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Testing strong-field gravity with space-based detectors Emanuele Berti, University of Mississippi/Caltech LISA Symposium X, Gainesville, May 21 2014 Outline 1) Weak-field tests of gravity • Laboratory and Solar System tests • Radiation from binary pulsars 2) Strong-field tests of gravity: why do we need (e/u)LISA? • Earth-based detectors: AdLIGO/Virgo, KAGRA, LIGO India, ET… • Pulsar timing: IPTA, SKA • Space-based detectors 3) Strong-field tests of gravity: “external” vs. “internal” tests q External tests: extensions of general relativity q Internal tests: the no-hair theorem q Ringdown tests: Consistency with inspiral / area theorem Nonlinearities Importance of localization/distance determination q Propagation: graviton mass q Can we beat binary pulsar constraints? One example: superradiance and floating orbits The Confrontation between General Relativity and Experiment 9 the wire and g were not quite parallel because of the centripetal acceleration on the apparatus due to the Earth’s rotation; the apparatus was rotated about the direction14 of the wire. In the Dicke Cli↵ord M. Will and Braginsky experiments, g was that of the Sun, and the rotation of the Earth provided the modulation of the torque at a period of 24 hr (TEGP 2.4 (a) [281]). Beginning in the late 1980s, numerous experiments were carried out primarily to search for a “fifth force” (see Section 2.3.1), but their null results also constituted tests of WEP. In the “free-fall GalileoThe first experiment” successful, performed high-precision redshift measurement was the series of Pound–Rebka–Snider 57 at the University of Colorado, the relative free-fall acceleration ofexperiments two bodies made of 1960 of uranium – 1965 that and measured the frequency shift of gamma-ray photons from Fe as copper was measured using a laser interferometric technique. Thethey “E¨ot-Wash” ascended or experiments descended car- the Je↵erson Physical Laboratory tower at Harvard University. The ried out at the University of Washington used a sophisticated torsionhigh accuracy balance tray achieved to compare – one the percent – was obtained by making use of the M¨ossbauer e↵ect to accelerations of various materials toward local topography on Earth,produce movable a narrow laboratory resonance masses, line whose shift could be accurately determined. Other experiments 13 the Sun and the galaxy [249, 19], and have reached levels of 3since10− 1960[2]. measured The resulting the shift upper of spectral lines in the Sun’s gravitational field and the change in ⇥ limits on ⌘ are summarized in Figure 1 (TEGP 14.1 [281]; for arate bibliography of atomic of clocks experiments transported up to aloft on aircraft, rockets and satellites. Figure 3 summarizes the 1991, see [107]). importantThe redshiftfoundaons experiments that have been performed of general since 1960 (TEGP 2.4 (c) [281]). relavity TESTS OF THE TESTS OF WEAK EQUIVALENCE PRINCIPLE LOCAL POSITION INVARIANCE40 Cli↵ord M. Will -8 Pound Rebka Millisecond Pulsar [Will, gr-qc/0510072] 10 E¨otv¨os 10 -1 Renner -9 Free-fall Null 10 Redshift -2 Fifth-force 10 Pound searches Snider 10-10 Boulder η Saturn Princeton E¨ot-Wash 10 -3 10-11 α H maser Moscow E¨ot-Wash 10-12 LLR 10 -4 THE PARAMETER (1+γ)/2 12 Cli↵ord M. Will Null Redshift -13 10 Solar spectra 1.10 a -a 2 Radio η= 1 10 -5 Clocks in rockets DEFLECTION (a +a )/2 10-14 1 2 spacecraft & planes OF LIGHT Optical 1900 1920 1940 1960 1970 1980 1990 2000 1960 1970 1980 1990 2000 1.05 2X10-4 VLBI YEAR OF EXPERIMENT YEAR OF EXPERIMENT TESTS OF 2 Figure 1: Selected tests of theLOCAL weak equivalence LORENTZ principle, INVARIANCE showing bounds on ⌘, which measures Δν/ν = (1+α)ΔU/c 1.00 fractional di↵erence in acceleration of di↵erent materials or bodies. The free-fall and E¨ot-Wash experiments were originally performed to search for a fifth force (green region, representing many experiments). The blue band shows evolving bounds on ⌘ for gravitatingFigure 3: Selected bodies fromWeak Equivalence Principle+ tests lunar of local laser position invariance via gravitational redshift experiments, showing ranging (LLR). -2 bounds on ↵, which measures degree of deviation of redshift from the formula ∆⌫/⌫ = ∆U/c2.In 10 JPL Michelson-Morley null redshift experiments, the bound is on the di↵erence in ↵ between di↵erent kinds of clocks. Hipparcos Joos )/2 0.95 TPA Local Lorentz Invariance+ γ 10-6 Centrifuge After almost 50 years of inconclusive or contradictory measurements, the gravitational redshift Living Reviews in Relativity of solarCavities spectral lines was finally measured reliably. During the early years of GR, the failure http://www.livingreviews.org/lrr-2006-3Local Posi8on Invariance= (1+ to measure this e↵ect in solar lines was siezed upon by some as reason to doubt the theory. -10 10 Brillet-Hall Unfortunately, theEinstein’s