arXiv subjects

Yun-Song Piao

Publications and source records attributed to Yun-Song Piao.

At least 73 records · Page 4Linked to original sources

arXiv2022

Towards hybrid inflation with $n_s=1$ in light of Hubble tension and primordial gravitational waves

Recently, it has been found that complete resolution of the Hubble tension might point to a scale-invariant Harrison-Zeldovich spectrum of primordial scalar perturbation, i.e. $n_s=1$ for $H_0\sim 73$km/s/Mpc. We show that for well-known slow-roll models, if inflation ends by a waterfall instability with respect to another field in the field space while inflaton is still at a deep slow-roll region, $n_s$ can be lifted to $n_s= 1$. A surprise of our result is that with pre-recombination early dark energy, chaotic $ϕ^2$ inflation, ruled out by Planck+BICEP/Keck in standard $Λ$CDM, can be revived, which is now well within testable region of upcoming cosmic microwave background B-mode experiments.

astro-ph.CO↗

arXiv2022

Towards early dark energy and $n_s$=1 with Planck, ACT and SPT

We investigate the constraints on early dark energy (EDE) by combining the most recent CMB observations available, ACT DR4, SPT-3G, and Planck2018 ($\ell_\text{TT,max}=1000$) data. This combined CMB dataset favors non-zero EDE fractions and large Hubble constants. The inclusion of BAO+Pantheon data has little effect on the results, leads to $H_0=71.6( 72.9)_{-1.5}^{+2.0}$ and $73.17(72.74)^{+0.55}_{-0.77}$ km/s/Mpc for axion-like EDE and AdS-EDE, respectively. The axion-like EDE can fit the data significantly better ($Δχ^2 \lesssim -10$) than $Λ$CDM, which is mainly driven by the ACT data. It is found again that if the current $H_0$ measured locally is correct, complete resolution of the Hubble tension seems to be pointing towards a scale invariant Harrison-Zeldovich spectrum of primordial scalar perturbation, i.e. $n_s=1$ for $H_0\sim 73$ km/s/Mpc.

astro-ph.CO↗

arXiv2022

Improved constraint on primordial gravitational waves in light of the Hubble tension and BICEP/Keck

The Hubble tension that the standard $Λ$CDM model is suffering from can be resolved with pre-recombination early dark energy. We present the first constraint on the tensor-to-scalar ratio $r$ in corresponding Hubble-tension-free cosmologies using the most recent BICEP/Keck cosmic microwave background (CMB) B-mode polarization data. We find, combining BICEP/Keck with Planck18 CMB and baryon acoustic oscillation data, that the models with larger Hubble constant $H_0$ will have tighter upper bound on $r$, and resolution $H_0\sim73$ km/s/Mpc of the Hubble tension tightens the upper bound to $r<0.028\ (95\%\text{C.L.})$, $25\%$ tighter than the $Λ$CDM constraint $r<0.036$. We clarify the origin of this tightening bound.

astro-ph.CO↗

arXiv2022

Populating the landscape in an inhomogeneous universe

The primordial Universe might be highly inhomogeneous. We perform the 3+1D Numerical Relativity simulation for the evolution of scalar field in an initial inhomogeneous expanding Universe, and investigate how it populates the landscape with both de Sitter (dS) and AdS vacua. The simulation results show that eventually either the field in different region separates into different vacua, so that the expanding dS or AdS bubbles (the bubble wall is expanding but the spacetime inside AdS bubbles is contracting) come into being with clear bounderies, or overall region is dS expanding with a few smaller AdS bubbles (which collapsed into black holes) or inhomogeneously collapsing.

gr-qc↗

arXiv2021

Testing AdS early dark energy with Planck, SPTpol and LSS data

The Hubble tension might be resolved by injecting a new energy component, called Early Dark Energy (EDE), prior to recombination. An Anti-de Sitter (AdS) phase around recombination can make the injected energy decay faster, which thus allows a higher EDE fraction (so larger $H_0$) while prevents degrading the CMB fit. In this work, we test the AdS-EDE model with CMB and Large-Scale Structure (LSS) data. Our CMB dataset consists of low-$\ell$ part of Planck TT spectrum and SPTpol polarization and lensing measurements, since this dataset predicts the CMB lensing effect consistent with $Λ$CDM expectation. Combining it with BAO and Pantheon data, we find the bestfit values $H_0=71.92$ km/s/Mpc and $H_0=73.29$ km/s/Mpc without and with the SH0ES prior, respectively. Including cosmic shear and galaxy clusters data, we have $H_0=71.87$ km/s/Mpc and $S_8=0.785$, i.e. only $1.3σ$ discrepancy with direct $S_8$ measurement.

