Just How Important Is WMAP?

We had some prospective grad students visiting Irvine the other day.  I said something about WMAP and one of the prospectives said "What's WMAP?"  Coming from a lay man I can understand, but a physics major!  I was in shock.  Here's why:

First off, "Since 2000, the three most highly cited papers in all of physics and astronomy are WMAP scientific papers." (Emphasis added.)

Second, today WMAP is just a relevant as ever.  I draw your attention to the most cited papers in 2009.  (The first one, Review of Particle Physics, isn't a research article but an "encyclopedia" people quote for values of things like constants. We've discussed this book before.)

In 2009, the #1, #3, #9 and #10 most cited research articles were the WMAP papers.  To be in physics and to not know about WMAP, to me, means you are living under a rock.  There is no experiment in this last decade producing more follow up scientific research!

I point you to two other interesting observations from that list:

First, the #4, #5 and #6 most cited papers from 2009 are other cosmology papers.  That means 7 of the top 10 most cited papers, tracked by Spires, are cosmology papers.  Cosmology is hot baby!

Second, the #2 most cited paper last year is the famous paper by Juan Maldacena introducing the ADS/CFT correspondance.  As pointed out by Peter Woit, at this rate this paper will surpass "Weinberg’s 1967 paper as the most heavily cited particle physics paper of all time."

A Special Footnote In The WMAP 7 Paper

The, in my opinion, most important new WMAP 7 paper is the one on the  Cosmological Interpretation.  That's where we get all the good stuff: constraints on vital cosmological parameters and theory behind them, etc.  One such observable, primordial non-Gaussianity, has recieved a lot of attention recently as it is a direct probe of inflation.

And guess what?  We, myself and my collaborators, (I'm a coauthor on a second paper in addition to the first author one) got a special footnote on pg. 27 stating we've found a new way to do this:

Recently, the “skewness power spectrum” has been proposed as a new way to measure f_local and other non-Gaussian components such as the secondary anisotropies and point sources (Munshi & Heavens 2009; Smidt et al. 2009; Munshi et al. 2009; Calabrese et al. 2009...The skewness power spectrum method provides a means to visualize the shape of various bispectra as a function of multipoles.

It's not the highest honor in the world; many people were cited in the new WMAP 7 papers. But it at least makes you feel your work was recognized as being interesting.

Anyone Going To The AAS Meeting?

I'll be attending the AAS meeting this next week.  Let me know if any of you are going as I may see you there.  This is my first AAS meeting and I am excited to be in DC again.

Any predictions on what may be announced? I have no evidence, but the WMAP 7 year results should be published soon.  I wonder if they will be announced there.

Newscientist on WMAP Results

This is a big enough it deserves more than one post. Especially because it shows the first evidence of the the "Cosmic Neutrino Background." Also, the results can claim with confidence: neutrinos and dark matter were present in the early universe. This is big because it is further evidence that neutrinos and dark matter are different things.

From New Scientist:

We are all submerged in a sea of almost undetectable particles left over from the first few seconds of the big bang, according to the latest observations from a NASA satellite. The Wilkinson Microwave Anisotropy Probe (WMAP) has confirmed the theory that the universe is filled with a fluid of cold neutrinos that remain almost entirely aloof from ordinary matter.

Cosmologists think that in the hot, dense, young universe, neutrinos should have been created in high-energy particle collisions. About two seconds after the big bang, the cauldron of colliding particles would have cooled down so much that most would not have had enough energy to interact strongly with neutrinos. The neutrinos would then have "de-coupled" from other matter and radiation.

In theory, they should still be buzzing around, a soup of slippery particles that by today has been chilled to a temperature of only 1.9 ° Celsius above absolute zero.

Now WMAP has found evidence of this cosmic gazpacho. The spacecraft, launched in 2001, has been building up a picture of the cosmic microwave background radiation, which carries a detailed imprint of the state of the universe 380,000 years after the big bang. In particular, it reveals the pattern of density fluctuations in space, the "texture" of the early universe.

