Gravitational Radiation

At right are the results of a numerical simulation of a black hole/black hole collision and the gravitational radiation which is produced (much as the manner in which water waves are generated by disturbances and in the manner in which EM waves are generated by electrical charges).

Reading: Chapter 22 (Neutron Stars and Black Holes)

In the 1600s, Issac Newton developed his Universal Theory of Gravitation and his three laws of motion. This way of looking at the Universe works quite nicely for the motions of the planets and most of our everyday experiences. However, under certain circumstances, this picture is inadequate. To fix-up some problems, Albert Einstein developed his Special and General Theories of Relativity which brought a new persepctive to our thinking about space, time, and gravitation.
To get a feel for some of Einstein's ideas, we must start thinking about the space-time of the Universe. Suppose that I tell you that Astronomy 122 meets in 100 Willamette Hall. Is this enough to get you to class? Well, no, because I didn't tell you when the class meets, namely, 14:00-14:50 on MWF. In order for you to show up for class, you must know not only where it meets but also when it meets. This is true for all events in the Universe; you must know not only where the event occurs (its spatial position) but also when the event occurs (its temporal position). The space and time positions are equally important and we should think about events in the Universe in terms of their space-time positions.
An interesting property of this space-time is that it has structure* and is not rigid.*

Question: What are some consequences of viewing the Universe in this manner?

The path an object rolling on a table top follows is determined by the shape of the surface (it rolls on the table top). The table top defines the space-time for the rolling object. For objects rolling in the Universe, a similar idea holds in that the paths of objects follow the shape of the shape-time.
Locally, the space-time in this room is fairly flat and so, unless you push on an object, its free motion (unforced motion) is in a straight line. If I were to place a large chunk of mass into the room, the mass would distort the shape of the space-time. In two-dimensions, this is easy to visualize. Imagine a rubber sheet onto which you place a bb. The bb causes a depression to form in the rubber sheet.

This is analogous to what mass does to the structure of space-time. It causes a depression to form so that if an object rolls toward it, it falls into the pit and is captured. (This, by the way, is how Einstein envisioned how gravity works. Mass distorts the space-time causing particles to roll toward the mass. Note that the objects follow the shape of the space-time and in this sense are following an unforced motion! That is, there is no gravitational force, objects are simply following their natural motions.)
Return to the rubber sheet analogy. If I drop a bb on the sheet and it bounces, ripples in the sheet are produced which propagate away from the disturbance. These ripples in the space-time are referred to as gravitational waves.

Gravitational waves from compact stars and other Celestial Objects. To the right is shown the Laser Interferometer Gravitational-Wave Observatory (LIGO) located in Hanford, WA which announced the discovery of gravitational waves in 2015 from GW150914, produced by the merger of two orbiting black holes (Abbott et al. 2016). This important work earned the 2017 Nobel Prize in Physics for Rainer Weiss, Barry C. Barish and Kip S. Thorne for "decisive contributions to the LIGO detector and the observation of gravitational waves." Note that there was an earlier indirect detection of gravitational waves for which the Nobel Prize in Physics 1993 was aswarded to Russell A. Hulse and Joseph H. Taylor Jr. for "the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation."

Comment: The opening of the sky to the study of grqavitational waves was huge, but opening ot the sky to the combination of gravitational and the Electro-magnetic (EM) spectrum and the study of other forms of emissions from Celestial objects would be huge. Exampales for such GW/EM sourfes are supernovae, $gaqmma;-ray bursters (merging neutron stars or black holes), unstable rotating neutron stars, close binary white dwarf stars... .
The exciting discovery of gravitational waves from merging neutron stars in the form of a γ-ray burst has verified that indeed, gravitational waves with electromagnetic radiation are produced by a broad range of astrophysical sources. The recent detetion by LIGO of a γ-ray burst source has substantially enhanced our understanding of the heavy element production in the Universe.

Possible sources for gravity waves are supernovae, merging neutron stars or black holes, unstable rotating neutron stars, ... . The exciting discovery of gravitational waves from merging neutron stars has verified that indeed, gravitational waves are produced by a broad range of astrophysical sources, in particualr, new sources of gravitational waves that produced large amounts of electromagnetic radiation.