astro-ph.CO↗

arXiv2021

Implication of the Hubble tension for the primordial Universe in light of recent cosmological data

In prerecombination resolutions of the Hubble tension, such as early dark energy, new physics before recombination shifts the values of relevant cosmological parameters so that the models can fit with cosmic microwave background and baryon acoustic oscillations observations as well as $Λ$CDM does. In this paper, we clarify how the parameter shifts are related with $δH_0$, particularly we find the shift of primordial scalar spectral index scales as ${δn_s}\simeq 0.4{δH_0\over H_0}$ by performing the Monte Carlo Markov chain analysis with the Planck2018+BAO+Pantheon+R19+Keck Array/BICEP dataset. A novel point of our result is that if the current $H_0$ measured locally is correct, complete resolution of the Hubble tension seems to be pointing to a scale invariant Harrison-Zeldovich spectrum, i.e. $n_s= 1$ for $H_0\sim 73$km/s/Mpc.

astro-ph.CO↗

arXiv2021

Intermittent null energy condition violations during inflation and primordial gravitational waves

Primordial null energy condition (NEC) violation would imprint a blue-tilted spectrum on gravitational wave background (GWB). However, its implications on the GWB might be far richer than expected. We present a scenario, in which after a slow-roll (NEC-preserving) inflation with Hubble parameter $H\simeq H_{inf1}$, the Universe goes through an NEC-violating period and then enters subsequent slow-roll inflation with a higher $H$ ($=H_{inf2}\gg H_{inf1}$). The resulting primordial gravitational wave spectrum is nearly flat at the cosmic microwave background band, as well as at the frequency $f\sim 1/{\rm yr}$ but with higher amplitude (compatible with the recent NANOGrav result). It is also highlighted that for the multi-stage inflation if the NEC violations happened intermittently, we might have a Great Wall-like spectrum of the stochastic GWB at the corresponding frequency band.

gr-qc↗

arXiv2021

Is the NANOGrav signal a hint of dS decay during inflation?

As suggested by the swampland conjectures, de Sitter (dS) space might be highly unstable if it exists at all. During inflation, the short-lived dS states will decay through a cascade of the first-order phase transition (PT). We find that the gravitational waves (GWs) yielded by such a PT will be "reddened" by subsequent dS expansion, which may result in a slightly red-tilt stochastic GWs background at low-frequency band, compatible with the NANOGrav 12.5-yr result.

astro-ph.CO↗

arXiv2020

$T_0$ censorship of early dark energy and AdS vacua

Present-day temperature $T_0$ of cosmic microwave background has been precisely measured by the FIRAS experiment. We identify that the early dark energy (EDE) (non-negligible around matter-radiation equality) scenario can remain compatible with the FIRAS result, while lifting the Hubble constant $H_0$. We perform Monte Carlo Markov chain analysis to confirm our observations. We also present an $α$-attractor Anti-de Sitter (AdS) model of EDE, in which the AdS depth is consistently varied in the Monte Carlo Markov chain analysis. We found that our datasets weakly hinted the existence of an AdS phase near recombination with $H_0\sim 73$km/s/Mpc at 1$σ$ region in the best-fit model.

astro-ph.CO↗

arXiv2020

Searching for gravitational wave echoes in GWTC-1 and O3 events

Gravitational wave (GW) echoes, if they exist, would be a probe to the near-horizon physics of black hole. In this brief report, we performed the Monte Carlo Markov Chain analysis to search for echo signal in all GWTC-1 and O3 GW events. We focus on the Insprial-Merger-Ringdown-Echo (IMRE) waveform, and apply the Bayesian model selection to compare the IMRE result with IMR's (no echo). We find no statistically significant ($<1σ$ combined) evidence for the GW echoes and only individual GW events with the echoes at $1\sim 2σ$ significance.

gr-qc↗

arXiv2020

Pre-inflation and Trans-Planckian Censorship

We investigate the implication of Trans-Planckian Censorship Conjecture (TCC) for the initial state of primordial perturbations. It is possible to set the state of perturbation modes in the infinite past as the Minkowski vacuum, only if the pre-inflationary era is past-complete. We calculate the evolution of the perturbation modes in such a pre-inflationary era and show that at the beginning of inflation the perturbation modes with wavelengths much shorter than the Hubble scale (but still larger than the Planck length scale) will behave as they are in the Bunch-Davis state. Therefore, a past-complete pre-inflationary evolution may automatically prepare the initial state required for the inflationary perturbations at the CMB window while obeying the TCC.

