3,712 research outputs found
Measuring the properties of extragalactic dust and implications for the Hubble diagram
Scattering and absorption of light by a homogeneous distribution of
intergalactic large dust grains has been proposed as an alternative,
non-cosmological explanation for the faintness of Type Ia supernovae at z\s im
0.5. We investigate the differential extinction for high-redshift sources
caused by extragalactic dust along the line of sight.
Future observations of Type Ia supernovae up to , e.g. by the
proposed SNAP satellite, will allow the measurement of the properties of dust
over cosmological distances. We show that 1% {\em relative} spectrophotometric
accuracy (or broadband photometry) in the wavelength interval 0.7--1.5 m
is required to measure the extinction caused by ``grey'' dust down to magnitudes.
We also argue that the presence of grey dust is not necessarily inconsistent
with the recent measurement of the brightness of a supernova at (SN
1997ff), in the absence of accurate spectrophotometric information of the
supernova.Comment: Accepted by A&
Implications of Two Type Ia Supernova Populations for Cosmological Measurements
Recent work suggests that Type Ia supernovae (SNe) are composed of two
distinct populations: prompt and delayed. By explicitly incorporating
properties of host galaxies, it may be possible to target and eliminate
systematic differences between these two putative populations. However, any
resulting {\em post}-calibration shift in luminosity between the components
will cause a redshift-dependent systematic shift in the Hubble diagram.
Utilizing an existing sample of 192 SNe Ia, we find that the average luminosity
difference between prompt and delayed SNe is constrained to be . If the absolute difference between the two populations is 0.025 mag,
and this is ignored when fitting for cosmological parameters, then the dark
energy equation of state (EOS) determined from a sample of 2300 SNe Ia is
biased at . By incorporating the possibility of a two-population
systematic, this bias can be eliminated. However, assuming no prior on the
strength of the two-population effect, the uncertainty in the best-fit EOS is
increased by a factor of 2.5, when compared to the equivalent sample with no
underlying two-population systematic. To avoid introducing a bias in the EOS
parameters, or significantly degrading the measurement accuracy, it is
necessary to control the post-calibration luminosity difference between prompt
and delayed SN populations to better than 0.025 mag.Comment: 4 pages, 4 figures; New figures added, some old figures removed; The
effect of the uncertainty in the two population model on parameter estimation
discussed; Reflects version accepted for publication in Astrophys. J. Let
Double-valuedness of the electron wave function and rotational zero-point motion of electrons in rings
I propose that the phase of an electron's wave function changes by when
the electron goes around a loop maintaining phase coherence. Equivalently, that
the minimum orbital angular momentum of an electron in a ring is
rather than zero as generally assumed, hence that the electron in a ring has
azimuthal zero point motion. This proposal provides a physical explanation for
the origin of electronic `quantum pressure', it implies that a spin current
exists in the ground state of aromatic ring molecules, and it suggests an
explanation for the ubiquitousness of persistent currents observed in
mesoscopic rings
Hubble parameter reconstruction from a principal component analysis: minimizing the bias
A model-independent reconstruction of the cosmic expansion rate is essential
to a robust analysis of cosmological observations. Our goal is to demonstrate
that current data are able to provide reasonable constraints on the behavior of
the Hubble parameter with redshift, independently of any cosmological model or
underlying gravity theory. Using type Ia supernova data, we show that it is
possible to analytically calculate the Fisher matrix components in a Hubble
parameter analysis without assumptions about the energy content of the
Universe. We used a principal component analysis to reconstruct the Hubble
parameter as a linear combination of the Fisher matrix eigenvectors (principal
components). To suppress the bias introduced by the high redshift behavior of
the components, we considered the value of the Hubble parameter at high
redshift as a free parameter. We first tested our procedure using a mock sample
of type Ia supernova observations, we then applied it to the real data compiled
by the Sloan Digital Sky Survey (SDSS) group. In the mock sample analysis, we
demonstrate that it is possible to drastically suppress the bias introduced by
the high redshift behavior of the principal components. Applying our procedure
to the real data, we show that it allows us to determine the behavior of the
Hubble parameter with reasonable uncertainty, without introducing any ad-hoc
parameterizations. Beyond that, our reconstruction agrees with completely
independent measurements of the Hubble parameter obtained from red-envelope
galaxies.Comment: Modified to match journal versio
Brans-Dicke model constrained from Big Bang nucleosynthesis and magnitude redshift relations of Supernovae
The Brans-Dicke model with a variable cosmological term () has
been investigated with use of the coupling constant of .
