In addition, gravitational waves could have been 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.3 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.
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.
Our targeted studies of variations in the brightness and position of different objects indicate that we have only just begun to explore lively variations in the cosmos. If we study the temporal behavior of the sky in systematic ways and over wide ranges of the electromagnetic spectrum, we are sure to discover new and unexpected phenomena. In the highest-energy portion of the electromagnetic spectrum, where the universe shows its greatest variability, the value of viewing large areas of the sky repeatedly on short timescales has been amply demonstrated by the breakthrough capabilities of the Fermi Gamma-ray Space Telescope. The impact of such surveys will be broad and deep, and the committee gives just a few illustrative examples of what the future holds.
In our own solar system, new temporal surveys will discover and characterize a vast population of relic objects in the outer reaches of the solar system. These Kuiper belt objects, of which Pluto is the nearest large example, are the icy residue left over from the formation of our solar system about 4.5 billion years ago. As such, they are the fossil record of events that we can otherwise only theorize about.