arXiv subjects

De-Chang Dai

Publications and source records attributed to De-Chang Dai.

At least 19 recordsLinked to original sources

arXiv2026

The Signals of Doomsday I: False Higgs vacuum decay signatures

The measured standard model parameters indicate that we might live in a false Higgs vacuum, though with a very long lifetime. However, small black holes can serve as catalysers and significantly speed up the phase transition. In fact, bubbles of true vacuum might already exist in our universe. If the propagation of the bubble walls slows down due to interaction with the surrounding matter and plasma, these signals can reach us before the bubble wall hits us. Using the vacuum mismatch method, we calculate the spectrum of the Higgs particles produced by such a bubble until the terminal velocity is reached. In addition, we show that frictional dissipation at the terminal wall velocity generates a large population of thermally produced Higgs particles, which continues even after the mismatch channel shuts off. Since the Higgs is neutral, a good part of the final decay products (after hadronization, annihilation and decay of unstable particles) will be photons and neutrinos, which will then act as a long-range signature. For the conservative set of parameters used here, the thermal channel produces a macroscopically large burst of high energy neutrinos and photons from Higgs decays, which could be detectable from sufficiently nearby bubbles with current or upcoming multi messenger facilities.

hep-ph↗

arXiv2025

Kerr black holes as circular polarizers

We study the retrograde second caustics of extremal Kerr black holes, where the intensity of the light beam is infinitely magnified. We find that the caustics of different polarized beams are split by as much as $10^{-3}$rad by an external black hole for a suitable range of parameters. A lensing black hole at several lys away separates the polarized beams about $10^{12}$m apart. This splitting is larger than the radius of the Earth. Therefore, an observer on Earth would see different circularly polarized light according to their location. The polarization will change while the detector is wandering around. Thus, the polarization of light beams can be an important quantity in retrolensing observations.

gr-qc↗

arXiv2025

Entropy created when colliding particles fall into a black hole

If two particles collide in the vicinity of a black hole horizon, their center of mass energy is practically unlimited, so another black hole with a large mass and thus entropy can be created. The resulting black hole can then merge with the original one. If the black hole is created very close to the horizon, its energy will be highly redshifted for an asymptotic observer. However, its entropy is not redshifted. We demonstrated that the newly created entropy can be higher than the Bekenstein-Hawking entropy of the final black hole, though we neglect that a certain amount of energy can escape to infinity, carrying away part of the entropy produced in the process. This is a counter-example to the statement that the black hole thermal entropy counts all the states inside the black hole. Unlike similar examples, this colliding process does not involve exotic matter, alternative theories of gravity, nor artificial ad hoc gluing of two different spacetimes.

gr-qc↗

arXiv2024

Searching for small primordial black holes in planets, asteroids and here on Earth

Small primordial black holes could be captured by rocky planets or asteroids, consume their liquid cores from inside and leave hollow structures. We calculate the surface density and surface tension of a hollow structure around a black hole and compare them with the density and compressive strength of various materials that appear in nature to find the allowed parameter space. For example, granite or iron can support a hollow asteroid/planetoid/moon of the size of up to $0.1 R_\oplus$. Along the same lines, future civilizations might build spherical structures around black holes to harvest their energy. Using the strongest material that we currently know how to make (multiwall carbon nanotube), to withstand gravity of one solar mass black hole, the shell must be constructed at distances larger than $10^4 R_\odot$. Alternatively, a fast black hole can leave a narrow tunnel in a solid object while passing through it. For example, a $10^{22}$g black hole should leave a tunnel with a radius of $0.1$ micron, which is large enough to be seen by an optical microscope. We could look for such micro-tunnels here on Earth in very old rocks, or even glass or other solid structures in very old buildings. While our estimate gives a very small probability of finding such tunnels, looking for them does not require expensive equipment and long preparation, and the payoff might be significant.

gr-qc↗

arXiv2024

The spin correlation of fermion pairs created by a Kerr black hole gravitational potential

We study the properties of massive fermions created and scattered by a rotating Kerr black hole. The helicities of the scattered fermions can vary during propagation. A fermion with a right-handed helicity can become either right or left-handed after interacting with the gravitational potential. This implies that measuring characteristics of an escaping particle is insufficient to reconstruct all the characteristics of its infalling partner. This further means the helicities of a particle pair created by the gravitational potential are not fully entangled. Since spin and helicity share many common features, it is likely that the same is true for spins of spontaneously created particles.

gr-qc↗

arXiv2024

Studies on particle creation during the universe expansion with a laser system

While two highly intensive laser beams collide, they create a region where the refractive index varies so quickly that photons are created. The variance of the refractive index is analog to the universe scale factor variance. Therefore, this laser system can be an analog to the expansion of the universe. We find that several hundreds of photons can be created under feasible conditions. This system can demonstrate the particle creation during inflation or other similar periods.

