prepublication copy - The Department of Astronomy & Astrophysics ...

FIGURE 2‐2 The source 3C 75, shown here in x‐rays (blue) and radio waves (pink), is a rare example where
two galaxies have been caught in the act of merging. Not only do their stars merge, but their central black
holes ‐ each producing a pair of jets containing gas moving outward with speed close to that of light ‐ will
do likewise in perhaps a few hundred million years. Many similar mergers involving smaller black holes in
the nuclei of younger galaxies are thought to have taken place. When black holes coalesce, they create
intense bursts of gravitational radiation. (Credit: X‐ray: NASA/CXC/AIfA/D.Hudson & T.Reiprich et al.;
Radio: NRAO/VLA/NRL.)
galaxies form and evolve will be possible. We are on the verge of a new era of discovery in gravitational
wave astronomy.
In addition, gravitational waves could be created by exotic processes occurring in the young
universe and would have been propagating freely to us ever since. Several speculative sources such as
cosmic strings and abrupt changes in the form that the contents of the universe assumed—“phase
changes,” like the change from water to ice—have been suggested but the truth is that we do not quite
know what to expect. A possible way to see if there are any measurable signals with wavelengths of
roughly light years employs very precise radio measurements of naturally occurring cosmic “clocks”
called pulsars. 2 Spread across the sky, the separations between these cosmic clocks will change as a long
wavelength gravitational wave passes by, potentially measurably changing the arrival times of their radio
pulses.
Opening the Time Domain: Making Cosmic Movies
By eye, the universe appears static apart from the twinkling of starlight caused by Earth’s
atmosphere. In fact, it is a place where dramatic things happen on timescales we can observe—from a
tiny fraction of a second to days to centuries. Stars in all stages of life rotate, pulsate, and undergo
activity cycles while many flare, accrete, lose mass, and erupt, and some die in violent explosions. Binary
neutron stars and black holes merge, emitting, in addition to bursts of radiation, gravity waves.
Supermassive black holes in the centers of galaxies swallow mass episodically and erupt in energetic
outbursts. Some objects travel rapidly enough for us to measure their motion across the sky.
2 The 1993 Nobel Prize in Physics was awarded to two American astronomers for their work on binary pulsars.
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