Travelling at nearly the speed of light, neutrinos should have discouraged matter from forming tight clumps, and so smoothed out the texture of the universe slightly.

Only detector

The WMAP data clearly show this smoothing effect, implying that those fast-flowing neutrinos formed about 10% of all the energy in the 380,000-year-old universe. "This confirms the theory," says Eiichiro Komatsu of the University of Texas in Austin, US, lead author of a study about the result.

In 2005, another analysis also provided evidence for a cosmic neutrino background, but it relied on combining WMAP data from other sources, and making some assumptions about other cosmological parameters, says Komatsu. Now that WMAP has collected five years' worth of data, it is enough to show firm evidence of the neutrino background on its own.

The neutrinos are too weak to be detected individually. "These neutrinos cannot be detected on the ground; you need the CMB to do it," Komatsu told New Scientist.

Other neutrinos, for example those generated in the Sun's core, can be detected on Earth, often in large tanks of water buried deep underground, where an occasional neutrino is unlucky enough to hit an atomic nucleus. But cosmic background neutrinos have only a millionth of the energy of a typical solar neutrino, making them even more ethereal.

To stop a substantial fraction of solar neutrinos, you would already need a lead shield a light year thick, says Komatsu. How about cosmic background neutrinos? "I'd estimate you would need a block of lead that is thicker than the entire universe."

5th Year WMAP Results Released

For those of you who don't know, WMAP is a satellite taking very precise measurements of the CMB. Lots of cosmological parameters have been determined form it. Physics, like inflationary LambdaCDM models have also been confirmed by it.

Also, to show how big this is, the WMAP's predecessor COBE was responsible for Nobel Prizes. WMAP also has produced the most cited papers in all science since 2003 and the third year results were the Science breakthrough of the year.

Every odd year WMAP has released data. The first and third year data has been previously released and now the 5th year data has been released.

To quote Sean Carroll:

The biggest piece of news isn’t that the results have overturned any foundations, but that the concordance model with dark matter, dark energy, and ordinary matter continues to work. The WMAP folks have produced an elaborate cosmological parameters table that runs the numbers for different sets of assumptions (with and without spatial curvature, running spectral index, etc), and for different sets of data (not just WMAP but also supernovae, lensing, etc). Everything is basically consistent with a flat universe comprised of 72% vacuum energy, 23% dark matter, and 5% ordinary matter. The perturbations are close to scale-free, but still seem to be a little larger on long wavelengths than shorter ones. Probably the most fun result is that there is, for the first time, evidence from the CMB that neutrinos exist! Good to know.

Big Empty Cold Spot Found in the Universe

From the BBC:

Astronomers have found an enormous void in space that measures nearly a billion light-years across.

It is empty of both normal matter - such as galaxies and stars - and the mysterious "dark matter" that cannot be seen directly with telescopes. The "hole" is located in the direction of the Eridanus constellation and has been identified in data from a survey of the sky made at radio wavelengths. The discovery will be reported in a paper in the Astrophysical Journal.
Previous sky surveys that have traced the large-scale structure of the nearby Universe have long shown, for example, how the clustering of galaxies is strung into vast filaments and sheets that are separated by great gaps.

But the void discovered by a University of Minnesota team is about 1,000 times the volume of what would be expected in typical cosmic gaps. "It's hard even for astronomers to picture how big these things are," conceded Minnesota's Professor Lawrence Rudnick. "If you were to travel at the speed of light, it would take you several years to get to the nearest stars in our own Milky Way galaxy; but if you were to go to this hole and enter one side, you'd have to travel for a billion years before you would get to the other side," he told BBC News. The void is roughly 6-10 billion light-years away and takes a sizable chunk out of the visible Universe in its direction.

There is more to the story. It goes on to explain that they found this because it corresponds to a "cold spot" from the CMB. They pointed the VLA towards this spot and were unable to detect any matter, light or dark. They think it has to do with dark energy. The originating paper that will be published in the Astrophysical Journal. You can find it here (arXiv).