Models of the Primordial Standard Clock

Abstract

Oscillating massive fields in the primordial universe can be used as Standard Clocks. The ticks of these oscillations induce features in the density perturbations, which directly record the time evolution of the scale factor of the primordial universe, thus if detected, provide a direct evidence for the inflation scenario or the alternatives. In this paper, we construct a full inflationary model of primordial Standard Clock and study its predictions on the density perturbations. This model provides a full realization of several key features proposed previously. We compare the theoretical predictions from inflation and alternative scenarios with the Planck 2013 temperature data on Cosmic Microwave Background (CMB), and identify a statistically marginal but interesting candidate. We discuss how future CMB temperature and polarization data, non-Gaussianity analysis and Large Scale Structure data may be used to further test or constrain the Standard Clock signals. IntroductionObservationally distinguishing the inflation scenario from other possible alternative scenarios, as the origin of the Big Bang, remains an outstanding challenge for modern astrophysics and cosmology. The simplest inflation models [1-10] have received strong support from observational results on Cosmic Microwave Background (CMB) and Large Scale Structures (LSS) [11][12][13]. In the meanwhile, there are also various attempts to construct alternative scenarios to explain the same observational results, see [14][15][16][17] for recent discussions. One of the reasons that such attempts are still possible is that there are only two parameters in the Standard Model of cosmology that are related to the primordial scenario, namely the amplitude and spectral index of the density perturbations. Therefore, an important research activity is to explore signatures beyond the Standard Model of cosmology, both in theories and most importantly in experiments.While all observational signatures beyond the Standard Model are extremely valuable, not many of them can be used to model-independently distinguish inflation from the alternatives. Many signatures, if discovered, provide information that are helpful to distinguish different models within one scenario.So far, there are two kinds of observational signals that are known to be capable of model-independently distinguishing the inflation scenario from the alternatives.The first and well-known one is the primordial gravitational wave [18][19][20], namely the tensor mode, which may be detected in terms of the B-mode polarization in the CMB [21][22][23]. The tensor mode records the magnitude of the Hubble rate during the primordial epoch, therefore if detected distinguishes scenarios with fast-evolving scale factors, such as inflation, from scenarios with slowly-evolving scale factors, such as Ekpyrosis [24]. However, phenomenologically, it is difficult to use the tensor mode to model-independently distinguish scenarios that all have fast-evolving scale factors and generate scale-invariant tensor modes. An example is...

Planck2015 results

“…On the other hand, sharp features or corners in an inflationary potential can temporarily drive the inflaton away from slow-roll; these large changes in the field and derivatives can create evenly-spaced oscillations, to be discussed in the next subsection. However, in multifield models residual oscillations after corner-turning can also lead to logspaced oscillations, just as in the resonance models (Chen 2011;Achúcarro et al 2011;Battefeld et al 2013;Chen et al 2015). A preliminary search for bispectrum resonance signals was performed in the first Planck analysis (Planck Collaboration XXIV 2014) and our purpose here is to substantially increase the frequency range and number of models investigated.…”

Preheating after Higgs inflation: Self-resonance and gauge boson production

“…In this case weakly coupled gauge sectors might leave observational imprints through oscillations of the background during inflation. A search for "primordial clocks" [86,87] in these models is beyond the scope of the present work, because they do not arise in SM Higgs inflation, but could provide a useful tool for exploring gauge field phenomena in broader classes of nonminimally coupled inflation.…”

Lensing anomaly and oscillations in the primordial power spectrum

“…Here we explore inflationary models that might give rise to oscillatory features in the primordial power spectrum that might account for the A L anomaly. Oscillatory features generated during inflation usually have an oscillation frequency which has a logarithmic or linear dependence on k. On one hand, the logarithmic dependence could be either because there is an oscillating modulation in the Lagrangian that depends linearly on the inflaton which is slowly rolling [11,12] or because an extra massive field is oscillating around its minimum in which case it oscillates with a constant frequency and linearly in the cosmic time [13][14][15]. Also, successive turns in the multi-field inflationary trajectory yield a logarithmic dependence in k [16].…”