Accelerating Universe and the Scalar-Tensor Theory

To understand the accelerating universe discovered observationally in 1998, we develop the scalar-tensor theory of gravitation originally due to Jordan, extended only minimally. The unique role of the conformal transformation and frames is discussed particularly from a physical point of view. We show the theory to provide us with a simple and natural way of understanding the core of the measurements, Lambda(obs) similar to t(0)(-2) for the observed values of the cosmological constant and today's age of the universe both expressed in the Planckian units. According to this scenario of a decaying cosmological constant, Lambda(obs) is this small only because we are old, not because we fine-tune the parameters. It also follows that the scalar field is simply the pseudo Nambu-Goldstone boson of broken global scale invariance, based on the way astronomers and astrophysicists measure the expansion of the universe in reference to the microscopic length units. A rather phenomenological trapping mechanism is assumed for the scalar field around the epoch of mini-inflation as observed, still maintaining the unmistakable behavior of the scenario stated above. Experimental searches for the scalar field, as light as similar to 10(-9) eV, as part of the dark energy, are also discussed.