Steve Padin: Tracking the Density and Properties of Dark Energy
04 Jun 2007 - Interviews, Logistics, Atmosphere & Space, Antarctic
Dr. Steve Padin
© Steve Padin / Steve Padin
The International Polar Foundation recently interviewed senior scientist Dr. Stephen Padin, from the University of Chicago, whom has been project manager for the South Pole Telescope (SPT) project over the past four years and whom is currently wintering over at the South Pole, making sure that the SPT is working properly. The SPT was built within the International Polar Year 2007-2008 framework to detect dark matter density and properties, by studying the temperature anisotropy of the Cosmic Microwave Background radiation in our Universe.
Let's start with the history of dark energy. How did the idea first come about?
The name "dark energy" comes from Turner and Huterer in 1998. It was introduced to describe, in very general terms, the contribution to the density of the Universe causing the expansion to accelerate.
The idea has connections back to Einstein's work on cosmology. He had invented a cosmological constant that made his models of the Universe static. In the 1920s, Hubble discovered that the Universe was expanding. The cosmological constant then essentially disappeared, except as a curiosity in graduate courses on cosmology.
Observations of type 1a supernovae in the late 1990s showed that the expansion of the Universe was accelerating, providing the first direct evidence for something like the cosmological constant. At about the same time, measurements of CMB anisotropy showed ordinary matter accounting for only about a third of the density of the universe, with the cosmological constant or, more generally, dark energy accounting for the remaining two thirds. The best current measurement of the density contributions is 4% ordinary matter, 23% dark matter and 73% dark energy.
Evidence for the dark matter component has been around for decades because we can see its gravitational effect on the motions of stars in galaxies.
We don't know what it is (brown dwarfs, black holes and new types of particles are some possibilities) but we're sure it's there.
The dark energy component is even less well understood. Dark energy is not very dense, it is uniformly distributed throughout the Universe (unlike ordinary and dark matter which clump into galaxies), it exerts a negative pressure that causes the expansion of the Universe to accelerate and it doesn't appear to interact through any of the fundamental forces except gravity.
Dark energy has been described as a "hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the Universe". What does dark energy mean for the future of the Universe?
Right now, dark energy is speeding up the expansion of the Universe. What happens in the future will be determined by whether or not dark energy remains dominant. At this point we really don't know. If the acceleration continues, we'll end up an isolated galaxy in essentially empty space. By then the Milky Way will have merged with Andromeda and the other galaxies in our local group. Eventually, all the stars will run out of fuel, and the Universe will be cold and dark.
Don't worry. It will take a while.
The South Pole Telescope is a 10-meter submillimeter-wave telescope designed to conduct large surveys with high sensitivity to low surface brightness emission such as Sunyaev-Zel'dovich Effect (SZE) and CMB temperature and polarization anisotropy. What does this say about dark energy?
Dark energy affects the rate at which structures form in the Universe. It was most important at redshifts in the range 0.2 to 2, so a measurement of the density of galaxy clusters in this range will tell us much about the role of dark energy. A large enough survey of galaxy clusters should allow us to say something about the properties of dark energy, in particular its equation of state. That's the ratio of pressure to energy density and it will help us figure out what dark energy is. If that ratio turns out to be one, then dark energy is something like the cosmological constant.
To measure the density of clusters, we first have to find them. We use the CMB as a backlight, and we measure the spectral distortion caused by hot electrons in the cluster. That's the SZE. The technique is powerful because the brightness of the SZE signal is essentially independent of the distance to the cluster, so we can detect clusters that are far away.
Optical and X-ray surveys on the other hand have to deal with the decreasing brightness of clusters with distance. Having found the clusters, we need to know their distances, and for that we'll make targeted optical observations to measure the cluster redshifts.
The temperature and polarization anisotropy of the CMB also contains information about the density and properties of dark energy, the density of ordinary and dark matter, and the spectrum and amplitude of density fluctuations in the early Universe. The SPT will measure CMB temperature anisotropy, focusing on small angular scales which have not yet been explored.
