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COSMICBACKGROUND GRAVITATIONALWAVE RADIATION AND PROSPECTSFORITSDETECTION

Allan JoelAndersonPlanetary Geodesy and Geophysics, The University of Uppsala, 5—75590 Uppsala,Sweden

ABSTRACT

Calculatedenergy fluxes of gravitational waves from various astrophysicalsources,when taken togetherwith estimatesof their respectiveeventratesandspatialdistributions, indicatethat an incoherentisotropicbackgroundof gravitational waves. This radiation appearsto be most intensein the spectralrange from10_i to iO~Hz. Gravitational wave detectorsdesignedto operatein space,which cover this waveband,should see this radiation at measuredlevels of spatial strain near or below 10_18 8L/t. Severalpossibilities for operatingsuch detectorsare discussedin this paper.

INTRODUCTION

Since 1969, observationsdesignedto detect gravitational waves havebeen carried out using deepspacetracking techniques. Theseexperimentsbegan their operationswith sensitivitiesapproachingiO’~andimprovements are projected to 10_16 sometime in the 1990 timeframe. Gravitational wave fluxcalculations from hypothetical sourcesare highly uncertain at this stage, however most astrophysicalcalculationsof gravitational wave amplitudesset expectedlevels below 10_iT. One major consequenceof these types of calculationsis that such radiation would most likely be seenas an incoherentisotropicbackground.

Specific procedureshavebeendesignedto searchfor this incoherentbackgroundusing current spacecrafttracking experiments. Recently, severalproposalshavebeenmadeto deploy multi-arm interferornetersinspacewhich would be sensitivities to this incoherentbackgroundat levels below 10_IT.

GRAVITATIONAL WAVE FLUX ESTIMATES

Over the past decade,a number of papers have appearedthat have reviewed our current knowledge ofgravitational wave productionin various astrophysicalenvironments. Figure 1 gives a summaryof theseestimates,hereshownin termsof the wave flux andspectrafor the respectivesource. The diagonallinesindicate the spatial strain that would result from these fluxes and frequencies. The ribbed lines showobservationallimits that can be set through direct or indirect observationsas well as thoseprojectedforfuture experiments. Table 1 is a legend that identifies items marked in Figure 1.

INDIRECT OBSERVATIONS

Limits to the amount of gravitational wave energy in various regions of the spectrumcan be set byindirect observations. Here we have considered4 kinds of measurements.

1) Results from binary pulsar timing experiments.2) Statistics of quiet pulsar timing variations.3) Large scale microwave backgroundvariations.4) Small scale microwave backgroundvariations.

Current limits on a cosmic backgroundof gravitationalwaves have beenset by the observedhomogeneityof the 3 degreeKelvin electromagneticbackground. Theselimits setbounds to theextremelow frequencyend of the spectrumwhich relate to generationmechanismsat the very earliest epochsof the cosmos.These observations have been made by the Soviet spacecraft experiment RELICK /1/ and byexperimentersat Princetonand Berkeley. An improvedversion of this experiment is to be made on theNASA COBE mission sometime early in the 1990’s.

(9)103

(9)104 A. J. Anderson

Recently, the measurementof the stability of the receivedbinary pulsar timing experimentshave allowedus to set an upperlimit on the amount of gravitational wave energy in the very low frequencyband forperiodsbetween1 month and 10 years/2/. In addition, the timing of an ensembleof observedpulsarsalso would allow us to set an upper bound at somewhatlower frequenciesand with somewhatgreateruncertainty using the present measurements.

B 1O9~(44/fl • .14 -16 ~18 -20 -22

~ /~A ~ A / i, /-2 PAI9~I

1

1°c

-10 PAll ¼.

~ ~4 ~ l~ 1~

10910 (‘~~)

Fig. 1. Isotropic Cosmic Gravitational Wave Background

SOURCES LDIIITS SIT BY INDIRECT OBSERVATIONS OIRECT OBSERVATIONS

8 • Binary 5~Sta0S NP • Binary Pulsar Timing VOYAGER• Voyager Spacecraft Analysis (1981)

BHB • Black Holes Binaries QP • Quiet Pulsar Timing GALILEO • Galileo 0 Sand Estimates (1996)

CS • Cosmic Strings MB • Microwave Background (large scale) SMILE • Space Microwave InterfermileterL using TORSS and small ion

06 — Dwarf Binaries MB

5 • Microwave Backgrouud (smell scale) propulsion probes (1998)

01 • 100 5eV SPACE LASER INTERFERBIETER — Full DoubleDragfree Laser Light Fringe

QCD • 100 15eV Space Interferometer (u 2000)

QI • Superinassive Pulsars

Q2 • Rapid Massive Collapses

Q4 • Relativistic Star Clusters

010 • Quasars with lO

10M BK

SMBH • Supermassine Black Holes

PAlO — Parametric 1019 GaY

PAll • Parametric 1017 0eV

SN • Super Nova Explosions

1987a • 1987a Estimates

1°c • l°K Gravitons — Plnnck Scale

Interactions

Table 1. Key to Figure 1

Detectionof CosmicBackgroundRadiation (9)105

DIRECT OBSERVATIONS

Spacecrafttrackingexperimentscurrently operatein the spectralbandwith periodsbetween100 to 20000seconds. Limitations imposed on current experimentsby interplanetaryplasms,troposphericfluctuations,and clock stability do not allow sensitivitiesmuch beyond a few tens of closure density to be reached.Several experimentalproposalshavebeenmade /3/ that would eliminate theseeffects and allow futurespaceexperimentsto be carriedout with sensitivitieswell below that necessaryfor what is believedto bedetectioncapability. The more ambitiousof theseproposals,the laserinterferometerwould be capableofdetectingbackgroundat 10.12 of the cosmicclosure, while themicrowaveinterferometerhasa capability of1O~. Both of these experimentsare still at the proposalstageand require study before~more definitelimits can be placed on their respectivesensitivities. While the microwave /4/ interferometercouldoperateas early as the mid 1990’s, the laser interferometerwould not likely be deployedbefore a numberof technological obstaclesare overcome,particularly in the drag-freedesign component.

ACKNOWLEDGEMENTS

I thank Richard Matzner and RobertZimmermanfor useful discussionson the astrophysicalgravitationalradiation source estimates. I thank Peter Bender and Jim Faller for information on the laserinterferometer concept.

REFERENCES

1. Project RELICK (this symposium)2. Taylor, J, Science, November 1987.3. Anderson, A.J., A SpaceborneMulti-Arm Interferometerfor VLF Gravitational Wave Detection (The

SMILE Project), in (M.J. Reed and J.M. Moran , eds.) The Impact of VLBI on AstrophysicsandGeophysics, IAU Symposium No. 129, Kluwer Academic Publishers,pp. 321-322, 1986.

4. Bender, P., J. Faller, (this symposium).