共查询到20条相似文献,搜索用时 31 毫秒
1.
Robert C. Nichol 《General Relativity and Gravitation》2008,40(2-3):249-267
In this review, I outline the use of galaxy correlations to constrain cosmological parameters. As with the cosmic microwave
background (CMB), the density of dark and baryonic matter imprints important scales on the fluctuations of matter and thus
the clustering of galaxies, e.g., the particle horizon at matter-radiation equality and the sound horizon at recombination.
Precision measurements of these scales from the baryon acoustic oscillations (BAO) and the large scale shape of the power
spectrum of galaxy clustering provide constraints on Ω
m
h
2. Recent measurements from the Sloan Digital Sky Survey (SDSS) and 2dF Galaxy Redshift Survey (2dFGRS) strongly suggest that
Ω
m
< 0.3. This forms the basic evidence for a flat Universe dominated by a Cosmological Constant (Λ) today (when combined with
results from the CMB and supernova surveys). Further evidence for this cosmological model is provided by the late-time Integrated
Sachs–Wolfe (ISW) effect, which has now been detected using a variety of tracers of the large scale structure in the Universe
out to redshifts of z > 1. The ISW effect also provides an opportunity to discriminate between Λ, dynamical dark energy models and the modification
of gravity on large scales. 相似文献
2.
Renée Hlozek Marina Cortês Chris Clarkson Bruce Bassett 《General Relativity and Gravitation》2008,40(2-3):285-300
We discuss degeneracies between dark energy and cosmic parameters using a fully non-perturbative and non-parametric approach.
This allows us to examine the knock-on bias induced in the reconstructed dark energy equation of state, w(z), if there is a bias in the cosmic curvature or dark matter content. Assuming perfect Hubble, distance and volume measurements,
we show that for z > 1, the bias in w(z) is up to two orders of magnitude larger than the corresponding errors in Ω
k
or Ω
m
. 相似文献
3.
Varun Sahni 《Pramana》1999,53(6):937-944
I briefly review the observational evidence for a small cosmological constant at the present epoch. This evidence mainly comes
from high redshift observations of Type 1a supernovae, which, when combined with CMB observations strongly support a flat
Universe with Ω
m
+ ΩA ⋍ 1. Theoretically a cosmological constant can arise from zero point vacuum fluctuations. In addition ultra-light scalar
fields could also give rise to a Universe which is accelerating driven by a time dependent Λ-term induced by the scalar field
potential. Finally a Λ dominated Universe also finds support from observations of galaxy clustering and the age of the Universe. 相似文献
4.
We have obtained a generalization of the hydrodynamic theory of vacuum in the context of general relativity. While retaining
the Lagrangian character of general relativity, the new theory provides a natural alternative to the view that the singularity
is inevitable in general relativity and the theory of a hot Universe. We show that the macroscopic source-sink motion as a
whole of ordinary (dark) matter that emerges during the production of particles out of the vacuum can be a new source of gravitational
vacuum polarization (determining the variability of the cosmological term in general relativity). We have removed the well-known
problems of the cosmological constant by refining the physical nature of dark energy associated precisely with this hydrodynamically
initiated variability of the vacuum energy density. A new exact solution of the modified general relativity equations that
contains no free (fitting) parameter additional to those available in general relativity has been obtained. It corresponds
to the continuous and metric-affecting production of ultralight dark matter particles (with mass m
0 = (ħ/c
2) $
\sqrt {12\rho _0 k}
$
\sqrt {12\rho _0 k}
≈ 3 × 10−66 g, k is the gravitational constant) out of the vacuum, with its density ρ0, constant during the exponential expansion of a spatially flat Universe, being retained. This solution is shown to be stable
in the regime of cosmological expansion in the time interval −∞ < t < t
max, when t = 0 corresponds to the present epoch and t
max= 2/3H
0
cΩ0m
≈ 38 × 109 yr at Ω0m
= ρ0/ρc ≈ 0.28 (H
0 is the Hubble constant, ρc is the critical density). For t > t
max, the solution becomes exponentially unstable and characterizes the inverse process of dark matter particle absorption by
the vacuum in the regime of contraction of the Universe. We consider the admissibility of the fact that scalar massive photon
pairs can be these dark matter particles. Good quantitative agreement of this exact solution with the cosmological observations
of SnIa, SDSS-BAO, and the decrease in the acceleration of the expansion of the Universe has been obtained. 相似文献
5.
