arXiv subjects

Yun-Song Piao

Publications and source records attributed to Yun-Song Piao.

At least 37 records · Page 2Linked to original sources

arXiv2025

Can the universe experience an AdS landscape since matter-radiation equality?

Though an anti-de Sitter (AdS) vacuum, corresponding to a negative cosmological constant (NCC), can be not responsible for the acceleration of current universe, it might coexist with one evolving positive dark energy component at low redshift, as well as with early dark energy around the recombination to solve the Hubble tension. In this paper, we investigate the scenario with one AdS vacuum around the recombination and one at low redshift, and from both current observational and theoretical perspectives preliminarily explore the possibility that the universe experienced a landscape with multiple AdS vacua since matter-radiation equality.

gr-qc↗

arXiv2025

Can the sound horizon-free measurement of $H_0$ constrain early new physics?

The sound horizon-independent $H_0$ extracted by using galaxy clustering surveys data through, e.g., EFTofLSS or ShapeFit analyses, is considered to have the potential to constrain the early new physics responsible for solving the Hubble tension. Recent observations, e.g. DESI, have shown that the sound horizon-independent measurement of $H_0$ is consistent with $Λ$CDM. In this work, we clarify some potential misuses and misinterpretations in these analyses. On the one hand, imposing some prior from other cosmological probes is often used to strengthen the constraints on the results, however, these priors are usually derived using the assumption of $Λ$CDM, it is not suitable to apply these so-called $Λ$CDM priors (e.g., the $n_s$ prior from CMB), which would bias the results, to early new physics because these early new physics are usually accompanied by shifts of the $Λ$CDM parameters. On the other hand, the constraints on $H_0$ in the sound horizon-independent EFTofLSS analysis arise from not only the shape of the power spectrum ($k_\text{eq}$-based $H_0$), but also the overall amplitude (when combined with CMB lensing observations) and the relative amplitudes of the BAO wiggles, thus besides $k_\text{eq}$ other information may also play a role in constraining $H_0$. We also make forecasts for an Euclid-like survey, which suggest that ongoing observations will also have difficulty constraining early new physics.

astro-ph.CO↗

arXiv2025

Dark energy and lensing anomaly in Planck CMB data

In this paper, we investigate the impact of the lensing anomaly in Planck cosmic microwave background (CMB) data on the nature of dark energy (DE). We constrain the state equation ($w_0,w_a$) of DE with the lensing scaling parameter $A_L=1$ and varying $A_L$, using the Planck PR3 and two updated Planck PR4 likelihoods, CamSpec and HiLLiPoP respectively, combined with DESI baryon acoustic oscillation (BAO) and Pantheon+ supernova data. As expected, when $A_L$ is allowed to vary, the evolving DE is not preferred due to the degeneracy between $w_0,w_a$ and $A_L$. In particular, we also consider replacing DESI BAO data with pre-DESI BAO in our analysis, and observe that DESI BAO appears to exacerbate the lensing anomaly, which is caused by the smaller matter density $Ω_m$ it prefers, however, this effect can be offset by the shifts in $w_0$ and $w_a$ preferring the evolving DE. Our work indicates that the lensing anomaly in Planck data is worth carefully reconsidering when new cosmological survey data is combined with CMB.

astro-ph.CO↗

arXiv2025

Phase transition in a doubly holographic model of closed $\mathbf{dS_{2} }$ spacetime

Double holography has been proved to be a powerful method in comprehending the spacetime entanglement. In this paper we investigate the doubly holographic construction in ${\mathrm{dS_{2}} }$ spacetime. We find that in this model there exists a new extremal surface besides the Hartman-Maldacena surface and the island surface, which could lead to a more complex phase structure. We then propose a generalized mutual entropy to interpret the phase transition. However, this extremal surface has a subtle property that the length of a part of the geodesic is negative when this saddle is dominant. This is because the negative part of the geodesic is within the horizon of the bulk geometry. We move to the ${\mathrm{AdS_{2}} }$ spacetime and find this subtlety still exists. We purpose a simple solution to this issue.

hep-th↗

arXiv2025

Search for the non-linearities of gravitational wave background in NANOGrav 15-year data set

