|Apr25-12, 10:47 PM||#1|
From Boomerang to Planck: reading the ancient light
Many of us, probably most, know Smoot and Mather got the 2006 Nobel "for their discovery of the blackbody form and anisotropy of the cosmic microwave background radiation."
But there is more to the story, other scientists other instruments other features of the early universe discovered in the data.
So we could try having a thread that covers some other parts of the story besides the familiar chapters of COBE and WMAP. I know only small part of it so I'm asking for help. Please contribute anything you know to help fill out the picture.
I think there is more that we can learn from this, or at least I can in any case. And this is the year that the PLANCK mission data will become available---mapping the ancient light with even finer resolution than WMAP, including polarization as well as spectrum and temperature.
It could be worthwhile building some awareness in preparation for when the Planck data comes out.
I think there's an interest in the human angle too so any anecdotes or if you knew any of the personalities involved and want to share that would be all to the good.
In the beginning people didn't use spacecraft to observe the CMB, among other means, they actually used balloons to carry the instruments aloft. Here's a bit from the Wikipedia article on COBE that mentions some earlier projects:
==quote http://en.wikipedia.org/wiki/Cosmic_Background_Explorer ==
...During the long gestation period of COBE, there were two significant astronomical developments. First, in 1981, two teams of astronomers, one led by David Wilkinson of Princeton and the other by Francesco Melchiorri of the University of Florence, simultaneously announced that they detected a quadrupole distribution of CMB using balloon-borne instruments. This finding would have been the detection of the black-body distribution of CMB that FIRAS on COBE was to measure. In particular, the Florence group claimed a detection of intermediate angular scale anisotropies at the level 100 microkelvins  in agreement with later measurements made by the BOOMERanG experiment. However, a number of other experiments attempted to duplicate their results and were unable to do so.
Second, in 1987 a Japanese-American team led by Andrew Lange and Paul Richards of UC Berkeley and Toshio Matsumoto of Nagoya University...
Boomerang was another balloon-borne experiment, that was carried out later. It may have been the first to gather enough information about the "angular power spectrum" (the angular scale of the blotchy speckly temperature variation) to deduce things like the approximate average flatness of the universe.
No reason to stick to chronological order. The Planck mission looks to be the most exciting and information packed one yet. We could just as well start by gathering some basic facts and links about that.
|Apr25-12, 11:20 PM||#2|
Here are some links on Boomerang:
Boomerang operated at an altitude of 37 kilometers (120,000 feet).
The instrument payload was over 3000 pounds including cryogenics. It traveled a large circle around the South Pole, using prevailing winds that blow from west to east in the Antarctic.
Here is a sample paper:
Mapping the CMB Sky: The BOOMERANG experiment
You can get a bunch more just by searching arxiv with the lead investigators' names
Long before Boomerang and COBE there were experiments such as one in the 1970s using a U2 aircraft. These did not get up so high and get as good resolution. About all the U2 mapping could tell was the "dipole". The hotspot in the constellation Leo and the coldspot opposite it in the sky. This shows the speed and direction of our motion relative to the ancient matter of the universe and the ancient light emitted by it.
Ned Wright has assembled some history about the discovery of the dipole:
Boomerang and COBE produced the first complex maps, mottled and speckled with hot and cold fluctuations. Anyone more familiar with the history please correct me if I'm mistaken.
|Apr26-12, 01:24 AM||#3|
In everyday language, what did Boomerang do? And how does "measuring the angular size of the first few acoustic peaks" translate into determining that the geometry of the world is approximately flat?
That would be good to understand. Here's a popular account:
It is rather typical both in what it says and what it doesn't say. It mentions mapping the CMB and determining how big the fluctuations look in the sky---their angular size. And it says largescale flatness of spatial geometry was deduced from this. But it doesn't tell how you deduce your way from one to the other! It would be great if somebody, or several people, would offer their explanations of that, as much as possible in everyday language.
Boomerang was one of the projects which in 1998 provided evidence for dark matter and a positive cosmological constant (often referred to as "dark energy"). What actually was that evidence, in the case of Boomerang? It was its support for geometric flatness. There was not enough visible matter to account for overall spatial flatness (the density of matter/energy affects curvature.) So there had to be one or more unseen components to balance the budget.
So how does measuring the sizes of the largest temperature fluctuations (besides the dipole) tell you space is approximately flat? Angles of triangles adding up to near 180 degrees. The volume of an astronomically large ball growing as the cube of its radius, etc.
I exclude the dipole from consideration because it does not tell us anything about the early universe. It is just evidence of our own motion. The other fluctuations (with the dipole removed from the data) are actually a picture of the early universe: of its density and temperature variations at a certain moment in its history when it was filled nearly uniformly with a cloud of glowing hot gas. The ancient light is that glow. You can imagine the kind of geometric detective work that goes into interpreting that picture: the microwave sky.
