iTHEMS Cosmology Forum n°2 - Stochastic gravitational waves: fossils from the early universe

Friday 27 Sept 2024, 09:00 → 18:00 Asia/Tokyo

435-437 Meeting Room (RIKEN Wako Campus)

iTHEMS Cosmology Forum Workshop is a series of short workshops, each focused on an emerging topics in cosmology. The targeted 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 second workshop is devoted to explanations of the stochastic gravitational wave background (SGWB). The SGWB is a significant detection of GW from pulsar timing arrays. The origin of this background could be either supermassive black holes or primordial. Surprisingly, both of these early Universe mechanisms are not well understood. This one-day workshop gathers both the observational and theoretical aspects of this growing topic.

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:

13th September - Registration deadline 

27th September - Workshop Day

Invited Speakers:

Keitaro Takahashi (Kumamoto University)

Kazuyuki Sugimura (Hokkaido University)

Ryusuke Jinno (Kobe University)

Yuichiro Tada (Nagoya University)

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.

09:00 Opening remarks and coffee

1 (Keynote) Opening a new door: Nano-Hertz Gravitational-Wave Astronomy

Pulsar timing array is an experiment to detect gravitational waves with a frequency of nanohertz by accurate long-term observation of pulsars. It will not only open a new door for cosmology and astrophysics but also pave the way for multi-wavelength gravitational-wave astronomy. However, pulsar timing array suffers from various systematic errors such as dispersion delays caused by interstellar plasma and uncertainty in the position and motion of solar system objects, which inhibit detection of gravitational wave signals. Recently, there have been signs of gravitational-wave background through appropriate modeling of systematic errors and correlation between pulsars. The estimated power spectrum of gravitational wave background is consistent with one from supermassive black hole binaries, but other possibilities such as secondary generation due to density fluctuations and inflation are not excluded. In this talk, I cover the principles of pulsar timing arrays, data analysis methods and the physical interpretation of recent results. In addition, I introduce the Indian Pulsar Timing Array, which is a joint project by Japan and India.

Speaker: Dr Keitaro Takahashi (Kumamoto University)

2 Supermassive black hole formation: insights from astrophysical simulations

Supermassive black holes (SMBHs), with masses exceeding 10^6
solar masses (Msun), are believed to reside at the centers of nearly
all galaxies and significantly influence their host galaxies through
the so-called "co-evolution" of galaxies and SMBHs. However, the
origin of SMBHs remains a mystery and a major challenge in modern
astronomy. In this talk, I will first review the current status of
cosmological simulations trying to reproduce the co-evolution of
galaxies and SMBHs. Then, I will present numerical studies that focus
on the formation of SMBHs with masses around 10^6 Msun, which are
considered the starting point of the subsequent growth toward heavier
SMBHs.

Speaker: Dr Kazuyuki Sugimura (Hokkaido University)

11:45 Lunch

3 First-order phase transitions and gravitational wave production in the early Universe

Speaker: Dr Ryusuke Jinno (Kobe University)

14:30 Discussion and Coffee

4 Scalar-induced gravitational waves as a cosmological phonograph

Our universe has experienced phase transitions/crossovers several times throughout its thermal history. The gravitational wave (GW) as a cosmological phonograph can record and tell us about such history thanks to its high penetration ability. In particular, GWs sourced by oscillations of density contrasts called scalar-induced GWs (SIGWs), have sensitivity not only to the background expansion rate but also to the plasma sound speed and hence could serve more information on crossovers. Furthermore, they are closely related to the exotic astrophysical object: primordial black holes. In this talk, the potential capabilities of SIGWs will be presented with a view to future observations.

Speaker: Dr Yuichiro Tada (Nagoya University)

16:30 Panel Discussion