| 1. |
- Castello, Sveva, et al.
(author)
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A cosmological underdensity does not solve the Hubble tension
- 2022
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In: Journal of Cosmology and Astroparticle Physics. - : IOP Publishing. - 1475-7516. ; :7
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Journal article (peer-reviewed)abstract
- A potential solution to the Hubble tension is the hypothesis that the Milky Way is located near the center of a matter underdensity. We model this scenario through the Lemaître-Tolman-Bondi formalism with the inclusion of a cosmological constant (ΛLTB) and consider a generalized Gaussian parametrization for the matter density profile. We constrain the underdensity and the background cosmology with a combination of data sets: the Pantheon Sample of type Ia supernovae (both the full catalogue and a redshift-binned version of it), a collection of baryon acoustic oscillations data points and the distance priors extracted from the latest Planck data release. The analysis with the binned supernovae suggests a preference for a -13 % density drop with a size of approximately 300 Mpc, interestingly matching the prediction for the so-called KBC void already identified on the basis of independent analyses using galaxy distributions. The constraints obtained with the full Pantheon Sample are instead compatible with a homogeneous cosmology and we interpret this radically different result as a cautionary tale about the potential bias introduced by employing a binned supernova data set. We quantify the level of improvement on the Hubble tension by analyzing the constraints on the B-band absolute magnitude of the supernovae, which provides the calibration for the local measurements of H0. Since no significant difference is observed with respect to an analogous fit performed with a standard ΛCDM cosmology, we conclude that the potential presence of a local underdensity does not resolve the tension and does not significantly degrade current supernova constraints on H0.
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| 2. |
- Högås, Marcus, et al.
(author)
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Analytic Solution of the Magnetic Field and Inductance in a Coaxial Short Circuit
- 2015
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In: IEEE Transactions on Instrumentation and Measurement. - 0018-9456 .- 1557-9662. ; 64:6, s. 1582-1587
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Journal article (peer-reviewed)abstract
- In this paper, the analytic solution of the magnetic field and the inductance in a coaxial short circuit is derived from Maxwell's equations with the appropriate boundary conditions on the short circuit. Helmholtz equation is thus derived for the magnetic field and is solved by a mode matching technique. By integrating the absolute square of the magnetic field the inductance is obtained. The solution is discussed in the light of earlier approximations and solutions and is evaluated both theoretically and through measurements.
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| 3. |
- Högås, Marcus, et al.
(author)
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Constraints on bimetric gravity from Big Bang nucleosynthesis
- 2021
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In: Journal of Cosmology and Astroparticle Physics. - : IOP Publishing. - 1475-7516. ; :11
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Journal article (peer-reviewed)abstract
- Bimetric gravity is a ghost-free and observationally viable extension of general relativity, exhibiting both a massless and a massive graviton. The observed abundances of light elements can be used to constrain the expansion history of the Universe at the period of Big Bang nucleosynthesis. Applied to bimetric gravity, we readily obtain constraints on the theory parameters which are complementary to other observational probes. For example, the mixing angle between the two gravitons must satisfy θ≲ 18^∘ in the graviton mass range ≳ 10-16 eV/c2, representing a factor of two improvement compared with other cosmological probes.
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| 4. |
- Högås, Marcus, et al.
(author)
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Constraints on bimetric gravity. Part I. Analytical constraints
- 2021
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In: Journal of Cosmology and Astroparticle Physics. - : IOP Publishing. - 1475-7516. ; 2021:05
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Journal article (peer-reviewed)abstract
- Ghost-free bimetric gravity is an extension of general relativity, featuring a massive spin-2 field coupled to gravity. We parameterize the theory with a set of observables having specific physical interpretations. For the background cosmology and the static, spherically symmetric solutions (for example approximating the gravitational potential of the solar system), there are four directions in the parameter space in which general relativity is approached. Requiring that there is a working screening mechanism and a nonsingular evolution of the Universe, we place analytical constraints on the parameter space which rule out many of the models studied in the literature. Cosmological solutions where the accelerated expansion of the Universe is explained by the dynamical interaction of the massive spin-2 field rather than by a cosmological constant, are still viable.
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| 5. |
- Högås, Marcus, et al.
(author)
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Constraints on bimetric gravity. Part II. Observational constraints
- 2021
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In: Journal of Cosmology and Astroparticle Physics. - : IOP Publishing. - 1475-7516. ; :5
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Journal article (peer-reviewed)abstract
- Ghost-free bimetric gravity is a theory of two interacting spin-2 fields, one massless and one massive, in addition to the standard matter particles and fields, thereby generalizing Einstein's theory of general relativity. To parameterize the theory, we use five observables with specific physical interpretations. We present, for the first time, observational constraints on these parameters that: (i) apply to the full theory, (ii) are consistent with a working screening mechanism (i.e., restoring general relativity locally), (iii) exhibit a continuous, real-valued background cosmology (without the Higuchi ghost). For the cosmological constraints, we use data sets from the cosmic microwave background, baryon acoustic oscillations, and type Ia supernovae. Bimetric cosmology provides a good fit to data even for large values of the mixing angle between the massless and massive gravitons. Interestingly, the best-fit model is a self-accelerating solution where the accelerated expansion is due to the dynamical massive spin-2 field, without a cosmological constant. Due to the screening mechanism, the models are consistent with local tests of gravity such as solar system tests and gravitational lensing by galaxies. We also comment on the possibility of alleviating the Hubble tension with this theory.
