iTHEMS Cosmology Forum n°4 - Evolving Cosmos: new physical insights from new spectroscopic data

4 Aug 2025, 09:30 → 5 Aug 2025, 17:20 Asia/Tokyo

435-437 Meeting Room (RIKEN Wako Campus)

iTHEMS Cosmology Forum Workshop is a series of short workshops, each focusing on an emerging topics in cosmology. The target audience is cosmologists, high-energy physicists and astronomers interested in learning about the subject, not just those who have already worked on the topic. The goal of the workshop is to provide working knowledge of the topic and leave dedicated time for discussions to encourage mutual interactions among participants.

The fourth workshop is dedicated to new physics discoveries enabled by new spectroscopic data. Nearly three decades after the discovery of accelerated expansion, there is at last compelling data pointing away from the simple cosmological constant. The results of new data hint at evolving dark energy, but the statistical significance and physical interpretation are both far from clear. Furthermore, another anticipated new physics measurement of the neutrino mass has also proven difficult. With this workshop, we aim to interrogate both the statistical evidence for new physics as well as the theoretical implications if these new results are confirmed.

This forum will consist of two days. 

The workshop will be in English.

The workshops are organised by the iTHEMS Cosmology Forum study group, which is the successor of the Dark Matter Working Group at RIKEN iTHEMS.

Important dates:

July 18 - Registration deadline 

August 4th, 5th - Workshop Days

Invited Speakers:

Sesh Nadathur (University of Portsmouth)

Andrei Cuceu (LBNL)

Gerrit Farren (LBNL)

Antonio De Felice (YITP)

Linda Blot (IPMU)

Wen Yin (TMU)

Organisers:

Shuntaro Aoki, Nagisa Hiroshima, Derek Inman, Amaury Micheli, Hironao Miyatake, Kai Murai, Ryo Namba

Co-hosted by:

• RIKEN Center for Interdisciplinary Theoretical and Mathematical Sciences (iTHEMS)
• JST ASPIRE program "RIKEN-Berkeley mathematical quantum science initiative" (PI: Tetsuo Hatsuda, Partner PI: Wick Haxton)

Support:

No VISA support can be provided.

Limited support for local expenses may be available for postdocs and non-permanent faculty members. Students may be supported if presenting a poster. Please contact us directly.

Monday 4 August

09:30 Coffee and Opening Remarks

1 Keynote Talk - Dark energy and cosmology from DESI Data Release 2

More than 25 years after the discovery of the accelerated expansion of the Universe, the physical nature of dark energy remains elusive. The Dark Energy Spectroscopic Instrument (DESI) is the first of a new generation of “Stage-IV” cosmology survey experiments aiming to improve this understanding. By precisely mapping the positions of over 50 million galaxies and quasars, DESI is measuring the expansion history of the Universe over the last 11 billion years. I will describe the experiment and discuss the cosmological results from the first 3 years of data, from baryon acoustic oscillations (BAO) and the full shape of the clustering power spectrum. These include exciting hints of an anomaly in the cosmological constant model of dark energy, and unprecedented constraints on the neutrino mass scale. I will describe the nature of the data constraints and comment on the implications for fundamental physics models.

Speaker: Dr Sesh Nadathur (University of Portsmouth)

Nadathur-iTHEMS-DESI-talk.pdf

11:15 Discussion

11:45 Lunch

2 Invited Talk - The Cosmological Constant Problem, Time-Varying Dark Energy, and DESI

The cosmological constant problem, the most serious fine-tuning problem for the standard model parameters, remains a major puzzle, especially in light of Weinberg’s no-go theorem, which forbids natural solutions if dark energy is constant.
Recent DESI observations suggest time variation in dark energy, opening a possible loophole. I present a model that realizes this idea to make the tuning technically natural.
Interestingly the prediction points to a dark energy evolution consistent with DESI DR1 and DR2.

