We now move on to discuss the second face of the problem of the cosmological constant, which was highlighted recently by the discovery of the acceleration of the universe. This chapter will first review briefly how searching for “dark energy” has come finally to a spatially flat universe well described by a cosmological constant Λ of a size smaller than but nearly comparable to the critical density. For a number of reasons, we consider that this Λ is not a true constant but is mimicked most naturally by a scalar field.

In section 5.1, we sketch what the development has been like mainly on the observational front, culminating in the conclusion that we have an accelerating universe.

As a possible theoretical model discussed recently, we first review in section 5.2 the results of “quintessence,” a name mainly indicating a cosmological scalar field. Since this is a phenomenological approach that is not necessarily constrained rigorously by the scalar–tensor theory, our focus is mainly on the assumed inverse-power potential. A primary concern is the question of how naturally the initial conditions for the scalar field can be chosen. A relevant question is that of whether the scalar-field energy falls off in the same way as the ordinary matter density (“scaling”), or approaches the latter starting from different values (“tracking”).

As was emphasized in the preceding chapter, the way the scalar field enters the arena of the scalar–tensor theory is not simple. It does so through what is known as a nonminimal coupling term. This is a unique feature shared by those models qualified to be called scalar–tensor theories in the sense conceived by Jordan. In spite of the simplicity of wanting to implement a variable gravitational “constant,” this term is a somewhat contrived technical device that tends to obscure other issues of physical significance. One of the emphases in this chapter is placed on revealing them beyond mathematical manipulations.

Among several versions, or models, of the scalar–tensor theory, the one due to Brans and Dicke might be viewed as a “prototype.” This model, which is based on certain assumptions made for the sake of simplicity, is in fact over-simplified from the point of view of theoretical models of the modern unification program. Also for some other reasons, this model may not be accepted as fully realistic. Nevertheless, a prototype has its own merit that deserves careful and comprehensive understanding. In this chapter we introduce the original BD model as a basis of the subsequent developments.

Section 1 is an elementary but technical introduction to the prototype BD model as a basis of the whole discussion that follows.

We now turn to thermal detectors, the second major class of detector listed in Chapter 1. Unlike all detector types described so far, these devices do not detect photons by the direct excitation of charge carriers. They instead absorb the photons and convert their energy to heat, which is detected by a very sensitive thermometer. The energy that the photons deposit is important to this process; the wavelength is irrelevant, that is, the detector responds identically to signals at any wavelength so long as the number of photons in the signal is adjusted to keep the absorbed energy the same. Thus, the wavelength dependence of responsivity is flat and as broad as the photon-absorbing material will allow. Because the absorber is decoupled from the detection process, it can be optimized fully, and quantum efficiencies can be as high as 90–100%. Bolometers based on semiconductor or superconductor temperature sensors are the most highly developed form of thermal detector for low light levels and are the detector of choice for many applications, especially in the submillimeter spectral range. They are also used as energy-resolving X-ray detectors. For the highest possible performance, such detectors need to be cooled to below 1 K. Bolometers manufactured by etching miniature structures in silicon and silicon nitride provide new possibilities for very high performance with large pixels and also for detector arrays.

The general principles derived in Chapter 10 are equally valid for submillimeter- and millimeter-wave receivers. Performance attributes that limit the general usefulness of infrared heterodyne receivers, such as limited bandpass and diffraction-limited throughput, cease to be serious limitations as the wavelength of operation increases. Heterodyne receivers are therefore the preferred approach for high-resolution spectroscopy in the submillimeter spectral region, and their usefulness is expanded as the wavelength increases into the millimeter regime. At wavelengths longer than a few millimeters, they are used to the exclusion of all other kinds of detectors.

Basic operation

The operational principles of heterodyne receivers were described in Section 10.1, and the operation of the components that follow the mixer in a submillimeteror millimeter-wave receiver is essentially identical to the systems discussed in Chapter 10. Such components can be used for amplification, frequency conversion, and detection. Often, much of the expense in a heterodyne receiver system is in the “backend” spectrometer (for example, filter bank or autocorrelator) and in other equipment that processes the IF signal. Because these items can be identical from one system to another, it is common to use a single set of them as back ends with many different receiver “front ends” that together can operate over a broad range of signal frequencies.

This book provides a comprehensive overview of the important technologies for photon detection from the millimeter-wave through the ultraviolet spectral regions. The reader should gain a good understanding of the similarities and contrasts, the strengths and weaknesses of the multitude of approaches that have been developed over a century of effort to improve our ability to sense photons. The emphasis is always upon the methods of operation and physical limits to detector performance. Brief mention is sometimes made of the currently achieved performance levels, but only to place the broader physical principles in a practical context.

Writing is a process of successive approximations toward poorly defined goals. A second edition not only brings a book up to date, it also allows reconsideration of the goals and permits a new series of approximations toward them. Specific goals for this edition are to:

• Provide a bridge from general physics into the methods used for photon detection;

• Guide readers into more detailed and technical treatments of individual topics;

• Give a broad overview of the subject;

• Make the book accessible to the widest possible audience.

Based on the extensive survey of the literature that accompanied preparation of this edition, these goals have led to a unique book.