Open Questions: The Cosmic Microwave Background

BOOMERANG Image of the Cosmic Microwave Background
See BOOMERANG Home Page for more.

Introduction

COBE project

BOOMERanG project

MAXIMA project

Origin of the CMB

CMB anisotropies

Polarization of the CMB

Lowest harmonics of the CMB

Important findings from CMB research

Open questions

Recommended references: Web sites

Recommended references: Magazine/journal articles
Recommended references: Books

Introduction

What it all comes down to is that a very hot ionized gas of hydrogen and helium is not transparent to light. This is because photons of any energy can interact with free electrons. And in a sufficiently hot gas, there are plenty of free electrons, since they have too much energy to be bound into atoms. But as soon as the gas cools enough that electrons can be bound into hydrogen and helium atoms, they cease to interact strongly with photons. This is because quantum mechanics allows bound electrons to absorb or emit photons of only certain discrete frequencies.

The rapidly cooling early universe reached this critical temperature about 300,000 years after the big bang. At this point, matter and light "decoupled" from each other. From that time on, light was virtually uninhibited from traveling any distance without being absorbed. Which means that we can see that very same light today. It is what we call the cosmic microwave background.

Although light photons had much shorter wavelengths at the time (about ---), we see it red-shifted by a factor of xxx, giving it a wavelength of ---, which is in what we now call the microwave band. Ordinary microwave receivers can "see" this light -- and what they see is no less than what the universe itself looked like at the age of 300,000 years. If our eyes were sensitive to microwave energy and could distinguish variations of a few dozen parts per million, the image at the top of this page is what we would see (rougly an 1800 square degree portion of it in the southern hemisphere).

From the appearance of the universe at that age, we can deduce some very significant facts about it, and test our theories about how it came to be the way it was/is. The most important thing about it that we can see is that it is an example of what physicists call a radiating black body. In fact, it is the most perfect example we know of. The salient fact about a black body is that the spectrum of light that it emits is completely determined by its temperature. That is, we can describe everything about the spectrum simply by specifying the temperature. It is therefore natural to talk about the microwave background that we observe in terms of temperature rather than wavelength (since it is includes a whole spectrum of wavelenghts, not just one). At the time the light was emitted, the corresponding temperature was about 2700° K, but now, due to the redshift, it is only 2.73° K.

The next most significant thing which we observe about the microwave background is that it is almost, but very importantly not quite, entirely homogeneous. That is, there are fluctuations in the temperature that we observe, but they are extremely small in amplitude -- about 300 millionths of a degree, or 3 parts in 10,000. However, it is the spatial size of these fluctuations which is the most significant thing. The distance from the peak of one fluctuation to the next is an angle of about 1 degree of the sky (twice the angular size of the Moon).

Temperature fluctuations are so important because they correspond to fluctuations in the density of matter. We can in effect "see" regions of greater or lesser matter density, just as we can see more or less dense regions of a cloud. This is important, in turn, because we know just about what the minimum size of such fluctuations should be.

Recommended references: Web sites

Site indexes

Web Resources for Cosmology and the CMB: From the Cosmic Background Imager site.
CMB Resources: From Wayne Hu's site.
Max Tegmark's CMB analysis center: Experiments: Contains many links to specific CMB experiments and other CMB pages. Also has graphs summarizing results of many experiments. Part of Max Tegmark's site.
CMB Experiments: Links provided by Martin White to many different experiments and general information related to the CMB.

Sites with general resources

Microwave Anisotropy Probe (MAP): Home page of the NASA project. The site is most easily navigated through the site map.
Planck Science Team Home: Home page of the ESA Planck mission that will "map the structure of the Cosmic Microwave Background, in unprecedented detail. Planck will constrain cosmological models and examine the birth of large-scale structure in the universe."
Planck Overview: Set of pages that provides general information about the scientific mission of the Planck satellite. Part of the ESA Space Science site.
Cosmic Background Explorer (COBE): Home page of the NASA project.
Arcminute Cosmology Bolometer Array Receiver: Home page describing an instrument, known as ACBAR for short, installed at the South Pole to measure small temperature differences in the CMB.
Degree Angular Scale Interferometer: DASI is a 13-element interferometer designed to measure temperature and polarization anisotropy of the CMB over a large range of scales with high sensitivity. Its results have confirmed the long predicted existence of polarization anisotropy.
Cosmic Background Imager: The CBI is a special-purpose radio telescope designed to study the cosmic microwave background radiation. It is located at an altitude of 5080 m in the Chilean Andes. The Web site contains a description of the instrument, research papers, and external links.
Wayne Hu: Home page of a cosmologist specializing in the cosmic microwave background, with many helpful resources, including introductory material, as well as more intermediate and advanced information on the physics of microwave background anisotropies.
CMB Astrophysics Research Program: Astrophysicist George Smoot's group. Information about projects such as COBE, MAXIMA, and BOOMERanG. An earlier version of this site is here.
BOOMERanG - Balloon Observations of Millimetric Extragalactic Radiation: Project home page. Includes a fact sheet which explains a little about the CMB and the BOOMERanG project.
MAXIMA - Millimeter Anisotropy Experiment Imaging Array: Project home page.
Julian Borrill's Homepage: Describes various research projects which investigated the the CMB.

