An axion-like scalar field environment effect on binary black hole merger

The environment, such as an accretion disk, could modify the signal of the gravitational wave from astrophysical black hole binaries. In this article, we model the matter field around intermediatemass binary black holes by means of an axion-like scalar field and investigate their joint evolution. In detail, we consider equal mass binary black holes surrounded by a shell of axion-like scalar field both in spherically symmetric and non-spherically symmetric cases, and with different strengths of the scalar field. Our result shows that the environmental scalar field could essentially modify the dynamics. Firstly,in the spherically symmetric case, with increase of the scalar field strength, the number of circular orbits for the binary black hole is reduced. This means that the scalar field could significantly accelerate the merger process. Secondly, once the scalar field strength exceeds a certain critical value, the scalar field could collapse into a third black hole with its mass being larger than that of the binary. Consequently,the new black hole that collapses from the environmental scalar field could accrete the binary promptly and the binary collides head-on with each other. In this process, there is almost no quadrupole signal produced, and, consequently, the gravitational wave is greatly suppressed. Thirdly, when the scalar field strength is relatively smaller than the critical value, the black hole orbit could develop eccentricity through accretion of the scalar field. Fourthly, during the initial stage of the inspiral, the gravitational attractive force from the axion-like scalar field could induce a sudden turn in the binary orbits, hence resulting in a transient wiggle in the gravitational waveform. Finally, in the non-spherical case, the scalar field could gravitationally attract the binary moving toward the center of mass for the scalar field and slow down the merger process.

Revealing the amplitude of primordial curvature perturbations on small scales from primordial black hole binaries for gravitational wave detection

Primordial black holes (PBHs) have been intensively investigated in cosmology as they are considered a potential candidate to be used as sources of the observed gravitational wave (GW) signals [1]. A substantial increase in primordial curvature perturbations on small scales is one of the natural methods to generate PBHs (smaller than a single Mpc)[2]. Hence, the PBH scenario of observed GW events by LIGO/VIRGO [3] offers an opportunity to analyze the primordial curvature perturbations on small scales, which have not yet been explored systematically because of lack of corresponding observations.

The Gravitational-wave physics Ⅱ: Progress

It has been a half-decade since the first direct detection of gravitational waves, which signifies the coming of the era of the gravitational-wave astronomy and gravitational-wave cosmology. The increasing number of the detected gravitational-wave events has revealed the promising capability of constraining various aspects of cosmology, astronomy, and gravity. Due to the limited space in this review article, we will briefly summarize the recent progress over the past five years, but with a special focus on some of our own work for the Key Project "Physics associated with the gravitational waves" supported by the National Natural Science Foundation of China. In particular,(1) we have presented the mechanism of the gravitational-wave production during some physical processes of the early Universe, such as inflation, preheating and phase transition, and the cosmological implications of gravitational-wave measurements;(2) we have put constraints on the neutron star maximum mass according to GW170817 observations;(3) we have developed a numerical relativity algorithm based on the finite element method and a waveform model for the binary black hole coalescence along an eccentric orbit.

The LISA-Taiji Network:Precision Localization of Coalescing Massive Black Hole Binaries

We explore a potential LISA-Taiji network to fast and accurately localize the coalescing massive black hole binaries.For an equalmass binary located at redshift of 1 with a total intrinsic mass of 10^(5)M_(⊙),the LISA-Taiji network may achieve almost four orders of magnitude improvement on the source localization region compared to an individual detector.The precision measurement of sky location from the gravitational-wave signal may completely identify the host galaxy with low redshifts prior to the final black hole merger.Such identification of the host galaxy is vital for the follow-up variability in electromagnetic emissions of the circumbinary disc when the binary merges to a new black hole and enables the coalescing massive black hole binaries to be used as a standard siren to probe the expansion history of the Universe.

Oscillatory behaviors near a black hole triple point

Studying the thermodynamics and phase transition of black holes is of great help in revealing the nature of black holes and gravity.Since the establishment of the four laws of black hole thermodynamics and discovery of the HawkingPage phase transition,the black hole phase transition has attracted great attention,and important progress has been made in viewing the cosmological constant as the thermodynamic pressure[1,2],which enables various interesting observations on the black hole phase transition in the so-called extended phase space.Of note,a characteristic phenomenon,the existence of the triple point,has been extensively investigated.

