Open Questions: The Cosmic Microwave Background
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
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.
Web Resources for Cosmology and the CMB
- From the
Cosmic Background Imager site.
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.
Max Tegmark's site.
- 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
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
- 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
research papers, and
- 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
BOOMERanG - Balloon Observations of Millimetric Extragalactic
- Project home page. Includes a
fact sheet which explains a little about the CMB and the BOOMERanG
MAXIMA - Millimeter Anisotropy Experiment Imaging Array
- Project home page.
Julian Borrill's Homepage
- Describes various research projects which investigated the
Surveys, overviews, tutorials
Cosmic microwave background radiation
- Article from
Tests of the Big Bang: The CMB
- Brief overview of the CMB at NASA's
An Introduction to the Cosmic Microwave Background
- A beginning level overview of the subject, by
Cosmic Microwave Background: Intermediate Tutorials
- Continuation of
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,
This includes a
layman's introduction, a
tour of CMB physics, some
external links, and more advanced material.
- 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
"scorecard" rating the success of various cosmological theories
at meeting observational tests. The
frequently asked questions (and answers) page is especially
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
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
"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
Cosmological model gets a boost
- March 2001 news article from
Physics World, about
measurements of the power spectrum of the cosmic microwave
The universe is flat - official
- April 2000 news article from
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
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
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
- May 2001 press release from Dartmouth College that discusses
the cosmic microwave background, by Robert Caldwell. Related
press release is
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
- LBL article dated April 26, 2000, describing the BOOMERanG
Earliest images reveal universe is flat
- April 27, 2000 news article about BOOMERANG results.
Planck by Planck
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
Scientific American, September 2002
- An Illuminating Journey
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.
- 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?
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
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 Magnificent Mission
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
A flat Universe from high-resolution maps of the cosmic microwave
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
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.
- 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.
Copyright © 2002 by Charles Daney, All Rights Reserved