The recently reported signal of common red noise between pulsars by several pulsar timing array collaborations has been thought as evidence of the stochastic gravitational wave background (SGWB) due to the Helling-Downs correlation. In this Letter, we search for the non-Gaussianity of SGWB through its non-linear effect on the overlap reduction function in NANOGrav 15-year data set. In particular, we focus on a folded component to SGWB whose amplitude is quantified with a single parameter $α$ in the unpolarized case. The resulting Bayes factor of $1.68 \pm 0.01$ ($1.78 \pm 0.01$ in the case of signals from SMBHBs) indicates that there is no evidence of such a non-Gaussianity of SGWB in the NANOGrav 15-year data yet. If it is detected in future PTA experiments, it will impact our understanding on the origin of the detected SGWB.

gr-qc↗

arXiv2025

Broad primordial power spectrum and $μ$-distortion constraints on primordial black holes

Supermassive black holes (SMBHs) might originate from supermassive primordial black holes (PBHs). However, the hypothesis that these PBHs formed through the enhancement of the primordial curvature perturbations has consistently faced significant challenges due to the stringent constraints imposed by $μ$-distortion in the cosmic microwave background (CMB). In this work, we investigate the impact of non-Gaussianity on $μ$-distortion constraint in the context of broad power spectra. Our results show that, under the assumption of non-Gaussian curvature perturbations, a broad power spectrum may lead to weaker $μ$-distortion constraints compared to the Gaussian cases. Our findings highlight the potential of the broad power spectrum to alleviate the $μ$-distortion constraints on supermassive PBHs under large non-Gaussianity.

astro-ph.CO↗

arXiv2024

Prepare inflationary universe via the Euclidean charged wormhole

In this paper, we present a wavefunction of the universe, which correspond to an Euclidean charged wineglass (half)-wormholes semiclassically, as a possible creation for our inflationary universe. We calculate the Euclidean action of the charged wormhole, and find that the initial state of universe brought by such an Euclidean charged wormhole can exhibit a high probability weight for a long period of inflation. We compare our result with that of axion wormholes, and evaluate the potential of other corresponding Euclidean configurations as the pre-inflationary initial states.

gr-qc↗

arXiv2024

Sub-Eddington accreting supermassive primordial black holes explain Little Red Dots

The James Webb Space Telescope (JWST) has uncovered an abundant population of compact, extremely red, and X-ray weak objects at $z\gtrsim4$, knows as ``Little Red Dots" (LRDs). These objects exhibit spectral energy distributions that resemble both active galactic nuclei (AGN) and stellar population templates. However, whether dominated by AGN activity or compact star formation, the high redshifts and masses/luminosities of LRDs, coupled with their significant abundance, present potential challenges to the standard $Λ$CDM model. In this work, we proposes a novel cosmic interpretation of this anomaly, suggesting that these LRDs are likely massive galaxies seeded by supermassive primordial black holes (SMPBHs) came into being in the very early universe. We analyze 434 known LRDs from the 0.54 ${\rm deg}^2$ COSMOS-Web survey and test the hypothesis that they originated from SMPBHs assuming sub-Eddington accretion. According to our result, SMPBHs actually could lead to the existence of more LRDs, even at higher redshifts ($z>8$).

astro-ph.GA↗

arXiv2024

Supermassive primordial black holes for the GHZ9 and UHZ1 observed by the JWST

The high redshift ($z>10$) galaxies GHZ9 and UHZ1 observed by the James Webb Space Telescope (JWST) are very massive and have exceptionally high black hole-to-star mass ratios with the central black hole masses $M\gtrsim 10^7\rm~M_\odot$. In this paper, we explore the possibility that they are seeded by the supermassive primordial black holes (SMPBHs), which came into being in the very early universe, with initial masses $\sim 10^7\rm~M_\odot$. We present the self-similar accretion solutions for SMPBHs, and find that the mass growth of SMPBHs during pregalactic era may be negligible. These SMPBHs, when the redshift $z\lesssim 20$, can accelerate seeding high-redshift galaxies and their baryonic content, and consequently explain the central supermassive black holes (SMBHs) of high-redshift massive galaxies through sub-Eddington accretion. According to our results, SMPBHs actually could lead to the existence of more massive SMBHs at higher redshifts compared to other SMBH seed scenarios, specially SMBHs with masses $M\gtrsim 10^7~\rm M_\odot$ at $z>20$ might only origin from SMPBHs, thus the corresponding observation can serve as a potential probe to PBHs.

