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Astrophysics and CosmologyThe Committee on Physics of the Universe released a recent report entitled: "The Eleven Science Questions for the New Century". Although all eleven of them are fundamental and highly exciting, we limit here to a discussion of only four of them. These are:
The Physics Department at Florida State University is making significant contributions in all these areas which depend on the synergism between scientific disciplines as diverse as earth and space-based observations, terrestrial experiments, theoretical modeling, and computer simulations. Click here for Astrophysics and Cosmology faculty and links.
A variety of astrophysical measurements [galactic rotation curves, galactic binding, mappings of the cosmic microwave background radiation (CMB)] all point to the fact that nuclear matter only comprises a tiny fraction of the total matter and energy density of the universe. In addition to nuclear matter, there is also a pervasive dark energy component responsible for the apparent accelerating expansion of the universe, and a cold dark matter (CDM) component, which is responsible for large scale structure formation in the universe. The CDM is thought to be made up of weakly interacting massive particles (WIMPs) such as the lightest superparticle (LSP) which occurs in supersymmetric theories of particle physics. Supersymmetry is a quantum mechanical extension of Einstein's relativity theory, which links force particles with matter particles, and allows for the incorporation of gravity into particle physics. In supersymmetric theories, it is possible to estimate the relic density of LSPs left over from the Big Bang, by solving the Boltzmann equation as formulated for a Friedmann-Robertson-Walker universe. At FSU, we have constructed state-of-the-art computer programs for estimating the LSP relic density. By comparing the relic density against cosmological observations, it is possible to find preferred parameter choices for supersymmetric particle physics models. It should also be possible to produce the new supersymmetric matter particles at giant particle colliders such as the Fermilab Tevatron and CERN Large Hadron Collider. It may also be possible to directly detect WIMP particles via assorted cryogenic detectors, usually located deep underground to shield against cosmic rays. Our group at FSU has been involved in theoretical estimations of WIMP-nucleus collision rates so that the results from experiments can be directly compared against predictions from supersymmetric particle physics models. How were the elements from iron to uranium made?Nuclear Astrophysics is the research field which explains the origin of the elements and the source of much of the energy in the universe. We know that chemical or biological processes can not convert an atom from one element to another. Only nuclear reactions or radioactive decays can change the charge and mass of an atomic nucleus and therefore convert between elements. The relative abundance of the elements and isotopes we find in nature therefore represents a signature of the nuclear processes and a record of their history from the beginning of the universe until today.
We know that Hydrogen was formed shortly after the big bang and that nuclear processes fused some of this Hydrogen into Helium and Lithium about 3 minutes after the big bang. However, big bang nucleosynthesis could not proceed farther because there are no stable nuclei with an atomic mass of 5 or 8. When Hydrogen condensed and formed stars, it ignited in thermo-nuclear fusion reactions which produce most of the energy radiated by stars. During their normal lifecycle, stars process their initial Hydrogen as fuel toward heavier elements, starting with Helium and proceeding over all elements up to Iron. Because Iron lies at the peak of the curve of nuclear binding energy, the fusion process ends at Iron and the star dies. The relatively large natural abundance of Iron on Earth is an indication that our planet was formed out of matter that already came out of burnt-out stars. In fact, its raw material was probably processed in several generations of stars before it formed our solar system. The heavier elements beyond Iron can not be formed during this "normal" lifecycle of a star. In order to account for these heavier elements, which obviously exist on Earth, we have to find out which astrophysical processes can lead to the creation of elements heavier than Iron. One such process is slow neutron capture, called the s-process, which occurs on the shells of AGB stars. The other processes must happen on a faster pace than is possible in normal star burning. This leads us to think of stellar explosions as an important source of heavier elements. Two mechanisms were identified and are subjects of ongoing research: First the so-called "r-process" of rapid neutron capture, which is thought to occur in type II supernova events. The second mechanism is the "rp-process" of rapid proton capture, which is thought to be the cause of Nova explosions and X-ray bursts. These two models have explained some very significant properties of stellar explosions, but in many cases their predictions have large uncertainties. The relevant stellar processes involve reactions of short-lived nuclei with life times of minutes or seconds. These isotopes do not exist on earth and therefore could not be studied in experiments in the past. That is about to change. Several large-scale facilities around the world are being planned in order to measure the nuclear reactions of short--lived, exotic isotopes. At FSU's superconducting Accelerator laboratory, we are building a new facility called RESOLUT, which will be dedicated to the field of Nuclear Astrophysics. The experimental program at Resolut is to measure the reaction properties of the exotic nuclei which fuel stellar explosions. For a number of technical reasons, it is very difficult to produce beams of short--lived nuclei at energies relevant for nuclear astrophysics experiments. The experiment has to create an exotic species of nuclei, separate it in mass and charge and transport it to a detector setup, which detects its interactions with other matter. The beam from the existing superconducting accelerator bombards a target. Nuclei in the beam react with the target nuclei to form a range of exotic nuclear species, which take along the momentum of the primary beam. The exotic products leave the target and are re-focused into a superconducting accelerator module, which is used to manipulate the reaction products to have the same velocity. The subsequent magnetic spectrometer disperses the reaction products according to the charge to mass ratio. The desired reaction product is then selected and refocussed on a secondary reaction setup, where its interactions will be detected. Are there new states of matter at exceedingly high density and temperature?Two wonderful pictures taken by the Chandra X-ray Observatory. The picture on the left is Chandra's first light: Cassiopeia A; the 320 year old remnant of an exploding star. The picture on the right is 3C58, the remnant of a supernova witnessed on Earth in 1181 AD. It reveals that the pulsar has a temperature much lower than expected. This suggests that an exotic state of matter might harbor the core of the star.  
