Research & Motivation

With respect to astrophysics and cosmology, we live in truly fascinating times. Upcoming galaxy surveys will allow us to map a large part of our Universe, looking far back in time, with unprecedented precision. Moreover, the observation of gravitational waves offers an exiting alternative window into our Universe. In particular, recent observations of pulsar timing arrays have found evidence for the existence of a stochastic gravitational wave background. With these observational advances, we will have the opportunity to learn more about the Universe than ever before, which will help us to address long-standing questions in cosmology: What is the nature of dark energy, this mysterious component causing the accelerated expansion of space? Does Einstein's theory of general relativity work well on cosmological scales, or do we need to include new physics described by a modified theory of gravity? Moreover, what is the origin of the stochastic gravitational wave backgroud that seems to permeate spacetime?

As we will gain an enormous amount of high-precision cosmological data in the next two decades, we have a good chance to solve these mysteries – or at least come a great step closer to doing so! However, this goal cannot be achieved without overcoming a number of observational and theoretical challenges. In fact, determining which gravity theory is best compatible with cosmological data is a highly non-trivial task. Given the plethora of modified gravity theories and the high computational cost of cosmological data analysis, testing each theory individually is infeasible. Alternatively, by first identifying model-independent observables, gravity modification can be detected robustly and independently from the governing theory of gravity. For this reason, one focus of my research lies on developing such model-independent tests of gravity. Moreover, I aim to explore synergies between pulsar timing arrays and other probes of our Universe, as this can give us novel insights into the origin of the stochastic gravitational wave background.

My complete list of publications can be find below. If you have some more specific questions about my research, or ideas for collaborations, feel free to contact me – I'd be happy to hear from you!

List of Publications

  1. Does dark matter fall in the same way as standard model particles? A direct constraint of Euler's equation with cosmological data, N. Grimm, C. Bonvin and I. Tutusaus,
    Undergoing peer review, arXiv:2502.12843

  2. The impact of large-scale galaxy clustering on the variance of the Hellings-Downs correlation: theoretical framework, N. Grimm, M. Pijnenburg, G. Cusin and C. Bonvin,
    JCAP 03 (2025) 011, arXiv:2404.05670

  3. New measurements of EG: Testing General Relativity with the Weyl potential and galaxy velocities, N. Grimm, C. Bonvin and I. Tutusaus,
    Phys. Rev. Lett. 133 (2024) 211004 , arXiv:2403.13709

  4. The impact of large-scale galaxy clustering on the variance of the Hellings-Downs correlation: numerical results, N. Grimm, M. Pijnenburg, G. Cusin and C. Bonvin,
    Undergoing peer review, arXiv:2411.08744

  5. Measurement of the Weyl potential evolution from the first three years of dark energy survey data, I. Tutusaus, C. Bonvin and N. Grimm,
    Nature Communications 15 (2024) 9295, arXiv:2312.06434

  6. Gravitational Redshift Constraints on the Effective Theory of Interacting Dark Energy, S. Castello, M. Mancarella, N. Grimm, D. Sobral-Blanco, I. Tutusaus and C. Bonvin,
    JCAP 05 (2024) 003, arXiv:2311.14425

  7. Combining chirp mass, luminosity distance and sky localisation from gravitational wave events to detect the cosmic dipole, N. Grimm, M. Pijnenburg, S. Mastrogiovanni, C. Bonvin, S. Foffa and G. Cusin,
    MNRAS 526 (2023) 3, 4673–4689, arXiv:2309.00336

  8. Rescuing constraints on modified gravity using gravitational redshift in large-scale structure, S. Castello, N. Grimm and C. Bonvin,
    Phys. Rev. D 106 (2022) 8, 083511, arXiv:2204.11507

  9. Non-Gaussianity in the squeezed three-point correlation from the relativistic effects, J. Yoo, N. Grimm and E. Mitsou,
    JCAP 08 (2022) 050, arXiv:2204.03002

  10. General relativistic effects in weak lensing angular power spectra, N. Grimm and J. Yoo,
    Phys. Rev. D 104 (2021) 8, 083548 , arXiv:2012.06368

  11. Cutting out the cosmological middle man: General Relativity in the light-cone coordinates, E. Mitsou, G. Fanizza, N. Grimm and J. Yoo,
    Class. Quant. Grav. 38 (2021) 5, 055011 , arXiv:2009.14687

  12. Galaxy Power Spectrum in General Relativity, N. Grimm, F. Scaccabarozzi, J. Yoo, S. G. Biern and J.-O. Gong,
    JCAP 11 (2020) 064, arXiv:2005.06484

  13. Cosmological Information Contents on the Light-Cone, J. Yoo, E. Mitsou, N. Grimm, R. Durrer and A. Refregier,
    JCAP 12 (2019) 015, arXiv:1905.08262

  14. Jacobi Mapping Approach for a Precise Cosmological Weak Lensing Formalism, N. Grimm and J. Yoo,
    JCAP 07 (2018) 067, arXiv:1806.00017

  15. Gauge-Invariant Formalism of Cosmological Weak Lensing, J. Yoo, N. Grimm, E. Mitsou, A. Amara and A. Refregier,
    JCAP 04 (2018) 029, arXiv:1802.03403