gr-qc↗

arXiv2024

Superluminal propagation along the brane in space with extra dimensions

We demonstrate that a model with extra dimensions formulated in Csaki et al. (Phys Rev D 62, 045015), which fatefully reproduces Friedmann-Robertson-Walker (FRW) equations on the brane, allows for an apparent superluminal propagation of massless signals. Namely, a massive brane curves the spacetime and affects the trajectory of a signal in a way that allows a signal sent from the brane through the bulk to arrive (upon returning) to a distant point on the brane faster than the light can propagate along the brane. In particular, the signal sent along the brane suffers a greater gravitational time delay than the bulk signal due to the presence of matter on the brane. While the bulk signal never moves with the speed greater than the speed of light in its own locality, this effect still enables one to send signals faster than light from the brane observer's perspective. For example, this effect might be used to resolve the cosmological horizon problem. In addition, one of the striking observational signatures would be arrival of the same gravitational wave signal at two different times, where the first signals arrives before its electromagnetic counterpart. We used GW170104 gravitational wave event to impose a strong limit on the model with extra dimensions in question.

gr-qc↗

arXiv2023

Shedding new light on the absence of fermionic superradiance and maximal infalling rate of fermions into a black hole

Using the complete classification of the bases in the rotating black hole background we separate superradiance from the Hawking effect. We first find that there is spontaneous particle creation for fermions by the potential outside the black hole horizon for the frequencies inside the superradiant regime, i.e. $ω<kΩ_H$. However, these particles do not enhance the total flux from the black hole. For the superradiance particle to became real, its negative energy counterpart has to be canceled by the positive energy Hawking radiation mode at the horizon. Since due to the Pauli's principle this cancellation must be one-to-one, the superradiance effect cannot add anything to the total black hole flux. For an extremal black hole, the Hawking temperature is zero, horizon is not populated with thermal modes, and fermions can be emitted through the superradiance mechanism. On the other hand, a macroscopic flux of fermions infalling to the black hole is the opposite process of Hawking radiation. A positive energy-infalling particle must cancel out a negative energy thermal mode at the horizon, which leaves a net positive energy mode that crosses the horizon. Since there is finite thermal particle density at the horizon, this implies that there is a maximal fermion infalling rate which is also controlled by the Hawking temperature.

gr-qc↗

arXiv2023

Separating the superradiant emission from the Hawking radiation from a rotating black hole

Emission of particles created in the background of a rotating black hole can be greatly amplified taking away rotational energy of a black hole. This amplification affects both particles created near the horizon (due to the Hawing effect), and particles created near the potential barrier far from the horizon. Only the latter effect is called the superradiance in the strict sense. We explicitly calculate the superradiant emission for scalar particles and compare it with the total scalar particle emission (Hawking radiation plus superradiance) to clarify some confusion in the literature. We clearly show that these two emissions are not the same. In particular, superradiance persists even for extremal black holes whose Hawking temperature is zero.

gr-qc↗

arXiv2022

On black holes as macroscopic quantum objects

The relative flow of the Schwarzschild vs. the proper time during the classical evolution of a collapsing shell in the Schwarzschild coordinates practically forces us to interpret black hole formation as a highly non-local quantum process in which a shell/anti-shell pair is created within the incipient horizon, thus canceling out the original collapsing shell exactly at the horizon. By studying quantum fields in the black hole background, we reveal similar non-local effects. Among other things, the outgoing member of the Hawking pair very quickly becomes entangled with the black hole geometry (and not its partner), which is in contrast with the usual assumption that the Hawking pair is maximally entangled according to the local geometry near the horizon. Also, an infalling wave affects the black hole geometry even before it crosses the horizon. Finally, we find that a particle takes a finite amount of time to tunnel in and out of the black hole horizon, and thus avoids infinite blue and redshift in processes happening exactly at the horizon. These findings strongly support the picture of a black hole as a macroscopic quantum object.

gr-qc↗

arXiv2022

Interaction of cosmological domain walls with large classical objects, like planets and satellites, and the flyby anomaly

Cosmological domain walls can be formed as a result of symmetry breaking at any epoch during the evolution of our universe. We study their interaction with a classical macroscopic object, like Earth or a satellite in Earth's orbit. We set up an action that includes the interaction term between the massive classical object and the scalar field that the domain wall is made of. We use numerical calculations to solve the coupled equations of motion which describe the crossing between the domain wall and the classical object. Depending on the strength of the interaction, relative velocity and size, the object can be either stopped by the wall, or it can pass through it inducing deformations in the wall that cost energy. At the same time, the coupling to the scalar filed might change the object's mass during the crossover. The fact that satellites in Earth's orbit (or planets in Sun's orbit) can change their mass and/or lose energy interacting with walls can be used as a new domain wall detection probe. For example, a typical velocity precision of a satellite is about $0.5$ mm/s, which directly puts an upper limit on its mass change to $ΔM/M \lessapprox 5\times 10^{-17} $. Alternatively, a known satellite flyby anomaly can easily be explained as an interaction with a closed domain wall. We also show that the presence of matter modifies the scalar filed potential and can locally create a bubble of the true vacuum, and thus trigger the decay of the false vacuum. For a critical bubble which is able to expand, such an interaction with the domain wall must be strong enough.

