Gravitational waves. What are gravitational waves (GWs)? N. K. Johnson-McDaniel. Wintersemester 2013

Transcription

1 Gravitational waves N. K. Johnson-McDaniel TPI, FSU Jena Wintersemester 2013 What are gravitational waves (GWs)? Intuitively, ripples in spacetime that carry energy and (angular) momentum away from an isolated source; the gravitational analogue of electromagnetic radiation, so sourced by accelerated masses. (We ll see the more rigorous version later.) But, unlike E&M radiation, extremely weak. While one can generate GWs by literally waving one s hands, that radiation would be completely undetectable (as would any gravitational radiation one can conceive of being generated in the solar system). This is why we don t feel like we are swimming in molasses from gravitational radiation reaction when we move around. GWs of any appreciable magnitude can only be generated by astronomical objects, primarily compact objects like neutron stars and black holes (and white dwarfs, though these are not nearly so compact).

2 Other important qualitative properties of GWs and detectors Despite extremely close formal analogies with E&M radiation, GWs are often more like sound (acoustic radiation) than electromagnetic radiation. In particular, since they are generated by bulk motion of the source, they generally have wavelengths of about the size of the source or larger, and thus can t be used to form an image. They are phase-coherent, unlike most E&M radiation, so we can directly observe the field (as opposed to some sort of energy), which only falls off as 1/r, instead of 1/r 2. Thus, if we double our detector s sensitivity, we increase its reach by nearly an order of magnitude! (2 3 = 8) Gravitational wave detectors are naturally all-sky monitors, unlike most astronomical observatories, which have to work to increase their field of view. See Sec. 6 in Flanagan and Hughes for more detailed discussion. Why an entire course on GWs? These days, mostly because of experiment and astrophysics. There are multimillion dollar international collaborations devoted to detecting gravitational waves (e.g., LIGO/Virgo/KAGRA/IndIGO and LISA [elisa/ngo])....and ever since observations of the Hulse-Taylor pulsar indirectly demonstrated the existence of GWs, they have been a staple of descriptions of the evolution of compact binaries and even, lately, galactic dynamics (through the kicks that can be generated by merging black hole binaries). But on the more theoretical/mathematical side, the question of whether gravitational waves even exist in GR (and if they really carry away energy and angular momentum from the source), and how to characterize them has motivated many advances in our understanding of general relativity, as you will hear about from David.

3 Why? (cont.) Returning to observation, while the direct detection of gravitational waves will provide all sorts of interesting tests of fundamental gravitational physics (by probing strong-field gravity, as well as just detecting the waves themselves), gravitational waves are also a powerful tool for astrophysics, from studying compact objects, to such bread-and-butter astrophysics as star formation, binary evolution, and galactic mergers. In particular, since GWs tell us what is going on with the bulk motions of mass, they would let us probe the supernova (SN) mechanism (E&M observations only observe the surface, and even neutrinos take a little while to diffuse out). They ll also generally help us learn more about cold matter at supernuclear densities, from observations of neutron stars. Additionally, since GWs interact so weakly, like neutrinos, we can observe them from sources (e.g., some SNe and white dwarf binaries in our own galaxy) that are completely obscured by dust and thus unobservable electromagnetically. Why? (cont.) Of course, since GWs are so weak, the signals one is trying to detect are almost always hidden in the noise, so one needs highly accurate templates to detect the signals, and in particular to extract the properties of the source from the signal. Building such templates has driven much theoretical work over the past few decades. This course will present the basics of GWs, give elementary tools for computations of the waves emitted by a source, and then discuss the detection of these waves (and the sources expected for current and planned detectors), and how to infer properties of the source from them. And while the flagship strong-field sources (most notably coalescing compact binaries) require exquisitely accurate templates generated by exacting analytical and numerical work, the tools we develop will be perfectly adequate to describe other, equally important (if perhaps not quite so glamorous) sources.

4 Why? (cont.) I We will focus on the GR side of things (with a little statistics at the end). However, one needs pretty much all of physics to describe the standard GW sources. Even for binary black holes, that paradigmatic vacuum GR system, one needs lots of nuclear physics and radiation hydrodynamics to describe the stars that explode to create stellar mass black holes, in addition to much laser physics for the detectors. I Indeed, to quote my Ph.D. advisor: My main research interest is the detection of gravitational waves. People call this relativity, but I wind up doing most of physics except relativity. B. J. Owen GW detectors: From the ground... I Since GWs are so weak, one requires a huge detector to observe them [plus lots of tough experimental work to reduce (and characterize) the noise]. I On the ground, there are currently the two LIGO detectors in the US, the Virgo detector in Italy, and the GEO600 detector in Hannover. All these detectors use the principle of laser interferometry. I Even with the LIGO detectors 4 km arms, the maximal changes in armlength that can be expected from a GW are m, well below the nuclear scale (10 15 m)! LIGO Hanford, 4 km Virgo (Pisa), 3 km GEO600 (Hannover), 600 m