Surveys, overviews, tutorials

Cosmic microwave background radiation: Article from Wikipedia. See also Sunyaev-Zeldovich effect.
Tests of the Big Bang: The CMB: Brief overview of the CMB at NASA's MAP site.
An Introduction to the Cosmic Microwave Background: A beginning level overview of the subject, by Wayne Hu.
Cosmic Microwave Background: Intermediate Tutorials: Continuation of Wayne Hu's tutorials on the CMB, with emphasis on sound waves in the fine angular scale structure of the temperature anisotropies.
The Physics of Microwave Background Anisotropies: Am excellent collection of resources related to the CMB, provided by Wayne Hu. This includes a layman's introduction, a tour of CMB physics, some external links, and more advanced material.
CMB Polarization: Very good information about polarization in the CMB, by Angelica de Oliveira-Costa. She also has a good page on making sky maps using CMB data.
The Cosmic Microwave Background: Good explanatory pages by Douglas Scott. Author has a page giving a "scorecard" rating the success of various cosmological theories at meeting observational tests. The frequently asked questions (and answers) page is especially worth reading.
Cosmic Background Radiation: Part of the Cambridge Cosmology site. Provides an explanation of the CMB and possible causes of fluctuations in the CMB.
The Cosmic Microwave Background: Good single-page overview, by Ben Wandelt.
Cosmic Microwave Background: Visual materials from a presentation by Neil Cornish on Measuring the Universe.
About WMAP and the Cosmic Microwave Background: February 2003 article from Space.com. Describes basic facts about the CMB and the MAP experiment.
WMAP data put cosmic inflation to the test: May 2006 article from Physics World, by Gary Hinshaw. "Measurements of the polarization of the cosmic microwave background open up a new window on the universe when it was just 10^-35 s old."
The cosmic microwave background: Summary of April 2003 article from Physics World, by Pedro Ferreira. "Measurements of the cosmic microwave background reveal a universe that is surprisingly easy to model but difficult to understand. Why and how have we got it so right?"
Tuning in to the early universe: August 2002 article from Physics World, by Joseph Silk. "The latest images of the cosmic microwave background reveal the seeds of galaxy formation."
Microwaves map cosmic origins: June 2001 article from Physics World, by Edwin Cartlidge. "Recent data from the cosmic microwave background add further weight to the inflationary big-bang model. But more precise measurements from NASA's MAP satellite, which is due to be launched this month, could challenge this theory."
Boomerang backs flat universe: June 2000 article from Physics World, by Joseph Silk. "The faint microwave glow left over from the big bang has been measured with unprecedented precision, giving astronomers a new insight into the nature of the universe."
Early universe comes into focus: May 2001 news article from Physics World, about recent measurements of temperature fluctuations in the cosmic microwave background that are consistent with theories of cosmic inflation.
Cosmological model gets a boost: March 2001 news article from Physics World, about measurements of the power spectrum of the cosmic microwave background.
The universe is flat - official: April 2000 news article from PhysicsWeb about measurements of the CMB by the Boomerang experiment.
Balloon sounds out the early universe: April 2000 news article in Science News about results of the BOOMERANG experiment that demonstrate nearly perfect flatness of the universe.
CMB Experiments Lecture: A presentation by John Carlstrom, in PDF format, from an October 2001 conference on cosmology.
Skewing the Cosmic Bell Curve: September 1999 Scientific American Science and the Citizen article, subtitled "Nonrandom features could sink inflation." The article explains how statistical information about the CMB can help confirm or refute inflationary theories.

