arXiv subjects

Yun-Song Piao

Publications and source records attributed to Yun-Song Piao.

At least 55 records · Page 3Linked to original sources

arXiv2024

On primordial universe in anti-de Sitter landscape

How the spacetime evolved non-perturbatively in a landscape with multiple anti-de Sitter (AdS) vacua, which is theoretically well-motivated, has always been a matter of concern. As a step towards this issue, we perform (3+1)D numerical relativity simulations for the inhomogeneous universe in an AdS landscape, and find that large inhomogeneity of scalar field can develop into not only sphere-like bubbles, but also novel tube-like structures. It is observed that the bubble or tube wall (across the potential barrier) likely inflates as a quasi-dS space, while the different regions separated by the walls are in different AdS vacua and will collapse towards singularity.

gr-qc↗

arXiv2024

Bounded islands in dS$_{2}$ multiverse model

The cosmological event horizons are observer-dependent, which might bring a paradox. As an example, in dS$_{2}$ multiverse model there are entanglement islands in crunching regions encoding the information of regions near future infinity of inflating or Minkowski bubbles, however, for two observers in different bubbles, since their island regions overlap, both observers will be able to get access to the information encoded in the overlapping region, indicating a violation of no-cloning theorem. In this paper, we present a different resolution to this paradox. Based on the Petz $\mathrm{R\acute{e}nyi}$ mutual information, we show that besides the quantum extremal surfaces there might be another boundary for the island in corresponding spacetime so that the island regions are bounded by ``division points" rather than extending to the rest of the entire spacetime. We also discuss the implications of our result.

hep-th↗

arXiv2024

Broken blue-tilted inflationary gravitational waves: a joint analysis of NANOGrav 15-year and BICEP/Keck 2018 data

Recently, the pulsar timing array (PTA) collaborations have reported the evidence for a stochastic gravitational wave background (SGWB) at nano-Hertz band. The spectrum of inflationary gravitational wave (IGW) is unknown, which might exhibit different power law at different frequency-bands, thus if the PTA signal is primordial, it will be significant to explore the underlying implications of current PTA and CMB data on IGW. In this paper, we perform a joint Markov Chain Monte Carlo analysis for a broken power-law spectrum of IGW with the NANOGrav 15-year and BICEP/Keck 2018 data. It is found that though the bestfit spectral tilt of IGW at PTA band is $n^\text{PTA}_\text{T} =2.42^{+0.32}_{-0.91}$, at CMB band the bestfit is $n^\text{CMB}_\text{T} =0.55^{+0.37}_{-0.10}$ while a detectable amplitude of $r$ with $n^\text{CMB}_\text{T} \simeq 0$ is still compatible. The implication of our results for inflation is also discussed.

astro-ph.CO↗

arXiv2024

Primordial extreme mass-ratio inspirals

The coalescence of stellar-mass primordial black holes (PBHs) might explain some of the gravitation waves (GWs) events detected by LIGO-Virgo-KAGRA. On the other hand, observational hints for supermassive PBHs (SMPBHs) have been accumulated. Thus it can be expected that stellar-mass PBHs might be gravitationally bounded to SMPBHs ($\sim10^{6}-10^9M_\odot$) in the early Universe, and both constituted primordial extreme mass-ratio inspirals (EMRIs). In this work, we initiate the study of the merger rate for primordial EMRIs. The corresponding intrinsic EMRI rate at low redshift may be comparable to that of astrophysical model, $10-10^4$yr$^{-1}$, which the space-based detector LISA has the capability to detect, but significantly raises with redshift. Though equal mass binaries also inevitably form, we find that under certain conditions the primordial EMRIs can be the most prevalent GW sources, and thus potentially a new probe to PBH.

astro-ph.CO↗

arXiv2024

Impact of the Hubble tension on the $r$-$n_s$ contour

The injection of early dark energy (EDE) before the recombination, a possible resolution of the Hubble tension, will not only shift the scalar spectral index $n_s$ towards $n_s=1$, but also be likely to tighten the current upper limit on tensor-to-scalar ratio $r$. In this work, with the latest CMB datasets (Planck PR4, ACT, SPT and BICEP/Keck), as well as BAO and SN, we confirm this result, and discuss its implication on inflation. We also show that if we happen to live with EDE, how the different inflation models currently allowed would be distinguished by planned CMB observations, such as CMB-S4 and LiteBIRD.

astro-ph.CO↗

arXiv2024

Implications of GWTC-3 on primordial black holes from vacuum bubbles

The population of black holes inferred from the detection of gravitational waves by the LIGO-Virgo-KAGRA Collaboration has revealed interesting features in the properties of black holes in the Universe. We analyze the GWTC-3 dataset assuming the detected black holes in each event had an either astrophysical or primordial origin. In particular, we consider astrophysical black holes described by the fiducial \textsc{Power Law + Peak} distribution and primordial black holes whose mass function obeys a broken power law. These primordial black holes can be generated by vacuum bubbles that nucleate during inflation. We find that astrophysical black holes dominate the events with mass less than $\sim 30M_\odot$, whereas primordial black holes are responsible for the massive end, and also for the peak at $\sim 30M_\odot$ in the mass distribution. More than half of the observed events could come from primordial black hole mergers. We also discuss the implications on the primordial black hole formation mechanism and the underlying inflationary model.

