Mikjel Thorsrud

• Normann, Ben David; Ricciardone, Angelo; Hervik, Sigbjørn & Thorsrud, Mikjel (2022). A study of inhomogeneous massless scalar gauge fields in cosmology. International Journal of Modern Physics, Conference Series. ISSN 2010-1945. s. 1378–1883. doi: 10.1142/9789811258251_0199. Fulltekst i vitenarkiv Vis sammendrag

  Why is the Universe so homogeneous and isotropic? We summarize a general study of a γ-law perfect fluid alongside an inhomogeneous, massless scalar gauge field (with homogeneous gradient) in anisotropic spaces with General Relativity. The anisotropic matter sector is implemented as a j-form (field-strength level), where j ∈ {1, 3}, and the spaces studied are Bianchi space-times of solvable type. Wald’s no-hair theorem is extended to include the j-form case. We highlight three new self-similar space-times: the Edge, the Rope and Wonderland. The latter solution is so far found to exist in the physical state space of types I,II, IV, VI0, VIh, VII0 and VIIh, and is a global attractor in I and V. The stability analysis of the other types has not yet been performed. This paper is a summary of [1], with some remarks towards new results which will be further laid out in upcoming work.

• Thorsrud, Mikjel; Normann, Ben David & Pereira, Thiago dos Santos (2020). Extended FLRW models: dynamical cancellation of cosmological anisotropies. Classical and Quantum Gravity. ISSN 0264-9381. 37(6), s. 1–37. doi: 10.1088/1361-6382/ab6f7f. Fulltekst i vitenarkiv
• Thorsrud, Mikjel (2019). Bianchi models with a free massless scalar field: invariant sets and higher symmetries. Classical and Quantum Gravity. ISSN 0264-9381. 36(23). doi: 10.1088/1361-6382/ab45b3. Fulltekst i vitenarkiv
• Thorsrud, Mikjel (2018). Balancing anisotropic curvature with gauge fields in a class of shear-free cosmological models. Classical and Quantum Gravity. ISSN 0264-9381. 35(9). doi: 10.1088/1361-6382/aab65a. Fulltekst i vitenarkiv
• Normann, Ben David; Hervik, Sigbjørn; Ricciardone, Angelo & Thorsrud, Mikjel (2018). Bianchi cosmologies with p-form gauge fields. Classical and Quantum Gravity. ISSN 0264-9381. 35(9). doi: 10.1088/1361-6382/aab3a7. Fulltekst i vitenarkiv
• Ramazanov, Sabir; Rubtsov, Grigory; Thorsrud, Mikjel & Urban, Federico (2017). General quadrupolar statistical anisotropy: Planck limits. Journal of Cosmology and Astroparticle Physics (JCAP). 2017(3). doi: 10.1088/1475-7516/2017/03/039. Fulltekst i vitenarkiv
• Thorsrud, Mikjel (2015). Quintessence with Kaluza-Klein type couplings to matter and an isotropy-violating vector field, The Thirteenth Marcel Grossmann Meeting: On Recent Developments in Theoretical and Experimental General Relativity, Astrophysics, and Relativistic Field Theories. World Scientific. ISSN 9789814612142. s. 1561–1563. doi: 10.1142/9789814623995_0224. Fulltekst i vitenarkiv Vis sammendrag

  We study the dynamics of a scalar field with Kaluza-Klein type couplings to cold dark matter and an isotropy-violating vector field. The vector coupling, f2(ϕ)F2, has been studied thoroughly in the context of inflation recently. We generalize the model to a dark energy context and study the cosmological consequences. We find a rich set of exact anisotropic power-law solutions and identify a strong vector coupling regime where the anisotropy is controllable and all solutions are close to the ΛCDM limit.