Equivalence Principle measurement is not simple. Solar spectral lines are subject to the “limb e↵ect”, PSR 1937+21 δ a variation of spectral line wavelengths between the center of the solar disk and its edge or1.05 “limb”; this e↵ect is actually a Doppler shift caused by complex convective and turbulent motions in the SHAPIRO 10-14 Hughes Drever photosphere and lower chromosphere, and is expected to be minimized by observing at the solar TIME Voyager NIST Implies gravity is a metric theory: DELAY 10-18 Harvard 1.00 Living Reviews ingravity is Relativity space3me curvature U. Washington 10-22 http://www.livingreviews.org/lrr-2006-3 Viking Cassini (1X10-5) δ = 1/c2 - 1 Best test of space8me curvature: 0.95 10-26 1900 1920 1940 1960 1970 1980 1990 2000 2010 Cassini bound 1920 1940 1960 1970 1980 1990 2000 YEAR OF EXPERIMENT YEAR OF EXPERIMENT Figure 2: Selected tests of local Lorentz invariance showing the bounds on the parameter δ, which measures the degree of violation of Lorentz invariance in electromagnetism. The Michelson–Morley, Joos, Brillet–Hall and cavity experiments test the isotropy of the round-trip speed of light. The centrifuge, two-photon absorption (TPA) and JPL experiments test the isotropy of light speed Figure 5: Measurements of the coefficient (1 + γ)/2 from light deflection and time delay measure- using one-way propagation. The most precise experiments test isotropy of atomic energy levels. ments. Its GR value is unity. The arrows at the top denote anomalously large values from early 1 The limits assume a speed of Earth of 370 km s− relative to the mean rest frame of the universe. eclipse expeditions. The Shapiro time-delay measurements using the Cassini spacecraft yielded an 3 agreement with GR to 10− percent, and VLBI light deflection measurements have reached 0.02 percent. Hipparcos denotes the optical astrometry satellite, which reached 0.1 percent. Living Reviews in Relativity http://www.livingreviews.org/lrr-2006-3 Living Reviews in Relativity http://www.livingreviews.org/lrr-2006-3 Indirect detec3on of gravitaonal waves: the binary pulsar 1993 Nobel Prize to Hulse and Taylor: discovery of the binary pulsar 1913+16 -3 Strongest test of GR: PSR J0348+0432, P=2.46hr, v/c=2x10 [Antoniadis+, 1304.6875] [Weisberg+Taylor] Probes and Tests of Strong-Field Gravity with Observations in the Electromagnetic Spectrum 13 Strong field: gravitaonal field vs. curvature; probing vs. tesng 10-10 -13 Neutron Stars 10 Black Holes 10-16 -19 ) 10 -2 10-22 (cm Grav Prob B 2 -25 c AGN 3 10 Eclipse 10-28 Hulse-Taylor =GM/r -31 Over the Horizon ξ 10 10-34 Moon Mercury -37 10 10-40 10-15 10-12 10-9 10-6 10-3 100 2 ε=GM/rc [Psal8s, Living Reviews in Relavity] Figure 2: Tests of general relativity placed on an appropriate parameter space. The long-dashed line represents the event horizon of Schwarzschild black holes. Living Reviews in Relativity http://www.livingreviews.org/lrr-2008-9 Gravitaonal-wave tests Energy flux: E.g. scalar-tensor theories predict dipole radiaon because (iner8al mass)≠(gravitaonal mass) Polarizaon: Up to six polarizaon states Propagaon: If e.g. mgraviton≠0, gravitaonal waves would travel slower than EM waves Massive black hole inspirals -26 can yield bounds mgraviton<∼10 eV (104-106 beaer than Solar System) Direct detecon: Strong-field dynamics [e.g. Gair+,1212.5575] Gravitational-wave astronomy: a timeline! Timeline: Earth-based detectors 2014: BICEP2 – inflationary GWs? 2015: AdLIGO LISA Pathfinder 2018: AdLIGO design sensitivity FIRST DETECTIONS? <~2022: First PTA detections? <2030 AdVirgo, KAGRA, LIGO India <2030? Einstein Telescope? 2034: ESA L3 launch – eLISA <2044: DECIGO? BBO? Decadal 2010: The panel recommends that the LISA mission be given the highest priority for a new start in the next decade, given the extensive technology development that has already been completed, the expected short 3me unl the LISA Pathfinder mission (LPF) launch, and the need to maintain momentum in the U.S. community and guarantee a smooth transi3on to a joint NASA-ESA mission. The panel recommends that NASA funding of LISA begin immediately, with con3nua3on beyond LPF con3ngent on the success of that mission. Timeline: pulsar ming arrays 2014: BICEP2 – inflationary GWs? 2015: AdLIGO LISA Pathfinder 2018: AdLIGO design sensitivity FIRST DETECTIONS? <~2022: First PTA detections? <2030 AdVirgo, KAGRA, LIGO India <2030? Einstein Telescope? 2034: ESA L3 launch – eLISA <2044: DECIGO? BBO? Decadal 2010: The panel recommends that the LISA mission be given the highest priority for a new start in the next decade, given the extensive technology development that has already been completed, the expected short 3me unl the LISA Pathfinder mission (LPF) launch, and the need to maintain momentum in the U.S. community and guarantee a smooth transi3on to a joint NASA-ESA mission.
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