Nature 411, 880 - 881 (2001); doi:10.1038/35082152

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Raking through the embers


Clues about the origin and fate of the Universe lie hidden in the microwave radiation left over from its early days. Tom Clarke examines the latest attempts to map the Big Bang's afterglow.

The first high-resolution images of the radiation that makes up the cosmic microwave background (CMB) earned rave reviews. In 1990, the American Astronomical Society gave an early presentation on the new images a standing ovation. Two years later, Stephen Hawking of the University of Cambridge described one result1 arising from the image data as the "discovery of the century".

Hawking was praising an analysis of a CMB image taken by NASA's Cosmic Background Explorer (COBE) satellite. The CMB provides a direct link back to the early Universe it is a cosmic 'baby picture' that faithfully reproduces every freckle and dimple of the infant Universe. COBE's data showed that not long after the Big Bang, matter was starting to clump together. Cosmologists believe that this uneven distribution of matter was the starting point for the formation of stars and galaxies.

But these results were just the beginning for CMB research. Thanks to plans for two successors to COBE one of which is scheduled to launch next week new, higher-resolution images will soon be available. Armed with these, cosmologists will be able to test their understanding of what happened after the Big Bang and, perhaps, explain why the Universe looks the way it does today. "We're entering the decade of CMB science," says Charles Lawrence, a CMB physicist at the Jet Propulsion Laboratory in Pasadena, California.

The CMB dates back to a time when the Universe was just 300,000 years old. Back then, the Universe was just a soup of photons, electrons, protons and helium nuclei atoms were yet to form. The electrons were locked in a frantic tango with the photons, constantly absorbing and re-emitting them.

But as the Universe expanded and cooled, protons and electrons began to pair up to form hydrogen atoms. This freed the photons, as electrons in hydrogen atoms are much more reluctant to interact with photons. Physicists describe this as the 'decoupling' of photons and electrons. The decoupled photons exist to this day as the CMB, a blanket of microwave radiation that pervades every corner of the sky. "It's our cleanest probe of the early Universe," says Max Tegmark, a cosmologist at the University of Pennsylvania in Philadelphia.

Treasure maps


The CMB contains a wealth of information about the state of the Universe at the moment of decoupling. COBE's big contribution was to reveal differences of around 1 part in 10,000 in the average energy, or temperature, of CMB photons found in different areas of the sky. COBE's maps (see picture, right) of the CMB's temperature show ripples of alternating hot and cold areas. These arise because at the moment of decoupling different parts of the Universe had different densities the hotter CMB photons came from the denser areas of the early Universe. The presence of these ripples added support to the idea that stars and galaxies formed from an uneven distribution of matter.



Twin peaks: Lyman Page feels that two missions investigating the CMB are better than one.

But COBE's sensors lacked the resolution needed to answer many important questions. A finer-grained analysis of the CMB will provide cosmologists with the missing details, and should help them decide which of the proliferating models describing the evolution of the Universe is correct.


The significance of the information tied up in the CMB is such that ground-based projects have rushed to "skim off the cream" of CMB data before the launch of the space missions, according to Lyman Page, a physicist at Princeton University in New Jersey. Unlike COBE, sensors on ground stations and balloons can only map a restricted area of the sky, but that has not stopped them from producing some important results.

Last year, the international Balloon Observations of Millimetric Extragalactic Radiation and Geophysics (BOOMERANG) experiment a sensitive microwave detector slung beneath a balloon which circled the Antarctic achieved the best picture yet of the CMB2. Analysis of these data confirmed that the Universe is 'flat' it will continue to expand for ever, rather than collapsing in a 'big crunch' as some models had predicted.

Vive la resolution

Blowing hot and cold: with the launch of NASA's MAP satellite (right) next week, cosmologists hope that the image data of temperature fluctuations in the cosmic microwave background produced by the COBE satellite (left) will be replaced by a much higher-resolution version (simulated, centre).