• Thorsrud, Mikjel; Mota, David & Urban, Federico (2014). Local observables in a landscape of infrared gauge modes. Physics Letters B. ISSN 0370-2693. 733, s. 140–143. doi: 10.1016/j.physletb.2014.04.028. Fulltekst i vitenarkiv Vis sammendrag

  Cosmological local observables are at best statistically determined by the fundamental theory describing inflation. When the scalar inflaton is coupled uniformly to a collection of subdominant massless gauge vectors, rotational invariance is obeyed locally. However, the statistical isotropy of fluctuations is spontaneously broken by gauge modes whose wavelength exceeds our causal horizon. This leads to a landscape picture where primordial correlators depend on the position of the observer. We compute the stochastic corrections to the curvature power spectrum, show the existence of a new local observable (the shape of the quadrupole), and constrain the theory using Planck limits.

• Thorsrud, Mikjel; Urban, Federico & Mota, David (2014). Statistics of anisotropies in inflation with spectator vector fields. Journal of Cosmology and Astroparticle Physics (JCAP). 2014(4). doi: 10.1088/1475-7516/2014/04/010. Fulltekst i vitenarkiv
• Barrow, J. D.; Thorsrud, Mikjel & Yamamota, Kei (2013). Cosmologies in Horndeski's second-order vector-tensor theory. Journal of High Energy Physics (JHEP). ISSN 1126-6708. doi: 10.1007/JHEP02(2013)146. Fulltekst i vitenarkiv
• Jimenez, Jose Beltran; Durrer, Ruth; Heisenberg, L. & Thorsrud, Mikjel (2013). Stability of Horndeski vector-tensor interactions. Journal of Cosmology and Astroparticle Physics (JCAP). 2013. doi: 10.1088/1475-7516/2013/10/064. Fulltekst i vitenarkiv
• Thorsrud, Mikjel; Mota, David & Hervik, Sigbjørn (2012). Cosmology of a scalar field coupled to matter and an isotropy-violating Maxwell field. Journal of High Energy Physics (JHEP). ISSN 1126-6708. doi: 10.1007/JHEP10(2012)066. Fulltekst i vitenarkiv
• Hervik, Sigbjørn; Mota, David & Thorsrud, Mikjel (2011). Inflation with stable anisotropic hair: is it cosmologically viable? Journal of High Energy Physics (JHEP). ISSN 1126-6708. doi: 10.1007/JHEP11(2011)146. Fulltekst i vitenarkiv

Se alle arbeider i NVA

• Thorsrud, Mikjel (2019). Gravitasjon på skrivebordet. Fulltekst i vitenarkiv
• Thorsrud, Mikjel (2018). Shear-Free Cosmological Solutions And Their Stability. Fulltekst i vitenarkiv
• Thorsrud, Mikjel (2015). Horndeski’s Vector-Tensor Theory. Fulltekst i vitenarkiv Vis sammendrag

  Horndeski formulated a most general vector-tensor theory that reduces to ordinary electrodynamics in a flat spacetime, in which the dynamical equations are of second order and the vector field respects the gauge symmetry. The theory is remarkably simple containing only a single free parameter which controls the strength of the vector's coupling to gravity. In this talk I will give a short introduction to the theory and report on our attempts to understand its stability and phenomenological consequences, focusing mainly on a cosmological context.

• Urban, Federico; Mota, David & Thorsrud, Mikjel (2014). Bubble observers in Bubbland [ARISF, Paris, pp. 255-258 (2014)]. Fulltekst i vitenarkiv Vis sammendrag

• Thorsrud, Mikjel (2013). Inflation with vector interactions. Fulltekst i vitenarkiv Vis sammendrag

  Vector fields are usually assumed to decay away exponentially fast during inflation and therefore ignored. Nevertheless, with an appropriate coupling to the inflaton, vectors may play an interesting role. In this talk I will first give an introduction to inflation with the f^2F^2 interaction and review some of its rich phenomenology. Next I will focus on the role of super horizon vector fluctuations and their contribution to the primordial power spectrum. Contrary to the contribution from the background vector, their contribution is not deterministically determined by background parameters. Instead the huge cosmic variance associated with large scale fluctuations requires a stochastic approach and the prediction is in terms of a probability distribution function (PDF) which we estimate by Monte-Carlo simulations.