gr-qc↗

arXiv2022

Milky Way and M31 rotation curves: $Λ$CDM vs. MOND

We analyze the existing rotation-curve data of the Milky Way and M31 galaxies that extends to very large distances and low accelerations. We find a systematic downward trend in the weak acceleration (large distances) segment of the radial acceleration. A similar downward trend has been noticed in the $Λ$CDM EAGLE simulation, while the deviation from the generic MOND prediction would need to be ascribed to an external field effect, or possibly a post facto selected acceleration function $μ(x)$.

astro-ph.GA↗

arXiv2022

Quantum Gravity and Phenomenology: Dark Matter, Dark Energy, Vacuum Selection, Emergent Spacetime, and Wormholes

We discuss the relevance of quantum gravity to the frontier questions in high energy phenomenology: the problems of dark matter, dark energy, and vacuum selection as well as the problems of emergent spacetime and wormholes. Dark matter and dark energy phenomenology, and the problem of vacuum selection are discussed within the context of string theory as a model of quantum gravity. Emergent spacetime and wormholes are discussed in a more general context of effective theories of quantum gravity.

hep-th↗

arXiv2021

Bulk entropy is crucial to validate the second law of the extended black hole thermodynamics

The extended black hole thermodynamics in which the cosmological constant plays the role of pressure significantly enriches the phase structure of the theory. In order to understand the extended black hole thermodynamics more precisely, we let the value of the cosmological constant vary dynamically via tunneling from one vacuum to another in a black hole induced vacuum decay. In this process, entropy of the matter/energy released by a black hole is crucial to validate the second law of thermodynamics. In other words, without taking this bulk entropy into account, entropy associated with the black hole and cosmological horizons may not always increase. Since the bulk entropy is not represented by the black hole and the cosmological horizons, this result calls for a more careful interpretation of the holographic principle in which environmental effects are taken into account.

gr-qc↗

arXiv2021

Variance of Newtonian constant from local gravitational acceleration measurements

We use IGETS absolute gravitational acceleration measurement data to study the gravitational acceleration variance. The relative variance of $δg /g$ in 22 years is less than $4\times 10^{-8}$. Since $δG /G\lessapproxδg /g $, this implies the relative variance of Newtonian constant is less than $3\times 10^{-9}$ based on an sine-like oscillation hypothesis. This limit is at least 4 orders of magnitude better than the existing $G$ measurements. The scattered values of reported $G$ measurements coming from different experiments are most probably coming from systematic errors associated with these experiments and not due to intrinsic time variation of $G$. We also find that $\dot{ G} /G<5.61\times 10^{-10} \text{yr}^{-1}$ based on a linear hypothesis. This is the best terrestrial result so far.

gr-qc↗

arXiv2020

How to form a wormhole

We provide a simple but very useful description of the process of wormhole formation. We place two massive objects in two parallel universes (modeled by two branes). Gravitational attraction between the objects competes with the resistance coming from the brane tension. For sufficiently strong attraction, the branes are deformed, objects touch and a wormhole is formed. Our calculations show that more massive and compact objects are more likely to fulfill the conditions for wormhole formation. This implies that we should be looking for wormholes either in the background of black holes and compact stars, or massive microscopic relics. Our formation mechanism applies equally well for a wormhole connecting two objects in the same universe.

gr-qc↗

arXiv2020

Testing ER=EPR

We discuss a few tests of the ER=EPR proposal. We consider certain conceptual issues as well as explicit physical examples that could be experimentally realized. In particular, we discuss the role of the Bell bounds, the large N limit, as well as the consistency of certain theoretical assumptions underlying the ER=EPR proposal. As explicit tests of the ER=EPR proposal we consider limits coming from the entropy-energy relation and certain limits coming from measurements of the speed of light as well as measurements of effective weights of entangled states. We also discuss various caveats of such experimental tests of the ER=EPR proposal.

hep-th↗

arXiv2020

A sensitive search for wormholes

The realm of strong classical gravity and perhaps even quantum gravity are waiting to be explored. In this letter we consider the recently detected triple system composed of two stars and a non-accreting black hole. Using published observations of this system we conduct the most sensitive test to date for whether the black hole is actually a wormhole by looking for orbital perturbations due to an object on the other side of the wormhole. The mass limit obtained on the perturber is $\sim4$ orders of magnitude better than for observations of S2 orbiting the supermassive black hole at Sgr A*. We also consider how observations of a pulsar could test for whether the black hole in a pulsar-black hole binary is a wormhole. A pulsar in a similar orbit to S2 would be $\sim10$ orders of magnitude more sensitive than observations of S2. For a nominal pulsar-black hole binary of stellar masses, with orbital size similar to that of the Hulse-Taylor binary pulsar, one year of observations could set a mass limit on a perturber that is $\sim6$ orders of magnitude better than observations of a pulsar around Sgr~A*. A range of limits between the pulsar-Sgr~A* and Hulse-Taylor cases could be obtained for a possible population of pulsar-black hole binaries that may exist near the galactic center.

gr-qc↗