astro-ph.GA↗

arXiv2024

Climbing over the potential barrier during inflation via null energy condition violation

The violation of the null energy condition (NEC) may play a crucial role in enabling a scalar field to climb over high potential barriers, potentially significant in the very early universe. We propose a single-field model where the universe sequentially undergoes a first stage of slow-roll inflation, NEC violation, and a second stage of slow-roll inflation. Through the NEC violation, the scalar field climbs over high potential barriers, leaving unique characteristics on the primordial gravitational wave power spectrum, including a blue-tilted nature in the middle-frequency range and diminishing oscillation amplitudes at higher frequencies. Additionally, the power spectrum exhibits nearly scale-invariant behavior on both large and small scales.

astro-ph.CO↗

arXiv2024

Towards primordial gravitational waves and $n_s=1$ in light of BICEP/Keck, DESI BAO and Hubble tension

Recent observational data seem to show a $\gtrsim 3σ$ evidence for an evolving dark energy (DE) against the cosmological constant, so the standard $Λ$CDM model. In this paper, we perform the search for the primordial gravitational waves with the potential pre-recombination solutions to the Hubble tension, using recent DESI baryon acoustic oscillation measurements combined with BICEP/Keck cosmic microwave background (CMB) B-mode polarization, Planck CMB and Pantheon supernova data, which reveal that the low bound of the tensor-to-scalar ratio $r$ is $> 1.5σ$ non-zero with the bestfit $r_{0.05}\sim 0.01$ and the scalar spectral index $n_s= 1$ (both $|r_{0.05}-0.01|$ and $|n_s-1|\sim {\cal O} (0.001)$). In particular, we observe the unnoticed impact of CMB B-mode polarization data for constraining the nature of DE, which together with early dark energy solutions to the Hubble tension is calling for the return of post-recombination $Λ$CDM.

astro-ph.CO↗

arXiv2024

Parity-violating primordial gravitational waves from null energy condition violation

We investigate the parity-violating effects in primordial gravitational waves (GWs) due to null energy condition (NEC) violation in two very early universe scenarios: bounce-inflation and intermediate NEC violation during inflation. In both scenarios, we numerically solve the power spectra of parity-violating primordial GWs generated by coupling the background field and the spectator field with the Nieh-Yan term, respectively. We find that the background field can significantly enhance parity-violating effects at scales corresponding to the maximum of the GW power spectra. In contrast, the parity-violating effects produced by the spectator show significantly weaker observability even if the coupling constant is large. Therefore, in NEC-violating scenarios, the significant observable parity-violating effects in primordial GWs primarily arise from the physics directly related to NEC violation. This result highlights the potential of primordial GWs as crucial tools for exploring NEC-violating and parity-violating physics.

astro-ph.CO↗

arXiv2024

Can recent DESI BAO measurements accommodate a negative cosmological constant?

Anti-de Sitter vacuum, which correspond to a negative cosmological constant (CC), is theoretically important and well-motivated. However, whether it exists in reality or not has always been a controversial issue. In this paper, we perform the search for the negative CC using recent Dark Energy Spectroscopic Instrument (DESI) baryon acoustic oscillation measurements combined with Planck cosmic microwave background and Pantheon Plus supernova data. Though we did not find the evidence for negative CC, we observed the indication for it, the negative CC is preferred at $>68\%$ significance level, while such a negative CC can make the state equation of evolving dark energy component (coexisting with negative CC) $w\geqslant -1$. Our work highlights the potential of upcoming cosmological surveys to search for the negative CC.

astro-ph.CO↗

arXiv2024

Generating enhanced primordial GWs during inflation with intermittent violation of NEC and diminishment of GW propagating speed

