Standard Clock in primordial density perturbations and cosmic microwave background

Abstract

Standard Clocks in the primordial epoch leave a special type of features in the primordial perturbations, which can be used to directly measure the scale factor of the primordial universe as a function of time a(t), thus discriminating between inflation and alternatives. We have started to search for such signals in the Planck 2013 data using the key predictions of the Standard Clock. In this Letter, we summarize the key predictions of the Standard Clock and present an interesting candidate example in Planck 2013 data. Motivated by this candidate, we construct and compute full Standard Clock models and use the more complete prediction to make more extensive comparison with data. Although this candidate is not yet statistically significant, we use it to illustrate how Standard Clocks appear in Cosmic Microwave Background (CMB) and how they can be further tested by future data. We also use it to motivate more detailed theoretical model building.

Neutrino signatures in primordial non-gaussianities

“…There is yet another line of reasoning that motivates the study of neutrinos in inflation. As was pointed out in [35][36][37][38][39][40][41][42], the nonanalytical oscillatory behavior in the squeezed bispectrum generated by heavy fields also directly records the evolution of the scale factor a(t) as a function of time in the primordial era. Therefore it is potentially a useful discriminator of different scenarios of primordial universe, which can be either inflationary or not.…”

Searching for primordial localized features with CMB and LSS spectra

“…This motivates a search for such kind of features produced by inflationary scenarios beyond canonical single field. 1 Several mechanisms that produce oscillatory features have been realized, such as a transient reduction in the speed of sound [4][5][6], step inflaton potential , different initial vacuum states [28][29][30][31], multifield dynamics [32][33][34][35][36][37], or phenomenological superimposed oscillations in the primordial power spectrum [38][39][40][41][42][43][44][45][46]. In this work we focus on searching for oscillatory features in the scenario of a transient reduction in the speed of sound.…”

Unique Fingerprints of Alternatives to Inflation in the Primordial Power Spectrum

“…These fields oscillate like harmonic oscillators. It has been shown that these harmonic oscillations leave their imprints in the density perturbations and directly record the background a(t) [10][11][12][13]. We call these massive fields the primordial standard clocks.…”

“…But, can we find any observable that directly measures a(t)? It has been shown that such observables indeed exist in principle [10][11][12][13].…”

“…The oscillations may be excited classically or quantum mechanically. In the former case [10,11], an extra ingredient, namely some sharp feature, is required. On the other hand, quantum excitation of standard clocks is universal, and it has been shown that the clock signals exist in all inflationary models, and possibly in many models of alternative scenarios too [13].…”