Knocking on Heaven's Door - Lisa Randall [175]
[ FIGURE 73 ] Zero, positive, and negative curvature on two-dimensional surfaces. The universe, too, can be curved, but in four-dimensional spacetime that is difficult to draw.
This flatness of the universe was a huge victory for inflationary cosmology. Had it not been true, inflation would have been ruled out. The WMAP measurements were also a victory for science. When theorists first proposed the detailed measurements of the microwave background that would eventually tell us about the geometry of the universe, everyone thought it interesting enough to throw out to the science community, but far too difficult technically to achieve any time soon. Within the decade, confounding all expectation, observational cosmologists made the necessary measurements and gave us amazing insights into how the universe has evolved. WMAP is still providing new results, performing detailed measurements of the variation in temperature across the sky. The Planck satellite in operation today is measuring these fluctuations more precisely still. The CMB measurements have proven to be a prime resource of insight into the early universe and will most likely continue to be so.
Recent detailed studies of the cosmic radiation left throughout the sky have led to other enormous leaps in our quantitative knowledge of the universe and its evolution. The details of the radiation have provided rich information about the matter and energy that surrounds us. In addition to telling us the conditions when the light first started heading toward us, the CMB tells us about the universe through which the light had to travel. If the universe had changed in the last 13.75 billion years, or if its energy were different than expected, relativity tells us that it would have affected the path that the light-ray took and consequently the measured properties of the radiation that was measured. Since it is such a sensitive probe of the energy content of the universe today, the microwave background gives information about what the universe contains. This includes the dark matter and dark energy we will now consider.
HEART OF DARKNESS
In addition to successfully confirming inflationary theory, CMB measurements presented a few major mysteries that cosmologists, astronomers, and particle physicists now want to address. Inflation tells us that the universe should be flat but it doesn’t tell us where the energy required to make it flat now resides. Nonetheless, based on Einstein’s equations of general relativity, we can calculate the energy needed for the universe to be flat today. It turns out that known visible matter alone provides a mere four percent of the energy required.
An additional puzzle that had already indicated the need for something new concerned the tininess of the fluctuations in temperature and density that COBE had measured. With only visible matter and such tiny perturbations, the universe wouldn’t have lasted long enough for the perturbations to have grown large enough for structure to have formed. The existence of galaxies and clusters of galaxies in conjunction with the tininess of the measured fluctuations pointed to the existence of matter that no one had yet directly seen.
In fact, scientists had already known that a new type of matter known as dark matter should exist well before COBE’s microwave radiation results. Other observations that we will get to soon that had already indicated additional unseen matter must exist. This mysterious stuff, which became known as dark matter, exerts