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Kavli Cosmologists Probe Cosmological Mystery of Dark Energy With South Pole Telescope

Something is pulling the universe apart. What is it, and where will it take us from here? Scientists at the Kavli Institute for Cosmological Physics, University of Chicago, seek answers to those questions with the newly-commissioned South Pole Telescope.

Frigid and bone-dry, with six straight months of night each year, the South Pole is a forbidding place to live or work. But for largely the same reasons, it’s one of the best spots on the planet for surveying the faint cosmic microwave background (CMB) radiation left over from the Big Bang. The 10-meter microwave South Pole Telescope (SPT), which began operating in February 2007, is studying the CMB to gather clues about the birth, evolution and eventual fate of the universe.

The SPT project, led by researchers at the Kavli Institute for Cosmological Physics, University of Chicago, aims to help solve one cosmological mystery in particular – that of dark energy. Little is known about this force, other than that it works against gravity and appears to have sped up the expansion of the universe. Unlike energy as we know it (and measure it), dark energy does not seem to act through any of the fundamental forces of nature other than gravity.

It can’t be detected directly, for instance, through light or other manifestations of the electromagnetic force. The evidence for dark energy is indirect. Its existence was first posited in 1998 by scientists seeking to explain unexpected data from distant supernovae. Since then, research using the Hubble Space Telescope and other instruments has traced the impact of dark energy to about nine billion years ago, when the universe was five billion years old and galaxies started flying away from one another at a faster pace.

Understand the Past; Predicting the Future
From studying the CMB and what it tells them about the geometry of the universe, scientists estimate that dark energy makes up 70% to 75% of the universe’s entire mass and energy combined. This is about three times as much as dark matter, which cannot be detected by light or other electromagnetic radiation but exerts a powerful gravitational pull on galaxies. Only about 4% of the cosmos is ordinary matter, the stuff we are made of and the stuff we can see.

So whatever dark energy is, its effect is stronger than anything else on large scales. It also may determine the future of the universe. It might gain strength and end the universe by pulling all matter apart – even atomic nuclei (cosmologists call this the “big rip”). Or it might weaken and allow gravity to re-pack the universe, in a so-called “big crunch,” resulting in something like the infinitely dense condition at the point of the Big Bang. Or perhaps it will simply let the current expansion continue until most stars and galaxies are too distant to be seen.

What can the SPT tell us about the past and future of dark energy? John E. Carlstrom, director of KICP and the S. Chandrasekhar Distinguished Professor in Astronomy and Astrophysics at the University of Chicago, says the telescope is examining clusters of galaxies to learn what role dark energy played in their evolution. “One of the important things we need to learn about dark energy is what influence it has had on structure,” Carlstrom says. If scientists can learn how the density of clusters changed over time, he says they can determine “constraints on the equation of state of dark energy.” That is, they can get a more precise idea of whether dark energy is taking us toward a big rip, a big crunch or something in between.

The telescope is looking specifically for the Sunyaev-Zel’dovich (SZ) effect, a distortion of the CMB radiation caused by the highly energized gas of galaxy clusters. When photons originating from the CMB traverse the clusters, they interact with electrons and tend to scatter, creating slight variations in temperature -- shadows against the microwave background – that the SPT detects with a battery of 1,000 sensors chilled to near absolute zero.

The SPT will survey about a fifth of the entire southern sky and is expected to detect thousands of clusters. Analyzing follow-up data from optical telescopes, the scientists will determine the mass, distance and age of the clusters. They will then map the clusters in space and time to see how their density and structure evolved over billions of years under the competing pulls of gravity and dark energy. They hope to learn how much power dark energy exerted in the early universe, how it evolved to dominate the universe now, and by extension, how much power it may wield in the future.

Back to the Big Bang
The SPT’s activity will not end with this survey of galaxy clusters. Another project in the works will use the telescope to scan the CMB for tiny fluctuations in its polarization. Like visible light, the microwave radiation from the Big Bang has waves moving in electromagnetic fields at different angles, some up-and-down and other side-to-side. Observations with another South Pole instrument, the degree angular scale interferometer (DASI), have confirmed that the CMB is polarized as expected from prevailing theories about the physics of the Big Bang. Researchers now want to use the more sensitive SPT to look for minute variations in the CMB polarization that mark the presence of huge gravity waves.

Stephan Meyer, associate director of KICP and Professor in Astronomy and Astrophysics at the University of Chicago, says these waves are “a reasonable fraction of the size of the universe” in length and would have been generated in the “inflationary epoch” of the Big Bang. This was the time when the universe was just 10-50 seconds old and matter had not yet coalesced into neutrons and protons. “We don’t really understand the physics of that era,” Meyer says. A new set of sensors, able to detect polarization as well as heat, is being built by the University of Chicago and should be ready for installation on the SPT by the austral summer (the northern winter) of 2009-10.

