投稿者: MoriyaTakashi

Secondary gravitational waves induced by primordial curvature perturbations

Speaker: Takahiro Terada (Nagoya University)

Abstract:

The direct detection of gravitational waves (GWs) marked the beginning of the era of GW and multi-messenger astronomy. In addition to astrophysical sources, we can consider the stochastic GW background as a powerful probe of the early Universe, thereby promoting the development of GW cosmology.  In this talk, I will focus on secondary GWs induced by primordial curvature perturbations, which are generated at second order in cosmological perturbation theory. These GWs carry information about small-scale primordial fluctuations that are inaccessible to cosmic microwave background and large-scale structure observations.  I will explain their physical origin and cosmological significance. For instance, such GWs can probe inflationary models—especially those predicting primordial black holes—as well as the thermal history of the Universe. I will also discuss the relevance of secondary GWs in light of recent results from pulsar timing arrays and highlight prospects for future observations.

Formation and Evolution of Filamentary Clouds Associated with High-Mass Star Formation in the Magellanic Clouds

Speaker: Kazuki Tokuda (Kagawa Univ.)

Abstract:

Recent ALMA observations have achieved ~0.1 pc resolution in the Large and Small Magellanic Clouds (LMC and SMC), enabling the identification of filamentary molecular structures that are thought to be ubiquitous in Galactic star-forming regions. We find that the most luminous high-mass protostars (> 5 × 10^4 L_sun) in the LMC are often embedded in hub-filament systems, and some exhibit coherent, asymmetric head-tail structures extending beyond a single GMC, suggesting triggering by global-scale gas flows. In contrast, high-mass star-forming clouds in the SMC are not always filamentary. This morphological diversity may arise from thermal properties specific to the low-metallicity environment, highlighting the role of metallicity in shaping molecular cloud structures.

The Past and Future of CMB Observations

Speaker: Yuji Chinone (KEK/QUP)

Abstract:

The cosmic microwave background (CMB) radiation has been a cornerstone of modern cosmology. Over the past several decades, CMB observations have revolutionized our understanding of the universe, providing precise measurements of its age, composition, geometry, and evolution. Initial studies focused on temperature fluctuations, but more recently, measurements of linear polarization—particularly the curl-like B-mode pattern—have become central. These observations aim to uncover the smoking-gun evidence of primordial gravitational waves and probe physics beyond the Standard Model.
In this talk, I will begin with an overview of CMB observations and their applications in cosmology, highlighting key scientific milestones. I will then focus on the current and near-future landscape, with particular attention to experiments such as POLARBEAR and the Simons Observatory. I will discuss recent achievements and ongoing efforts to detect primordial B-mode polarization, constrain inflationary models, and explore new physics, including the search for axion-like particles.
Finally, I will outline the long-term vision for CMB research, including next-generation ground-based and space missions, and the role they will play in shaping the future of observational cosmology.

The search for massive stars with black-hole companions

Speaker: Soetkin Janssens (RESCEU, U. Tokyo)

Abstract:

Massive stars play an important role in the evolution of galaxies. With masses larger than eight times the mass of the Sun, these stars regulate galaxy-wide feedback and star formation. They end their lives with a violent supernova, leaving behind a compact object — neutron star of black hole. The formation of black holes is however insufficiently understood, largely due to a limited number observations. While over ten million black holes are predicted to reside in the Milky Way, only very few have been discovered so far. Moreover, in the past years, several black-hole imposters were discovered, highlighting the difficulty of the black-hole search. However, this search is far from over and the coming years may be very promising. While our current methods are very slowly becoming fruitful, a new method is on the way that may uncover hundreds of black holes at once based on astrometric data from the Gaia space telescope. This will reveal how black holes form and will help us to better understand the evolution towards gravitational wave mergers. 

Tracing Extreme Star Formation: Chemical Abundances in Early Galaxies Across Cosmic Time

Speaker: Yuki Isobe (Cambridge University)

Abstract:

Chemical abundance ratios in the interstellar medium of galaxies trace the accumulated contributions of stars with varying masses, serving as key indicators of early star formation. In particular, nitrogen-to-oxygen (N/O) and iron-to-oxygen (Fe/O) ratios have generally been thought to increase with galaxy age: massive stars initially eject oxygen via core-collapse supernovae, while later contributions from lower-mass stars enhance nitrogen and iron abundances. However, this standard evolutionary scenario has been challenged by recent spectroscopic observations using the James Webb Space Telescope (JWST) and large ground-based telescopes. This talk will review these findings across cosmic time, particularly z~0 extremely metal-poor emission-line galaxies with ~solar Fe/O ratios and z>5 galaxies with supersolar N/O ratios. I will also discuss a connection between nitrogen enhancement, dense gas, and active galactic nuclei, providing new insights into early chemical enrichment, clustered star formation, and massive black hole seeding in the early Universe.