Without much public notice, the team running the Wilkinson Microwave Anisotropy Probe (WMAP) recently released results from the satellite's "seven-year data set," updating the five-year data released in 2008.

WMAP has been mapping the sizes and strengths of the slight irregularities in the cosmic microwave background radiation filling the sky. The microwave background is the "wallpaper" on the sky behind everything else seen in the universe. The slight temperature irregularities written on it (seen on the all-sky map here) tell much about the cosmic conditions just 380,000 years after the Big Bang, when the universe first became transparent to its own radiation — and before, right back to the Big Bang itself.

The two more years of data have further beaten down the statistical uncertainties in the cosmic background map, allowing analysts to refine what it tells us about the cosmos as a whole. If the new, revised results didn't make much news, it's because they show modern cosmology to be steady on course. The better data only firm up confidence in what we already thought we knew.

Some high points:

The canonical age of the universe is now 13.75 plus or minus 0.11 billion years since the Big Bang, compared to 13.73 ± 0.12 billion years from the 5-year data set. That's now an age uncertainty of only 0.8% (all uncertainties are expressed at the "1-sigma," or 68%, confidence level). By comparison, astronomy books in your public library probably say the universe is "between 10 and 20" billion years old.

The Hubble constant, the rate at which the universe is expanding today, is 70.4 ± 1.4 km per second per megaparsec. Books in your library probably say the Hubble constant is "between 50 and 100."

Combining the new values for these and many other parameters, the 7-year WMAP data tightens up the standard model of cosmology by 50% overall, according to a NASA statement. This includes WMAP's detection of the expected polarization patterns around warm and cool spots in the microwave background.

A key prediction of inflationary-universe theory — which explains how the universe grew to be the way we see it, based on quantum events that happened in the first 10^–32 second after the Big Bang — is also firmed up. The simplest versions of inflation predict that quantum fluctuations during that early instant did not produce equally strong irregularities on all size scales, the way nature often works. Instead they should have produced slightly weaker fluctuations at larger scales. WMAP finds exactly this. The so-called "scalar spectral index" is clearly tilted, with a value not of exactly 1.00, but of 0.96 ± 0.01 by the new refinement. This is a big deal for boosting confidence that inflation really happened.

Space, as far away as we can see, is flat to a new degree of precision. The total density of normal matter, dark matter, and dark energy adds up to 1.0023 ± 0.0055 of the critical flatness density. This is consistent with a value of exactly 1 (flat space) to a precision of just half a percent!

The breakdown of the basic components of the universe is now:

Ordinary matter, 4.56 ± 0.16 percent

Nonbaryonic dark matter, 22.7 ± 1.4 percent

Dark energy, 72.8 ± 0.5 percent.

The dark energy's "equation of state," a parameter known as w, is –0.980 ± 0.053, consistent with a value of exactly –1 to a precision of about 5%. This is the value that w would have if the dark energy is an inherent, constant property of any given volume of space, regardless of how much the space may have expanded in the past. This property matches Albert Einstein's idea of a "cosmological constant," which he inserted into his equations in 1917, and pretty much rules out the idea that the dark energy is some kind of "quintessence" existing in space that thins out as space expands.

For the first time, WMAP has extracted evidence from the microwave background that directly reveals primordial helium emerging from the Big Bang — not just hydrogen. This was totally expected, but it's nice to see it confirmed. This is the first direct evidence of helium existing before the first stars.

The first stars and/or quasars turned on at a time corresponding to a redshift (z) of 10.4 ± 1.2, dating the start of the "reionization era" at 460 ± 80 million years after the Big Bang — in agreement with more direct astronomical evidence.

One reason why these improved results didn't make much news could be that Europe's newer and greater Planck probe of the microwave background is up and working. But its first results probably won't be available for about another year.

Here's a NASA press release with more information.

Here's the team's summary paper on WMAP's 7-year results. For the table of the new best values for all the basic cosmic parameters, scroll to page 39.

Cool tool for building your own universe to see if you can make one that matches the WMAP data. Suitable for kids — this makes it look simple!

Posted by Alan MacRobert, February 14, 2010

related content: Cosmology news

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