Gravitational waves from the early Universe Part 1 Sachiko Kuroyanagi (Nagoya University) 26 Aug 2017 Summer Institute 2017
What is a Gravitational Wave?
What is a Gravitational Wave?
11 Feb 2016 We have detected gravitational waves. We did it! Hanford Washington Livingston, Louisiana
Detection on 14 Sep 2015
It was from a binary black hole GWs from Black hole binary 1.3billion light years away 36 + 29 = 62 solar mass black hole
Detection with a interferometer
If a gravitational wave pass through
Note: what is strain? h D 0 1 1 2 h D max/min = D 0 [1 ± 1 2 h] LIGO: strain Fabry-Perot D0 h = (4km 200) 10-21 ~ 10-15 m ~ size of atomic nucleus
Observation run 2: 30 Nov 2016-25 Aug 2017 More detections GW150914 36Msun + 29Msun All from black hole binaries GW151226 14Msun + 7.5Msun Observation run 1: 12 Sep 2015-19 Jan 2016 GW170104 31Msun + 19Msun
Hints for black hole formation history Test of strong gravity ~400Mpc ~900Mpc Event rate
More GW sources are expected Astrophysical Neutron star binaries Supernovae Pulsars
More GW sources are expected Astrophysical Cosmological Inflation Neutron star binaries Reheating Supernovae Phase transitions Pulsars Cosmic strings My talk
Observing the early Universe
The most distant galaxy (March 2016): GN-z11 light was emitted 13.4 billion years ago 31.9 billion light-year away now 0.4 billion years after the birth old galaxies new galaxies The further the galaxy, the older the signal
The oldest signal: Cosmic Microwave background (CMB) lights from 13.8 billion years ago 0.38 million years after the birth
Can we see further? No (currently) The temperature of the Universe was high in the past high 3000K low 3K electron photon photons are scattered by electrons
Can we see further? Gravitational waves can! Gravitational waves do not interact with electrons
Advantages of gravitational wave observation Only gravitational waves can directly bring us the information of the early Universe! log(time) 380,000 years 13.8 billion years Cosmic Microwave Background now Inflation light gravitational waves observer reheating cosmic phase transitions Full of information on high energy physics!
Multi wavelength observation of GWs
Sensitivities of gravitational wave experiments GWs from inflation
Ground-based interferometers LIGO-India (2024) Advanced-LIGO (2015) KAGRA (2018) full design sensitivity in 2019 Advanced-VIRGO (2017) 3km Started observation on 1st August 2017 full design sensitivity in 2021 Test run with room temperature has done in March 2016 Cryogenic test run is planned in March 2018 full design sensitivity in 2022
Interferometers in space LISA ESA + NASA launch in 2034 pathfinder (test for technology) in 2015 better sensitivity than expected B-DECIGO proposed in Japan 2020 s DECIGO 2030 s 40 s? Triangle formation 4 better angular resolution correlation analysis for GW background
How to detect a stochastic background GWs from the early Universe random phase no directional dependence strain very similar to noise Cross Correlation time [s] detector1 s 1 (t) =h(t)+n 1 (t) detector2 s 2 (t) =h(t)+n 2 (t) s: observed signal h: gravitational waves n: noise no correlations 0 GW signal We need multiple detectors
Indirect detection of GWs Pulsar timing array SKA (Square Kilometer Array) 2020- International project for radio telescope Pulsar (= rotating neutron star) precise period GWs change the arrival time of the pulses B-mode polarization in CMB Many ground-based and space project LiteBIRD 2022- GWs induece polarizations in the Cosmic Microwave background
large detector size small GWs from inflation
Generation of GWs in the early Universe
Equation for GWs in the expanding Universe Einstein equation G µ =8 GT µ Curvature of the space-time = Gravity Matter
Equation for GWs in the expanding Universe Einstein equation G µ =8 GT µ Geometry Flat Universe tensor perturbations (=GWs) a(t) : scale factor 1 ḧ ij +3Hḣij a 2 2 h ij = 16 G ij H ȧ a H affects GW evolution anisotropic stress : transverse-traceless part of T µ
Hubble expansion rate Friedmann equation: H 2 = 8 G 3 K a 2 H ȧ a : Hubble parameter = r + m + density H 2 radiation a -4 matter a -3 sum of energy densities radiation matter r a 4 m a 3 dark energy? constant scale factor : a
Meaning of H H ȧ a speed of expansion d H ch 1 : Hubble horizon region of causality x v = Hx v<c we have information of the object v>c no information To be precise L H = Z t 0 cdt a(t) d H for radiation- and matter-dominated Universe
What happened in the early Universe? From Cosmological observations radiation- matterdark energy? dominated dominated (relativistic particles) (non-relativistic particles) observer log(time) 380,000 years 13.8 billion years
What happened in the early Universe? accelerated expansion inflation reheating mechanism to heat the Universe cold dense and hot observer log(time) 380,000 years 13.8 billion years
What happened in the early Universe? The grand unification theory predicts separation of forces inflation reheating cosmic phase transitions observer log(time) 380,000 years 13.8 billion years
What happened in the early Universe? cosmic superstrings GUT scale string: 10 18 kg/cm ~1000 Mt.Fuji inflation reheating cosmic phase transitions cosmic strings observer log(time) 380,000 years 13.8 billion years
Equation for GWs GW generation 1 ḧ ij +3Hḣij a 2 2 h ij = 16 G ij 1. Non-negligible initial condition Inflation quantum fluctuations 2. Sourced by matter component of the Universe Preheating rapid particle productions Phase transition bubble collisions Cosmic strings heavy string objects generated in phase transition
Equation for GWs GW generation 1 ḧ ij +3Hḣij a 2 2 h ij = 16 G ij wavelength of GWs at generation < Hubble horizon size f/a < H Hubble horizon = region of causality 1 H / 2 / T 2 rad a / T f / T 2. Sourced by matter component of the Universe Preheating rapid particle productions Phase transition bubble collisions Cosmic strings heavy string objects generated in phase transition
Sensitivities of gravitational wave experiments GWs from inflation
Sensitivities of gravitational wave experiments GWs from inflation Reheating T~10 7 GeV ~10 10 GeV Reheating change the expansion rate
Sensitivities of gravitational wave experiments T~10 4 GeV ~10 7 GeV GWs from inflation Reheating exotic model of the expansion can enhance the amplitude
frequency at the generation ~ (Hubble horizon) -1 present f / T past GWs from rapid particle productions Preheating T~10 9 GeV ~10 15 GeV
frequency at the generation ~ (Hubble horizon) -1 present f / T past Electroweak phase transition T~100GeV GWs from bubble collisions
frequency at the generation ~ (Hubble horizon) -1 present f / T past tension Gμ~10-10 Gμ~10-12 cosmic string heavy string objects originating from phase transition or superstring theory
Summary GWs can become a powerful probe of the very early Universe! log(time) Inflation 380,000 years Current observation 13.8 billion years now Observations observer reheating cosmic phase transitions more details on inflation and cosmic strings in part 2