| Topic: |
Science > Physics |
| User: |
"Robert Karl Stonjek" |
| Date: |
25 Feb 2006 02:12:18 PM |
| Object: |
Article: On Cosmic Magnetic Fields |
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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. 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 . 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. As new work 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. 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.
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.
Full Text from ScienceWeek
http://scienceweek.com/2006/sw060303-1.htm
--=20
Posted by
Robert Karl Stonjek
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<BODY>
<DIV>On Cosmic Magnetic Fields</DIV>
<DIV> </DIV>
<DIV>The following points are made by Ruth Durrer (Science 2006 =
311:787):</DIV>
<DIV> </DIV>
<DIV>1) Observing astrophysical magnetic fields is difficult. =
Nonetheless,=20
fields of surprisingly consistent amplitudes on the order of microgauss =
have=20
been discovered in many galaxies and clusters of galaxies. So far, the=20
generation of these fields has remained a mystery. For a long time, =
researchers=20
tried to conceive of a mechanism by which tiny primordial fields would =
be=20
created in the early Universe . Later, during gravitational collapse, =
such=20
fields could be amplified -- for example, by means of a dynamo mechanism =
-- and=20
thereby lead to the observed fields in galaxies and clusters. Even if =
the seed=20
fields needed for dynamo amplification were as small as 10^(-25) G or =
smaller,=20
these primordial seed fields have been shown to be severely constrained =
by the=20
gravity wave background that they induce. As new work reports, there is =
another=20
possibility. The authors show that second-order cosmological =
perturbations=20
necessarily generate magnetic fields that are of the right order to be =
amplified=20
by the dynamo mechanism into the currently observed fields in galaxies =
and=20
clusters.</DIV>
<DIV> </DIV>
<DIV>2) This is an exciting proposal. It implies that tiny magnetic =
fields on=20
the order of 10^(-22) G are present even in intergalactic space. =
Furthermore,=20
the clustering properties of magnetic fields carry an imprint of the =
primordial=20
fluctuation spectrum from cosmic inflation. If true, magnetic fields =
might come=20
to play a very important role in cosmology, comparable to the cosmic =
microwave=20
background (CMB) anisotropies. In analyzing these fields, we might learn =
about=20
the physics of cosmic inflation that occurs at very high energies, =
probably=20
about 10 orders of magnitude higher than the energy that will be reached =
at the=20
Large Hadron Collider, the world's largest particle accelerator now =
under=20
construction at CERN in Geneva.<BR></DIV>
<DIV>3) During ordinary cosmic inflation, a period where the temperature =
may=20
reach T ~ 10^(14) GeV, no magnetic fields are generated, because=20
electromagnetism cannot be induced by the expansion of the Universe. =
Only when=20
introducing additional quantum fields can one generate magnetic fields =
during=20
inflation. On the other hand, at the electroweak phase transition, which =
takes=20
place at a temperature of Tew ~ 200 GeV, where the electromagnetic and =
weak=20
nuclear forces separate, magnetic fields can be generated and their =
amplitude=20
might be correlated to the baryon number produced during the transition. =
Within=20
the standard model, both mechanisms, the production of baryons and of =
magnetic=20
fields, are too inefficient to explain the observations, but in slightly =
generalized (e.g., supersymmetric) versions, the production of =
sufficient seed=20
fields might be possible.<BR><BR>4) The most problematic aspect of such=20
primordial scenarios is, however, that the galactic scales we are =
interested in=20
are far larger than the Hubble scale at the time of magnetic field =
generation.=20
This leads to very substantial generation of gravitational waves during =
horizon=20
crossing. This is in conflict with the helium abundance in the Universe, =
which=20
requires that at the time of nucleosynthesis, when T ~ 0.1 MeV, the =
gravity wave=20
contribution to the energy density of the Universe must have been less =
than=20
about 10%. This limit is severely violated by nearly all of the proposed =
models,=20
because the gravity wave energy density usually has a very blue =
spectrum. In=20
order to obtain sufficient magnetic field amplitudes on large scales, =
the models=20
violate the constraints required by nucleosynthesis on small =
scales.</DIV>
<DIV> </DIV>
<DIV>Full Text from ScienceWeek<BR><A=20
href=3D"http://scienceweek.com/2006/sw060303-1.htm">http://scienceweek.co=
m/2006/sw060303-1.htm</A></DIV>
<DIV><BR>-- <BR>Posted by<BR>Robert Karl Stonjek</DIV></BODY></HTML>
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| User: "Robert Karl Stonjek" |
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| Title: Re: Article: On Cosmic Magnetic Fields |
25 Feb 2006 05:25:15 PM |
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Dark Magnetic Flux?
.
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| User: "ma1ibu" |
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| Title: Re: Article: On Cosmic Magnetic Fields |
26 Feb 2006 05:05:15 AM |
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The energy in the galaxy goes round and round,
round and round,
round and round,
the energy in the galaxy goes round and round,
and why are we surprised a magnetic pole exists at right-angles to it?
.
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| User: "ma1ibu" |
|
| Title: Re: Article: On Cosmic Magnetic Fields |
26 Feb 2006 05:30:16 AM |
|
|
The energy in the galaxy goes round and round,
round and round,
round and round,
the energy in the galaxy goes round and round,
and why are we surprised a magnetic pole exists at right-angles to it?
.
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