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ScienceWeek
ASTROPHYSICS: ON COSMIC MAGNETIC FIELDS
The following points are made by Ruth Durrer (Science 2006 311:787):
1) Observing astrophysical magnetic fields is difficult. Nonetheless, fields of surprisingly consistent amplitudes on the order of microgauss have been discovered in many galaxies and clusters of galaxies [1]. So far, the generation of these fields has remained a mystery. For a long time, researchers tried to conceive of a mechanism by which tiny primordial fields would be created in the early Universe [2]. Later, during gravitational collapse, such fields could be amplified -- for example, by means of a dynamo mechanism -- and thereby lead to the observed fields in galaxies and clusters. Even if the seed fields needed for dynamo amplification were as small as 10^(-25) G or smaller, these primordial seed fields have been shown to be severely constrained by the gravity wave background that they induce [3, 4]. As new work[5] reports, there is another possibility. The authors show that second-order cosmological perturbations necessarily generate magnetic fields that are of the right order to be amplified by the dynamo mechanism into the currently observed fields in galaxies and clusters.
2) This is an exciting proposal. It implies that tiny magnetic fields on the order of 10^(-22) G are present even in intergalactic space. Furthermore, the clustering properties of magnetic fields carry an imprint of the primordial fluctuation spectrum from cosmic inflation. If true, magnetic fields might come to play a very important role in cosmology, comparable to the cosmic microwave background (CMB) anisotropies. In analyzing these fields, we might learn about the physics of cosmic inflation that occurs at very high energies, probably about 10 orders of magnitude higher than the energy that will be reached at the Large Hadron Collider, the world's largest particle accelerator now under construction at CERN in Geneva.
3) During ordinary cosmic inflation, a period where the temperature may reach T ~ 10^(14) GeV, no magnetic fields are generated, because electromagnetism cannot be induced by the expansion of the Universe. Only when introducing additional quantum fields can one generate magnetic fields during inflation [2]. On the other hand, at the electroweak phase transition, which takes place at a temperature of Tew ~ 200 GeV, where the electromagnetic and weak nuclear forces separate, magnetic fields can be generated and their amplitude might be correlated to the baryon number produced during the transition. Within the standard model, both mechanisms, the production of baryons and of magnetic fields, are too inefficient to explain the observations, but in slightly generalized (e.g., supersymmetric) versions, the production of sufficient seed fields might be possible [2].
4) The most problematic aspect of such primordial scenarios is, however, that the galactic scales we are interested in are far larger than the Hubble scale at the time of magnetic field generation. This leads to very substantial generation of gravitational waves during horizon crossing. This is in conflict with the helium abundance in the Universe, which requires that at the time of nucleosynthesis, when T ~ 0.1 MeV, the gravity wave contribution to the energy density of the Universe must have been less than about 10%. This limit is severely violated by nearly all of the proposed models, because the gravity wave energy density usually has a very blue spectrum. In order to obtain sufficient magnetic field amplitudes on large scales, the models violate the constraints required by nucleosynthesis on small scales [3,4].
References (abridged):
1. L. M. Widrow, Rev. Mod. Phys. 74, 775 (2002)
2. For an overview, see D. Grasso, H. R. Rubinstein, Phys. Rep. 348, 163 (2001) and references therein.
3. C. Caprini, R. Durrer, Phys. Rev. D 65, 023517 (2002)
4. C. Caprini, R. Durrer, Phys. Rev. D 72, 088301 (2005)
5. K. Ichiki K. Takahashi, H. Ohno, H. Hanayama, N. Sugiyama, Science 311, 827 (2006)
Science http://www.sciencemag.org
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Related Material:
GALAXY CLUSTERS AND SUPERNOVA HEATING IN THE EARLY UNIVERSE
Notes by ScienceWeek:
Contemporary cosmology distinguishes two kinds of matter, "ordinary matter" and "dark matter". In general, a baryon is a nuclear particle (e.g., a proton) built from 3 quarks (fundamental particles that combine to make up protons, neutrons, and mesons), and so-called "ordinary matter" is baryonic. In this context, the term "dark matter" refers to material whose presence can be inferred from its effects on the motions of stars and galaxies, but which cannot be seen directly because it emits little or no radiation. It is believed that as much as 90 percent of the mass in the Universe may exist as some form or dark matter, although the proposed percentage of dark matter varies widely with different cosmological models.
The term "supernova" refers to a class of violently exploding stars whose luminosity after eruption suddenly increases millions or billions of times its normal level, the supernova explosion a cataclysmic event associated with the essential end of the active (energy-generating) life of the star.
The following points are made by G.M Voit and G.L. Bryan (Nature 2001 414:425):
1) Clusters of galaxies are believed to contain approximately 10 times as much dark matter as baryonic matter. The dark component therefore dominates the gravitational potential of a cluster, and the baryons confined by this potential radiate x-rays with a luminosity that depends mainly on the gas density in the core of the cluster.
2) Predictions of the properties of these x-rays based on models of cluster formation do not, however, agree with observations. If the models ignore the condensation of cooling gas into stars and also ignore feedback from the associated supernovae, the models overestimate the x-ray luminosity because the simulated density of the core gas is too high. An early episode of uniformly distributed supernova feedback could rectify this by heating the uncondensed gas and therefore making it more difficult for the gas to compress into the core. But such a process seems to require an implausibly large number of supernovae.
3) The authors demonstrate how radiative cooling of intergalactic gas and subsequent supernova heating combine to eliminate highly-compressible low-entropy gas from the intracluster medium. This brings the galaxy cluster core entropy and x-ray luminosities of clusters into agreement with observations in a way that depends little on the efficiency of supernova heating in the early Universe.
Nature http://www.nature.com/nature
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ASTROPHYSICS: ON BLACK HOLES IN THE EARLY UNIVERSE
The following points are made by Xiaohui Fan (Science 2003 300:752):
1) When did the first generation of galaxies and quasars form? How did these first sources of light end the cosmic "dark ages"? And what is the relation between the star formation in the first galaxies and the initial growth of supermassive black holes in the first quasars?
2) The most distant galaxies and quasars with confirmed redshifts are at approximately z = 6.5. At these high redshifts, the Universe was less than 1 billion years old and the first generations of galaxies and black holes were forming. At the other end of cosmic history, Hubble Space Telescope observations have shown that most, if not all, galaxies contain supermassive black holes. The masses of the black holes are tightly correlated with the velocity dispersions and masses of their host galaxies. This result suggests that the evolution of the black holes and the galaxies are connected, such that the process responsible for the assembly of the galaxy also feeds the growth of the black holes.
3) Optically bright quasars represent the critical phase of black hole evolution when it is acquiring most of its mass. The luminous quasars at redshifts of >6 likely represent black holes with several billion solar masses (M) residing in a halo of ~10^(13) M -- an amazing feat of early structure formation. A growing body of evidence suggests that high-redshift quasars are accompanied by intense star-formation activities on a galaxy scale.
4) With the help of gravitational lensing, the system studied by Carilli et al (Science 2003 300:773) provides the best case-study to date of the simultaneous formation of a supermassive black hole in a luminous quasar and a young star-forming galaxy at high redshift. Future high-resolution observations of high-redshift quasars, combined with more detailed understanding of black hole population from local systems, will help us to eventually understand the relation of black hole and quasar formation to star formation, and the role of black holes in the formation of galaxies.
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