astro-ph.CO↗

arXiv2024

Testing the $n_s-H_0$ scaling relation with Planck-independent CMB data

In early dark energy (EDE) resolution of Hubble tension, the spectral index $n_s$ of primordial scalar perturbation follows a scaling relation ${δn_s}\simeq 0.4\frac{δH_0}{H_0}$, where $H_0$ is the Hubble constant. However, this $n_s-H_0$ relation was obtained based on the datasets including Planck cosmic microwave background (CMB) data. In this paper, we investigate this scaling relation with Planck-independent CMB data, i.e. ACT and SPT-3G combined with WMAP(+BAO+Pantheon), respectively. Our results show that the WMAP+SPT-3G dataset also follows this scaling relation, while the WMAP+ACT dataset seems to favor smaller $n_s$, which is related to the fact that the critical redshift $z_c$, at which EDE is excited, favored by the WMAP+ACT dataset is lower and closer to the recombination time.

astro-ph.CO↗

arXiv2024

Trapped early dark energy

As a prospective resolution of the Hubble tension, early dark energy (EDE) suffers from the coincidence problem, why EDE is active just at matter-radiation equality (equivalently why the slope of EDE potential is required to approximately equal to the Hubble parameter at that time). In this paper we present a dark-matter-trapped EDE mechanism, by which the bound on the slope of EDE potential can be relieved. We show how this mechanism can work, and discuss the possibility that after inflation ended EDE can settle down at the initial conditions required by observations.

gr-qc↗

arXiv2023

Towards supermassive primordial black holes from inflationary bubbles

The bubbles that nucleated during slow-roll inflation can be supercritical, i.e. their radii are larger than the Hubble horizon of de Sitter spacetime inside the bubble (an inflating baby universe inside it), and thus naturally develop to the supermassive primordial black holes (SMPBHs) with a multi-peaks mass function. In this paper, we further investigate relevant phenomenology. After slow-roll inflation ended, the bubbles may be not only supercritical, but also subcritical. It is showed that it seems unlikely for the subcritical bubbles to collapse to SMPBHs. Theoretically, however, before they collapsed such bubbles might have a probability of up-tunnelling to the supercritical ones and thus contribute to SMPBHs. We present a mechanism for the origin of initial clustering of SMPBHs, which can significantly magnify the merger rate of SMPBH binaries, and show the possibility that the merging of such SMPBH binaries explains recent NANOGrav signal.

astro-ph.CO↗

arXiv2023

Merger rate of supermassive primordial black hole binaries

The probability that the primordial black hole (PBH) binaries formed in the early Universe can be affected by the Hubble expansion of background, which is non-negligible when the number density of PBHs is very low (it is actually this case for supermassive PBHs). In this paper, taking into account the effect of cosmic expansion on the comoving distance of PBH pairs, we worked out the merger rate of PBHs with any extended mass function. The torques by all PBHs and linear density perturbations are also considered. It is found that the merger rate of PBH, $M\gtrsim 10^6M_\odot$, binaries is significantly lower for $f_\text{pbh}\lesssim 0.01$ than expected.

astro-ph.CO↗

arXiv2023

Return of Harrison-Zeldovich spectrum in light of recent cosmological tensions

The spectral index $n_s$ of scalar perturbation is the significant initial condition set by inflation theory for our observable Universe. According to Planck results, current constraint is $n_s = 0.965\pm 0.004$, while an exact scale-invaiant Harrison-Zeldovich spectrum, i.e. $n_s=1$, has been ruled out at $8.4σ$ significance level. However, it is well-known that the standard $Λ$CDM model is suffering from the Hubble tension, which is at $\sim 5σ$ significance level. This inconsistency likely indicates that the comoving sound horizon at last scattering surface is actually lower than expected, which so seems to be calling for the return of $n_s=1$. Here, in light of recent observations we find strong evidence for a $n_s=1$ Universe. And we show that if so, it would be confirmed conclusively by CMB-S4 experiment.

astro-ph.CO↗

arXiv2023

Does the fraction of dark matter diminish with early dark energy?

In pre-recombination early dark energy (EDE) resolutions of the Hubble tension, the rise of Hubble constant value $H_0$ is usually accompanied with the exacerbation of so-called $S_8$ tension. Inspired by the swampland conjecture, we investigate what if a fraction $f_*$ of dark matter is coupled to EDE, $m_{cdm}\sim \exp{(-c{|Δϕ_{ede}|\over M_{pl}})}$ with $c\sim {\cal O}(1)$. We perform the MCMC analysis for the relevant EDE models with PlanckCMB, BAO, Pantheon and SH0ES dataset, as well as DES-Y1 data, and find that such a fraction helps to alleviate the $S_8$ tension. However, though $c\gtrsim 0.1$ is allowed for a very small $f_*$, which suggests that a small fraction of dark matter has ever faded with EDE, $c\sim0$ is also consistent.

