Gravitational Wave Astronomy Dr. Giles Hammond Institute for Gravitational Research SUPA, University of Glasgow Universität Jena, August 2010
Estimate of Strain Amplitude The magnitude of the metric stretch in the xx direction is or, using Kepler’s 3rd law, which gives modulated at 2
How to Make a Gravitational Wave h ~ 10-37 1000 kg Case #1: In the lab! M = 1000 kg R = 1 m f = 1000 Hz r = 300 m 1000 kg
How to make a Gravitational Wave that can be Detected Case #2: A 1.4 solar mass binary pair M = 1.4 M R = 60 km f = 200 Hz r = 10Mpc h ~ 10-21
GW and EM Waves I EM waves interact strongly with matter while GWs interact weakly The weak interaction of GWs is both blessing and curse: It means that they propagate with essentially zero absorption, making it possible to probe astrophysics that is hidden or dark to electromagnetic observations It also means that detecting GWs is very difficult. Also, because many of the best sources are hidden or dark, they are very poorly understood.
GW and EM Waves II EM radiation typically has a wavelength smaller than the size of the emitting system => can be used to form an image of the source This is because EM radiation is usually generated by moving charges in the environment of some larger source—e.g. hot plasma The wavelength of GWs is typically comparable to or larger than the size of the radiating source. GWs are generated by the bulk dynamics of the source itself As a consequence, GWs cannot be used to form an image Instead, GWs are best thought of as analogous to sound—with imaging/source location requiring multiple detectors.
GW and EM Waves III Gravitons in a GW are phase coherent while photons in EM waves are usually phase-incoherent. Each graviton is generated from the same bulk motion of matter or of spacetime curvature Each photon is normally generated by different, independent events involving atoms, ions or electrons. An important consequence of this coherency is that the direct observable of gravitational radiation is the strain h, a quantity that falls off with distance as 1/r. Most electromagnetic observables are some kind of energy flux, and so fall off with a 1/r2 law
GW and EM Waves IV It is useful to categorize GW sources (and the methods for detecting their waves) by the frequency band in which they radiate. Broadly speaking, we may break the GW spectrum into four rather different bands:
GW and EM Waves IV For compact sources, the GW frequency band is typically related to the source’s size R and mass M. Here the source size R means the length scale over which the source’s dynamics vary; for example, it could be the actual size of a particular body or the separation of the members of a binary. The ‘natural’ GW frequency is; Because (the Schwarzschild radius of a mass M), we can estimate an upper bound for the frequency of a compact source: HF are stellar mass, LF are 103-106 Msun or else widely separated
17 / sec · · Joseph Taylor Russell Hulse ~ 8 hr Existence proof: PSR 1913+16
Gravitational Wave Astronomy Pulsar timing ~10-8 Hz CMB B-mode polarization ~10-16 Hz Ground-based detectors ~100Hz Spacecraft tracking ~10-2 Hz
A New Probe of the Universe Gravitational Waves will give us a different, non electromagnetic view of the universe, and open a new spectrum for observation. This will be complementary information, as different from what we know as hearing is from seeing. Radio x-ray g-ray CMB IR GRBs EXPECT THE UNEXPECTED! GW sky??
Gravitational Waveform Gravitational Wave Sources • ‘Coalescing Binary Systems’ • Neutron stars, black holes • ‘chirped’ waveform http://web.mit.edu/sahughes/www/sounds.html
Gravitational Wave Sources • ‘Coalescing Binary Systems’ • Neutron stars, black holes • ‘chirped’ waveform
Credit: Chandra X-ray Observatory Gravitational Wave Sources • ‘Bursts’ • asymmetric core collapse supernovae • ???? (sources we haven’t thought about Mass energy converted to GW’s
Gravitational Wave Sources • ‘Neutron Stars’ • 1017 kg/m3 (1.4 MSUN in sphere of radius 10 km) • – Rapidly rotating : • Periods from seconds to milliseconds • – Highly magnetised (conservation of magnetic flux) • Magnetic fields ranging from: • – 1011 T - “magnetars” • – 108 T - normal young neutron star
Gravitational Wave Sources • ‘Continuous Sources’ • Spinning neutron stars • “monotone” waveform • Probe internal structure A bumpy neutron star has a quadrupole moment => Continuous source of GW’s
Gravitational Wave Sources • ‘Cosmic GW background’ • residue of the Big Bang (like CMBR) • probes back to 10-21 s after the birth of the universe • stochastic, incoherent background
Gravitational Wave Sources ‘Gamma Ray Bursts’ • Intense flashes of gamma rays from (mostly) extra-galactic sources • GRBs are extremely luminous events in the Universe • Long (> 2 s) and short duration (< 2 s) • Long GRBs are associated with star forming galaxies • Short GRBs are less well understood • Soft gamma repeaters magnatars