gr-qc↗

arXiv2020

Positivity in the effective field theory of cosmological perturbations

Requiring the existence of a unitary, causal and local UV-completion places a set of positivity bounds on the corresponding effective field theories (EFTs). We discuss the obstructions and possibility in applying the positivity bound to cosmology, in particular the EFT of cosmological perturbations. Taking a $c_T=1$ beyond-Horndeski EFT as an illustrative example, we derive such bounds, which incorporate the cosmological correction of order $H^2/Λ^2$, $Λ$ being the cutoff scale. The derived bounds are applied to slow-roll inflation with beyond Horndeski operators. It is found that the cosmological positivity bounds may be either stronger or weaker than their flat space counterpart.

hep-th↗

arXiv2020

Is the Hubble tension a hint of AdS phase around recombination?

Anti-de Sitter (AdS) vacua, being theoretically important, might have an unexpected impact on the observable universe. We find that in early dark energy (EDE) scenarios the existence of AdS vacua around recombination can effectively lift the CMB-inferred $H_0$ value. As an example, we study a phenomenological EDE model with an AdS phase starting at the redshift $z\sim2000$ and ending shortly after recombination (hereafter the universe will settle down in a $Λ>0$ phase until now), and obtain a best-fit $H_0=72.74$ km/s/Mpc without degrading the CMB fit compared with the standard $Λ$CDM model.

astro-ph.CO↗

arXiv2020

Trans-Planckian censorship of multistage inflation and dark energy

We explore the bound of the trans-Planckian censorship conjecture on an inflation model with multiple stages. We show that if the first inflationary stage is responsible for the primordial perturbations in the cosmic microwave background window, the $e$-folding number of each subsequent stage will be bounded by the energy scale of the first stage. This seems to imply that the lifetime of the current era of accelerated expansion (regarded as one of the multiple inflationary stages) might be a probe for distinguishing inflation from its alternatives. We also present a multistage inflation model in a landscape consisting of anti-de Sitter vacua separated by potential barriers.

gr-qc↗

arXiv2019

Time crystals in primordial perturbations

Cosmological time crystal (TC) corresponds to a matter state where the periodic motion of field forms a limit cycle in its phase space. We explore what would happen if it existed in inflationary phase. It is found that the limit cycle responsible for TC will inevitably cause the periodic oscillation of the primordial perturbation spectrum. The oscillatory patterns of the spectrum depend on the TC parameters, and so encode the crystalline patterns of TC.

gr-qc↗

arXiv2019

Bounce in GR and higher-order derivative operators

Recent progress seems to suggest that one must modify General Relativity (GR) to stably violate the null energy condition and avoid the cosmological singularity. However, with the higher-order derivative operators of scalar field (a subclass of the degenerate higher-order scalar-tensor theory), we show that at energies well below the Planck scale, fully stable nonsingular cosmologies can actually be implemented within GR.

gr-qc↗

arXiv2019

Mixing of gravitational wave echoes

Gravitational wave (GW) echoes, if they exist, would be a probe to the near-horizon quantum structure of black hole (BH), which has motivated the searching for the echo signals in GW data. We point out that the echo phenomenology related with the potential structure might be not so simple as expected. In particular, if the near-horizon regime of BH is modelled as a multiple-barriers filter, the late-time GW ringdown waveform will exhibit the mixing of echoes, even the superpositions. As a result, the amplitudes of successive echoes might not drop sequentially.

gr-qc↗

arXiv2019

On echo intervals in gravitational wave echo analysis

Gravitational wave echoes, if they exist, could encode important information of new physics from the strong gravity regime. Current echo searches usually assume constant interval echoes (CIEs) a priori, although unequal interval echoes (UIEs) are also possible. Despite of its simplicity, the using of CIE templates need to be properly justified, especially given the high sensitivity of future gravitational wave detectors. In this paper, we assess the necessity of UIE templates in echo searches. By reconstructing injected UIE signals with both CIE and UIE templates, we show that the CIE template may significantly misinterpret the echo signals if the variation of the interval is greater than the statistical errors of the interval, which is further confirmed by a Bayesian analysis on model stelection. We also forecast the constraints on the echo intervals given by future GW detectors such as Advanced LIGO and Einstein Telescope.

gr-qc↗