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| 6. |
- Högås, Marcus, et al.
(author)
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Generalized Vaidya solutions in bimetric gravity
- 2020
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In: Classical and quantum gravity. - : IOP Publishing. - 0264-9381 .- 1361-6382. ; 37:14
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Journal article (peer-reviewed)abstract
- In general relativity, the endpoint of spherically symmetric gravitational collapse is a Schwarzschild-[(A)dS] black hole. In bimetric gravity, it has been speculated that a static end state must also be Schwarzschild-[(A)dS]. To this end, we present a set of exact solutions, including collapsing massless dust particles. For these, the speculation is confirmed.
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| 7. |
- Högås, Marcus, 1991-
(author)
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Gravitational collapse in bimetric gravity
- 2019
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Licentiate thesis (other academic/artistic)abstract
- Einstein’s theory of general relativity has been the prevailing theory of classical gravity for over a century. Its observational consistency is unparalleled. There are however a few (arguably) serious short-comings. First, the cosmological standard model, within which general relativity plays a crucial role, implies that the main part of the matter content of the universe is unknown (dark matter). Second, Einstein’s theory cannot be quantized and predicts singularities where the theory breaks down. Hence, there are strong reasons to consider extended theories of gravitation. Bimetric gravity is such a theory. Besides a massless spin-2 field, this theory exhibits a massive spin-2 field.To be competitive, bimetric gravity should describe observational data at least as well as general relativity. Therefore, it is of uttermost importance to analyze the solutions to the equations of motion. In this licentiate thesis, we analyze gravitational collapse in spherical symmetry. We examine an exact solution where the matter content consists of massless and pressureless particles (dust). One of the main conclusions is that the end state is a Schwarzschild space-time which is also a solution to Einstein’s equations. Besides exact solutions, we perform numerical simulations of gravitational collapse in spherical symmetry. Starting with initial data close to a corresponding solution in general relativity, the evolution stays close and there is no evidence of a physical instability. We also investigate the role of symmetries and topology for bimetric gravity.
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| 8. |
- Högås, Marcus, 1991-, et al.
(author)
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Hubble tension and fifth forces
- 2023
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In: Physical Review D. - 2470-0010 .- 2470-0029. ; 108:12
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Journal article (peer-reviewed)abstract
- Fifth forces are ubiquitous in modified theories of gravity. In this paper, we analyze their effect on the Cepheid-calibrated cosmic distance ladder, specifically with respect to the inferred value of the Hubble constant (H0). We consider a variety of effective models where the strength, or amount of screening, of the fifth force is estimated using proxy fields related to the large-scale structure of the Universe. To quantify the level of tension between the local distance ladder and the Planck value for H0, we calculate the probability of obtaining a test result at least as extreme as the observed one, assuming that the model is correct (the p-value). For all models considered, the level of agreement is ≳20%, relieving the tension compared to the concordance model, exhibiting an agreement of only 1%. The alleviated discrepancy comes partially at the cost of an increased tension between distance estimates from Cepheids and the tip of the red-giant branch (TRGB). Demanding also that the consistency between Cepheid and TRGB distance estimates is not impaired, some fifth force models can still accommodate the data with a probability ≳20%. This provides incentive for more detailed investigations of fundamental theories on which the effective models are based and their effect on the Hubble tension.
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| 9. |
- Högås, Marcus, 1991-, et al.
(author)
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Impact of symmetron screening on the Hubble tension : New constraints using cosmic distance ladder data
- 2023
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In: Physical Review D. - 2470-0010 .- 2470-0029. ; 108:2
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Journal article (peer-reviewed)abstract
- Fifth forces are ubiquitous in modified theories of gravity. To be compatible with observations, such a force must be screened on Solar System scales but may still give a significant contribution on galactic scales. If this is the case, the fifth force can influence the calibration of the cosmic distance ladder, hence changing the inferred value of the Hubble constant H0. In this paper, we analyze symmetron screening and show that it generally increases the Hubble tension. On the other hand, by doing a full statistical analysis, we show that cosmic distance ladder data are able to constrain the theory to a level competitive with Solar System tests—currently the most constraining tests of the theory. For the standard coupling case, the constraint on the symmetron Compton wavelength is λC≲2.5 Mpc. Thus, distance ladder data constitutes a novel and powerful way of testing this, and similar, types of theories.
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| 10. |
- Högås, Marcus, et al.
(author)
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Influence on measured conductor AC resistance of high voltage cables when the screen is used as return conductor
- 2015
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In: Proceedings Jicable'15. ; , s. F2.17-
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Conference paper (other academic/artistic)abstract
- At Jicable 2011 a measuring system of conductor AC and DC resistance using synchronous sampling was presented. In the method the current circuit is made coaxial to minimize the influence of external magnetic fields and to minimize error due to mutual coupling to the voltage circuit. However, it has been questioned if the current in the screen induces additional power loss in the conductor due to eddy currents which will influence the measured AC resistance. In this paper we use a first order approximation of an iterative method to show that the additional power loss is negligible.
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