5 GW detectors:...to Earth orbit... Due to seismic noise, one cannot go below 1 Hz with ground-based GW detectors (and the current detectors do not go below 10 Hz, even with upgrades). Thus, to access the rich mhz regime, including GWs from galactic binaries and coalescing supermassive black hole binaries, one needs to go into space. The LISA mission was the standard space-based GW detector design for many years, but recently has been downscaled to an ESA-only mission (elisa/ngo), with a somewhat shorter armlength (likely 10 6 km vs. the original km). It will be preceded by the technology demonstration mission LISA Pathfinder. LISA LISA Pathfinder GW detectors:...and throughout the galaxy One can also search for low-frequency (nhz) GWs (from, e.g., ultramassive black hole binaries with periods of 1 year) using radio observations of pulsars (generally at distances of 100s of pc) one looks for correlated changes in arrival time of the pulses from many pulsars in different directions, which could only be caused by a gravitational wave. However, unlike all the other GW detectors, pulsar timing only observes sources that are moving slowly enough that most relativistic effects will be unobservable. Schematic of pulsar timing from the Manchester group.

6 GW detector status LIGO and Virgo are currently being upgraded to Advanced sensitivity (about an order of magnitude improvement) expected to be operational in They will be joined later by other, similar ground-based detectors in Japan (KAGRA, formerly LCGT) and India (IndIGO) somewhat later ( 2017) to make a world-wide network of detectors, around the time first detections are expected with the Advanced instruments (based on models for the population of sources). GW detector status (cont.) Timeline from the IndIGO website.

7 GW detector status (cont.) elisa/ngo s funding status is still not confirmed, though the optimistic timeline has it launch in 2028, and it is still possible that the original LISA design could be resurrected (with contributions from NASA or some other agency). Regardless, LISA Pathfinder is slated to fly in ET, DECIGO, BBO, etc. are even further in the future. Pulsar timing is taking data now, and only improving in sensitivity (and baseline). (There ll be a dramatic improvement in sensitivity when the Square Kilometre Array comes online in 2020.) The GW spectrum and various detectors [Figure from T. Creighton s website] Converting from wavelength to the more common representation in terms of frequency, note that λ = {10 27, 10 16, 10 11, 10 9, 10 6, 10 3 } m f = { Hz, 30 nhz, 3 mhz, 0.3 Hz, 300 Hz, 300 khz}

8 An illustration of the GW data analysis challenge Example of some realistic compact binary signals hidden in ground-based detector noise from the companion to PRD 88, (2013). (With LISA, there s the possibility of being able to pick out the brightest SMBHB signals in the data by eye, though there are other data analysis challenges for LISA...) What has GW science already achieved? While there have not yet been any direct GW detections, LIGO and Virgo have set some interesting upper limits at their initial and Enhanced sensitivities. GRBs and SGRs In particular, they showed that two gamma-ray bursts (GRBs) whose sky positions were coincident with (relatively) nearby galaxies (M31 [the Andromeda Galaxy], our sister galaxy, and M81) were not produced by (quasicircular) compact binary coalescences in those galaxies. They are thought most likely to come from soft gamma repeater (SGR) flares if they indeed originated in those galaxies. (SGR flares are thought to be caused by a large-scale rearrangement of the magnetic field of a magnetar, a highly magnetized neutron star with a field strength of G.) Error box for potential GRB in M31.

9 What has GW science already achieved? (cont.) Deformations of pulsars They also constrained the contribution of gravitational waves to the Crab pulsar s spin-down power (the amount of rotational energy it has to lose to account for the observed increase in its period) to be 1%. Alternatively, they have constrained its quadrupole ellipticity to be 10 4, a factor of 10 below the limit placed by electromagnetic observations of the spin-down. This corresponds to a surface deformation of 5 30 cm (EOS-dependent) on a star with a radius of 10 km (!). Cas A NS and nebula, Chandra Crab pulsar and nebula, Chandra (LIGO and Virgo have also placed similar, though less stringent, direct constraints for other pulsars, notably the Vela pulsar and the Cas A central object. However, for most pulsars, the constraints from the electromagnetically observed spin-down are still well below the direct GW constraints.) Conclusions As we have seen briefly here and will explore in the rest of the course, GW observations promise to be a powerful tool for both astrophysics and fundamental physics, giving one views of the universe and strong gravity that are inaccessible with any other messenger (photons, cosmic rays, or neutrinos), perhaps most notably with supernovae and compact object binaries (plus if we are very fortunate the early universe). Gravitational waves also are an important driving force in compact binary evolution, in that they are what finally drives the binary to coalescence (while efficiently circularizing the orbit); such coalescence of binaries containing neutron stars is thought to lead to GRBs. The neutron-rich ejecta from these mergers also enriches the interstellar medium with heavy r-process elements. Also, in certain circumstances, the GW emission during merger can impart a substantial kick to the binary, even enough to eject it from its host galaxy in extreme cases. (This is a teaser for later!)

10 Conclusions (cont.) However, the experimental challenge in making the first direct detections of GWs is immense. We will not touch on most of the issues here, though we will discuss the very basics of detector design and data analysis. In particular, we will show how one is able to estimate the accuracy with which GW detectors will be able to measure the parameters of various sources.