Hideki Tanimura's Homepage

Hideki Tanimura (Cosmologist)

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About me

Status: Postdoc@IAS(Institut d'Astrophysique Spatique, Orsay, France)

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, and Inflation

Contact: hideki.tanimura at ias.u-psud.fr

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.

1, 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. The primary focus of my PhD research has been to study the properties of gas in large scale structure using the SZ effect as a tracer. I am continuing this line of inquiry in my postdoctoral work at the ByoPiC project (ByoPiC: The Baryon Picture of the Cosmos).

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

(Left) The average Planck SZ map stacked against 262,864 LRG (luminous red galaxies) pairs in a coordinate system where one LRG is located at (X, Y) = (-1, 0) and the other is at (X, Y) = (+1, 0). The square region, -3 < X < +3 and -3 < Y < +3, comprises 151~151 pixels. (Right) The residual SZ map after the best-fit circular halo signals are subtracted.

2, Massive neutrinos free-stream and act as hot dark matter, reducing the growth of large-scale structure, compared to a pure cold dark matter model. The small-scale SZ power spectrum is sensitive to neutrino mass. In addition, galaxy clusters are excellent probes of structure formation, and therefore can be complimentary probes of neutrino mass

3, In my view, the most compelling question in cosmology today is: did inflation occur? And if so, at what energy scale? The latest CMB data from Planck, WMAP, SPT, ACT, and other experiments strongly support a ƒ©CDM cosmology. This picture requires that something like inflation took place in the very early universe. If this occurred at the scale of Grand Unification, we expect a relic gravitational wave abundance that would be strong enough to imprint a measurable B-mode pattern in the CMB polarization field. Several ongoing and planned experiments are hoping to observe this signal in the near future. However, galactic foreground emission is expected to be much stronger than the B-mode signal over much or all of the sky, even at the minimum of the foreground frequency window. The dominant source of the polarized emission at frequencies below ~40 GHz is galactic synchrotron emission, but it is relatively poorly measured at this time. My PhD supervisor, Dr. Gary Hinshaw@UBC is ongoing an experiment to produce maps of total intensity and linear polarization at 10 GHz with a new telescope that would initially be sited at the DRAO in Penticton, BC. This data would probe galactic synchrotron emission could be used with other surveys to model and subtract galactic foreground emissions to obtain more accurate CMB data. In my MSc research, I quantified the improvement that 10 GHz data could bring to unpolarized CMB measurements. In my PhD research, I updated this study to include polarization, especially in light of Planckfs much improved dust polarization measurements. The results can be used to forecast the foreground-cleaned sensitivity of planned B-mode experiments, such as LiteBIRD, COrE, and/or PIXIE.


Angular power spectra of CMB with varying tensor-to-scalar ratio. Total intensity, E modes, primordial B modes and lensing B modes are shown.

(Left): Stokes Q spectra estimated by the MCMC fitting including 10 GHz. The solid lines are the input spectra and the dash lines are the output spectra by the MCMC fitting. (Right): Spectra obtained by the MCMC fitting not including 10 GHz.

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Science Accueil
Coursera (Physics)

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