The dynamical system of multiple scalar fields in FRW universe with different spatial curvature have been analyzed in this paper. In the radiation-dominated phase, the constant curvature factor k does not work on the cosmic dynamical behaviors, including the scaling solution, energy density parameter and equation-of-state parameter. These aspects are affected by curvature factor k in the matter-dominated phase. In the special scalar field-dominated phase, the energy density parameter normalization restricts the Universe is spatial flat and the curvature factor k is not present in the dynamics. In this paper, the Universe is closed in the matter-dominated phase, and flat in the scalar field-dominated phase. The spatial flatness and the w ϕ =−1 in the third phase are coincide with the current observations. 相似文献
6.
Vinod B Johri 《Pramana》2002,59(3):L553-L561
We present a realistic scenario of tracking of scalar fields with varying equation of state. The astrophysical constraints
on the evolution of scalar fields in the physical universe are discussed. The nucleosynthesis and the galaxy formation constraints
have been used to put limits on Ωφ and estimate ɛ during cosmic evolution. Interpolation techniques have been applied to estimate ɛ ⋍0.772 at the present epoch.
The epoch of transition from matter to quintessence dominated era and consequent onset of acceleration in cosmic expansion
is calculated and taking the lower limit Θ
n
/0
=0.2 as estimated from SN
e
I
a
data, it is shown that the supernova observations beyond redshift z=1 would reveal deceleration in cosmic expansion. 相似文献
7.
Yang Zhang 《General Relativity and Gravitation》2003,35(4):689-696
Recent observations of large scale structure of the Universe, especially that of Type Ia supernovae, indicate that the Universe is flat and is accelerating, and that the dominant energy density in the Universe is the cosmic dark energy. We propose a model in which the cosmic effective Yang-Mills condensate familiar in particle physics plays the role of the dark energy that causes the acceleration of the Universe. Since the quantum effective Yang-Mills field in certain states has the equation of state p
y
= –
y
, when employed as the cosmic matter source, it naturally results in an accelerating expansion of the Universe. With the matter components (
m
1/3) being added into the model, the composition of YM condensate and matter components can give rise to the desired equation of state w –2/3 for the Universe. 相似文献
8.
Jianbo Lu Lixin Xu Molin Liu Yuanxing Gui 《The European Physical Journal C - Particles and Fields》2008,58(2):311-324
We use recent data: the 192 ESSENCE type Ia supernovae (SNe Ia), the 182 Gold SNe Ia, the three-year WMAP, the SDSS baryon
acoustic peak, the X-ray gas mass fraction in clusters and the observational H(z) data, to constrain models of the accelerating universe. Combining the 192 ESSENCE data with the observational H(z) data to constrain the parameterized deceleration parameter, we obtain the best-fit values of the transition redshift and
current deceleration parameter z
T=0.632−0.127+0.256 and q
0=−0.788−0.182+0.182. Furthermore, using the ΛCDM model and two model-independent equations of state of the dark energy, we find that the combined
constraint from the 192 ESSENCE data and four other cosmological observations gives smaller values for Ω
0m and q
0, but a larger value for z
T than the combined constraint from the 182 Gold data with four other observations. Finally, according to the Akaike information
criterion it is shown that the recently observed data equally support three dark energy models: ΛCDM, w
de(z)=w
0 and w
de(z)=w
0+w
1ln (1+z). 相似文献
9.
Teppo Mattsson 《General Relativity and Gravitation》2010,42(3):567-599
Motivated by the observed cosmic matter distribution, we present the following conjecture: due to the formation of voids and
opaque structures, the average matter density on the path of the light from the well-observed objects changes from Ω
M
≃ 1 in the homogeneous early universe to Ω
M
≃ 0 in the clumpy late universe, so that the average expansion rate increases along our line of sight from EdS expansion
Ht ≃ 2/3 at high redshifts to free expansion Ht ≃ 1 at low redshifts. To calculate the modified observable distance–redshift relations, we introduce a generalized Dyer–Roeder
method that allows for two crucial physical properties of the universe: inhomogeneities in the expansion rate and the growth
of the nonlinear structures. By treating the transition redshift to the void-dominated era as a free parameter, we find a
phenomenological fit to the observations from the CMB anisotropy, the position of the baryon oscillation peak, the magnitude–redshift
relations of type Ia supernovae, the local Hubble flow and the nucleosynthesis, resulting in a concordant model of the universe
with 90% dark matter, 10% baryons, no dark energy, 15 Gyr as the age of the universe and a natural value for the transition
redshift z
0 = 0.35. Unlike a large local void, the model respects the cosmological principle, further offering an explanation for the
late onset of the perceived acceleration as a consequence of the forming nonlinear structures. Additional tests, such as quantitative
predictions for angular deviations due to an anisotropic void distribution and a theoretical derivation of the model, can
vindicate or falsify the interpretation that light propagation in voids is responsible for the perceived acceleration. 相似文献
10.
