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Register a new userImproved constraint on the primordial gravitational-wave density using recent cosmological data and its impact on cosmic string models
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The production of a primordial stochastic gravitational-wave background by processes occuring in the early Universe is expected in a broad range of models. Observing this background would open a unique window onto the Universes evolutionary history. Probes like the Cosmic Microwave Background (CMB) or the Baryon Acoustic Oscillations (BAO) can be used to set upper limits on the stochastic gravitational-wave background energy density $Omega_{GW}$ for frequencies above $10^{-15}$ Hz. We perform a profile likelihood analysis of the Planck CMB temperature anisotropies and gravitational lensing data combined with WMAP low-$ell$ polarization, BAO, South Pole Telescope and Atacama Cosmology Telescope data. We find that $Omega_{GW}h_{0}^{2} < 3.8 times 10^{-6}$ at 95% confidence level for adiabatic initial conditions which improves over the previous limit by a factor 2.3. Assuming that the primordial gravitational waves have been produced by a network of cosmic strings, we have derived exclusion limits in the cosmic string parameter space. If the size of the loops is determined by gravitational back-reaction, string tension values greater than $sim 4 times 10^{-9}$ are excluded for a reconnection probability of $10^{-3}$.
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Gravitational waves (GWs) are one of the key signatures of cosmic strings. If GWs from cosmic strings are detected in future experiments, not only their existence can be confirmed but also their properties might be probed. In this paper, we study the determination of cosmic string parameters through direct detection of GW signatures in future ground-based GW experiments. We consider two types of GWs, bursts and the stochastic GW background, which provide us with different information about cosmic string properties. Performing the Fisher matrix calculation on the cosmic string parameters, such as parameters governing the string tension $Gmu$ and initial loop size $alpha$ and the reconnection probability $p$, we find that the two different types of GW can break degeneracies in some of these parameters and provide better constraints than those from each measurement.
We do a complete calculation of the stochastic gravitational wave background to be expected from cosmic strings. We start from a population of string loops taken from simulations, smooth these by Lorentzian convolution as a model of gravitational back reaction, calculate the average spectrum of gravitational waves emitted by the string population at any given time, and propagate it through a standard model cosmology to find the stochastic background today. We take into account all known effects, including changes in the number of cosmological relativistic degrees of freedom at early times and the possibility that some energy is in rare bursts that we might never have observed.
We investigate the idea that current cosmic acceleration could be the consequence of gravitational leakage into extra dimensions on cosmological scales rather than the result of a non-zero cosmological constant, and consider the ability of future gravitational-wave siren observations to probe this phenomenon and constrain the parameters of phenomenological models of this gravitational leakage. In theories that include additional non-compact spacetime dimensions, the gravitational leakage intro extra dimensions leads to a reduction in the amplitude of observed gravitational waves and thereby a systematic discrepancy between the distance inferred to such sources from GW and EM observations. We investigate the capability of a gravitational space interferometer such as LISA to probe this modified gravity on large scales. We find that the extent to which LISA will be able to place limits on the number of spacetime dimensions and other cosmological parameters characterising modified gravity will strongly depend on the actual number and redshift distribution of sources, together with the uncertainty on the GW measurements. A relatively small number of sources ($sim 1$) and high measurement uncertainties would strongly restrict the ability of LISA to place meaningful constraints on the parameters in cosmological scenarios where gravity is only five-dimensional and modified at scales larger than about $sim 4$ times the Hubble radius. Conversely, if the number of sources observed amounts to a four-year average of $sim 27$, then in the most favourable cosmological scenarios LISA has the potential to place meaningful constraints on the cosmological parameters with a precision of $sim 1%$ on the number of dimensions and $sim 7.5%$ on the scale beyond which gravity is modified, thereby probing the late expansion of the universe up to a redshift of $sim 8$.
The recent observations from CMB have imposed a very stringent upper-limit on the tensor/scalar ratio $r$ of inflation models, $r < 0.064$, which indicates that the primordial gravitational waves (PGW), even though possible to be detected, should have a power spectrum of a tiny amplitude. However, current experiments on PGW is ambitious to detect such a signal by improving the accuracy to an even higher level. Whatever their results are, it will give us much information about the early Universe, not only from the astrophysical side but also from the theoretical side, such as model building for the early Universe. In this paper, we are interested in analyzing what kind of inflation models can be favored by future observations, starting with a kind of general action offered by the effective field theory (EFT) approach. We show a general form of $r$ that can be reduced to various models, and more importantly, we show how the accuracy of future observations can put constraints on model parameters by plotting the contours in their parameter spaces.
General Relativity provides us with an extremely powerful tool to extract at the same time astrophysical and cosmological information from the Stochastic Gravitational Wave Backgrounds (SGWBs): the cross-correlation with other cosmological tracers, since their anisotropies share a common origin and the same perturbed geodesics. In this letter we explore the cross-correlation of the cosmological and astrophysical SGWBs with Cosmic Microwave Background (CMB) anisotropies, showing that future GW detectors, such as LISA or BBO, have the ability to measure such cross-correlation signals. We also present, as a new tool in this context, constrained realization maps of the SGWBs extracted from the high-resolution CMB {it Planck} maps. This technique allows, in the low-noise regime, to faithfully reconstruct the expected SGWB map by starting from CMB measurements.
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