astro-ph.CO↗

arXiv2023

Shape of CMB lensing in the early dark energy cosmology

Recently, the cosmological tensions, $H_0$ and $S_8$ in particular, have inspired modification of both pre- and postrecombination physics simultaneously. Early dark energy is a promising pre-recombination solution of the $H_0$ tension, known to be compatible with the cosmic microwave background (CMB). However, the compatibility of early dark energy, as well as general early resolutions, with the CMB is no longer obvious if the late Universe is also modified. Aside from cosmological parameters, the main channel through which late Universe physics affects CMB observables is gravitational lensing. We employed a new method of sampling functions using the Gaussian Process in the Monte Carlo Markov Chain analysis to constrain the shape of the CMB lensing potential. We obtained the early Universe (CMB) only constraints on the full shape of the CMB lensing potential, with the late-time Universe being marginalized over. It is found that CMB data prefers a lensing potential shape that is $Λ$CDM-like at $80\lesssim L\lesssim400$ but with enhanced amplitude beyond this range. The obtained shape constraints can serve as a CMB-compatibility guideline for both late and early Universe model building that modifies the lensing potential.

astro-ph.CO↗

arXiv2023

Supermassive primordial black holes in multiverse: for nano-Hertz gravitational wave and high-redshift JWST galaxies

Recently, observational hints for supermassive black holes have been accumulating, which has inspired ones to wonder: Can primordial black holes (PBHs) be supermassive, in particular with the mass $M\gtrsim 10^{9}M_\odot$? A supercritical bubble (with an inflating baby universe inside it) that nucleated during inflation can develop into a PBH in our observable Universe. Here, we find that when the inflaton slowly passes by a neighboring vacuum, the nucleating rate of supercritical bubbles would inevitably attain a peak, so the mass distribution of multiverse PBHs, and the mass of peak can be up to $M\gtrsim 10^{11}M_\odot$. Thus our mechanism naturally provides a primordial origin of supermassive BHs.

gr-qc↗

arXiv2023

Implication of island for inflation and primordial perturbations

It is usually thought that the efolds number of inflation must be bounded by its de Sitter entropy, otherwise we will have an information paradox. However, in light of the island rule for computing the entanglement entropy, we show that such a bound might be nonexistent, while the information flux of primordial perturbation modes the observer after inflation is able to detect follows a Page curve. In corresponding eternally inflating spacetime, it seems that our slow-roll inflation patch is accompanied with a neighbourly collapsed patch (eventually developing into a black hole) so that its Hawking radiation might be just our primordial perturbations. Accordingly, the perturbation spectrum we observed will present a ``Page-like" suppression at large scale.

hep-th↗

arXiv2023

Probing higher-spin particles with gravitational waves from compact binary inspirals

Under the framework of gravitational effective field theory, we propose a theory agnostic strategy of searching for higher-spin particles with gravitational waves from compact binary inspirals. Using this strategy, we analyze gravitational wave signals from the binary black hole merger events GW151226 and GW170608, as well as the binary neutron star merger event GW170817. We find that the existence of higher-spin particles with mass ranged from $10^{-12} {\rm eV}$ to $10^{-11} {\rm eV}$ is strongly disfavored by these events unless the particles precisely combine within a supersymmetric supermultiplet. We argue that the gravitational effective field theory also provides a framework to search for signals beyond GR from other GW observations, such as extreme-mass-ratio-inspirals.

gr-qc↗

arXiv2023

Alleviating both $H_0$ and $S_8$ tensions: early dark energy lifts the CMB-lockdown on ultralight axion

The existence of ultralight axion (ULA) with mass $\mathcal{O}( 10^{-26}\text{eV})$ is not favored by the CMB observations in the standard $Λ$CDM model. We show that the inclusion of early dark energy (EDE) will lift the CMB-lockdown on such ULA, and possibly other forms of dark matter beyond cold dark matter. By performing Monte Carlo Markov Chain analysis, it is found that, as opposed to $Λ$CDM, the AdS-EDE cosmology (with an Anti-de Sitter phase around recombination) now allows the existence of axion with mass $10^{-26}$ eV and predicts $6\%$ of the matter in our Universe to be such ULA, which can also help alleviating the $S_8$ tension in EDE.

astro-ph.CO↗

arXiv2022

Testing dark energy after pre-recombination early dark energy

In the studies on pre-recombination early dark energy (EDE), the evolution of Universe after recombination is usually regarded as $ΛCDM$-like, which corresponds that the equation of state of dark energy responsible for current accelerated expansion is $w=-1$. However, in realistic models, $w$ might be evolving. We consider the parametrizations of $w$ with respect to the redshift $z$ in Axion-like EDE and AdS-EDE models, respectively. We performed the Monte Carlo Markov chain analysis with recent cosmological data, and found that the bestfit $w(z)$ is compatible with $w_0=-1,w_a=0$ (the cosmological constant) and the evolution of $w$ is only marginally favored, which so has little effect on lifting the bestfit value of ${H_0}$.

astro-ph.CO↗