How can one be sure of the existence of dark energy, when doubt has been expressed by ESA's X-ray observatory in December 2003 (they noticed a difference between today's clusters of galaxies and those present in the Universe around 7 thousand million years ago)?
The XMM X-ray observations in 2003 found that galaxy clusters were brighter than expected in the past and there were fewer clusters than expected. Both observations implied a higher matter density and little dark energy. On the other hand, dark energy is supported by a variety of independent observations: type 1a supernova observations show that the expansion of the universe is accelerating; CMB observations indicate that about 70% of the total density is dark energy; and measurements of galaxy clustering on large scales also support a low matter density and a large dark energy component. None of this is yet sufficient for us to be sure about dark energy, but the evidence in favour is growing.
Why is it important that this telescope be built at the South Pole?
The CMB is brightest at millimeter wavelengths, so that's where we make our observations. Unfortunately, millimeter-wave radiation is strongly absorbed by water vapor in the earth's atmosphere, so we must observe from sites that are dry, and high enough to get above most of the water. The South Pole is an exceptional millimeter-wave site, probably the best on the planet. The altitude is about 10,000ft, which is a few times the scale height for water, and it's so cold that there is very little water in the atmosphere - it's all frozen out under our feet.
What have your observations revealed so far?
We've only just started our large survey of galaxy clusters using the SZE, so it's too early to say anything about cosmology. During the coming austral winter, when observing conditions at the South Pole are at their best, we'll survey enough sky to see if the cluster density is consistent with the picture of dark energy we have from CMB anisotropy and supernova observations. Listen out for exciting news.
What is the relationship between CMB measurements, as part of cosmological observations, and particle physics? Can CMB measurements allow physicists to look back in time?
The primary link between the CMB and particle physics is through tests of inflation - the process that expanded the Universe from sub-atomic to cosmic scales at the beginning of the Big Bang. Particle physics has to explain inflation, but we likely can't build laboratory experiments to explore this, so for now cosmology observations are the only way to test new ideas.
CMB temperature fluctuations allow us to measure the geometry of the Universe and the spectrum of density perturbations that were produced from quantum fluctuations during inflation. These are direct probes of the very beginning of the Universe. Enormous progress has been made in the past decade and we now know that the geometry of the Universe is flat at about the 2% level. Flatness is an important prediction of inflation. The spectrum of density perturbations is also consistent with inflation, but we don't yet have accurate enough measurements for a real test.
Measurements of CMB polarization fluctuations are an exciting new field that will probe the background of gravitational waves generated by inflation. The polarization signals are very weak, maybe at the microkelvin level in the 3K background radiation, but this is a key test of inflation. Several instruments, including BICEP and QUaD at the South Pole, are already making observations.
Dark energy also has strong connections to particle physics. The quantum-mechanical energy of empty space is a good candidate for dark energy. Unfortunately, calculations of this vacuum energy density currently yield results that are far from what we measure. Since dark energy is very diffuse, we probably can't do laboratory experiments with it. We can only detect its effect on very large scales, so again cosmology observations play a critical role.
What are the project's long term expectations?
Over the next couple of years, we'll survey a few thousand square degrees of sky, finding galaxy clusters with masses greater than about a thousand times that of the Milky Way. We'll also start measuring redshifts for all these clusters. At the end of the project, we should have a measurement of the density of clusters and associated constraints on the nature of dark energy. There's also a lot of astrophysics to do along the way, particularly in understanding the completeness of the survey. Some of our observing time will be dedicated to observing CMB temperature fluctuations on small angular scales, and we also have plans to field a polarimeter to measure polarization fluctuations.
The SPT was designed as a submillimeter telescope, capable of operating at wavelengths as short as 0.2mm. It is a unique facility, combining a large collecting area, submillimeter performance and an excellent observing site. In the long term, its role will likely be expanded to include very high frequency observations that will improve our understanding of star and galaxy formation.
By: Lise Johnson