Parameters inherent in this model are constrained from comparison between Big
Bang nucleosynthesis and the observed abundances. Furthermore, the magnitude
redshift () relations are studied for with and without another
constant cosmological term in a flat universe. Observational data of Type Ia
Supernovae are used in the redshift range of . It is found that our
model with energy density of the constant cosmological term with the value of
0.7 can explain the SNIa observations, though the model parameters are
insensitive to the relation.Comment: Submitted to A&A, 4 pages, 3 figure
Time-dependent radiative transfer with PHOENIX
Aims. We present first results and tests of a time-dependent extension to the
general purpose model atmosphere code PHOENIX. We aim to produce light curves
and spectra of hydro models for all types of supernovae. Methods. We extend our
model atmosphere code PHOENIX to solve time-dependent non-grey, NLTE, radiative
transfer in a special relativistic framework. A simple hydrodynamics solver was
implemented to keep track of the energy conservation of the atmosphere during
free expansion. Results. The correct operation of the new additions to PHOENIX
were verified in test calculations. Conclusions. We have shown the correct
operation of our extension to time-dependent radiative transfer and will be
able to calculate supernova light curves and spectra in future work.Comment: 7 pages, 12 figure
Vacuum energy and Universe in special relativity
The problem of cosmological constant and vacuum energy is usually thought of
as the subject of general relativity. However, the vacuum energy is important
for the Universe even in the absence of gravity, i.e. in the case when the
Newton constant G is exactly zero, G=0. We discuss the response of the vacuum
energy to the perturbations of the quantum vacuum in special relativity, and
find that as in general relativity the vacuum energy density is on the order of
the energy density of matter. In general relativity, the dependence of the
vacuum energy on the equation of state of matter does not contain G, and thus
is valid in the limit when G tends to zero. However, the result obtained for
the vacuum energy in the world without gravity, i.e. when G=0 exactly, is
different.Comment: LaTeX file, 7 pages, no figures, to appear in JETP Letters, reference
is adde
On thermodynamic and quantum fluctuations of cosmological constant
We discuss from the condensed-matter point of view the recent idea that the
Poisson fluctuations of cosmological constant about zero could be a source of
the observed dark energy. We argue that the thermodynamic fluctuations of
Lambda are much bigger. Since the amplitude of fluctuations is proportional to
V^{-1/2}, where V is the volume of the Universe, the present constraint on the
cosmological constant provides the lower limit for V, which is much bigger than
the volume within the cosmological horizon.Comment: 4 pages, version submitted to JETP Letter
The Deepest Supernova Search is Realized in the Hubble Ultra Deep Field Survey
The Hubble Ultra Deep Field Survey has not only provided the deepest optical
and near infrared views of universe, but has enabled a search for the most
distant supernovae to z~2.2. We have found four supernovae by searching spans
of integrations of the Ultra Deep Field and the Ultra Deep Field Parallels
taken with the Hubble Space Telescope paired with the Advanced Camera for
Surveys and the Near Infrared Multi Object Spectrometer. Interestingly, none of
these supernovae were at z>1.4, despite the substantially increased sensitivity
per unit area to such objects over the Great Observatories Origins Deep Survey.
We present the optical photometric data for the four supernovae. We also show
that the low frequency of Type Ia supernovae observed at z>1.4 is statistically
consistent with current estimates of the global star formation history combined
with the non-trivial assembly time of SN Ia progenitors.Comment: 24 pages (6 figures), submitted to the Astronomical Journa
The Rate of Type Ia Supernovae at High Redshift
We derive the rates of Type Ia supernovae (SNIa) over a wide range of
redshifts using a complete sample from the IfA Deep Survey. This sample of more
than 100 SNIa is the largest set ever collected from a single survey, and
therefore uniquely powerful for a detailed supernova rate (SNR) calculation.
Measurements of the SNR as a function of cosmological time offer a glimpse into
the relationship between the star formation rate (SFR) and Type Ia SNR, and may
provide evidence for the progenitor pathway. We observe a progressively
increasing Type Ia SNR between redshifts z~0.3-0.8. The Type Ia SNR
measurements are consistent with a short time delay (t~1 Gyr) with respect to
the SFR, indicating a fairly prompt evolution of SNIa progenitor systems. We
derive a best-fit value of SFR/SNR 580 h_70^(-2) M_solar/SNIa for the
conversion factor between star formation and SNIa rates, as determined for a
delay time of t~1 Gyr between the SFR and the Type Ia SNR. More complete
measurements of the Type Ia SNR at z>1 are necessary to conclusively determine
the SFR--SNR relationship and constrain SNIa evolutionary pathways.Comment: 37 pages, 9 figures, accepted for publication in Astrophysical
Journal. Figures 7-9 correcte
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