One thing that helps is that we can deduce by what factor distances have expanded since the ancient light was released. The hot gas would have become transparent when it cooled to about 3000 kelvin. So the light started life as the thermal glow characteristic of things that temperature, and it is now the thermal glow characteristic of things cooler by factor of 1090. Distances must have grown by that factor.
Another thing that helps is we know what the speed of soundwaves was in that hot gas. So we can estimate what the actual sizes of the fluctuations in density and pressure should have been. And compare that with how they look in the sky---their angular size.
Hopefully I'll have more on this later.
|Apr26-12, 04:04 AM||#4|
From Boomerang to Planck: reading the ancient light
Good morning marcus,
I like this page:
It's a very good starting point for those who want to understand what the spectrum means but don't actually want to do the math - like me.
|Apr26-12, 06:11 AM||#5|
Thanks Ich. That material from Wayne Hu looks excellent!
|Apr26-12, 10:36 AM||#6|
This is a really good read, its a laymans accont of COBE and WMAP, not just the science but the people too:
I spoke to Heirnanya Peiris who is on the PLanck team a few weeks ago. I think she said the data will be out early 2013 but maybe I have that wrong. Shes preparing it now but is not allowed to discuss it til the release date.
I wonder if there will be anything new or will it just reconfirm WMAP with more accuracy?
Speaking to a few people in the field , they dont expect PLanck to detect the B mode, but I imagine they hope it will.
|Apr26-12, 12:00 PM||#7|
Thanks Skydive! It's great to have direct contacts like that shared here. I remember now: the Planck data release is planned for early 2013.
If I was going to recommend a first source to read I guess it would be a few pages of
especially the figures 10, 11, 13, and captions. It explains how the "Acoustic Peaks" were formed and why they are important. They are features that were predicted well before they were seen, and whose size-distribution we can predict, and which are bunched according to size.
Maybe somebody knows a better nuts-and-bolts primer on the Acoustic Peaks. I don't.
Pages 25, 27, 29.
Boomerang was the first mapper with enough resolution to locate the first few acoustic peaks (at least with enough confidence to deduce near flatness.) COBE came earlier but I don't think it quite did this. Brian or other please correct me if I'm mistaken about this.
The first acoustic peak comes at wavenumber 200, which means fluctuations with an angular size of about half a degree, about how big the full moon, how much of the sky it takes up.
The acoustic peaks are tiny features compared with the grosser mottling and blotching of the map. Look at figure 11. It shows the gross mottling (wavenumber < 100) separated out from the fine acoustic pulsations (wavenumber > 100).
Lineweaver is a master at presenting information for the intelligent general reader. To always have this article at your fingertips just remember to google "Lineweaver arxiv". He has a lot of highly cited articles in the Arxiv, but apparently this is the most popular so google knows what you want. To be extra sure you could google "Lineweaver cosmic arxiv". Either way you get the arxiv copy of "lineweaver inflation and the cosmic microwave background".
We still have to face the question of how in heaven charting the size-dstribution of these acoustic peaks enabled people to deduce something about the geometry of the universe.
|Jun9-12, 10:16 PM||#8|
This graphic helps one grasp how they deduce curvature from the skymap of CMB fluctuations:
Cepheid found the link.
A new member Jerusalem brought in the picture but without a link. I will copy Jerusalem's picture:
If we believe the confidence interval on this picture, Boomerang found Omega to be in the range [1.01, 1.13].
By modern standards this is WRONG. The 2010 WMAP report, the final report based on 7 years of data, gave the 95% confidence interval as [0.9916, 1.0133].
So it now includes Omega = 1 (exactly flat, zero overall curvature)
and it even includes a little territory < 1 (negative curvature, fluctuations look SMALLER than actual size)
|Jun9-12, 10:42 PM||#9|
I guess most people realize that this is where the issue is to be settled whether the standard LCDM universe is more likely finite spatial volume or more likely infinite volume with infinite amount of matter all existing simultaneously.
The Planck mission is currently taking in more data and the results will be presented in 2013, so we will have a different confidence interval for Omega. Hopefully something narrower and inside the WMAP [0.9916, 1.0133]
To get WMAP's google "komatsu wmap 7" and look on page 3 in Table 2.
they give the interval for Ωk and plain Ω= 1 - Ωk
so you can figure out that it is [0.9916, 1.0133]
It is in the rightmost column labeled WMAP+BAO+SN or some such thing.
saying "7" when you google just ensures that you get the final (7 year) report.
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