14 research outputs found
TASI Lectures on the Cosmological Constant
The energy density of the vacuum, Lambda, is at least 60 orders of magnitude
smaller than several known contributions to it. Approaches to this problem are
tightly constrained by data ranging from elementary observations to precision
experiments. Absent overwhelming evidence to the contrary, dark energy can only
be interpreted as vacuum energy, so the venerable assumption that Lambda=0
conflicts with observation. The possibility remains that Lambda is
fundamentally variable, though constant over large spacetime regions. This can
explain the observed value, but only in a theory satisfying a number of
restrictive kinematic and dynamical conditions. String theory offers a concrete
realization through its landscape of metastable vacua.Comment: 39 pages, 3 figure
Quintessence Cosmology and the Cosmic Coincidence
Within present constraints on the observed smooth energy and its equation of
state parameter, it is important to find out whether the smooth energy is
static (cosmological constant) or dynamic (quintessence). The most dynamical
quintessence fields observationally allowed are now still fast-rolling and no
longer satisfy the tracker approximation if the equation of state parameter
varies moderately with cosmic scale. We are optimistic about distinguishing
between a cosmological constant and appreciably dynamic quintessence, by
measuring average values for the effective equation of state parameter.
However, reconstructing the quintessence potential from observations of any
scale dependence appears problematic in the near future. For our flat universe,
at present dominated by smooth energy in the form of either a cosmological
constant (LCDM) or quintessence (QCDM), we calculate the asymptotic collapsed
mass fraction to be maximal at the observed smooth energy/matter ratio.
Identifying this collapsed fraction as a conditional probability for habitable
galaxies, we infer that the prior distribution is flat. Interpreting this prior
as a distribution over theories, rather than as a distribution over
unobservable subuniverses, leads us to heuristic predictions about the class of
future quantum cosmology theories and the static or quasi-static nature of the
smooth energy.Comment: Typos corrected, as presented at Cosmo-01 Workshop, Rovaniemi,
Finland and accepted for publication in Physical Review D. 9 pages, 4 figure
Bulk Viscous LRS Biachi-I Universe with variable and decaying
The present study deals with spatially homogeneous and totally anisotropic
locally rotationally symmetric (LRS) Bianchi type I cosmological model with
variable and in presence of imperfect fluid. To get the
deterministic model of Universe, we assume that the expansion in the
model is proportional to shear . This condition leads to , where ,\; are metric potential. The cosmological constant
is found to be decreasing function of time and it approaches a small
positive value at late time which is supported by recent Supernovae Ia (SN Ia)
observations. Also it is evident that the distance modulus curve of derived
model matches with observations perfectly.Comment: 11 pages, 4 figures and 1 table, Accepted for publication in
Astrophysics and Space Scienc
Detection Limits for Super-Hubble Suppression of Causal Fluctuations
We investigate to what extent future microwave background experiments might
be able to detect a suppression of fluctuation power on large scales in flat
and open universe models. Such suppression would arise if fluctuations are
generated by causal processes, and a measurement of a small suppression scale
would be problematic for inflation models, but consistent with many defect
models. More speculatively, a measurement of a suppression scale of the order
of the present Hubble radius could provide independent evidence for a
fine-tuned inflation model leading to a low-density universe. We find that,
depending on the primordial power spectrum, a suppression scale modestly larger
than the visible Horizon can be detected, but that the detectability drops very
rapidly with increasing scale. For models with two periods of inflation, there
is essentially no possibility of detecting a causal suppression scale.Comment: 8 pages, 4 figures, revtex, In Press Physical Review D 200
SARS-CoV-2-specific nasal IgA wanes 9 months after hospitalisation with COVID-19 and is not induced by subsequent vaccination
BACKGROUND: Most studies of immunity to SARS-CoV-2 focus on circulating antibody, giving limited insights into mucosal defences that prevent viral replication and onward transmission. We studied nasal and plasma antibody responses one year after hospitalisation for COVID-19, including a period when SARS-CoV-2 vaccination was introduced. METHODS: In this follow up study, plasma and nasosorption samples were prospectively collected from 446 adults hospitalised for COVID-19 between February 2020 and March 2021 via the ISARIC4C and PHOSP-COVID consortia. IgA and IgG responses to NP and S of ancestral SARS-CoV-2, Delta and Omicron (BA.1) variants were measured by electrochemiluminescence and compared with plasma neutralisation data. FINDINGS: Strong and consistent nasal anti-NP and anti-S IgA responses were demonstrated, which remained elevated for nine months (p < 0.0001). Nasal and plasma anti-S IgG remained elevated for at least 12 months (p < 0.0001) with plasma neutralising titres that were raised against all variants compared to controls (p < 0.0001). Of 323 with complete data, 307 were vaccinated between 6 and 12 months; coinciding with rises in nasal and plasma IgA and IgG anti-S titres for all SARS-CoV-2 variants, although the change in nasal IgA was minimal (1.46-fold change after 10 months, p = 0.011) and the median remained below the positive threshold determined by pre-pandemic controls. Samples 12 months after admission showed no association between nasal IgA and plasma IgG anti-S responses (R = 0.05, p = 0.18), indicating that nasal IgA responses are distinct from those in plasma and minimally boosted by vaccination. INTERPRETATION: The decline in nasal IgA responses 9 months after infection and minimal impact of subsequent vaccination may explain the lack of long-lasting nasal defence against reinfection and the limited effects of vaccination on transmission. These findings highlight the need to develop vaccines that enhance nasal immunity. FUNDING: This study has been supported by ISARIC4C and PHOSP-COVID consortia. ISARIC4C is supported by grants from the National Institute for Health and Care Research and the Medical Research Council. Liverpool Experimental Cancer Medicine Centre provided infrastructure support for this research. The PHOSP-COVD study is jointly funded by UK Research and Innovation and National Institute of Health and Care Research. The funders were not involved in the study design, interpretation of data or the writing of this manuscript
New Upper Limit on the Total Neutrino Mass from the 2 Degree Field Galaxy Redshift Survey
We constrain f(nu)equivalent toOmega(nu)/Omega(m), the fractional contribution of neutrinos to the total mass density in the Universe, by comparing the power spectrum of fluctuations derived from the 2 Degree Field Galaxy Redshift Survey with power spectra for models with four components: baryons, cold dark matter, massive neutrinos, and a cosmological constant. Adding constraints from independent cosmological probes we find f(nu)<0.13 (at 95% confidence) for a prior of 0.1<Omega(m)<0.5, and assuming the scalar spectral index n=1. This translates to an upper limit on the total neutrino mass m(nu,tot)<1.8 eV for "concordance" values of Omega(m) and the Hubble constant