1,207 research outputs found

### Inflation after Planck and BICEP2

We discuss the inflationary paradigm, how it can be tested, and how various
models of inflation fare in the light of data from Planck and BICEP2. We
introduce inflation and reheating, and discuss temperature and polarisation
anisotropies in the cosmic microwave background radiation due to quantum
fluctuations during inflation. Fitting observations of the anisotropies with
theoretical realisations obtained by varying various parameters of the
curvature power spectrum and cosmological parameters enables one to obtain the
allowed ranges of these parameters. We discuss how to relate these parameters
to inflation models which allows one to rule in or out specific models of
inflation.Comment: Slightly longer version of a plenary review talk at the XXI DAE-BRNS
High Energy Physics Symposium at IIT Guwahati, Dec.8-12, 2014. 14 pages, 7
fig

### Weak Lensing Probes of Modified Gravity

We study the effect of modifications to General Relativity on large scale
weak lensing observables. In particular, we consider three modified gravity
scenarios: f(R) gravity, the DGP model, and TeVeS theory. Weak lensing is
sensitive to the growth of structure and the relation between matter and
gravitational potentials, both of which will in general be affected by modified
gravity. Restricting ourselves to linear scales, we compare the predictions for
galaxy-shear and shear-shear correlations of each modified gravity cosmology to
those of an effective Dark Energy cosmology with the same expansion history. In
this way, the effects of modified gravity on the growth of perturbations are
separated from the expansion history. We also propose a test which isolates the
matter-potential relation from the growth factor and matter power spectrum. For
all three modified gravity models, the predictions for galaxy and shear
correlations will be discernible from those of Dark Energy with very high
significance in future weak lensing surveys. Furthermore, each model predicts a
measurably distinct scale dependence and redshift evolution of galaxy and shear
correlations, which can be traced back to the physical foundations of each
model. We show that the signal-to-noise for detecting signatures of modified
gravity is much higher for weak lensing observables as compared to the ISW
effect, measured via the galaxy-CMB cross-correlation.Comment: 16 pages, 8 figures; accepted for publication in Phys. Rev. D; v2:
references added; v3: clarifications and additions to the text in response to
refere

### The Small-Scale Power Spectrum of Cold Dark Matter

One of the best motivated hypotheses in cosmology states that most of the
matter in the universe is in the form of weakly-interacting massive particles
that decoupled early in the history of the universe and cooled adiabatically to
an extremely low temperature. Nevertheless, the finite temperature and horizon
scales at which these particles decoupled imprint generic signatures on their
small scales density fluctuations. We show that the previously recognized
cut-off in the fluctuation power-spectrum due to free-streaming of particles at
the thermal speed of decoupling, is supplemented by acoustic oscillations owing
to the initial coupling between the cold dark matter (CDM) and the radiation
field. The power-spectrum oscillations appear on the scale of the horizon at
thermal decoupling which corresponds to a mass scale of
\~10^{-4}*(T_d/10MeV)^{-3} solar masses for a CDM decoupling temperature T_d.
The suppression of the power-spectrum on smaller scales by the acoustic
oscillations is physically independent from the free-streaming effect, although
the two cut-off scales are coincidentally comparable for T_d~10MeV and a
particle mass of M~100GeV. The initial conditions for recent numerical
simulations of the earliest and smallest objects to have formed in the
universe, need to be modified accordingly. The smallest dark matter clumps may
be detectable through gamma-ray production from particle annihilation, through
fluctuations in the event rate of direct detection experiments, or through
their tidal gravitational effect on wide orbits of objects near the outer edge
of the solar system.Comment: Physical Review D, in pres

### Cosmic Neutrino Last Scattering Surface

Neutrinos decoupled from the rest of the cosmic plasma when the Universe was
less than one second old, far earlier than the photons which decoupled at
t=380,000 years. Surprisingly, though, the last scattering surface of the
neutrinos is much closer to us than that of the photons. Here we calculate the
properties of the last scattering surfaces of the three species of neutrinos.Comment: Important reference to earlier work of Bisnovatyi-Kogan and Seidov
added, and mis-spelling of Opher reference correcte

### Phenomenology of the CAH+ measure

The CAH+ measure regulates the infinite spacetime volume of the multiverse by
constructing a surface of constant comoving apparent horizon (CAH) and then
removing the future lightcones of all points on that surface (the latter
prescription is referred to by the "+" in the name of the measure). This
measure was motivated by the conjectured duality between the bulk of the
multiverse and its future infinity and by the causality condition, requiring
that the cutoff surfaces of the measure should be spacelike or null. Here we
investigate the phenomenology of the CAH+ measure and find that it does not
suffer from any known pathologies. The distribution for the cosmological
constant Lambda derived from this measure is in a good agreement with the
observed value, and the distribution for the number of inflationary e-foldings
satisfies the observational constraint. The CAH+ measure does not exhibit any
"runaway" behaviors at zero or negative values of Lambda, which have been
recently shown to afflict a number of other measures.Comment: 35 pages, including 6 figures and 2 appendices; v2 corrections in
Section 2.4, conclusions unchange

### Is a Massive Tau Neutrino Just What Cold Dark Matter Needs?

The cold dark matter (CDM) scenario for structure formation in the Universe
is very attractive and has many successes; however, when its spectrum of
density perturbations is normalized to the COBE anisotropy measurement the
level of inhomogeneity predicted on small scales is too large. This can be
remedied by a tau neutrino of mass 1\MeV - 10\MeV and lifetime $0.1\sec -
100\sec$ whose decay products include electron neutrinos because it allows the
total energy density in relativistic particles to be doubled without
interfering with nucleosynthesis. The anisotropies predicted on the degree
scale for ``$\tau$CDM'' are larger than standard CDM. Experiments at $e^\pm$
colliders may be able to probe such a mass range.Comment: 9 pages LaTeX plus 4 figures (available upon request)
FERMILAB--Pub--94/026-

### Neutrino Mass and Dark Energy from Weak Lensing

Weak gravitational lensing of background galaxies by intervening matter
directly probes the mass distribution in the universe. This distribution, and
its evolution at late times, is sensitive to both the dark energy, a negative
pressure energy density component, and neutrino mass. We examine the potential
of lensing experiments to measure features of both simultaneously. Focusing on
the radial information contained in a future deep 4000 square degree survey, we
find that the expected (1-sigma) error on a neutrino mass is 0.1 eV, if the
dark energy parameters are allowed to vary. The constraints on dark energy
parameters are similarly restrictive, with errors on w of 0.09. Much of the
restrictive power on the dark energy comes not from the evolution of the
gravitational potential but rather from how distances vary as a function of
redshift in different cosmologies

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