Reading ripples in the cosmic microwave background

New galaxy clusters found and polarizing finds pursued

VANCOUVER, Canada — Hidden in the peaks and valleys imprinted on the cosmic microwave background — the radiation leftover from the Big Bang — is a wealth of information not only about the early universe but the distribution of matter throughout the cosmos.

MICROWAVES IN THE SKY Astronomers used the South Pole Telescope to discern variations in the cosmic microwave background and discover three new galaxy clusters. The findings demonstrate that a particular feature of the cosmic microwave background can be used to trace the universe’s growth patterns and better determine the character of dark energy. K. Vanderlinde

On December 8, researchers at the Texas Symposium on Relativistic Astrophysics in Vancouver reported reading some of these imprints to identify three previously unknown galaxy clusters. The find bolsters using the cosmic microwave background as a tool for understanding how the universe’s galaxy composition has changed over time. This understanding is critical for analyzing the fingerprints of dark energy, the mysterious force that is revving up the rate at which the universe expands. 

On their journey to Earth, photons from the microwave background collide with huge clusters of galaxies, millions of light-years long, which are bathed in hot gas. When the photons strike energetic electrons in this hot gas, the photons gain energy. At microwave energies, therefore, the intensity of the microwave background ought to decrease in the direction of a galaxy cluster, because photons there would be kicked to higher energies. This energy shift, known as the Sunyaev-Zel’dovich, or SZ, effect, was predicted by Rashid Sunyaev and Yakov Zel’dovich in 1980.

Over the past decade, several research groups have confirmed the SZ effect when they pointed their microwave telescopes in the direction of known clusters. The new research is the first to use the SZ effect to identify previously unknown clusters. Using the 10-meter microwave South Pole Telescope, Thomas Crawford of the University of Chicago and his colleagues report discovering the three galaxy clusters by recording a decrement in the number of microwave background photons in a seemingly blank portion of the sky.

The findings, based on data taken over the last two years with the South Pole Telescope, demonstrate that microwave SZ surveys can be an effective tool for finding clusters, Crawford reported at the meeting.

Combined with visible-light surveys that can measure the distance to those new-found clusters, the SZ effect could reveal the abundance of clusters at different times in the universe. And that in turn may shed new light on the character of dark energy, which is ripping the cosmos apart at its seams (SN: 2/2/08, p. 74).

The evolution of clusters depends on the strength of dark energy, which acts as a repulsive force countering gravity’s tug. Early in the universe, when the cosmos was more compact and the density of matter was high, gravitational attraction would have won the tug-of-war with dark energy. But as the universe expanded and became more dilute, gravitational attraction would have weakened, allowing dark energy to take over. So at later times, clusters couldn’t form. By tracing the growth of clusters throughout cosmic history with the SZ effect, researchers now hope to determine the strength of dark energy and whether it varied over time.  

The three clusters discovered with the South Pole Telescope findings “are the very first step in constraining cosmology with an SZ cluster survey, but it’s a very early step,” notes Crawford. Researchers will need to find and gauge the distance to some 1,000 clusters to gain a better understanding of dark energy, he says. With additional data from the South Pole Telescope, as well as observations from similar devices now installed at sites ranging from the Atacama Desert in Chile to the United Kingdom, “I don’t see any reason why three [new clusters] can’t turn into something much closer to the real cosmologically constraining number,” Crawford adds.

The new findings “are beautiful,” a beaming Sunyaev, now director of the Max Planck Institute for Astrophysics in Garching, Germany, said at the conference.

Hints from polarized photons

Another team used a different telescope at the South Pole to analyze other sorts of wiggles in the cosmic microwave background. These researchers examined the polarization of microwave background photons — the tendency of groups of photons to vibrate in a specific direction, rather than in random orientations — in finer detail than ever before. The detailed polarization pattern detected by the team confirms that the standard model of cosmology is indeed correct, Ed Wu of Stanford University announced at the conference.

In that model, the universe underwent a tremendous growth spurt in its first tiny fraction of a second. The earliest seeds of galaxy formation would be imprinted on the microwave background as small hot and cold spots, the model suggests.

Clem Pryke of the University of Chicago, along with Wu and several other colleagues, used a 2.6-meter radio telescope called QUaD to examine the small-scale polarization of the microwave background.

QUaD uses the same mount as the now-defunct Degree Angular Scale Interferometer, which in 2002 was the first telescope to measure polarization of the microwave background (SN: 9/28/02, p. 195).   

The polarization that the QUaD team measured was imprinted on the cosmic microwave background 400,000 years after the birth of the universe. Until that time, the universe was hot enough for atomic nuclei and electrons to be separate. The free electrons bounced the photons from the microwave background back and forth, trapping the primordial light.

But at 400,000 years, the universe cooled sufficiently for electrons and ions to combine into atoms, for the first time allowing the photons from the cosmic microwave background to stream freely into space. Just before the photons began their journey, they scattered off the electrons one last time, polarizing the photons now seen, some 13.7 billion years later, by the QUaD telescope.

Theorist Dick Bond of the Canadian Institute for Theoretical Astrophysics at the University of Toronto calls the polarization measurements one of the pillars of cosmology. Before these measurements, “there was always the crazy possibility that all the up and downs we saw [in the cosmic microwave background] had nothing to do with the parameters we’ve derived,” such as the amount of ordinary matter in the cosmos and the tiny hot and cold spots that laid out the distribution of galaxies in the modern-day universe. “So the fact is that the polarization pattern is a very strong positive for the whole scenario” of how cosmologists understand the early universe, Bond says. 

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