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By Root 619 0

radiation contain a gargantuan wealth of information about the physics of the early universe. They show how gravitational attraction led slightly denser than average regions to grow, some time between the hottest phase of the big bang and the moment when radiation was released in a sufficiently cooled-down universe. Compared to the total time passed since then, the allotted time until the release was not much; there was not much clumping, and so the density variations and anisotropies in the leftover radiation are small. But they are not zero, allowing crucial tests of our theoretical understanding. Using Einstein’s equations, which tell us how matter behaves in an expanding universe, one can start with the measured values to determine matter distribution at even earlier times. The result can then be compared with theoretical models, allowing predictions for how matter might have been distributed during the very hot phase of the big bang—close to the realm of quantum gravity.

All this involves extrapolations and is indirect, preventing the crystallization of a unique theoretical model. Still, the data are already precise enough to rule out some ideas and support others. In closest agreement with observations is a large class of models summarized under the name “inflation.” They posit that the universe expanded extremely rapidly for some period of time in the very hot phase, even accelerating the rate of its expansion. (As already mentioned, the universe entered another accelerated phase relatively recently, which is independent of the inflationary acceleration.) Accelerated expansion of space, an apparent repulsion of all masses in the universe, seems in conflict with the attractive nature of gravity, and so the physical mechanism causing inflation remains hotly debated. We will later come back to this question in more depth.

Further details of the anisotropies give insights into some parameters of importance for a characterization of the cosmos. For instance, the anisotropy spectrum—obtained by viewing the intensity distribution on the sky as a sum of single uniform oscillations—has one clearly dominant contribution. Its wavelength is a measure of the curvature of space given at a fixed time. Space-time and its spatial part do not have to be flat and planar, but could be curved like a sphere. In this case, space is said to have positive curvature. A planar space, by contrast, is flat and has zero curvature, while there is also the possibility of negative curvature whose space is saddlelike.

Curvature in general relativity now causes what we perceive as the gravitational force, also acting on light or the microwave radiation in the cosmic background by bending their rays. Depending on the curvature, radiation traveling to us since its release is influenced differently by the space it traverses; it may be focused or unfocused. When we watch the sky in its microwave background light, positive curvature acts like a lens held in between, magnifying the intensity variations and shifting the wavelength of the dominant intensity variation. From the measured wavelength, one finds that the spatial curvature must vanish almost completely: The space we live in is nearly flat on large scales. (To be sure, this refers only to the curvature of space, as a part of space-time. Space-time as a whole is curved by the matter it contains; only an empty space-time would be completely flat. Moreover, the current universe is spatially curved on smaller scales owing to the presence of large masses and their strong gravitational force; only on large spatial scales is space flat on average.)

From smaller hills beside the main maximum of the anisotropy spectrum—hills whose presence was already predicted in 1965 by the physicist and human rights activist Andrei Sakharov—additional properties can be inferred. Of interest is the total amount of matter that led to the gravitational clumping. On its own, this parameter does not mean very much, but it is important in comparison with the amount of matter needed, according to general relativity, for a