1. 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-G030556-00-M

2. 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 = 10 19 GeV t ~ 10 -43 sec

3. 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. 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. 5 The Gravitational Wave Signal  The spectrum:  The characteristic amplitude

6. 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 ~ 4.4 10 -9 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 10 -11 to 4.4 10 -9 Hz 

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

8. 8 Expected Signal Strength log Omega(f) -5 -10 -15 -6 -3 0 3 6 log f Slow-roll inflation Nucleosynthesis ?

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

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

11. 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 10 21 or 10 -18 meters Suspended Masses

12. 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. 13 Stochastic Background Signal auto-correlation

14. 14 Overlap Reduction Function

15. 15 Simultaneous Detection LIGO Hanford Observatory Livingston Observatory Caltech MIT

16. 16 LIGO Livingston Observatory

17. 17 LIGO Hanford Observatory

18. 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. 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. 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. 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 - 314 Hz) < 23  GW (40Hz - 314 Hz) < 72.4 90% CL Upper Limit LHO 2km-LLO 4km LHO 4km-LLO 4km Interferometer Pair

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

23. 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. 24 LISA: Sources and Sensitivity

25. 25 LISA: Astrophysical Backgrounds

26. 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. 27 Instrument’s sensitivity LIGO-I Advanced LIGO 3 rd generation LISA Correlation of 2 LISA’s Slow-roll inflation

28. 28 Sensitivity Primordial Stochastic Background

29. 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. 30 Astrophysical Signals Limit Sensitivity for Primordial Sources LISA - 1yr 10 -16 Extra- galactic NS-NS WD-WD Extra- galactic WD-WD 10 -13 10 -10

31. 31 Ultimate GW Stochastic Probes -6 -5 -4 -3 -2 -1 0 1 2 3 log f log Omega(f) -11 -12 -13 -14 -15 -16 LISA sensitivity limit (1yr) 3 rd generation sensitivity limit (1yr) WD-WD NS-NS BH-MBH NS ? CLEAN CORRUPTED

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

33. 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 ~10 -11 and 10 -13 in energy density  A future short arm space probe could probe the gap (0.1 – 1 Hz region looks cleanest)