iTHEMS Cosmology Forum n°3 - (P)reheating the primordial Universe

4 Mar 2025, 09:30 → 5 Mar 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 third workshop is devoted to the 'reheating' phase of the early Universe. Reheating bridges the gap between the (almost) empty universe at the end of cosmic inflation and the thermal state of particles, required for Big-Bang nucleosynthesis, and the events of the hot Big-Bang model as a whole, to unfold. It is expected to proceed in different stages starting with a violent parametric resonant creation of particles, dubbed preheating, followed by a redistribution of energy leading to a thermal state. This phase potentially hosts rich phenomenology such as the formation of topoligical defects e.g. solitons, generation of gravitiational wave, and so on. Yet, the very non-linear nature of reheating makes it notoriously hard to describe analytically, and even numerical simulations struggle to follow the whole sequence of events in a given model. Reheating studies have thus yet to reach the degree of compherensiveness and universality that the understanding of cosmic inflation has achieved.

This forum will consist of two events.  The first, on March 4th, will be in conference format comprising scientific talks on research trends in (P)Reheating.  The second, on March 5th, will be a tutorial on numerical aspects of reheating (both theory and hands-on with code) hosted by Tom Giblin of Kenyon College.

The workshop will be in English.

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

Important dates:

Feb. 25th - Registration deadline 

March 4th - Workshop Day

March 5th - Tutorial Day

Invited Speakers:

John T. Giblin - Kenyon College

Kyohei Mukaida - KEK

Seishi Enomoto - Yokohama National University

Pankaj Saha - KEK

Organisers:

Kohei Hayashi, Nagisa Hiroshima, Derek Inman, Amaury Micheli, Ryo Namba

Support:

No VISA support can be provided.

Limited support for local expenses might be available for postdoc and non-permanent faculty members only. Please contact us directly.

Tuesday 4 March

09:30 Coffee and Opening Remarks

1 Keynote Talk - A Snapshot of Preheating: success, challenges and possibilities

Speaker: Dr Tom Giblin (Kenyon College)

ITHEMS_Giblin.pdf

2 Perturbative Reheating and Thermalization

In this talk, we will discuss in detail the process of reheating that is dominated by the perturbative decay of inflaton.
Thought it appears to be merely a textbook exercise, we will see that the underlying physical processes are involved, which calls for e.g., the understanding of in-medium cascade of energetic particles.
We will also discuss how this refined understanding affects the predictions based on conventional picture.

Speaker: Dr Kyohei Mukaida (KEK)

cascadessm_kmukaida.pdf

12:15 Lunch

3 The exact WKB analysis as an analytic approach to preheating

Preheating is a crucial phase in the early universe where non-perturbative particle production occurs due to the oscillating backgroud field. While numerical methods have been widely used to study preheating dynamics, analytic approaches remain essential for gaining deeper theoretical insights and generalizing results across different models. In this talk, I will present the exact WKB analysis as a powerful analytical tool to investigate particle production during preheating.

Speaker: Dr Seishi Enomoto (Yokohama National University)

iTHEMS_Cosmology_Forum_3rd(20250304).pptx

14:15 Group photo

4 Poster session (with coffee)

Reheating Process after Axion Inflation - Tenta Tsuji
High frequency gravitational waves from Gauss-Bonnet inflation - Kamil Mudrunka

Speakers: Kamil Mudrunka (Tohoku University), Tenta Tsuji (KEK)

5 Reheating after Inflation: Characteristics and Constraints

Reheating, the period following inflation, marks the transition where the inflaton decays, populating the early universe with matter and radiation.
This phase remains relatively unexplored due to the absence of direct observational evidence and the complex, non-linear dynamics involved.
This talk will examine the characteristics and constraints of the reheating phase.
As reheating typically unfolds in multiple stages, I will describe the general features of the stages, drawing on some results from lattice simulations.
I will describe how the reheating phase can be linked to Cosmic Microwave Background (CMB) observables via its Gravitational Waves signature,
Finally, I will show how even subtle features in the inflationary potential at very small scales can significantly alter the dynamics of reheating and its potential observable signatures.

Speaker: Dr Pankaj Saha (KEK)

iTHEMESCosmo.pdf

16:00 Setup for Panel Discussion

6 Panel discussion

Speakers: Dr Naoya Kitajima (Tohoku University), Pankaj Saha (KEK), Seishi Enomoto, Tom Giblin (Kenyon College)

17:00 Closing Remarks

Wednesday 5 March

10:00 Welcome Coffee

7 Theory Session

Speaker: Dr Tom Giblin (Kenyon College)

ITHEMS_COMP_morning.pdf

12:00 Lunch and discussions

13:00 Free discussions and coffee

8 Practical Session

Speaker: Dr Tom Giblin

ITHEMS_COMP_afternoon.pdf