Press releases, news stories

Map reveals strange cosmos: March 3, 2002 BBC story, subtitled "The best map yet of the Cosmic Microwave Background (CMB) Radiation - the so-called echo of the Big Bang - shows the Universe may not be the same in all directions." It is based on a research paper by Max Tegmark.
The Universe May Be Flat But It Is Nevertheless Musical: June 5, 2001 article from LBL's Science Beat. Describes findings from the MAXIMA and BOOMERANG experiments which measured harmonics in the CMB power spectrum and provided good evidence supporting cosmic inflation.
A Revolution in Cosmology: A Closer Look at the Cosmic Microwave Background: May 2001 press release from Dartmouth College that discusses the cosmic microwave background, by Robert Caldwell. Related press release is here.
Balloon Measurements of the Cosmic Microwave Background Strongly Favor a Flat Cosmos: July 2000 news story from Physics Today on results from the Maxima and Boomerang experiments.
More Balloon Experiments Confirm That Universe Is 'Flat': May 10, 2000 article from the San Francisco Chronicle.
MAXIMA Finds Flat Universe: LBL article dated May 9, 2000, describing the MAXIMA findings. May 9, 2000.
MAXIMA experiment press release: From Berkeley, dated May 9, 2000.
New evidence supports flat Universe: May 9, 2000 news article about MAXIMA results.
Cosmologists Reveal First Detailed Images of Early Universe: Press release on BOOMERanG observations, issued April 26, 2000.
Cosmologists Publish First Detailed Images of Early Universe: UCSB Press release on BOOMERangG, April 26, 2000.
BOOMERanG Balloon Flight Sees a Flat Universe Filled With Dark Energy: LBL article dated April 26, 2000, describing the BOOMERanG findings.
Earliest images reveal universe is flat: April 27, 2000 news article about BOOMERANG results.

Recommended references: Magazine/journal articles

Planck by Planck
Ron Cowen
Science News, April 11, 2009: The upcoming ESA mission will study the cosmic microwave background radiation for clues about the dawn of time.
A Pixelated Cosmos
George Musser
Scientific American, September 2002
An Illuminating Journey
Ron Cowen
Science News, June 23, 2001, pp. 394-396: The cosmic microwave background has been studied for what it reveals about the universe at the time matter and light decoupled. Now it is also being studied to learn about how the universe has evolved since then.
[References]
Echoes from the Big Bang
Robert R. Caldwell; Marc Kamionkowski
Scientific American, January 2001, pp. 38-43: Much of our evidence for the nature of the Big Bang comes from detailed study of inhomogeneities in the cosmic microwave background (CMB). This may soon be supplemented by observation of the effects of gravitational waves on the CMB.
A Bang or a Whimper?
Ronald Ebert
Skeptic, Vol. 8, No. 3, 2000, pp. 48-53: At the start of a new millennium, the Big Bang theory has been very well accepted. The hypothesis of a period of "inflation" to solve the flatness and horizon problems is still in question, because it makes predictions about temperature fluctuations in the CMB which have been only partially verified. Much better measurements from the Microwave Anisotropy Probe and the Planck satellite should give conclusive answers.
Unveiling the Flat Universe
Diana Steele
Astronomy, August 2000, pp. 46-50: Data from the balloon-borne BOOMERANG experiment to measure the size of fluctuations in the cosmic background radiation suggest that the universe is flat and expanding at an accelerating rate. This flatness is further evidence for the inflationary model of the Big Bang. Some surprises in the observations, however, suggest we do not yet fully understand the ratios of ordinary (baryonic) matter, "dark matter", and "dark energy" needed to account for the expansion.
The Magnificent Mission
Tim Folger
Discover, May 2000, pp. 44-51: The Microwave Anisotropy Probe (MAP), scheduled for launch in November, 2000, will study the cosmic microwave background in far greater detail than the Cosmic Background Explorer (COBE). The inhomogeneities in the cosmic background which it will be able to chart should be able to settle many issues in the theory of the Big Bang.
A flat Universe from high-resolution maps of the cosmic microwave background radiation
P. De Bernardis, et al
Nature, April 27, 2000, pp. 955-959: Presentation of results of the BOOMERanG project.
The Cosmic Rosetta Stone
Charles L. Bennett, Michael S. Turner, Martin White
Physics Today, November 1997, pp. 32-38: Variations in the temperature of the cosmic background radiation of only a few millionths of a degree provide information on the origin and composition of the early universe.
Echo of the Big Bang
Gary Taubes
Discover, November 1997, pp. 110-117: The Microwave Anisotropy Probe (MAP), to be launched into Earth orbit at the L2 point in 2000, will make the most detailed study yet of the cosmic background radiation.

Recommended references: Books

George Smoot, Keay Davidson - Wrinkles in Time
William Morrow and Company, 1993: An elementary book about the COBE satellite project, which mapped the anisotropy of the cosmic background radiation. Smoot was a principal investigator in the project.

Home