X. M. Song Z. G. Huang H. Q. Lu Q. Q. Sun 《International Journal of Theoretical Physics》2008,47(4):865-872
In dilatonic cosmology model, we study the behavior of attractor solution in ω–ω′ plane, which is defined by the equation of state parameter for the dark energy and its derivative with respect to N (the logarithm of the scale factor a). This is a good method which is useful to the study of classifying the dynamical dark energy models including “freezing”
and “thawing” model. We find that our model belongs to “freezing” type model classified in ω–ω′ plane. We show mathematically the property of attractor solutions which correspond to ω
σ
=−1, Ω
σ
=1. The present values of energy density parameter
,
and
are 0.715001, 0.284972 and 0.00002706 respectively, which meet the current observations well. Finally, we can obtain that
the coupling between dilaton and matter affects the evolutive process of the Universe, but not the fate of the Universe. 相似文献
11.
Songbai Chen Xiangyun Fu Jiliang Jing 《The European Physical Journal C - Particles and Fields》2009,64(1):107-112
The unparticle has been suggested as a candidate of dark matter. We investigated the growth rate of the density perturbation
for unparticle dark matter in the flat Universe. First, we consider the model in which the unparticle is the sole dark matter
and find that the growth factor can be approximated well by f=(1+3ω
u
)Ω
u
γ
, where ω
u
is the equation of state of unparticle. Our results show that the presence of ω
u
modifies the behavior of the growth factor f. For the second model where the unparticle co-exists with cold dark matter, the growth factor has a new approximation f=(1+3ω
u
)Ω
u
γ
+α
Ω
m
and α is a function of ω
u
. Thus the growth factor of the unparticle is quite different from that of the usual dark matter. This information can help
us know more about unparticle and the early evolution of the Universe. 相似文献
12.
Ahmet M. Öztaş Michael L. Smith Jan Paul 《International Journal of Theoretical Physics》2008,47(9):2464-2478
We investigate universe expansion models as functions of emission frequency ratio decline rather than redshift z, using the latest on-line, self-consistent data from 192 supernovae. We present results for simpler and some current models
of cosmology, including those with dark energy (standard model) and a recent model correcting for the effect of a small time-dependent,
emission frequency increase with lookback. This new model, with a gentle lookback decline of the Planck constant, and the
standard model fit the data with similar confidence according to Bayesian Information Criteria. The standard model tends towards
solutions high in matter density while remaining flat, but models without dark energy tend towards dilute universes with significant
spacetime and curvature and a smaller Hubble constant. We conclude the normalized spacetime parameter, Ω
k
, should not be ignored and it includes the combined contributions of huge spacetime magnitude and curvature. 相似文献
13.
Writambhara Chakraborty Ujjal Debnath 《International Journal of Theoretical Physics》2009,48(1):232-247
In this work we have studied the possibility of obtaining cosmic acceleration in Brans-Dicke theory with varying or constant
ω (Brans-Dicke parameter) and with or without self-interacting potential, the background fluid being barotropic fluid or Generalized
Chaplygin Gas. Here we take the power law form of the scale factor and the scalar field. We show that accelerated expansion
can also be achieved for high values of ω for closed Universe. 相似文献
14.
A new relation for the density parameter Ω is derived as a function of expansion velocity υ based on Carmeli's cosmology.
This density function is used in the luminosity distance relation D
L. A heretofore neglected source luminosity correction factor (1 − (υ/c)2)−1/2 is now included in D
L. These relations are used to fit type Ia supernovae (SNe Ia) data, giving consistent, well-behaved fits over a broad range
of redshift 0.1 < z < 2. The best fit to the data for the local density parameter is Ωm = 0.0401 ± 0.0199. Because Ωm is within the baryonic budget there is no need for any dark matter to account for the SNe Ia redshift luminosity data. From
this local density it is determined that the redshift where the universe expansion transitions from deceleration to acceleration
is z
t = 1.095+0.264
−0.155. Because the fitted data covers the range of the predicted transition redshift z
t, there is no need for any dark energy to account for the expansion rate transition. We conclude that the expansion is now
accelerating and that the transition from a closed to an open universe occurred about 8.54 Gyr ago. 相似文献
15.
The focal switch of cosine-Gaussian (CsG) beams passing through a system with the aperture and lens separated is studied analytically
and numerically. It is shown that the focal switch of CsG beams can appear not only for the apertured case, but also for the
unapertured case. The necessary condition for the focal switch is that truncation parameter α > αc and the beam parameter β > βc, αc, βc being the corresponding critical values. There exists a maximum of the relative transition height Δz
sw as α varies, and Δz
sw increases with increasing β and decreasing N
w. The normalized axial intensity minimum I
min / I
max decreases with an increase of α and β, and I
min / I
max remains unchanged as N
w varies. 相似文献
16.
