Weekly meetings hosted by the "Gravitation Relativiste et
Cosmologie" (GreCO) group at IAP
Every Monday at 1.30pm in room 281 (2nd floor)
To suggest a paper please send a message to bertone.AT.iap.fr
Monday, 30 April 2007 @ 1.30pm, in room 281
Although
we recommend to read one or two papers from the list (which is by the
way based on your suggestions!), everyone is welcome to join the very informal discussion, and get
coffee in room 281
Fundamentalist
physics: why Dark Energy is bad for Astronomy
(Submitted on 18
Apr 2007)
Abstract:
Astronomers carry out observations to explore the diverse processes and
objects which populate our Universe. High-energy physicists carry out
experiments to approach the Fundamental Theory underlying space, time
and
matter. Dark Energy is a unique link between them, reflecting deep
aspects of
the Fundamental Theory, yet apparently accessible only through
astronomical
observation. Large sections of the two communities have therefore
converged in
support of astronomical projects to constrain Dark Energy. In this
essay I
argue that this convergence can be damaging for astronomy. The two
communities
have different methodologies and different scientific cultures. By
uncritically
adopting the values of an alien system, astronomers risk undermining
the
foundations of their own current success and endangering the future
vitality of
their field. Dark Energy is undeniably an interesting problem to attack
through
astronomical observation, but it is one of many and not necessarily the
one
where significant progress is most likely to follow a major investment
of
resources.
A
discriminating
probe of gravity at cosmological scales (Alberto)
(Submitted on 16
Apr 2007)
Abstract:
The standard cosmological model is based on general relativity and
includes
dark matter and dark energy. An important prediction of this model is a
fixed
relationship between the gravitational potentials responsible for
gravitational
lensing and the matter overdensity. Alternative theories of gravity
often make
different predictions for this relationship. We propose a set of
measurements
which can test the lensing/matter relationship, thereby distinguishing
between
dark energy/matter models and models in which gravity differs from
general
relativity. Planned optical, infrared and radio galaxy and lensing
surveys will
be able to measure $E_G$, an observational quantity whose expectation
value is
equal to the ratio of the Laplacian of the Newtonian potentials to the
peculiar
velocity divergence, to percent accuracy. We show that this will easily
separate alternatives such as $\Lambda$CDM, DGP, TeVeS and $f(R)$
gravity.
Download:
MiniBooNE
opens the box!
(press release April 11,
2007)
Results from Fermilab experiment
resolve long-standing neutrino question
BATAVIA, Illinois-Scientists of
the MiniBooNE1 experiment at the Department of Energy's
Fermilab2 today (April 11) announced their first findings.
The MiniBooNE results resolve questions raised by observations of the
LSND3
experiment in the 1990s that appeared to contradict findings of other
neutrino experiments worldwide. MiniBooNE researchers showed
conclusively that the LSND results could not be due to simple neutrino
oscillation, a phenomenon in which one type of neutrino transforms into
another type and back again.
The announcement significantly
clarifies the overall picture of how neutrinos behave. [read
more...]
Can N-body systems generate periodic
gravitational waves? (Luc)
(Submitted on 28 Sep 2006 ( v1), last revised 8
Feb 2007 (this version, v4))
Abstract:
None of N-body gravitating systems have been considered to emit
periodic
gravitational waves because of their chaotic orbits when N=3 (or more).
We
employ a figure-eight orbit as a specific model for a 3-body system in
order to
illustrate that some of triple stars are capable of generating periodic
waves.
This illustration would imply that a certain class of N-body
gravitating
systems may be relevant to the gravitational waves generation. We show
also
that the total angular momentum of this 3-body system is not carried
away by
gravitational waves. A waveform generated by this system is
volcano-shaped and
thus different from that of a binary system. Finally, by evaluating the
radiation reaction time scale, we give an order-of-magnitude estimate
of
merging event rates. The estimate suggests that figure-eight sources,
which
require carefully prepared initial states, may be too rare to detect.
Choreographic
solution
to the general relativistic three-body problem (Luc)
(Submitted on
14 Feb 2007 ( v1),
last revised 16 Apr 2007 (this version, v2))
Abstract:
We revisit the three-body problem in the framework of general
relativity. The
Newtonian N-body problem admits choreographic solutions, where a
solution is
called choreographic if every massive particles move periodically in a
single
closed orbit. One is a stable figure-eight orbit for a three-body
system, which
was found first by Moore (1993) and re-discovered with its existence
proof by
Chenciner and Montgomery (2000). In general relativity, however, the
periastron
shift prohibits a binary system from orbiting in a single closed curve.
Therefore, it is unclear whether general relativistic effects admit a
choreographic solution such as the figure eight. We carefully examine
general
relativistic corrections to initial conditions so that an orbit for a
three-body system can be closed and a figure eight. This solution is
still
choreographic. This illustration suggests that the general relativistic
N-body
problem also may admit a certain class of choreographic solutions.
Double Neutron Stars: Evidence For Two
Different Neutron-Star Formation Mechanisms (Jean-Pierre)
(Submitted on 10 Apr 2007 ( v1), last revised 26 Apr
2007 (this version, v2))
Abstract: Six of the eight
double neutron stars known in the Galactic disk have low
orbital eccentricities (< 0.27) indicating that their second-born
neutron stars
received only very small velocity kicks at birth. This is similar to
the case
of the B-emission X-ray binaries, where a sizable fraction of the
neutron stars
received hardly any velocity kick at birth (Pfahl et al. 2002). The
masses of
the second-born neutron stars in five of the six low-eccentricity
double
neutron stars are remarkably low (between 1.18 and 1.30 Msun). It is
argued
that these low-mass, low-kick neutron stars were formed by the
electron-capture
collapse of the degenerate O-Ne-Mg cores of helium stars less massive
than
about 3.5 Msun, whereas the higher-mass, higher kick-velocity neutron
stars
were formed by the collapses of the iron cores of higher initial mass.
The
absence of low-velocity single young radio pulsars (Hobbs et al. 2005)
is
consistent with the model proposed by Podsiadlowski et al. (2004), in
which the
electron-capture collapse of degenerate O-Ne-Mg cores can only occur in
binary
systems, and not in single stars.
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