1 What can gravitational waves do to probe early cosmology? Barry C. Barish Caltech “Kavli-CERCA Conference on the Future of Cosmology” Case Western Reserve University October 10-12, 2003 WMAP 2003 LIGO-G M

2 Signals from the Early Universe Cosmic Microwave background WMAP 2003 The smaller the cross-section, the earlier the particle decouples  Photons: T = 0.2 eV t = 300,000 yrs  Neutrinos: T = 1 MeV t ~ 1 sec  Gravitons: T = GeV t ~ sec

3 GW Stochastic Backgrounds Primordial stochastic backgrounds: relic GWs produced in the early Universe  Cosmology  Unique laboratory for fundamental physics at very high energy

4 GW Stochastic Backgrounds Astrophysically generated stochastic backgrounds (foregrounds): incoherent superposition of GWs from large populations of astrophysical sources  Populations of compact object in our galaxy and at high redshift  3D distribution of sources  Star formation history

5 The Gravitational Wave Signal  The spectrum:  The characteristic amplitude

6 What we know limits on primordial backgrounds The primordial abundance of 4 He is exponentially sensitive to the freeze-out temperature. This sets limit on the number of relic gravitons. The fluctuation of the temperature of the cosmic microwave background radiation. A strong background of gravitational waves at very long wavelengths produces a stochastic redshift on the frequencies of the photons of the 2.7 K radiation, and therefore a fluctuation in their temperature ms pulsars are fantastic clocks! Stability places limit on gravitational waves passing between the pulsar and us. Integrating for one year gives limit at f ~ Hz White dwarf and neutron star binary system has second clock, the orbital period! Change in orbital period can be computed in GR, simplifying fitting. Gives limit from to Hz 

7 Some theoretical “prediction” Strings Super-string Inflation Phase transition Theoretical Predictions Maggiore 2000

8 Expected Signal Strength log Omega(f) log f Slow-roll inflation Nucleosynthesis ?

9 Detection of Gravitational Waves Detectors in space LISA Gravitational Wave Astrophysical Source Terrestrial detectors Virgo, LIGO, TAMA, GEO AIGO

10 Astrophysics Sources frequency range  Gravitational Waves can be studied over ~10 orders of magnitude in frequency  terrestrial + space Audio band

11 Interferometer Concept  Laser used to measure relative lengths of two orthogonal arms As a wave passes, the arm lengths change in different ways…. …causing the interference pattern to change at the photodiode  Arms in LIGO are 4km  Measure difference in length to one part in or meters Suspended Masses

12 International Network  Network Required for: »Detection Confidence »Waveform Extraction »Direction by Triangulation LIGO Hanford, WA LIGO Livingston, LA GEO600 Hanover Germany TAMA300 Tokyo VIRGO Pisa, Italy + “Bar Detectors” : Italy, Switzerland, Louisiana, Australia

13 Stochastic Background Signal auto-correlation

14 Overlap Reduction Function

15 Simultaneous Detection LIGO Hanford Observatory Livingston Observatory Caltech MIT

16 LIGO Livingston Observatory

17 LIGO Hanford Observatory

18 What Limits LIGO Sensitivity?  Seismic noise limits low frequencies  Thermal Noise limits middle frequencies  Quantum nature of light (Shot Noise) limits high frequencies  Technical issues - alignment, electronics, acoustics, etc limit us before we reach these design goals

19 LIGO Sensitivity Livingston 4km Interferometer May 01 Jan 03 First Science Run 17 days - Sept 02 Second Science Run 59 days - April 03

20 Signals from the Early Universe  Strength specified by ratio of energy density in GWs to total energy density needed to close the universe:  Detect by cross-correlating output of two GW detectors: First LIGO Science Data Hanford - Livingston

21 Limits: Stochastic Search  Non-negligible LHO 4km-2km (H1-H2) instrumental cross- correlation; currently being investigated.  Previous best upper limits: »Garching-Glasgow interferometers : »EXPLORER-NAUTILUS (cryogenic bars): 61.0 hrs 62.3 hrs T obs  GW (40Hz Hz) < 23  GW (40Hz Hz) < % CL Upper Limit LHO 2km-LLO 4km LHO 4km-LLO 4km Interferometer Pair

22 Gravitational Waves from the Early Universe E7 S1 S2 LIGO Adv LIGO results projected

23 Interferometers in Space The Laser Interferometer Space Antenna (LISA)  The center of the triangle formation will be in the ecliptic plane  1 AU from the Sun and 20 degrees behind the Earth.

24 LISA: Sources and Sensitivity

25 LISA: Astrophysical Backgrounds

26 Detecting the Anisotropy (1) Break-up the yrs long data set in short (say a few hrs long) chunks (2) Construct the new signal The LISA motion is periodic (3) Search for peaks in S(t): is the observable

27 Instrument’s sensitivity LIGO-I Advanced LIGO 3 rd generation LISA Correlation of 2 LISA’s Slow-roll inflation

28 Sensitivity Primordial Stochastic Background

29 Stochastic Background Astrophysical Foregrounds White-dwarf binary systems (galactic and extra-galactic) (Hils et al, 1990; Schneider et al, 2001) Neutron star binary systems (galactic and extra-galactic) (Schneider et al, 2001) Rotating neutron stars (galactic and extra- galactic) (Giazzotto et al, 1997; Regimbau and de Freitas Pacheco, 2002) Solar mass compact objects orbiting a massive black hole (extra-galactic) (Phinney 2002) Super-massive black hole binaries (extra- galactic) (Rajagopal and Romani, 1995)

30 Astrophysical Signals Limit Sensitivity for Primordial Sources LISA - 1yr Extra- galactic NS-NS WD-WD Extra- galactic WD-WD

31 Ultimate GW Stochastic Probes log f log Omega(f) LISA sensitivity limit (1yr) 3 rd generation sensitivity limit (1yr) WD-WD NS-NS BH-MBH NS ? CLEAN CORRUPTED

32 Future experiments in the “gap” (?) A. Vecchio

33 Conclusions  Primordial Gravitational Wave Stochastic Background is potentially a powerful probe of early cosmology  Present/planned earth/space-based interferometers will begin to probe the sensitivity regime of interest.  They will either set limits constraining early cosmology or detect the stochastic background  They are ultimately limited to ~ and in energy density  A future short arm space probe could probe the gap (0.1 – 1 Hz region looks cleanest)