How things change over time has never been as strikingly demonstrated as in recent findings. If you go back to the distant era when the universe was only 380,000 years old, you find that neutrinos made up fully ten percent of the universe. Given that these sub-atomic particles moving at nearly the speed of light are so abundant today that millions of them pass through us every second, you’d think they compose a substantial portion of today’s universe, but they actually account for less than one percent. And the change in neutrino ratio is only the beginning.
For according to five years of recently released data from the Wilkinson Microwave Anisotropy Probe (WMAP), the early universe was composed of 12 percent atoms, 15 percent photons, almost no dark energy, and 63 percent dark matter. The contrast is stark, given WMAP estimates of the current cosmos: 4.6 percent atoms, 23 percent dark matter, 72 percent dark energy. And, of course, those greatly diminished neutrinos.
Image: WMAP data reveal that its contents include 4.6% atoms, the building blocks of stars and planets. Dark matter comprises 23% of the universe. This matter, different from atoms, does not emit or absorb light. It has only been detected indirectly by its gravity. 72% of the universe, is composed of “dark energy”, which acts as a sort of an anti-gravity. This energy, distinct from dark matter, is responsible for the present-day acceleration of the universal expansion. WMAP data is accurate to two digits, so the total of these numbers is not 100%. This reflects the current limits of WMAP’s ability to define Dark Matter and Dark Energy. Credit: NASA / WMAP Science Team
Usefully, the neutrino information jibes with theories based on the amount of helium we see today, which predict a cosmic neutrino background present when the helium was made. That, in turn, fits with measurements of neutrino properties made in particle accelerators here on Earth. The WMAP data release thus carries us forward, using observation that confirms theories and helps us stake out evidence for an era long vanished. The seven papers just submitted to the Astrophysical Journal about these data further enhance WMAP’s position as a breakthrough instrument.
Other WMAP results of interest: While the process that drives cosmic ‘inflation’ is still problematic (a rather serious understatement on my part), the new data make it possible to eliminate particular versions of inflation while leaving others viable. WMAP principal investigator Charles Bennett (Johns Hopkins) puts it this way: “The new WMAP data rule out many mainstream ideas that seek to describe the growth burst in the early universe.” The new constraints will power future investigations.
Image (click to enlarge): A representation of the evolution of the universe over 13.7 billion years. The far left depicts the earliest moment we can now probe, when a period of “inflation” produced a burst of exponential growth in the universe. (Size is depicted by the vertical extent of the grid in this graphic.) For the next several billion years, the expansion of the universe gradually slowed down as the matter in the universe pulled on itself via gravity. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. The afterglow light seen by WMAP was emitted ~400,000 years after inflation and has traversed the universe largely unimpeded since then. The conditions of earlier times are imprinted on this light; it also forms a backlight for later developments of the universe. Credit: NASA / WMAP Science Team
WMAP also indicates that the ‘cosmic fog’ produced when the first generation of stars began to shine first appeared when the universe was some 400,000 years old. What an era that would have been — the first stars put an end to the darkness, creating a fog of electrons in the surrounding gas that scattered microwaves. The microwaves WMAP looked at to help scientists make these assessments represent light that has lost energy over the subsequent 13.7 billion years of the universe’s expansion, but is still robust enough to tell an extraordinary tale.
For more, see this news release on the WMAP data and its implications. More on the papers in The Astrophysical Journal when they become available.