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  • Cosmic Origins

    THE FABRIC of the COSMOS, Brian Greene, 2004
    ```(annotated and with added bold highlights by Epsilon=One)
    Chapter 14 – Up in the Heavens and Down in the Earth
    Cosmic Origins
    As we saw in earlier chapters, the cosmic microwave background radiation has played a dominant role in cosmological research since its discovery in the mid-1960s. The reason is clear: in the early stages of the universe, space was filled with a bath of electrically charged particles — electrons and protons — which, through the electromagnetic force, incessantly buffeted photons this way and that. But by a mere 300,000 years after the bang (ATB), the universe cooled off just enough for electrons and protons to combine into electrically neutral atoms — and from this moment onward, the radiation has traveled throughout space, mostly undisturbed, providing a sharp snapshot of the early universe. There are roughly 400 million of these primordial cosmic microwave photons streaming through every cubic meter of space, pristine relics of the early universe.

    Figure 14.4 (a) Cosmic microwave background radiation data gathered by the COBE satellite. The radiation has been traveling through space unimpeded since about 300,000 years after the big bang, so this picture renders the tiny temperature variations present in the universe nearly 14 billion years ago. (b) Improved data collected by the WMAP satellite.

    Initial measurements of the microwave background radiation revealed its temperature to be remarkably uniform, but as we discussed in Chapter 11, closer inspection, first achieved in 1992 by the Cosmic Background Explorer (COBE) and since improved by a number of observational undertakings, found evidence of small temperature variations, as illustrated in Figure 14.4a. The data are gray-scale coded, with light and dark patches indicating temperature variations of about a few ten-thousandths of a degree. The figure's splotchiness shows the minute but undeniably real unevenness of the radiation's temperature across the sky.

    While an impressive discovery in its own right, the COBE, experiment also marked a fundamental change in the character of cosmological research. Before COBE, cosmological data were coarse. In turn, a cosmological theory was deemed viable if it could match the broad-brush features of astronomical observations. Theorists could propose scheme after scheme with only minimal consideration for satisfying observational constraints. There simply weren't many observational constraints, and the ones that existed weren't particularly precise. But COBE initiated a new era in which the standards have tightened considerably. There is now a growing body of precision data with which any theory must reckon successfully even to be considered. In 2001, the Wilkinson Microwave Anisotropy Probe (WMAP) satellite, a joint venture of NASA and Princeton University, was launched to measure the microwave background radiation with about forty times COBE's resolution and sensitivity. By comparing WMAP's initial results, Figure 14.4b, with COBE's, figure 14.4a, you can immediately see how much finer and more detailed a picture WMAP is able to provide. Another satellite, Planck, which is being developed by the European Space Agency, is scheduled for launch in 2007, and if all goes according to plan, will better WMAP's resolution by a factor of ten.

    The influx of precision data has winnowed the field of cosmological proposals, with the inflationary model being, far and away, the leading contender. But as we mentioned in Chapter 10, inflationary cosmology is not a unique theory. Theorists have proposed many different versions (old inflation, new inflation, warm inflation, hybrid inflation, hyperinflation, assisted inflation, eternal inflation, extended inflation, chaotic inflation, double inflation, weak-scale inflation, hypernatural inflation, to name just a few), each involving the hallmark brief burst of rapid expansion, but all differing in detail (in the number of fields and their potential energy shapes, in which fields get perched on plateaus, and so on). These differences give rise to slightly different predictions for the properties of the microwave background radiation (different fields with different energies have slightly different quantum fluctuations). Comparison with the WMAP and Planck data should be able to rule out many proposals, substantially refining our understanding.

    In fact, the data may be able to thin the field even further. Although quantum fluctuations stretched by inflationary expansion provide a compelling explanation for the observed temperature variations, this model has a competitor. The cyclic cosmological model of Steinhardt and Turok, described in Chapter 13, offers an alternative proposal. As the two three-branes of the cyclic model slowly head toward each other, quantum fluctuations cause different parts to approach at slightly different rates. When they finally slam together roughly a trillion years later, different locations on the branes will make contact at slightly different moments, rather as if two pieces of coarse sandpaper were being slapped together. The tiny deviations from a perfectly uniform impact yield tiny deviations from a perfectly uniform evolution across each brane. Since one of these branes is supposed to be our three-dimensional space, the deviations from uniformity are deviations we should be able to detect. Steinhardt, Turok, and their collaborators have argued that the inhomogeneities generate temperature deviations of the same form as those emerging from the inflationary framework, and hence, with today's data, the cyclic model offers an equally viable explanation of the observations.

    However, the more refined data being gathered over the next decade may be able to distinguish between the two approaches. In the inflationary framework, not only are quantum fluctuations of the (sic.) inflation field stretched by the burst of exponential expansion, but tiny quantum ripples in the spatial fabric are also generated by the intense outward stretching. Since ripples in space are nothing but gravitational waves (as in our earlier discussion of LIGO), the inflationary framework predicts that gravitational waves were produced in the earliest moments of the-universe. 8 These are often called primordial gravitational waves, to distinguish them from those generated more recently by violent astrophysical events. In the cyclic model, by contrast, the deviation from perfect uniformity is built up gently, over the course of an almost unfathomable length of time, as the branes spend a trillion years slowly heading toward their next splat. The absence of a brisk and vigorous change in the geometry of the branes, and in the geometry of space, means that spatial ripples are not generated, so the cyclic model predicts an absence of primordial gravitational waves. Thus, if primordial cosmological gravitational waves should be detected, it will be yet another triumph for the inflationary framework and will definitively rule out the cyclic approach.

    It is unlikely that LIGO will be sensitive enough to detect inflation's predicted gravitational waves, but it is possible that they will be observed indirectly either by Planck or by another satellite experiment called the Cosmic Microwave Background Polarization experiment (CMBPol) that is now being planned. Planck, and CMBPol in particular, will not focus solely on temperature variations of the microwave background radiation, but will also measure polarization, the average spin directions of the microwave photons detected. Through a chain of reasoning too involved to cover here, it turns out that gravitational waves from the bang would leave a distinct imprint on the polarization of the microwave background radiation, perhaps an imprint large enough to be measured.

    So, within a decade, we may get sharp insight into whether the bang was really a splat, and whether the universe we're aware of is really a three-brane. In the golden age of cosmology, some of the wildest ideas may actually be testable.
    Last edited by Reviewer; 10-14-2012, 09:34 PM.
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