to be a laboratory that offers access to regimes not available on Earth, helping us to both understand and discover new elements of the basic laws of nature.

Scientists can study the universe on the largest observable scales—more than 10 trillion, trillion times larger than the size of a person. The past decade has seen the confirmation from measurements of the truly remarkable discovery that the expansion of the universe is accelerating. In modern language, this acceleration is attributed to the effect of a mysterious substance called dark energy that accounts for 75 percent of the mass-energy of the universe today causing galaxies to separate at ever faster speeds. The remainder of the mass-energy is 4.6 percent regular matter and 20 percent a new type of matter, dubbed dark matter, that is believed to comprise new types of elementary particles not yet found in terrestrial laboratories. The effects of dark energy are undetectable on the scale of an experiment on Earth. The only way forward is to use the universe at large to infer the properties of dark energy by measuring its effects on the expansion rate and the growth of structure.

Amazingly, we can ask and hope to answer questions about the universe as it was very soon after the big bang. Recent observations of the microwave background are consistent with the theory that the universe underwent a burst of inflation when the expansion also accelerated and the scale of the universe that we see today grew from its infinitesimally small beginnings to about the size of a fist. Gravitational waves created at the end of the epoch of inflation can propagate all the way to us and carry information about the behavior of gravity and other forces during the first moments after the big bang. These waves can be detected through the distinctive polarization pattern1 that they impose on the relic cosmic microwave background radiation. Detection of this imprint would both probe fundamental physics at very high energies and bear witness to the birth of the universe.

Yet another opportunity to study fundamental principles comes from precisely observing the behavior of black holes. Black holes are commonly found in the nuclei of normal galaxies and are born when very massive stars end their stellar lives. Scientists have an exact theoretical description of space-time around black holes but do not know if this description is correct. One way to find out is to observe X-ray-emitting gas and stars as they spiral toward a black hole’s event horizon beyond which nothing, not even light, can escape. Another is to observe the jets that escape black holes with speeds close to that of light. However, the best test of all will come from measuring the gravitational radiation that is observed when moderate-mass black holes merge. We now have the software and the computing power to calculate the signals that should be seen and the technology to test the theory.

1

The cosmic microwave radiation signal can be decomposed into two components: an E-mode and a B-mode. Patterns in these polarization modes allow determination of conditions when the radiation was emitted.



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