**Welcome to Tsujikawa Lab **

In our lab, we are studying the evolution of Universe from the primordial era to today and the physics of strong gravitational objects like black holes and neutron stars. The basic theory dealing with such phenomena is Einstein's general relativistic theory of gravity. General relativity is known to be consistent with gravitational experiments in Solar System. However, the accuracy of this theory has not been precisely confirmed on microscopic scales like the beginning of Universe or macroscopic scales relevant to today's cosmic expansion.

If we apply general relativity to the very beginning of Universe, the theory predicts the presence of a big-bang singularity. When the size of Universe was smaller than the Planck length, it is commonly believed that gravity was unified with other three fundamental forces in Nature. In the region of such an extreme microscopic scale, we require a quantum theory of gravity to describe a space-time in a quantum mechanical way. There are several candidates for such generalized theories of gravity, e.g., string theory and loop quantum gravity. In spite of much effort, a unified theory of gravity has not been successfully constructed yet.

On the other hand, the precise measurements of Comic Microwave Temperature (CMB) anisotropies placed tight constraints on models of inflation--cosmic acceleration in the very early epoch. The phenomenon of inflation may have some relationship with unified theories of gravity mentioned above. During inflation, the primordial gravitational wave (tensor perturbations) was generated besides scalar perturbations, but the former was not detected yet. Still, the B-mode experiments for the detection of primordial gravitational waves are actively in progress. If they were detected in future observations, it is possible to approach the best model of inflation. This will allow us to reveal the physics around the beginning of Universe as well.

Moreover, today's cosmic acceleration was found in 1998 from the observations of distant type Ia supernovae. The origin of this late-time acceleration is called dark energy. Several independent observations showed that about 70% of today's energy density of Universe consists of dark energy, but its origin has been unknown since its first discovery. There may be a possibility that general relativity is subject to some modifications on scales relevant to the cosmic expansion. Indeed, the research of this viewpoint has been actively performed in literature. In addition, the growth of large-scale structures occurs mostly by a gravitational instability of weakly interacting particles called dark matter. Dark matter is also an unknown component, which shares about 25% of today's energy density of the Universe. To reveal the origins of dark energy and dark matter will be a most challenging problem for the physics in 21-st century.

In 2015, the gravitational wave from a black hole binary was detected, which was exactly 100 years after the first prediction by Einstein. Moreover, the gravitational wave from a neutron star binary was detected in 2017. The latter observation showed that the speed of gravity is very close to that of light. This put strong constraints on dark energy models in extended theories of general relativity. The gravitational waves emitted from those compact objects also allowed us to probe the physics in the strong gravity regime. The dawn of gravitational astronomy will enable us to understand the accuracy of general relativity and the possible deviation from it in many different cosmological scales and regimes.

There are still unknown mysteries in the Universe as explained above, but the future observations will give us hints to reveal their properties. We would like to solve such mysteries theoretically by exploiting upcoming observational data.

# NEWS & TOPICS

- The paper arXiv:2109.12288 (S. Panpanich, K. Maeda) was submitted. (2021.9.25)
- The paper arXiv:2109.12091 (T. Kitamura, S. Miyashita, Y. Sekino) was submitted. (2021.9.24)
- The paper arXiv:2107.11054 (B. Navascués, G. Marugán, S. Prado) was submitted. (2021.7.23)
- The paper arXiv:2107.08061 (S. Tsujikawa, C. Zhang, X. Zhao, A. Wang) was submitted. (2021.7.16)
- The paper arXiv:2106.12273 (S. Miyashita) was submitted. (2021.6.23)
- The paper arXiv:2106.11222 (J. B. Jiménez, D. Bettoni, D. Figueruelo, F. A. Teppa Pannia, S. Tsujikawa) was submitted. (2021.6.21)
- The paper arXiv:2106.05628 (B. Navascues, R. Jimenez-Llamas, G. A. Marugan) was submitted. (2021.6.10)
- The paper arXiv:2105.14661 (M. Minamitsuji, S. Tsujikawa) was submitted. (2021.5.31)
- The paper arXiv:2104.15002 (B. Navascues, G. A. Marugan) was submitted. (2021.4.30)
- Updated to new menbers (2021.4.1)
- The paper arXiv:2103.12342 (S. Tsujikawa) was submitted. (2021.3.23)
- The paper arXiv: 2102.06417 (V. Salzano, S. Tsujikawa et al) was submitted. (2021.2.12)
- The paper arXiv:2102.00124 (B. Navascues, G. Marugan) was submitted (2021.1.30)
- The paper arXiv:2012.12204 (J. B. Jimenez, D. Bettoni, D. Figueruelo, F. A. Teppa Pannia, S. Tsujikawa) was submitted. (2020.12.22)
- The paper arXiv:2011.11894 (T. Fujita, K. Murai, H. Nakatsuka, S. Tsujikawa) was submitted. (2020.11.24)
- Beatriz Elizaga Navascues joined our group as a JSPS fellow. (2020.11.16)
- The paper arXiv:2008.13350 (Ryotaro Kase, S. Tsujikawa) was submitted (2020.8.31)
- The webpage of our lab was made by Shinji Tsujikawa. (2020.07.30)
- Shinji Tsujikawa moved from Tokyo University of Science to Waseda University as a faculty member. (2020.04.01)