that the projection onto neutral templates would bias a large mass, without a very large loss of signal. This would, together with previous research done in the field, point towards a possible explanation for both aforementioned curiosities in the detector data so far. Finally we present discussions on how the analysis might be extended to unbound encounters and mergers carrying electric \textit{and} magnetic charges, presenting the $\mathcal{O}(v)$ order orbital dynamics and emissions for the latter.

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