But now the main focus of activity is poised to shift from ground- and balloon-based experiments to two satellites that promise to study the CMB with unprecedented accuracy. Space-based experiments have some big advantages. The new probes will take CMB research away from the noisy microwave sources on Earth, such as cellphones and radar systems, as well as avoiding the scattering effects of the atmosphere. And, like their forebear COBE, they will scan the entire sky, but this time at much higher resolutions.


Ripples with an angular resolution of less than 7 were invisible to COBE (1 is about twice the diameter of the Moon). BOOMERANG's limited sky view had a resolution of around 0.25. But if all goes well, next week will see the launch of NASA's Microwave Anisotropy Probe (MAP), which can conduct a full-sky survey of variations down to a scale of 0.3. And in 2007, the European Space Agency (ESA) plans to launch Planck, an even more sophisticated CMB probe which the agency hopes will map the CMB at a resolution of 0.17.


Mapping the past: set to launch in 2007, Planck will offer the highest-resolution CMB images.

The images that MAP and Planck will produce are eagerly awaited by cosmologists. According to the theory of how the variations in the CMB formed, ripples at different scales are linked to different fundamental properties of the early Universe.


Ripples at around the 1 scale have their origins in the oscillations that passed through the early Universe. Gravitational forces tried to pull the soup of matter inwards, but this was resisted by pressure arising from the movement of photons. The push and pull of these forces produced a series of reverberations and created patches of high and low density the cause of the small-scale ripples.

Analysis of these ripples will tell cosmologists about the structure of the soup of matter through which they once moved. Researchers hope that data from MAP and Planck will give them better estimates of properties of the early Universe, such as the density of protons and electrons, and the distribution of energy between matter and radiation.

These more accurate figures should help to eliminate some of the competing descriptions of how the Universe evolved. Most cosmologists are convinced that the Universe underwent a period of 'inflation', during which it was expanding faster than the speed of light, before decoupling. But measurements of many of the parameters in the competing inflationary theories have large errors associated with them, making it difficult to determine which one is correct. "You really have to get very precise if you want to back the theorists into a corner," says Philip Mauskopf, a member of the MAP team who is based at Cardiff University in Wales.

Double vision
Despite the importance of CMB data, the high cost of space missions does bring into question the need for two spacecraft. The probes were approved by their respective agencies within a month of each other in 1996. Some researchers suggest privately that the two agencies, having made CMB science their top astrophysical priority, lacked confidence in one other's selection procedures. Scientists affiliated with the MAP team say that they never believed that Planck being expensive and containing a number of untested technologies would survive the ESA's approval process.

Whatever the reasons, CMB scientists are happy with an outcome that gives them two spacecraft, allowing the teams to double-check their data. "The CMB is so important that two missions would be necessary anyway," says Page, who is a member of the BOOMERANG and MAP teams.

There is also a small possibility that new maps of the CMB will rule out the inflation theories altogether. The initial data from Earth-bound experiments seem to be in agreement with inflation, but there is still too much noise in the signal to rule out an error.

But a crucial test of the theories may need more data than MAP or Planck can provide. Inflation makes specific predictions about the polarization of CMB photons, which could be used to rule out rival theories. Planck will measure polarization, but only down to a scale of 10. More accurate measurements will be made by two balloon-based experiments extensions of the BOOMERANG and international Millimeter Anisotropy Experiment Imaging Array (MAXIMA) projects but only over restricted areas of the sky. Together, the satellite- and balloon-based polarization surveys will be an important step towards testing inflation but a high-resolution, full-sky survey will have to wait for a future, as yet unplanned mission.

Most researchers believe that inflation will survive the decade of CMB science. But they agree that the next ten years should see a cull of competing cosmological theories as researchers move towards a consensus on how the Universe evolved.

Web links

Microwave Anisotropy Probe

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Tom Clarke works in Nature's science writing team.



1. Smoot, G. et al. Astrophys. J. Lett. 396, L1-L4 (1992). | Article | ISI |
2. de Bernardis, P. et al. Nature 404, 955-959 (2000). | Article | PubMed | ISI |