# Anomalies in cosmology

# String Theory and Cosmology

# X-correlations

# Gravitation and Analogue Cosmology

### Anomalies in cosmology

##### Vivian Poulin, Mickael Rigault, Alain Blanchard, Pasquale Serpico

With the advent of “precision cosmology”, a number of anomalies of diverse statistical significance has emerged. These include, but are not limited to: the “Hubble tension” between measurements of the Hubble parameter with a variety of late-universe probes (cepheid-calibrated supernovae in particular) and its prediction from the LCDM model fit to early-universe data (Planck CMB data in particular); the “S8 tension” between the amplitude of the clustering measured via weak-lensing surveys and that predicted by the Planck/LCDM cosmology; a recent “growth factor” tension which reminds of the S8 tension; as well as older long-standing “cosmic dipole” anomalies, the famous “lithium problem”, and even some intriguing anomalies within Planck data. As of yet, it is not clear whether these anomalies are connected to each other, or even that they are cosmological in origins. The transverse task force “anomalies in cosmology” aims to gather the efforts of cosmologists interested in these growing (potential) issues of the LCDM model, in order to foster works to better characterize what sorts of systematic effects could be at play, or conversely, what sorts of new physics would be responsible for these anomalies. This will also serve as a platform to discuss with astrophysicists and particle physicists interested in these topics, whose expertise could help the cosmology community in understanding the origin of these anomalies.

### String Theory and Cosmology

#### David Andriot, Pasquale Serpico

In recent years, a renewed interest of the string theory community into reproducing valuable cosmological models has emerged, triggered by the new and precise cosmological observations, but also new theoretical constraints proposed in the context of the Swampland Program, or the development of new techniques such as the cosmological bootstrap. Research activities on the string theory side have highlighted the difficulties in reproducing the simplest cosmological models, namely single-field slow-roll inflation or having a positive cosmological constant, both in agreement with observations. More involved models are prefered, like multifield inflation or quintessence, to explain accelerated expansion phases of our universe, even though theoretical challenges remain for those as well. This situation requires closer interactions with cosmologists to test new models observationnally, and derive related constraints. For cosmologists, this is an opportunity to propose alternative models, as for instance prompted by the recent observational tensions in the standard cosmological model, and evaluate whether they could be string theory motivated.

### X-correlations

#### Thibaut Louis, Karim Benabed, Stephane Ilic

In the coming decade, the advent of the next generation of cosmological surveys will lead to a very accurate measurement of the matter density field of the Universe and its evolution through cosmic times. Large scale structure photometric and spectroscopic surveys will produce tomographics maps of the distribution of matter, using different tracers such as galaxy clustering or galaxy lensing. The next generation of CMB surveys will provide signal-dominated CMB lensing maps over a vast portion of the sky and will also provide tracers of the large scale structures using different secondary CMB anisotropies such as CIB, TSZ, kSZ. Future large scale radio surveys, such as SKA, will also provide other tracers of the LSS and open widows on their evolution at different redshifts.

Each of these tracers provide an incomplete measurement of the underlying distribution of the large scale structures, and suffers from different astrophysical or instrumental systematics. The combination of this different probes will yield the best cosmological information allowing us to both learn the large scale structure distribution and evolution and improve our knowledge of the biases of each of the probes, providing opportunity for ancillary astrophysics.

To reach this goal, several challenges will have to be tackled, such as the improvement of the modelling of each of those probes, their non idealities, as well as their statistical properties and in particular the modelling of their covariances. Solving those challenges will require the development of new analytical and numerical tools. It will also benefit from the expertise of the community in the building and analysis of large hydrodynamical simulation suites allowing to explore the different probes in synthetic data in a coherent way.

Thanks to the large and continuous involvement of the french cosmology community in large scale structures and CMB surveys, our collective expertise is important and well recognized. This GDR transverse group will build on the existing expertise in the french cosmology community, it will organise scientific workshops highlighting recent advances in the field and promote the work of permanent and non permanent researchers.

### Gravitation and Analogue Cosmology

#### Maxime Jacquet, Scott Roberton and Germain Rousseaux.

Analogue Gravity/Cosmology is an ever-expanding field that is experiencing continued success. Indeed, numerous field theory phenomena have been experimentally observed in the laboratory using analogue systems. We may cite three examples:

– Surface waves on inhomogeneous flows provide an historic example of an analogue system, in which the Hawking effect, rotational superradiance, and wormhole traversal have been observed in the laboratory. Institut Pprime is currently looking at the black hole laser effect based on their observation of the Hawking effect. Several new devices are being studied to mimic cosmological phenomena, such as the creation of particles during an inflationary phase, or the merging of analogue galaxies.

– Atomic Bose-Einstein condensates represent another historic platform for Analogue Gravity. The team at Institut d’Optique have used a modulated condensate to mimic and observe certain aspects of preheating physics. Recently, new theoretical developments have accompanied the development of a new experiment which should make it possible to highlight the quantum statistics of the emission in the system.

– Quantum fluids of light constitute a new platform for the quantum simulation of fields in curved space-time, allowing the observation of quantum statistics of fields in a finely controlled geometry. Two experiments are underway at the Kastler Brossel Laboratory, where black hole geometries of the Kerr and Schwarzschild type are studied and where it was recently shown that it is possible to excite, with vacuum fluctuations, the quasi-normal modes of the quantum field. Attempts to observe this effect are currently underway.

The task force will strengthen the links between different Analogue Gravity/Cosmology groups by creating a formal discussion forum, and will facilitate the exchange of doctoral students and post-docs, the organization of conferences, the preparation of grant proposals, etc. In addition, the increased proximity with the GDR COPHY theory groups will make it possible i) to bring the tools of relativity to the attention of condensed matter physicists, and ii) to make results of experiments known to the high-energy community.