Energy budget of cosmological first-order phase transition in FLRW background

We study the hydrodynamics of bubble expansion in cosmological first-order phase transition in the Friedmann-Lema??treRobertson-Walker(FLRW) background with probe limit. Different from previous studies for fast first-order phase transition in flat background, we find that, for slow first-order phase transition in FLRW background with a given peculiar velocity of the bubble wall, the efficiency factor of energy transfer into bulk motion of thermal fluid is significantly reduced, thus decreasing the previously-thought dominated contribution from sound wave to the stochastic gravitational-wave background.

GWs from S-stars Revolving Around SMBH at Sgr A

A preliminary estimation of gravitational waves(GWs) from the extreme-mass-ratio-inspirals(EMRIs)system in the Galactic Centre(GC) is given for the 37 observed S-stars revolving around the supermassive black hole(SMBH) at Sagittarius(Sgr) A*. Within this century, the total strain of the gravitational waveform calculated from the post-Newtonian(PN) method with eccentricity is well below the current planned sensitivity of pulsar-timing-array(PTA). New technology might be required in order to extract GW signal from this EMRIs system for future PTA detections.

When null energy condition meets ADM mass

We give a conjecture on the lower bound of the ADM mass M by using the null energy condition.The conjecture includes a Penrose-like inequality 3M≥_(k)A/(4π)+√A/4πand the Penrose inequality 2M≥√A/4πwith A the event horizon area andκthe surface gravity.Both the conjecture in the static spherically symmetric case and the Penrose inequality for a dynamical spacetime with spherical symmetry are proved by imposing the null energy condition.We then generalize the conjecture to a general dynamical spacetime.Our results raise a new challenge for the famous unsettled question in general relativity:in what general case can the null energy condition replace other energy conditions to ensure the Penrose inequality?

Cosmic inflation from broken conformal symmetry

A period of rapidly accelerating expansion is expected in the early Universe implemented by a scalar field slowly rolling down along an asymptotically flat potential preferred by the current data.In this paper,we point out that this picture of the cosmic inflation with an asymptotically flat potential could emerge from the Palatini quadratic gravity by adding the matter field in such a way to break the local gauged conformal symmetry in both kinetic and potential terms.

A diffraction phenomenon of gravitational waves:Poisson-Arago spot for gravitational waves

The gravitational wave, a new observational tool first observed in 2015 [1], offers a great opportunity to explore the nature of the universe. It is very closely relevant to some important issues in astrophysics, cosmology and fundamental physics [2].The properties of the gravitational wave, especially its wave nature.

A refined trans-Planckian censorship conjecture

We propose a refined version of trans-Planckian censorship conjecture(TCC), which could be elaborated from the strong scalar weak gravity conjecture combined with some entropy bounds. In particular, no fine-tuning on the inflation model-building is required in the refined TCC, and it automatically passes the tests from those stringy examples that support the original TCC.Furthermore, our refined TCC could be consistent with hilltop eternal inflation.

A novel approach to probing new physics with neutral Gauge Boson couplings

Even before reaching the energy threshold for producing any new elementary particle beyond the Standard Model(SM)[1],new physics effects can be systematically described by effective operators with mass-dimensions higher than 4.These new operators together with the SM form the so-called SM effective theory(SMEFT).Most analyses consider only dimension-6 operators.

Editorial

The hot big bang model is an amazing idea to explain the origin and the evolution of our universe. The Hubble constant (H0)characterizing the current expansion rate of the universe is one of the key parameters in the hot big bang model. A lot of efforts have been made to measure its value, and now an around 1% measurement accuracy is already available.

Primordial black holes and curvature perturbations from false vacuum islands

Recently,much attention has been focused on the false vacuum islands that are flooded by an expanding ocean of true-vacuum bubbles slightly later than most of the other parts of the world.These delayed decay regions will accumulate locally larger vacuum energy density by staying in the false vacuum longer than those already transited into the true vacuum.A false vacuum island with thus acquired density contrast of a super-horizon size will evolve locally from radiation dominance to vacuum dominance,creating a local baby Universe that can be regarded effectively as a local closed Universe.If such density contrasts of super-horizon sizes can ever grow large enough to exceed the threshold of gravitational collapse,primordial black holes will form similar to those collapsing curvature perturbations on super-horizon scales induced by small-scale enhancements during inflation.If not,such density contrasts can still induce curvature perturbations potentially observable today.In this paper,we revisit and elaborate on the generations of primordial black holes and curvature perturbations from delayed-decayed false vacuum islands during asynchronous first-order phase transitions with fitting formulas convenient for future model-independent studies.