• Thorsrud, Mikjel (2013). Infrared corrections to the primordial power spectrum in the f2(φ)F2 mechanism. Fulltekst i vitenarkiv
• Thorsrud, Mikjel (2013). Cosmologies in Horndeski's vector-tensor theory. Fulltekst i vitenarkiv Vis sammendrag

  In 1976 Horndeski formulated a most general vector-tensor theory in which the vector field respects the gauge symmetry and the resulting dynamical equations are of second order. In a flat spacetime background the theory reduces to ordinary electrodynamics. The theory is remarkably simple containing only a single free parameter which controlst he strength of the vector's coupling to gravity. Until very recently, the cosmological consequences of this theory remained unexplored. In this talk I will introduce you to the theory and tell you about our attempts to understand its consequences in a cosmological context.

• Thorsrud, Mikjel (2012). Quintessence with Kaluza-Klein type couplings to matter and an isotropy-violating vector field. Fulltekst i vitenarkiv Vis sammendrag

  We study the dynamics of a scalar field with Kaluza-Klein type couplings to cold dark matter and an isotropy-violating vector field. The vector coupling, f2(ϕ)F2, has been studied thoroughly in the context of inflation recently. We generalize the model to a dark energy context and study the cosmological consequences. We find a rich set of exact anisotropic power-law solutions and identify a strong vector coupling regime where the anisotropy is controllable and all solutions are close to the ΛCDM limit.

• Thorsrud, Mikjel (2012). Dynamical System Approach to Non-minimal Matter Couplings. Fulltekst i vitenarkiv
• Thorsrud, Mikjel (2012). Doubly Coupled Quintessence. Fulltekst i vitenarkiv Vis sammendrag

  Motivated by the couplings of the dilaton in four-dimensional effective actions, we investigate the cosmological consequences of a scalar field coupled both to matter and a Maxwell-type vector field. The vector field has a background isotropy-violating component. New anisotropic scaling solutions which can be responsible for the matter and dark energy dominated epochs are identified and explored. For a large parameter region the universe expands almost isotropically. Using that the CMB quadrupole is extremely sensitive to shear, we constrain the ratio of the matter coupling to the vector coupling to be less than 10-5. Moreover, we identify a large parameter region, corresponding to a strong vector coupling regime, yielding exciting and viable cosmologies close to the LCDM limit.

• Thorsrud, Mikjel (2011). Stability of Anisotropic Inflation. Fulltekst i vitenarkiv
• Thorsrud, Mikjel (2011). Inflation with Stable Anisotropic Hair. Fulltekst i vitenarkiv
• Thorsrud, Mikjel (2014). Cosmologies with Gauge Vector Interactions. Akademika forlag. Fulltekst i vitenarkiv
• Thorsrud, Mikjel (2010). Post-Newtonian methods and the gravito-electromagnetic analogy. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  In theoretical physics there is a well known analogy between general relativity and electrodynamics which is based on a linearization of Einstein's field equation. Such a kind of analogy is interesting since Einstein's equation (on component form) is extremely complicated mathematically and hard to gain physical insight into. It turns out though, that a lot of papers in the rich literature on the topic are in lack of a systematic method. The main purpose of this thesis is to study the analogy in a more consistent way using state of the art perturbative methods. The analysis is based on the socalled post-Newtonian approximation-scheme which provides a systematic way to expand any metric theory of gravity, and which also takes account for non-linear effects. By choosing suitable variables and gauge (coordinate) conditions, the post-Newtonian limit of general relativity is reformulated in a way which is appropriate for the discussion. The same kind of systematic expansion is also applied to electrodynamics. In this way the theories are compared in a consistent way beyond their lowest order approximations. This work, described in chapter 4, is basically just a comparison of the mathematical structure of the considered approximations of the theories. In the following chapter the perspective is extended by exploring the huge conceptual difference between the theories. Based on calculations the geometric significance of curvature in the post-Newtonian approximation of general relativity is investigated. The most interesting finding from this chapter is that the analogy turns out being stronger when gravitational phenomena are evaluated in a so-called local proper reference frame, which is quite interesting since in that case the theories are treated on a more equal footing conceptually.

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