Neutron stars are stellar remnants of one of the most cataclysmic events in the universe: a supernova explosion.The stellar remnant left behind after this violent explosion is a neutron star. Neutron stars, also known as pulsars because of their accurate rotation rates, are gravitationally bound, highly-condensed stellar objects having masses comparable to that of the Sun but with a typical radius of about 10 kilometers. Such exotic objects are expected to exhibit remarkable properties. Indeed pulsars, which are ideal probes of superdense matter, reach interior densities as high as ten times those found in normal nuclei, or 15 orders of magnitude higher than the density of water. Moreover, pulsars show exquisite rotation stability at rates of up to a thousand times per second. Finally, pulsars display magnetic fields that are more than a million times stronger than the highest field ever produced at the National High Magnetic Field Laboratory in Tallahassee. As expected from a system subject to such extreme conditions, the physical properties of neutron stars are not well understood. Yet it is widely agreed that the high-density environment at the core of neutron stars constitutes a fruitful ground for the exploration of exotic states of matter. Indeed Quantum Chromodynamics (or QCD) the fundamental theory of the strong interactions predicts that, at high density, nuclear matter will undergo a phase transition into deconfined quark matter. Equally exciting is the possibility that pulsars could be strange-quark stars: stars with quark-matter cores that contain, in addition to the familiar up and down quarks, a large fraction of the more massive strange quarks. The existence of stable strange-quark matter, more stable perhaps than Iron (the most stable of all nuclei) is fascinating indeed. While some of the claims made in the popular press may be premature, there is a real possibility that the neutron star in the supernova remnant 3C58 may be a strange-quark star. A very nice popular article reporting some of the research done at Florida State University was written by Adrian Cho of the New Scientist magazine (the British equivalent of Scientific American). It reads as follows: The star with a soft centreNew Scientist vol 170 issue 2294 - 09 June 2001, page 11
To try to understand neutron stars, researchers have abandoned telescopes for a new tool a billion billion times smaller - the nucleus of a lead atom. Neutron stars, like the one at the heart of the Crab Nebula, have a radius of only a dozen kilometres or so but weigh more than the Sun. Despite often pumping out X-rays or radio signals, it is hard to determine the structure of a neutron star just from its radiation. Researchers think that a neutron star is solid on the outside with a liquid centre. They want to know how thick the solid neutron crust is, as this affects many of the star's properties-from how fast it cools to how well it emits gravitational waves. Charles Horowitz of Indiana University in Bloomington and Jorge Piekarewicz of Florida State University in Tallahassee believe that they can get an idea of the crust's thickness by measuring the skin of neutrons that covers the nucleus of a lead atom. "We're trying to use lead-208 as a miniature surrogate," Horowitz says. A lead nucleus is a staggering 55 orders of magnitude less massive than a neutron star, but because both are nuclear matter they are governed by the same physics, which is enshrined in an "equation of state". Physicists do not know the exact form of the equation of state for either a neutron star or lead nucleus. But as Horowitz and Piekarewicz gradually modified the equations, they found a close correlation between the values they got for the star's crust and the nucleus's skin. "There is definitely a relationship there," says astrophysicist Jim Lattimer of the State University of New York at Stony Brook. "If we can tighten that relationship, it should be a useful tool." That tool may be put to use in a couple years at the Thomas Jefferson National Accelerator Facility in Newport News, Virginia. Physicists are planning to measure the neutron skin of lead-208 nuclei by bouncing electrons off it. Once they know the skin thickness, neutron stars will be a cinch. Faculty: Find more information about Astrophysics and Cosmology at: |
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