In the Friedmann cosmology, the deceleration of the expansion q plays a fundamental role. We derive the deceleration as a function of redshift q(z) in two scenarios: ΛCDM model and modified Chaplygin gas (MCG) model. The function for the MCG model is then fitted to the cosmological data in
order to obtain the cosmological parameters that minimize χ
2. We use the Fisher matrix to construct the covariance matrix of our parameters and reconstruct the q(z) function. We use Supernovae Ia, WMAP5, and BAO measurements to obtain the observational constraints. We determined the present
acceleration as q
0 = − 0.65 ±0.19 for the MCG model using the Union2 dataset of SNeIa, BAO, and CMB and q
0 = − 0.67 ±0.17 for the Constitution dataset, BAO and CMB. The transition redshift from deceleration to acceleration was found
to be around 0.80 for both datasets. We have also determined the dark energy parameter for the MCG model: Ω
X0 = 0.81 ±0.03 for the Union2 dataset and Ω
X0 = 0.83 ±0.03 using the Constitution dataset. 相似文献
17.
Hassan Amirhashchi Hishamuddin Zainuddin 《International Journal of Theoretical Physics》2010,49(11):2815-2828
The present study deals with Bianchi type III string cosmological models with magnetic field. The magnetic field is assumed
to be along z direction. Therefore F
12 is only the non-vanishing component of electromagnetic field tensor F
ij
. The expansion (θ) in the model is assumed to be proportional to the shear (σ). To get determinate solution in term of cosmic time, we have solved the fields equations in two cases (i) Reddy and (ii)
Nambu string. The physical and geometrical behaviour of these models is discussed. 相似文献
18.
19.
Gabriele Umberto Varieschi 《General Relativity and Gravitation》2010,42(4):929-974
We present an alternative cosmology based on conformal gravity, as originally introduced by H. Weyl and recently revisited
by P. Mannheim and D. Kazanas. Unlike past similar attempts our approach is a purely kinematical application of the conformal
symmetry to the Universe, through a critical reanalysis of fundamental astrophysical observations, such as the cosmological
redshift and others. As a result of this novel approach we obtain a closed-form expression for the cosmic scale factor R(t) and a revised interpretation of the space–time coordinates usually employed in cosmology. New fundamental cosmological parameters
are introduced and evaluated. This emerging new cosmology does not seem to possess any of the controversial features of the
current standard model, such as the presence of dark matter, dark energy or of a cosmological constant, the existence of the
horizon problem or of an inflationary phase. Comparing our results with current conformal cosmologies in the literature, we
note that our kinematic cosmology is equivalent to conformal gravity with a cosmological constant at late (or early) cosmological
times. The cosmic scale factor and the evolution of the Universe are described in terms of several dimensionless quantitites,
among which a new cosmological variable δ emerges as a natural cosmic time. The mathematical connections between all these quantities are described in details and
a relationship is established with the original kinematic cosmology by L. Infeld and A. Schild. The mathematical foundations
of our kinematical conformal cosmology will need to be checked against current astrophysical experimental data, before this
new model can become a viable alternative to the standard theory. 相似文献
20.
In this paper we put forward a running coupling scenario for describing the interaction between dark energy and dark matter.
The dark sector interaction in our scenario is free of the assumption that the interaction term Q is proportional to the Hubble expansion rate and the energy densities of dark sectors. We only use a time-variable coupling
b(a) (with a the scale factor of the universe) to characterize the interaction Q. We propose a parametrization form for the running coupling b(a)=b
0
a+b
e
(1−a) in which the early-time coupling is given by a constant b
e
, while today the coupling is given by another constant, b
0. For investigating the feature of the running coupling, we employ three dark energy models, namely, the cosmological constant
model (w=−1), the constant w model (w=w
0), and the time-dependent w model (w(a)=w
0+w
1(1−a)). We constrain the models with the current observational data, including the type Ia supernova, the baryon acoustic oscillation,
the cosmic microwave background, the Hubble expansion rate, and the X-ray gas mass fraction data. The fitting results indicate
that a time-varying vacuum scenario is favored, in which the coupling b(z) crosses the noninteracting line (b=0) during the cosmological evolution and the sign changes from negative to positive. The crossing of the noninteracting line
happens at around z=0.2–0.3, and the crossing behavior is favored at about 1σ confidence level. Our work implies that we should pay more attention to the time-varying vacuum model and seriously consider
the phenomenological construction of a sign-changeable or oscillatory interaction between dark sectors. 相似文献