We investigate constraints on the spectral index of primordial gravitational waves (GWs), paying particular attention to a blue-tilted spectrum. Such constraints can be used to test a certain class of models of the early Universe. We investigate obse
rvational bounds from LIGO+Virgo, pulsar timing and big bang nucleosynthesis, taking into account the suppression of the amplitude at high frequencies due to reheating after inflation and also late-time entropy production. Constraints on the spectral index are presented by changing values of parameters such as reheating temperatures and the amount of entropy produced at late time. We also consider constraints under the general modeling approach which can approximately describe various scenarios of the early Universe. We show that the constraints on the blue spectral tilt strongly depend on the underlying assumption and, in some cases, a highly blue-tilted spectrum can still be allowed.
The prospects for direct measurements of inflationary gravitational waves by next generation interferometric detectors inferred from the possible detection of B-mode polarization of the cosmic microwave background are studied. We compute the spectra
of the gravitational wave background and the signal-to-noise ratios by two interferometric detectors (DECIGO and BBO) for large-field inflationary models in which the tensor-to-scalar ratio is greater than the order of 0.01. If the reheating temperature $T_{rm RH}$ of chaotic inflation with the quadratic potential is high ($T_{rm RH}>7.9times10^6$ GeV for upgraded DECIGO and $T_{rm RH}> 1.8times 10^{6}$ GeV for BBO), it will be possible to reach the sensitivity of the gravitational background in future experiments at $3sigma$ confidence level. The direct detection is also possible for natural inflation with the potential $V(phi)=Lambda^4 [1-cos(phi/f)]$, provided that $f>4.2 M_{rm pl}$ (upgraded DECIGO) and $f>3.6 M_{rm pl}$ (BBO) with $T_{rm RH}$ higher than $10^8$ GeV. The quartic potential $V(phi)=lambda phi^4/4$ with a non-minimal coupling $xi$ between the inflaton field $phi$ and the Ricci scalar $R$ gives rise to a detectable level of gravitational waves for $|xi|$ smaller than the order of 0.01, irrespective of the reheating temperature.
We discuss how one can reconstruct the thermal history of the Universe by combining cosmic microwave background (CMB) measurements and gravitational wave (GW) direct detection experiments. Assuming various expansion eras to take place after the infla
tionary reheating and before Big-Bang Nucleosynthesis (BBN), we show how measurements of the GW spectrum can be used to break the degeneracies associated with CMB data, the latter being sensitive to the total amount of cosmic expansion only. In this context, we argue that the expected constraints from future CMB and GW experiments can probe a scenario in which there exists late-time entropy production in addition to the standard reheating. We show that, for some cases, combining data from future CMB and GW direct detection experiments allows the determination of the reheating temperature, the amount of entropy produced and the temperature at which the standard radiation era started.
We study future observational constraints on cosmic string parameters from various types of next-generation experiments: direct detection of gravitational waves (GWs), pulsar timing array, and the cosmic microwave background (CMB). We consider both G
W burst and stochastic GW background searches by ground- and space-based interferometers as well as GW background detection in pulsar timing experiments. We also consider cosmic string contributions to the CMB temperature and polarization anisotropies. These different types of observations offer independent probes of cosmic strings and may enable us to investigate cosmic string properties if the signature is detected. In this paper, we evaluate the power of future experiments to constrain cosmic string parameters, such as the string tension Gmu, the initial loop size alpha, and the reconnection probability p, by performing Fisher information matrix calculations. We find that combining the information from the different types of observations breaks parameter degeneracies and provides more stringent constraints on the parameters. We also find future space-borne interferometers independently provide a highly precise determination of the parameters.
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.