We investigate both the null energy condition (NEC) violating scenario and the $c_T$-diminishing scenario for generating enhanced power spectrum of primordial gravitational waves (GWs) during inflation, where $c_T$ is the propagating speed of primordial GWs. Both of these two scenarios can be realized stably with theories beyond Horndeski, hence can be uniformly implemented within the framework of the effective field theory. We calculate the power spectrum of primordial GWs by assuming that the inflationary Universe undergoes three phases, where the violation of NEC or the diminishment of $c_T$ occurs in the intermediate phase. A template of the spectrum is given for the NEC-violating scenario. We also discuss the underlying relation and discrepancy between these two scenarios with a disformal transformation.

gr-qc↗

arXiv2024

High-redshift JWST massive galaxies and the initial clustering of supermassive primordial black holes

In this paper, we show that the initial clustering of supermassive primordial black holes (SMPBHs) beyond a Poisson distribution can efficiently enhance the matter power spectrum, and thus the halo mass function. As a result, the population of initially clustered SMPBHs with $M_{\rm PBH}\sim 10^9M_\odot$ and the fraction of energy density $f_{\rm PBH}\sim 10^{-3}$ (consistent with current constraints on SMPBHs) has the potential to naturally explain high-redshift massive galaxies observed by the James Webb Space Telescope.

astro-ph.CO↗

arXiv2024

Impact of evolving dark energy on the search for primordial gravitational waves

Recent data seem to suggest a preference for the evolving dark energy (DE). However, if the case is actually so, and not caused by unknown systematics in data, it might impact our understanding about our Universe in an anomalous way due to the shifts of some primary parameters. As an example, we present the search for the primordial gravitational waves, based on the evolving DE. The joint analysis of recent BICEP/Keck cosmic microwave background (CMB) B-mode polarization data with Planck18 CMB, DESI baryon acoustic oscillations and PantheonPlus data shows that the bestfit tensor-to-scalar ratio is $r_{0.05}\sim 0.01$, and the lower bound of $r_{0.05}$ is $\sim 2σ$ non-zero.

astro-ph.CO↗

arXiv2024

Primordial black holes from null energy condition violation during inflation

Primordial black holes (PBHs) and the violation of the null energy condition (NEC) have significant implications for our understanding of the very early universe. We present a novel approach to generate PBHs via the NEC violation in a single-field inflationary scenario. In our scenario, the universe transitions from a first slow-roll inflation stage with a Hubble parameter $H = H_{\text{inf}1}$ to a second slow-roll inflation stage with $H = H_{\text{inf}2}\gg H_{\text{inf}1}$, passing through an intermediate stage of NEC violation. The NEC violation naturally enhances the primordial scalar power spectrum at a certain wavelength, leading to the production of PBHs with masses and abundances of observational interest. We also investigate the phenomenological signatures of scalar-induced gravitational waves (SIGWs) resulting from the enhanced density perturbations. Our work highlights the potential of utilizing a combination of PBHs, SIGWs, and primordial gravitational waves as a valuable probe for studying NEC violation during inflation, opening up new avenues for exploring the early universe.

gr-qc↗

arXiv2024

Dark energy in light of recent DESI BAO and Hubble tension

Recently, Dark Energy Spectroscopic Instrument (DESI) collaboration based on their first year data has reported a $\gtrsim 3σ$ evidence for an evolving dark energy (DE) against the cosmological constant (CC), so the standard $Λ$CDM model. However, it is necessary to access the impact of DESI data on the state equation $w_0$-$w_a$ of DE in the Hubble-tension-free cosmologies, where $w_0$ and $w_a$ is the parameters of state equation of DE. In this paper, using recent DESI BAO measurements combined with Planck CMB and Pantheon Plus dataset, we perform the Monte Carlo Markov Chain (MCMC) analysis for the $w_0w_a$CDM model with possible pre-recombination resolutions of the Hubble tension. It is found that though $w_0>-1$ and $w_a<0$ are still preferred, the CC is also $<2σ$ consistent, while the bestfit Hubble constant $H_0$ are higher than those with pre-DESI BAO data but without the further exacerbation of $S_8$ tension. According to our results, the resolutions of Hubble tension are likely to suppress the \textit{preference} of DESI for the evolving DE, thus the claim of ruling out the CC needs to be more cautious regarding not only the recent observational data but also the cosmological tensions.

astro-ph.CO↗