Carlstrom and Meyer have both made multiple trips to the South Pole since the mid-1990s. Meyer calls it “somewhat monotonic … there are no bugs, no kids” but he says there is “a brutal and stark beauty about it all.” Carlstrom points out that, as remote as the Pole is, it has “very well-developed infrastructure” thanks to the National Science Foundation and its Office of Polar Programs. Still, installing a 75-foot-tall, 280-ton telescope at the South Pole is a major logistical feat. Carlstrom notes with some pride that he and his team (he is principal investigator on the SPT project) took just three months in the austral summer of 2006-07 to assemble the SPT, insulate it and get it up and running.

The SPT was funded with $18.7 million from the NSF, along with additional support from the Kavli Foundation the Gordon and Betty Moore Foundation of San Francisco. The project manager is Steve Padin, Senior Scientist in Astronomy and Astrophysics at the University of Chicago.. Senior members of the SPT team include William Hozapfel, Adrian Lee and Helmuth Spieler of UC Berkeley; Joe Mohr of the University of Illinois at Urbana-Champaign; John Ruhl of Case Western Reserve University; Antony Stark of the Harvard-Smithsonian Astrophysical Observatory; Matt Dobbs of McGill University, and Erik Leitch of Jet Propulsion Laboratory.

Note for Dark Energy
In physical cosmology, dark energy is a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe. Assuming the existence of dark energy is the most popular way to explain recent observations that the universe appears to be expanding at an accelerating rate. In the standard model of cosmology, dark energy currently accounts for almost three-quarters of the total mass-energy of the universe.

Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously, and scalar fields such as quintessence or moduli, dynamic quantities whose energy density can vary in time and space. In fact, contributions from scalar fields, which are constant in space, are usually also included in the cosmological constant. The cosmological constant is thought to arise from the vacuum energy. Scalar fields which do change in space are hard to distinguish from a cosmological constant, because the change may be extremely slow.

High-precision measurements of the expansion of the universe are required to understand how the speed of the expansion changes over time. The rate of expansion is parameterized by the cosmological equation of state. Measuring the equation of state of dark energy is one of the biggest efforts in observational cosmology today.

Adding the cosmological constant to cosmology's standard FLRW metric leads to the Lambda-CDM model, which has been referred to as the "standard model" of cosmology because of its precise agreement with observations. Dark energy has been used as a crucial ingredient in a recent attempt to formulate a cyclic model for the universe.

A substance has positive pressure when it pushes outward on its surroundings. This is the usual situation for fluids. Negative pressure, or tension, exists when the substance instead pulls on its surroundings. A common example of negative pressure occurs when a solid is stretched to support a hanging weight.

According to the Friedmann-Lemaître-Robertson-Walker metric, which is an application of General Relativity to cosmology, the pressure within a substance contributes to its gravitational attraction for other things just as its mass density does. Negative pressure causes a gravitational repulsion.

The gravitational repulsive effect of dark energy's negative pressure is greater than the gravitational attraction caused by the energy itself (remember the Einstein relation of the identity of mass and energy divided by c squared). At the cosmological scale, it also overwhelms all other forms of gravitational attraction, resulting in the accelerating expansion of the universe.

One might wonder, how can pushing cause attraction? How can pulling cause repulsion? This sounds like a contradiction. The solution is:

The pushing of positive pressure (and the pulling of negative pressure) are non-gravitational forces which just move substances around within space without changing space itself.
But the gravitational attraction (or repulsion) they cause operates on space itself, decreasing (or increasing) the amount of space between things. It is this which determines the size of the universe.
There is no necessity that these two effects should act in the same direction. In fact, they act in opposite directions.
Think of a rubber band. When you pull on it, it pulls back. It is attempting to retract. When you try to make it smaller, it will push outward. It will try and expand. This rubber band is the universe. Because the positive energy is trying to expand the universe, the negative energy is pulling it back in, and vice versa.

The exact nature of this dark energy is a matter of speculation. It is known to be very homogeneous, not very dense and is not known to interact through any of the fundamental forces other than gravity. Since it is not very dense—roughly 10−29 grams per cubic centimeter—it is hard to imagine experiments to detect it in the laboratory. Dark energy can only have such a profound impact on the universe, making up 70% of all energy, because it uniformly fills otherwise empty space. The two leading models are quintessence and the cosmological constant.

Note for Cosmic Microwave Background Radiation
In cosmology, the cosmic microwave background radiation is a form of electromagnetic radiation discovered in 1965 that fills the entire universe. It has a thermal black body spectrum at a temperature of 2.725 kelvin. Thus the spectrum peaks in the microwave range at a frequency of 160.2 GHz, corresponding to a wavelength of 1.9 mm. Most cosmologists consider this radiation to be the best evidence for the Big Bang model of the universe.