Speaker: Dr Wen Yin (Tokyo Metropolitan University)

RIKENWenYin.pdf

3 Poster Session (with Discussion & Coffee!)

Yu Komiya - The phenomenology of domain walls and PBHs as probes of early universe dynamics
Tomoki Katayama - Bi-Horndeski Theory

Speakers: Tomoki Katayama (Sokendai), Yu Komiya (Yukawa Institute for Theoretical Physics, Kyoto University)

4 Invited Talk - Detecting clustering dark energy using cosmological observations

Clustering dark energy is a class of models in which the speed of propagation of the perturbations in the dark energy component is small compared to the speed of light, causing the perturbations to collapse on scales inside the horizon and affecting the process of cosmic structure formation. This presentation will go over the effects we expect to detect in cosmological observations and how we can use them to constrain the speed of sound of dark energy perturbations. I will then present a numerical approach to simulate such scenarios using the effective fluid description and I will show some preliminary results of the implementation of these methods in a cosmological simulation code.

Speaker: Dr Linda Blot (Kavli IPMU)

riken_talk.pdf

16:30 Discussion

Tuesday 5 August

09:30 Coffee and Discussion

5 Invited Talk - Baryon Acoustic Oscillations from DESI DR2

The Dark Energy Spectroscopic Instrument (DESI) is conducting a 5-year redshift survey of 40 million extra-galactic sources over 14,000 square degrees of the northern sky. One of its primary goals is to measure the cosmic expansion history across a wide range of redshifts using baryon acoustic oscillations (BAO). In this talk, I will present the latest DESI BAO results, which are based on the first three years of data. I will begin with an introduction to DESI and walk through the various steps of the analysis, from collecting galaxy spectra to making accurate and precise BAO measurements.

Speaker: Dr Andrei Cuceu (LBNL)

RIKEN_2025_DESI_DR2_BAO_Andrei_Cuceu.pdf

11:00 Group Photo

11:15 Discussion

11:45 Lunch

6 Invited Talk - Untangling multi-probe hints for beyond-ΛCDM physics

I will discuss the evidence for beyond-ΛCDM physics arising from the combination of different cosmological probes including DESI BAO, primary CMB and CMB lensing. I will review the information that can be extracted from the different probes. The tension between the parameters, particularly the matter fraction, inferred from individually fitting ΛCDM to each probes hints at the potential for new physics. However, I will show that although evolving dark energy is the only model found to fit the data when supernovae observations are included, several model extensions provide viable fits when these contested observations are excluded. Furthermore, it was recently pointed out that within the ΛCDM model a systematic error in the optical depth inferred from the primary CMB is also able to explain the observed discrepancies. Finally, I will look towards future avenues for untangling the complex picture arising from the combination of different probes and making headway in constraining a consistent cosmological model.

Speaker: Dr Gerrit Farren (LBNL)

slides_G_Farren_iTHEMS.pdf

14:00 Discussion (with coffee!)

7 Invited Talk - Spacetime & gravity after/during Planck/DESI/SH0ES

Recently, there has been considerable debate regarding potential evidence for the dynamical nature of dark energy (DE), particularly in light of Baryon Acoustic Oscillations (BAO) measurements released by DESI survey. In this work, we propose an agnostic test that simultaneously constrains the dark energy (DE) equation of state (EoS) and probes the possibility of a transition between the quintessence and phantom regimes, or vice versa. Our initial approach is independent of physical priors, allowing the data to determine which behavior best fits the parameters. We then consider a minimally modified gravity theory known as VCDM, into which we can map our initial approximation, placing it within a theoretically stable framework. To this end, we incorporate the most up-to-date datasets available, including BAO measurements from DESI-DR2, Type Ia Supernovae from the PantheonPlus, DESY5, and Union3 samples, as well as Cosmic Microwave Background (CMB) data from Planck. Our analysis reveals strong and statistically significant evidence for a quintessence-phantom transition across various data combinations. \textit{The strongest evidence is found for Planck+DESI+DESY5, with a significance exceeding ∼5σ in favor of a quintessence-phantom transition at z_†=0.493_{−0.081}^{+0.063}}. Beyond this redshift, the EoS remains within the phantom regime, whereas for z<z†, it favors the quintessence regime. Despite this strong indication, we find that such transitions do not resolve the H0 tension.

Speaker: Dr Antonio De Felice (YITP)

RIKEN-Planck-DESI-SH0ES-August-2025.pdf

15:30 Setup for Panel Discussion

8 Panel Discussion

16:45 Closing Remarks