South Pole Astronomy Dark Energy through the Telescope

04 Jun 2007 - Articles, Logistics, Atmosphere & Space, Antarctic

South Pole Telescope with Aurora

A telescope 10-metres in diameter, baptised the South Pole Telescope (SPT), has just been built at the South Pole. Within the activity framework of the International Polar Year 2007-2008, the SPT offers astronomers a new window into the Universe.

Located at the Amundsen-Scott American South Pole Station, SPT aims to track the evolution of structure in the Universe, providing a powerful measure of the global cosmological parameters (like matter density and cosmological constant) that govern the growth of the Universe's structure.

The Project Manager of SPT is Dr. Steve Padin, while the General Director is Dr. John E. Carlstrom, both from the University of Chicago. The SPT project brings together collaborations from 6 other American universities (University of California at Berkeley, Smithsonian Astrophysical Observatory, Case Western Reserve University, University of Illinois at Urbana-Champaign, Lawrence Berkeley National Laboratories, Jet Propulsion Laboratory) and one Canadian university (McGill University), all hoping to unveil the mystery covering dark energy composition.

Early Cosmology: Origins and Evolution of Space

The structure of our Universe is made up of two opposing energies: a first one called the negative Potential Gravitational Energy, amounting to the total mass of the Universe and acting to contract the universe, and a second one, the positive Kinetic Energy, acting to expand the Universe.

Astronomers have concluded through their observations that the Universe is currently expanding. This expansion could go on forever if the sum of the energies cited above remained positive; the expansion would slow down and subsequently be followed by Universal contraction if the sum of those energies were to become negative.

However, just as a high jumper on earth jumps away from the earth, it comes back down after a while, because the negative gravitational attraction of the earth is larger (in absolute value) than the positive kinetic energy of the jumper. It is very well possible that the Universe is now in a phase of expansion (just like the high jumper as he goes up), but that it will eventually shrink again (like the high jumper when he comes back down). But, such a statement can only be made for the Universe as a whole when one knows the entire mass of the Universe, and, this is not yet the case. It is thought that only a small fraction of the Universe has been observed until now.

Astronomers calculate the mass of spiral galaxies such as our own, the Milky Way, by determining their rotation curves, a plot of the rotation speed versus its distance from the centre of the galaxy. The mass within any given radius then follows from Newton's laws of motion. The orbital speed of stars and gas beyond 15 kiloparsecs (measure of the width of our galaxy, abbreviated as kpc) would then decrease when the distance from the galactic centre increases, in the same way that orbital speeds of the planets decrease when we move outwards from the sun. However, the true rotation turns out to be quite different. Radio observations have indicated that the rotation curves of many spiral galaxies remain flat (that is they do not decline, and may even rise a little).

Based on these observations, astronomers have concluded that a certain amount of mass and energy are "missing", an amount which could be responsible for keeping the Universe from everlasting expansion.

Consequently, the luminous portion of the Milky Way Galaxy, the region outlined by the globular clusters and by the spiral arms, is merely the "tip of the iceberg". Our galaxy is in fact very much larger. The luminous region is surrounded by an invisible dark halo, dwarfing the inner halo of stars and globular clusters, and extending far beyond the 15 kpc radius once thought to represent the limit of our galaxy. But what is the composition of this dark halo then?

At this point we don't know what dark mass and energy are, however most astronomers agree that it makes up over 70 % of the contents of our Universe and that it is this missing force which keeps the universe closed. Some astronomers even tend to think that the known mass of our Universe only accounts for 2.5 % of the total mass in the Universe. This would mean that 97.5% of the Universe would be filled with missing mass, or missing energy (according to the mass-energy equivalence put forward by Albert Einstein's formula E=mc²).

A Dark Universe

The SPT wishes to discover new clusters of galaxies by the signature they impart in the Cosmic Microwave Background (CMB). Indeed, high energy electrons distort the CMB, in which some of the energy of the electrons is then transferred to the low energy CMB photons. Observed distortions of the cosmic microwave background spectrum are used to detect the density perturbations of the universe. This effect, called the Sunyaev-Zel'dovich effect (SZE), allows dense clusters of galaxies to be observed.

The effect will be registered by the SPT in millimetre-wavelengths by a camera which has been cooled down to only 0.25 degree above absolute zero. By determining the number of galaxy clusters, we can obtain important information about how quickly structures were formed in the Universe and this, in turn, can shed light on the role of dark energy.

The telescope's large aperture and high sensitivity offer a great combination for observing the Universe. Starting March 2007, the project has been conducting a survey of over 4000 square degrees for galaxy clusters using the SZE, in order to help solve the equation concerning the state of dark energy.

South Pole Telescope by day

Why does the South Pole House So Many Astrophysical Experiments?

The SPT is funded by the National Science Foundation and the Kavli Foundation. The National Science Foundation, through its Office of Polar Programs (OPP), is granting .7M to the SPT for a period of 5 years. While the first five-year grant period will reach an end in 2007, it is expected that the full grant will be renewed for another five-year period.

The National Science Foundation (NSF) funds and manages the United States Antarctic Program (USAP), which was established in 1959 after the International Geophysical Year 1957-58. It coordinates almost all U.S. scientific research in the Antarctic, covering research in all fields of science and engineering.

USAP goals are: to understand the Antarctic and its associated ecosystems; to understand the region's effects on (and responses to) global processes such as climate; and to use Antarctica's unique features for carrying out the scientific research that cannot be done as well elsewhere. Research is done in Antarctica only when it cannot be performed at a more convenient location.

Various projects are currently underway in Antarctica in the field of astro-physics and astronomy: IceCube,South Pole Telescope, Bicep and Quad. The last two are CMB polarization experiments that are much smaller than SPT.

A first example showing the unique conditions of Antarctica are necessary for carrying out specific science projects: Ice Cube Neutrino Observatory. The IceCube neutrino observatory is a huge international project that bridges astronomy and high-energy physics. It indeed needs very clear and stable ice conditions to be able to detect the muons it is looking for. They use photo detectors buried up to 2.5 km deep under the ice sheet, in order to detect high-energy neutrinos, easily obscured by light and ordinary electromagnetic radiation.

The second example concerns of course the SPT. Situated at 90 degrees south, the South Pole Telescope lies in perfect conditions:

The weather at the South Pole is particular in that it makes the atmosphere very transparent.
The South Pole is unique in that during the winter, it is night 24 hours/day. The lack of sunrises and sunsets makes the atmosphere extremely stable.
The dryness in this part of the globe is such that the atmosphere is practically transparent.

This last point is very important because the problem most often encountered is that observations of the CMB can be overshadowed by atmospheric humidity. The humidity absorbs the millimetre and sub-millimetre wavelength light from the sky and makes it harder to detect the CMB. Another effect that water vapour has on observations of the CMB is that it emits light at these wavelengths and, since water vapour is not uniform, the light emitted makes some parts of the sky seem brighter than others, thus distorting observations. The telescope was built at a high altitude (over 3.2 kilometres), which further minimizes these effects as it is situated above the level of water vapour.

Many astronomy projects are carried out at the South Pole. As far as the Ice Cube project goes, it is clear that there is no better place than Antarctica for undertaking such an experiment. Considering millimetre-wavelength observations, such as required for studying the CMB by SPT, Bicep or Quad, aside from the obvious weather and atmospheric conditions, it is always best to undertake similar observations from a same point of reference, so as to be able to overlap and compare results. Our hopes are high that SPT will thus unlock some of the information contained within the missing Universe mass.

By: Lise Johnson