The cosmic microwave background is isotropic to roughly one part in 100,000: the root mean square variations are only 18 µK. The Far-Infrared Absolute Spectrophotometer (FIRAS) instrument on the NASA Cosmic Background Explorer (COBE) satellite has carefully measured the spectrum of the cosmic microwave background. FIRAS compared the CMB with a reference black body and no difference could be seen in their spectra. Any deviations from the black body form that might still remain undetected in the CMB spectrum over the wavelength range from 0.5 to 5 mm must have a weighted rms value of at most 50 parts per million (0.005%) of the CMB peak brightness. This made the CMB spectrum the most precisely measured black body spectrum in nature.

The cosmic microwave background is a prediction of Big Bang theory. In the theory, the early universe was made up of a hot plasma of photons, electrons and baryons. The photons were constantly interacting with the plasma through Thomson scattering. As the universe expanded, adiabatic cooling caused the plasma to cool until it became favourable for electrons to combine with protons and form hydrogen atoms. This happened at around 3,000 K or when the universe was approximately 379,000 years old (z=1088). At this point, the photons scattered off the now neutral atoms and began to travel freely through space. This process is called recombination or decoupling (referring to electrons combining with nuclei and to the decoupling of matter and radiation respectively).

The photons have continued cooling ever since; they have now reached 2.725 K and their temperature will continue to drop as long as the universe continues expanding. Accordingly, the radiation from the sky we measure today comes from a spherical surface, called the surface of last scattering. This represents the collection of points in space (currently around 46 billion light-years from the Earth—see observable universe) at which the decoupling event happened long enough ago (less than 400,000 years after the Big Bang, 13.7 billion years ago) that the light from that part of space is just reaching observers.

The big bang theory suggests that the cosmic microwave background fills all of observable space, and that most of the radiation energy in the universe is in the cosmic microwave background, which makes up a fraction of roughly 5×10-5 of the total density of the universe.

Two of the greatest successes of the big bang theory are its prediction of its almost perfect black body spectrum and its detailed prediction of the anisotropies in the cosmic microwave background. The recent Wilkinson Microwave Anisotropy Probe has precisely measured these anisotropies over the whole sky down to angular scales of 0.2 degrees. These can be used to estimate the parameters of the standard Lambda-CDM model of the big bang. Some information, such as the shape of the Universe, can be obtained straightforwardly from the cosmic microwave background, while others, such as the Hubble constant, are not constrained and must be inferred from other measurements.

About South Pole Telescope
The South Pole Telescope or (SPT) is a 10 meter diameter telescope located at the South Pole Antarctica. It is a microwave telescope that observes in a frequency range between 70 and 300 GHz. The primary science goal for SPT is to conduct a survey to find several thousand clusters of galaxies, which should allow interesting constraints on the Dark Energy density and its equation of state.

The project is a collaboration between the University of Chicago, the University of California-Berkeley, Case Western Reserve University, the University of Illinois at Urbana-Champaign, the Smithsonian Astrophysical Observatory, the University of Colorado-Boulder, and McGill University. It is funded by the National Science Foundation.

The South Pole is the premier observing site in the world for millimeter wavelength observations. The Pole's high altitude (2.8 km above sea level) means the atmosphere is thin, and the cold temperatures keep the amount of water vapor in the air low. This is particularly important for observing microwaves, which are absorbed by water vapor. At the South Pole the sun sets in mid-March and is followed by six months of total darkness. During this time the atmospheric conditions become extremely stable without the added turbulence caused by the daily rising and setting sun, a phenomenon akin to reducing the amount of twinkling of stars in the sky.

The focal plane for SPT is a 960 element bolometer array of superconducting Transition Edge Sensors (TES), which makes it one of the largest TES bolometer arrays ever built. The focal plane for SPT is split up into six pie-shaped wedges, each with 160 detectors. These wedges observe at three different frequencies: 90 GHz, 150 GHz, and 220 GHz. The modularity of the focal plane allows it to be broken into many different frequency configurations, however for the first year of observation two of the wedges observe at 90 GHz, three at 150 GHz, and one at 220 GHz. In the 150 GHz band, the array will be able image one square degree to about 30 microkelvins in about an hour.

The first key project for the SPT will be a 4000 square degree survey to search for clusters of galaxies via variations in the cosmic microwave background radiation, based on the Sunyaev-Zel'dovich effect (SZE). Given three years of observing the South Pole Telescope should find several thousand clusters of galaxies, which should allow interesting constraints on the Dark Energy density and its equation of state.

The Atacama Cosmology Telescope has similar, but complementary, science objectives.

The South Pole Telescope achieved first light on February 16, 2007, and began science observations in March 2007. The telescope continues to observe throughout the austral winter, with winter-overers Stephen Padin and Zak Staniszewski at its helm.