FEATURE ARTICLE
Observing the Beginning of Time
New maps of the cosmic background radiation may display evidence of the quantum origin of space and time
Craig Hogan
Images of Primordial Quanta
The most persuasive data are the sky maps of the primordial radiation. This cosmic background of light appears almost (but not perfectly) uniform in all directions, with a blackbody temperature of 2.725 degrees (Kelvin) above absolute zero. The gravitational effect of the inflaton perturbations, in addition to creating structures like galaxies, creates patterns on the sky of very slightly hot and cold spots. They are very subtle—temperature differences of less than one part in ten thousand—but the pattern preserves intact primordial information and so far, at the present level of experimental precision, agrees very well with what theory predicts.
About ten years ago, the Cosmic Background Explorer (COBE) satellite made the first observations that convincingly showed primordial structure of this kind. This historic map included the whole sky but was fairly noisy and still rather blurry, with a resolution of about seven degrees. In the past two years, experiments in exotic locations—on high-altitude balloons circling Antarctica and in the high-desert passes of the Andes—have returned low-noise maps of smaller patches of sky with much finer angular detail (Figure 6). A NASA spacecraft called the Microwave Anisotropy Probe (MAP) is now gathering data to produce an all-sky map. (For more information about the MAP mission, see http://map.gsfc.nasa.gov/index.html) Within a year, it should return an extraordinarily detailed map of the temperature fluctuations (Figure 7) as well as the polarization of the radiation. (Just as sunlight is polarized when it reflects off a surface, the microwave background photons were polarized by their encounters with free electrons in the early universe.) A few years from now, a European-led mission called the Planck Surveyor will create an even more detailed map, with even greater accuracy.
The new maps have enough resolution (down to much less than a degree) to show the early universe ringing like the head of a drum, or wiggling like the surface of a pond, in response to the small kick from the primordial perturbations. The precision of the data now allows measurement of many cosmological parameters, including the density of matter and the global curvature of space, to an accuracy of a few percent—the beginning of true "precision cosmology." We even have precise data about some parameters of the inflaton field, a new force of nature, which has not been observed in any other way.
In addition to the inflaton perturbations, quanta of another field are created during inflation—those of gravity itself. These may also leave imprints on spacetime and the background anisotropy today, in the form of large-scale gravitational waves. Neither gravitational waves nor gravitons have yet been observed, but they are a form of energy predicted in Einstein 's theory of spacetime. As it does with the inflaton quanta, inflation takes single graviton quanta and stretches them to great size. Starting with observing campaigns such as MAP, one strategy to tease apart the inflaton and the graviton fluctuations is to seek certain patterns of polarization that can only be created by gravitons. (It's likely that gravitational waves, though not individual quanta and probably not from inflation, will be soon detected, for the first time and at much higher frequencies, by new laser interferometers, such as LIGO, now coming into operation.)
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