3,846 research outputs found

### Testing Gravity Against Early Time Integrated Sachs-Wolfe Effect

A generic prediction of general relativity is that the cosmological linear
density growth factor $D$ is scale independent. But in general, modified
gravities do not preserve this signature. A scale dependent $D$ can cause time
variation in gravitational potential at high redshifts and provides a new
cosmological test of gravity, through early time integrated Sachs-Wolfe (ISW)
effect-large scale structure (LSS) cross correlation. We demonstrate the power
of this test for a class of $f(R)$ gravity, with the form $f(R)=-\lambda_1
H_0^2\exp(-R/\lambda_2H_0^2)$. Such $f(R)$ gravity, even with degenerate
expansion history to $\Lambda$CDM, can produce detectable ISW effect at z\ga
3 and l\ga 20. Null-detection of such effect would constrain $\lambda_2$ to
be $\lambda_2>1000$ at $>95%$ confidence level. On the other hand, robust
detection of ISW-LSS cross correlation at high $z$ will severely challenge
general relativity.Comment: 5 pages, 2 figures. Accepted to PRD. v2: Revised to address to more
general audience. v3: added discussion

### Dark Energy Accretion onto a Black Hole in an Expanding Universe

By using the solution describing a black hole embedded in the FLRW universe,
we obtain the evolving equation of the black hole mass expressed in terms of
the cosmological parameters. The evolving equation indicates that in the
phantom dark energy universe the black hole mass becomes zero before the Big
Rip is reached.Comment: 7 pages, no figures, errors is correcte

### Accelerated expansion in modified gravity with a Yukawa-like term

We discuss the Palatini formulation of modified gravity including a
Yukawa-like term. It is shown that in this formulation, the Yukawa term offers
an explanation for the current exponential accelerated expansion of the
universe and reduces to the standard Friedmann cosmology in the appropriate
limit. We then discuss the scalar-tensor formulation of the model as a metric
theory and show that the Yukawa term predicts a power-law acceleration at
late-times. The Newtonian limit of the theory is also discussed in context of
the Palatini formalism.Comment: 9 pages, 2 figures, to appear in IJMP

### Eternally inflating cosmologies from intersecting spacelike branes

Intersecting spacelike braneworld cosmologies are investigated. The time axis
is set on the scale parameter of extra space, which may include more than one
timelike metric. Obtained are eternally inflating (i.e. undergoing late-time
inflation) Robertson-Walker spacetime and extra space with a constant scale
factor. In the case of multibrane solutions, some dimensions are static or
shrink. The fact that the largest supersymmetry algebra contains 32
supercharges in 4 dimensions imposes a restriction on the geometry of extra
space.Comment: 19 page

### Model-Independent Distance Measurements from Gamma-Ray Bursts and Constraints on Dark Energy

Gamma-Ray Bursts (GRB) are the most energetic events in the Universe, and
provide a complementary probe of dark energy by allowing the measurement of
cosmic expansion history that extends to redshifts greater than 6. Unlike Type
Ia supernovae (SNe Ia), GRBs must be calibrated for each cosmological model
considered, because of the lack of a nearby sample of GRBs for
model-independent calibration. For a flat Universe with a cosmological
constant, we find Omega_m=0.25^{+0.12}_{-0.11} from 69 GRBs alone. We show that
the current GRB data can be summarized by a set of model-independent distance
measurements, with negligible loss of information. We constrain a dark energy
equation of state linear in the cosmic scale factor using these distance
measurements from GRBs, together with the "Union" compilation of SNe Ia, WMAP
five year observations, and the SDSS baryon acoustic oscillation scale
measurement. We find that a cosmological constant is consistent with current
data at 68% confidence level for a flat Universe. Our results provide a simple
and robust method to incorporate GRB data in a joint analysis of cosmological
data to constrain dark energy.Comment: 8 pages, 5 color figures. Version expanded and revised for
clarification, and typo in Eqs.(3)(4)(12) corrected. PRD, in pres

### Constraining Dark Energy and Cosmological Transition Redshift with Type Ia Supernovae

The property of dark energy and the physical reason for acceleration of the
present universe are two of the most difficult problems in modern cosmology.
The dark energy contributes about two-thirds of the critical density of the
present universe from the observations of type-Ia supernova (SNe Ia) and
anisotropy of cosmic microwave background (CMB).The SN Ia observations also
suggest that the universe expanded from a deceleration to an acceleration phase
at some redshift, implying the existence of a nearly uniform component of dark
energy with negative pressure. We use the ``gold'' sample containing 157 SNe Ia
and two recent well-measured additions, SNe Ia 1994ae and 1998aq to explore the
properties of dark energy and the transition redshift. For a flat universe with
the cosmological constant, we measure $\Omega_{M}=0.28_{-0.05}^{+0.04}$, which
is consistent with Riess et al. The transition redshift is
$z_{T}=0.60_{-0.08}^{+0.06}$. We also discuss several dark energy models that
define the $w(z)$ of the parameterized equation of state of dark energy
including one parameter and two parameters ($w(z)$ being the ratio of the
pressure to energy density). Our calculations show that the accurately
calculated transition redshift varies from $z_{T}=0.29_{-0.06}^{+0.07}$ to
$z_{T}=0.60_{-0.08}^{+0.06}$ across these models. We also calculate the minimum
redshift $z_{c}$ at which the current observations need the universe to
accelerate.Comment: 16 pages, 5 figures, 1 tabl

### 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

### Is there Evidence for a Hubble bubble? The Nature of Type Ia Supernova Colors and Dust in External Galaxies

We examine recent evidence from the luminosity-redshift relation of Type Ia
Supernovae (SNe Ia) for the $\sim 3 \sigma$ detection of a ``Hubble bubble'' --
a departure of the local value of the Hubble constant from its globally
averaged value \citep{Jha:07}. By comparing the MLCS2k2 fits used in that study
to the results from other light-curve fitters applied to the same data, we
demonstrate that this is related to the interpretation of SN color excesses
(after correction for a light-curve shape-color relation) and the presence of a
color gradient across the local sample. If the slope of the linear relation
($\beta$) between SN color excess and luminosity is fit empirically, then the
bubble disappears. If, on the other hand, the color excess arises purely from
Milky Way-like dust, then SN data clearly favors a Hubble bubble. We
demonstrate that SN data give $\beta \simeq 2$, instead of the $\beta \simeq 4$
one would expect from purely Milky-Way-like dust. This suggests that either SN
intrinsic colors are more complicated than can be described with a single
light-curve shape parameter, or that dust around SN is unusual. Disentangling
these possibilities is both a challenge and an opportunity for large-survey SN
Ia cosmology.Comment: Further information and data at
http://qold.astro.utoronto.ca/conley/bubble/ Accepted for publication in ApJ

### Naked Singularity in a Modified Gravity Theory

The cosmological constant induced by quantum fluctuation of the graviton on a
given background is considered as a tool for building a spectrum of different
geometries. In particular, we apply the method to the Schwarzschild background
with positive and negative mass parameter. In this way, we put on the same
level of comparison the related naked singularity (-M) and the positive mass
wormhole. We discuss how to extract information in the context of a f(R)
theory. We use the Wheeler-De Witt equation as a basic equation to perform such
an analysis regarded as a Sturm-Liouville problem . The application of the same
procedure used for the ordinary theory, namely f(R)=R, reveals that to this
approximation level, it is not possible to classify the Schwarzschild and its
naked partner into a geometry spectrum.Comment: 8 Pages. Contribution given to DICE 2008. To appear in the
proceeding

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