Welcome!

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Martin S. Sloth is a professor at University of Southern Denmark (SDU), and group leader for the Universe Origins group. He is the first professor of theoretical cosmology in Denmark. The pioneers of theoretical cosmology include the likes of Einstein, Penrose, Hawking, Guth, Weinberg, and Peebles. We mainly work on understanding the origin of the universe, dark matter, and dark energy. You can find more information about our research here. If you are interested in more popular descriptions of our research, you can look for some of our outreach activities here. Finally, on the group page, you can find a description of the group members.

E-mail: sloth@sdu.dk

Publications of M. S. Sloth: HEP-INSPIRE, Google Scholar.

M. S. Sloth on Linkedin

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In a new paper, “I demonstrate that soft graviton modes in de Sitter are the Goldstone modes of the spontaneously broken asymptotic symmetry group of de Sitter. I then show that any local measurement, including the effects of the environment, will collapse the symmetric state onto the broken state in the large volume limit. In any discussion involving observers, de Sitter is, therefore, best described globally by the broken phase, while local observers, in the small volume limit, can not discriminate between different degenerate global vacuum states, and are therefore best described by the symmetric state. This illuminates the physical nature of soft graviton modes in de Sitter.”

As a bonus, I also speculate about the possibility that a new type of crystals, with the same symmetry-breaking pattern as discussed in the paper, could be realized in the laboratory as an analog system of de Sitter and eternal inflation.

The paper is on arXiv:  https://arxiv.org/abs/2502.03520

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In an update of our recent paper with Aleksandr Chatrchyan, Florian Niedermann, and Vivian Poulin, we have included fits of Cold New Early Dark Energy (Cold NEDE) to the DESI BAO data. 

Using  CMB Planck Plik data, DESI BAO, and Pantheon+ data, we find around 2-sigma evidence for NEDE (see upper left panel below), which is enhanced to 6-sigma evidence when including SH0ES supernovae data (see upper right panel below). 

Without including SH0ES data, a value of H0=73 km/s/Mpc, compatible with SH0ES findings, is allowed within 2-sigma using Planck Plik, DESI BAO, and Pantheon+ data (see lower left panel below). Including SH0ES data, the Lambda-CDM preferred value of H0 less than 68 km/s/Mpc is excluded at 6-sigma in the Cold NEDE model.

For likelihoods with other dataset combinations, parameters, and discussion, see our full paper: https://arxiv.org/abs/2408.14537.

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In a new paper “we explore the possibility that exotic forms of dark matter could expose humans on Earth or on prolonged space travel to a significant radiation dose. The radiation exposure from dark matter interacting with nuclei in the human body is generally assumed to be negligible compared to other sources of background radiation. However, as we discuss here, current data allow for dark matter models where this is not necessarily true”. See: https://arxiv.org/abs/2411.10521

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In a new paper with Aleksandr Chatrchyan, Florian Niedermann, and Vivian Poulin (https://arxiv.org/abs/2408.14537), we generalize the treatment of the NEDE fluid in the Cold NEDE model and test it against updated CMB and Supernovae data. We show that in the Cold NEDE model, the NEDE fluid can, as previously anticipated, be described as a mix of stiff fluid and radiation consistently with data. 

The new CMB Planck NPIPE data and Pantheon+ supernova data slightly increase the tension between ΛCDM and SH0ES. This similarly slightly increases the residual tension in (N)EDE models when relieving the Hubble tension, with Cold NEDE doing slightly better than AxiEDE and other EDE-type models. On the other hand, we know from other studies that the recent DESI data pulls in the opposite direction toward reducing the residual tension by a similar amount. In conclusion, further improvements in data and NEDE model building provide a constructive and promising way forward. 

Finally, we have made a non-trivial test of the Cold NEDE model. In the Cold NEDE model, it is a theoretical prediction that the ratio between the Hubble rate at the time of the phase transition, H*, and the mass of the trigger field, m, should satisfy H*/m ~ 0.2. Being agnostic about the underlying model and fitting H* and m as free parameters to the data is a way of testing this specific prediction of Cold NEDE. It is spectacular that we find a peak in the probability distribution for H*/m between 0.20 and 0.22 as predicted (see figure below).

Screenshot 2024-08-28 at 09.01.28.png

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Two new analyses of the Hubble tension are based solely on data from the JWST telescope and serve as a check of the measurements using the Hubble Space Telescope (HST), which initially led to the tension in the first place. In the paper today (https://arxiv.org/abs/2408.11770), a value of H0 of 72.6 plus-minus 2.0 km/s/Mpc using JWST data alone is reported. This is also consistent with previous values reported by SH0ES using HST data and consistent with the slightly lower values reported by the CCHP team in another analysis based on JWST data alone (https://arxiv.org/abs/2408.06153) when accounting for the smaller subsample of supernovae used by CCHP. Restricting their analysis to the same smaller subsample as used by CCHP, the SH0ES team reproduces their values, a strong indication of overall agreement. It seems we are very close to a confirmation of the Hubble tension independent from the original HST data.

Figure from Riess et. al. 2024 [2408.11770]

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New paper with Mathias Garny, Florian Niedermann, and Henrique Rubira. We show that Hot NEDE provides a UV completion of SIDR and stepped models, bringing them in agreement with the BBN constraints when addressing the Hubble tension: https://arxiv.org/abs/2404.07256

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Exciting new results from DESI today on the large-scale structure of the universe and the measurement of baryonic acoustic oscillations (BAO). The first takeaway relating to New Early Dark Energy (NEDE) as a solution to the Hubble tension is the product of the drag-epoch sound horizon and the scaled Hubble constant of rdh = (101.8 ± 1.3) Mpc, slightly higher than previous measurements.

This might mean that one needs a smaller reduction of the sound horizon to achieve the same increase of H0—as illustrated by superimposing (by eye on the iPad) the constraint on the plot of Know and Millea from 2019 below — making it marginally easier to resolve the Hubble tension within NEDE.