Relic gravitational waves and their detection

Abstract

As strong evidence for inflation, the relic gravitational waves (RGW) have been extensively studied. Although, it has not been detected, yet some constraints have been achieved by the observations. Future experiments for the RGW detection are mainly two kinds: the CMB experiments and the laser interferometers. In this paper, we study these current constraints and the detective abilities of future experiments. We calculate the strength of RGW Ω g (k) in two methods: the analytic method and the numerical method by solving the inflationary flow equations. By the first method we obtain a bound Ω g < 3.89 × 10 −16 at ν = 0.1Hz, where we have used the current constraints on the scalar spectral index, the tensor-scalar ratio, furthermore, we have taken into account of the redshift-suppression effect, the accelerating expansion effect, the neutrino damping effect on the RGW. But the analytic expression of Ω g (k) depends on the specific inflationary models and only applies for the waves with very low frequencies. The numerical method is more precise for the high frequency waves and applies to any single-field inflationary model. It gives a bound Ω g < 8.62 × 10 −14 , which is independent of the inflationary parameters, and applies to any single-field slow-roll inflationary model. After considering the current constraints on the inflationary parameters, this bound reduces down to Ω g < 2 × 10 −17 . These two methods give the consistent conclusions: The current constraints on the RGW from LIGO, big bang nucleosynthesis, and pulsar timing are too loose to give any stringent constraint for the single-field inflationary models, and the constraint from WMAP are relatively tighter. The future laser interferometers are more effective for detecting the RGW with the smaller tensor-scalar ratio, but the CMB experiments are more effective for detecting the waves with the larger ratio. These detection methods are complementary to each other for the detections of RGW.

Probing the reheating temperature of the universe with a gravitational wave background

“…Another damping factor comes from the fact that the relativistic degrees of freedom g * does not remain constant and expansion rate is modified from simple power law a(t) ∝ T −1 in the early universe. This effect gives the damping factor [28,29] …”

“…Since the expansion rate depends on the equation of state of the universe, corresponding thermal history of the universe is imprinted in the gravitational wave spectrum at present. Although much work has been done which treats the spectrum and detection possibility of stochastic gravitational wave background of inflationary origin in the literature [12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30], the previous work did not take into account the modification of the gravitational wave spectrum around the reheating epoch and assumed nearly flat spectrum above the observationally interesting frequency ∼ 1Hz, except for a few works where it was claimed that the equation of state of the early universe can be probed by looking at the spectrum of the gravitational wave background [21] (this kind of study was also done and extended in [24]). Recently, we have pointed out that taking into account the reheating effect on the gravitational wave spectrum is essential both for the detection itself and the purpose of probing thermal history of the universe between BBN and inflation, with an emphasis on its impacts on particle physics [31].…”

Second-order cosmological perturbations. I. Produced by scalar-scalar coupling in synchronous gauge

“…[51] to general initial conditions at τ 0 . For a realistic cosmological model, the initial metric perturbations at the radiationmatter equality are important and determine the spectra of CMB abiotrophies and polarization [16]. S of (54) into Eq.…”

“…In the past the perturbations, both scalar and tensorial, have been extensively explored to linear order [1][2][3][4][5][6], which have been used in calculations of large scale structure [7], cosmic microwave background radiation (CMB) [8][9][10][11][12][13][14][15][16][17][18] and relic gravitational wave (RGW) [19][20][21][22][23][24][25][26][27][28][29]. In the era of precision cosmology, it is necessary to study the 2nd-order perturbation beyond the linear perturbation, to include nonlinear effects of gravity on CMB anisotropies and polarization [30,31], the non-Gaussianality of primordial perturbation [32], and on relic gravitational waves [33,34] etc.…”

Study of the near-thresholdωϕmass enhancement in doubly OZI-suppressedJ/ψ→γωϕdecays

“…The mass and width were determined to be M = γωφ decay mode is a doubly OZI suppressed process with a production rate that is expected to be suppressed relative to J/ψ → γωω or J/ψ → γφφ by at least one order of magnitude [2]. Possible interpretations of the ωφ threshold enhancement include a new type of resonance, such as a tetraquark state (with structure q 2 q 2 ) [3], a hybrid [4], or a glueball state [5] etc., a dynamical effect arising from intermediate meson rescattering [6], a manifestation of the f 0 (1710) below threshold [7], or a threshold cusp of an attracting resonance [8]. As of now none of these interpretations has either been established or ruled out by experiment.…”