the vacuum—is the correct explanation or if something more exotic is needed, as must be the case for the inflationary epoch, an earlier period of acceleration. It is even possible that a modification of Einstein’s general relativity will be needed. Either way, the implications for both astronomy and physics are profound.

Telescopes are time machines: because light travels across the cosmos at a finite speed, the most distant objects probe the furthest back in time. The 13.7-billion-year-old cosmic microwave background is seen in the millimeter band. The latest record holder (early 2010) for the most distant object is a gamma-ray burst that occurred 13.1 billion years ago when the universe was 0.6 billion years old. It was detected by a NASA Explorer program satellite called Swift, and its distance was measured by follow-up observations from telescopes on the ground. In the coming decade, powerful new observatories on the ground and in space will allow us to push back to still earlier times and glimpse the end of the cosmic dark ages signaled by the formation of the first-ever luminous sources in the universe—the first generation of stars.

Closer to home, the past decade has seen the discovery of well over 400 planets orbiting nearby stars. Although the existence of extrasolar planets had long been anticipated, the astonishing discovery is that the planets and their orbits seem to be nothing like our own. In the coming decade, new facilities on the ground and in space will enable us to detect potentially life-bearing planets similar to Earth.

Looking forward, the most promising areas for revolutionary discoveries are highlighted in the following subsections. This is indeed a special time in history. The unexpected can be expected with confidence.

The Discovery of Habitable Planets

We are rapidly building our knowledge of nearby analogs to our own solar system’s planets, most recently with the launch of NASA’s Kepler mission. The salient feature of the planetary menagerie of which we are currently aware is its diversity—in every measureable sense—of the properties of the planets as well as the properties of the stars around which they orbit. We are also improving our understanding of the planet formation process, and ALMA is expected to unveil the birthing of new worlds.

Until now detection methods have only been able to discover massive planets rivaling the giants in our solar system (Figure 2.1 upper) or larger objects (Figure 2.1 lower). The most profound discovery in the coming decade may be the detection of potentially habitable Earth-like planets orbiting other stars. To find evidence that life exists beyond our Earth is a longstanding dream of humanity, and it is now coming within our reach.

The search for life around other stars is a multi-stage process. Although JWST may be able to take the first steps, more complex and specialized instrumentation

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