Hideki Tanimura's Homepage

Hideki Tanimura (Cosmologist)

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About me

Status: Postdoc Fellow at Kavli IPMU, Univ of Tokyo, Kashiwa, Chiba, Japan

Research: Observational cosmology

Topic: The evolution of the large-scale structure of the Universe (cosmic-web, filamentary structure and clusters of galaxies), Cosmic microwave background radiation, Dark matter, and Inflation

Contact: hideki.tanimura at ipmu.jp

My research interests

The aspect of cosmology that attracts me the most is the prospect that we can use the universe as a laboratory to learn about fundamental physics. The existence of dark matter and dark energy clearly points to physics beyond the Standard Model of particle physics, though we are far from understanding what that physics is. Upcoming cosmological observations are likely to test it.

The evolution of large scale structure, from its very simple gaussian initial conditions to the magnifcent cosmic web, poses many challenges to our understanding. On the very largest scales, the evolution is relatively simple, but as we drill down to smaller scales, structures become nonlinear and baryonic physics begins to become important. One of the key tracers of large scale structure are clusters of galaxies: these objects are the most massive bound systems in the universe and they roughly mark the transition from the simple larger scales to the more complicated smaller scales. Baryonic physics is known to play an important role in the evolution of cosmic structure from roughly cluster scales on down, but observational probes of baryonic gas on these scales are difficult to come by. One such probe is the Sunyaev-Zel'dovich (SZ) effect: the inverse Compton scattering of CMB photons by free electrons along the line of sight. A key feature of the SZ effect is that its strength is independent of the redshift of the scattering gas, making it a powerful probe of gas evolution.

(Left) Sunyaev-Zel'dovich Effect and (Right) The spectral distortion of the CMB spectrum by the SZ effect

My main research works (see Publications for detail):

1, Cosmology with the SZ effect

I estimated the cosmological parameters with the SZ effect. The SZ measurement was consistent with the KiDS and DES weak-lensing measurements. But these SZ and WL measurements show a slight tension with the Planck CMB measurement (so called S8 tension).

2, Beyond the standard cosmological model with the SZ effect

To solve the S8 tension between the Planck CMB measurement and low-redshift probes, I extended the É©CDM model, including a decaying dark matter (DDM) model. Two DDM models were tested in my study: one DDM model, where DM decays into a form of noninteracting dark radiation (DR) and the other model, where the DM decays into warm dark matter (WDM) and DR.

(Figure) Impact of DDM model on the SZ power spectrum

3, Cosmic velocity field with CNN

I used a machine learning approach (CNN) to estimate the cosmic velocity field at the position of galaxy clusters. My network is trained to learn the correlation between the velocity field and the galaxy distribution using the Magneticum hydrodynamical simulations.

(Figure) True and estimated line-of-sight velocities with CNN without RSD (left) and with RSD (Right)

4, First detection of warm/hot plasma between galaxies with the SZ effect

Using the Planck SZ map and the SDSS DR12 catalogue of LRGs, I detected warm/hot plasma between pairs of LRGs for the first time by stacking the SZ map aligned with the LRG pairs and estimated the plasma density and temperature.

(Left) The Planck SZ map stacked against 262,864 LRG (luminous red galaxies) pairs. (Right) The residual SZ signals between the LRG pairs after subtracting halo contributions.

5, First detection of warm/hot plasma and dark matter in the cosmic web with SZ, X-ray and CMB lensing

I detected warm/hot plasma in the cosmic-web filaments using the Planck SZ map, ROSAT X-ray map, and eROSITA eFEDS map and estimated the plasma density and temperature. In addition, I estimated the DM density through the detection of the CMB lensing signals.

(Left) ROSAT X-ray map overlaid with the cosmic-web filaments. (Right) eROSITA X-ray map overlaid with the cosmic-web filaments.

Other links

Science Accueil
Coursera (Physics)

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