(fwd) WMAP results

Andrzej Marecki amr w astro.uni.torun.pl
Śro, 12 Lut 2003, 15:34:23 CET

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Subject: WMAP results
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MAP (now WMAP -- see below) had its press conference and has released
a bunch of papers on the first-year sky maps.  Technical papers are at


and a pretty picture and less-technical explanation of the results is


The most exciting thing about the results is that there are no big
surprises!  The angular power spectrum, which is the primary bit of
science you get out of a microwave background sky map, is very
consistent with what has become the "standard model" of cosmology: a
flat inflation-based model containing dark matter and dark energy.
The power spectrum is well measured past the second acoustic peak and
into the third one.  The temperature-polarization cross-correlation
signal was also seen at about the expected level.

Although surprises in the data would have been fun, the lack of
surprises is actually extremely remarkable, considering (a) how
strange the "standard model" is and (b) how recently cosmology
has become a mature scientific discipline.

I haven't read the papers yet -- there are 13 of them, some quite
long!  But here are a couple of highlights from scanning abstracts and

1. The data let you constrain cosmological parameters much more precisely
than before.  In particular, the age of the Universe is 13.7 +/- 0.2 Gyr.
(Not that long ago, we couldn't say more than "12 to 20 Gyr"!)  The
Universe is flat: Omega total is 1.02 +/- 0.02.  The densities of
various forms of matter and energy are pinned down quite well too.

One reason WMAP is such an improvement over previous data is that
it's a single data set covering both large and small scales.  To
get limits on parameters from previous data, you had to stitch together
COBE on large scales with other experiments on small scales.  Since
each experiment has its own calibration errors, that means you don't
measure the heights of the peaks all that well.

2. The TE polarization correlation showed up as expected.  This
strongly confirms that the initial fluctuations were adiabatic.  That
matters, because adiabatic fluctuations are a strong prediction of
inflation, and large-scale adiabatic fluctuations are hard to produce
in any way other than inflation.

3. The large-angular-scale polarization gives information about the
way the Universe reionized after the time of decoupling.  This is
an area about which very little was known before now: reionization
had to have happened some time, but the data didn't say when.
Now we know: the optical depth to reionization is 0.17 +/- 0.04,
which means that the Universe reionized at an age of about 200000 years.
This is presumably when the first stars formed -- their UV
radiation is what presumably reionized the Universe.

4. There is a hint that the spectral index may vary with scale, from
n>1 on large scales to n<1 on small scales.  That is, the primordial
spectrum of fluctuations may not have been a pure power law.  That's
only significant at the 2-sigma level, so it may go away.  If it's
true, it gives useful information on discriminating between specific
inflationary models.

That's all I'll say for now.  There's a ton of information in this
data set, and people will be poring over it for a long time to come,
but those are the main things that jumped out at me after a quick

On a non-scientific note, they changed MAP to WMAP.  The W stands
for Wilkinson, a member of the MAP team who died last year.
David Wilkinson was a leader in observational cosmology for
decades, and this is a very fitting tribute to him.  


[E-mail me at name w domain.edu, as opposed to name w machine.domain.edu.]
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Andrzej Marecki                | 
Torun Centre for Astronomy     |   e-mail: amr w astro.uni.torun.pl
N. Copernicus University       |   WWW:    http://www.astro.uni.torun.pl
ul. Gagarina 11                |   tel: +48 56 6113032
PL-87-100 Torun, POLAND        |   fax: +48 56 6113009

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