Finding gravitational waves from the early Universe Eiichiro Komatsu - PowerPoint PPT Presentation

Finding gravitational waves from the early Universe Eiichiro Komatsu [Max Planck Institute for Astrophysics] Colloquium, AEI Potsdam, February 7, 2020

Full-dome movie for planetarium Director: Hiromitsu Kohsaka

E and B mode • E mode : Polarisation directions parallel or perpendicular to the wavevector • B mode : Polarisation directions 45 degree tilted with respect to the wavevector

Parity • E mode : Parity even • B mode : Parity odd

Parity • E mode : Parity even • B mode : Parity odd

Temperature fluctuations from sound waves E-mode polarisation from sound waves B-mode polarisation from gravitational lensing B-mode from GW

Power spectrum, explained

Seven orders of magnitude in power Temperature from sound waves in “just” 25 years E-mode from sound waves B-mode from gravitational lensing B-mode from GW

CMB Stages Approximate raw experimental noise (µK) Space based experiments Detectors are a big challenge, − 1 Stage − I − ≈ 100 detectors 10 Approximate raw experimental sensitivity ( µ K) Stage − II − ≈ 1,000 detectors Stage − III − ≈ 10,000 detectors WMAP Stage − IV − ≈ 100,000 detectors − 2 10 then Planck now − 3 10 CMB − S4 − 4 10 2000 2005 2010 2015 2020 Year Figure by Clem Pryke for 2013 Snowmass documents 4

Advanced Atacama South Pole Telescope “3G” Cosmology Telescope What comes next? BICEP/Keck Array CLASS

Not just gravitational waves… H0liCOW Collaboration Ground-based CMB polarisation experiments measuring the E-mode polarisation from sound waves precisely will provide independent assessments of H 0 inferred from CMB, which has been derived mostly from temperature anisotropy so far.

Another two orders of magnitude Temperature from sound waves in the next 10–15 years E-mode from sound waves B-mode from gravitational lensing B-mode from GW We want this!!

JAXA ESA + participations from USA, Canada, Europe 2025– [proposed] LiteBIRD 2028– Polarisation satellite dedicated to measure CMB polarisation from primordial GW, with a few thousand TES bolometers in space

JAXA ESA + participations from USA, Canada, Europe 2025– [proposed] LiteBIRD 2028– Selected! May 21: JAXA has chosen LiteBIRD as the strategic large-class mission. We will go to L2!

A Remarkable Story • Observations of the cosmic microwave background and their interpretation taught us that galaxies, stars, planets, and ourselves originated from tiny fluctuations in the early Universe • But, what generated the initial fluctuations?

Mukhanov & Chibisov (1981); Hawking (1982); Starobinsky (1982); Guth & Pi (1982); Bardeen, Turner & Steinhardt (1983) Leading Idea • Quantum mechanics at work in the early Universe • “ We all came from quantum fluctuations ” • But, how did quantum fluctuations on the microscopic scales become macroscopic fluctuations over large distances? • What is the missing link between small and large scales?

Starobinsky (1980); Sato (1981); Guth (1981); Linde (1982); Albrecht & Steinhardt (1982) Cosmic Inflation Quantum fluctuations on microscopic scales Inflation! • Exponential expansion (inflation) stretches the wavelength of quantum fluctuations to cosmological scales

Inflationary Predictions ζ • Fluctuations we observe today in CMB and the matter distribution originate from quantum fluctuations during inflation scalar Mukhanov&Chibisov (1981) Guth & Pi (1982) mode Hawking (1982) Starobinsky (1982) Bardeen, Steinhardt&Turner h ij (1983) • There should also be ultra long-wavelength gravitational waves generated during inflation Grishchuk (1974) Starobinsky (1979) tensor mode

We measure distortions in space • A distance between two points in space d ` 2 = a 2 ( t )[1 + 2 ⇣ ( x , t )][ � ij + h ij ( x , t )] dx i dx j • ζ : “curvature perturbation” (scalar mode) • Perturbation to the determinant of the spatial metric • h ij : “gravitational waves” (tensor mode) • Perturbation that does not alter the determinant X h ii = 0 i

Measuring GW • GW changes distances between two points X d ` 2 = d x 2 = � ij dx i dx j ij d ` 2 = X ( � ij + h ij ) dx i dx j ij

Laser Interferometer Mirror Mirror detector No signal

Laser Interferometer Mirror Mirror detector Signal!

LIGO detected GW from a binary blackholes, with the wavelength of thousands of kilometres But, the primordial GW affecting the CMB has a wavelength of billions of light-years !! How do we find it?

Detecting GW by CMB Isotropic electro-magnetic fields

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