International Journal of Modern Physics D

Mirror matter is a stable self-collisional dark matter candidate. If parity is a conserved unbroken symmetry of nature, there could exist a parallel hidden (mirror) sector of the universe composed of particles with the same masses and obeying the same physical laws as our (visible) sector, except for the opposite-handedness of weak interactions. The two sectors interact predominantly via gravity, therefore mirror matter is naturally "dark". Here I review the cosmological signatures of mirror dark matter, concerning thermodynamics of the early universe, big bang nucleosynthesis, primordial structure formation and evolution, cosmic microwave background and large scale structure power spectra. Besides gravity, the effects on primordial nucleosynthesis of the kinetic mixing between photons and mirror photons are considered. Summarizing the present status of research and comparing theoretical results with observations/experiments, it emerges that mirror matter is not just a viable, but a promising dark matter candidate.

The principle of extreme physical information is a tool for deriving various equations of physics. Physical information is defined to be the difference between the Fisher information and the bound information specific to every physical system. Using the extreme physical information, equations are obtained for the probability of finding the location of a physical entity (a system of particles or a field) in space–time. Those equations turn out to be the relevant physical equations (Dirac's, Maxwell's equations etc.). A peculiarity of the method is that this only works if some of the coordinates that specify the whereabout of the physical entity in space–time are chosen to be imaginary. I will argue that this peculiarity is a direct effect of gravity.

We present the quantum spin as a novel test tool for probing directly the Planck scale space–time foam of quantum gravity. Quantum fluctuations of spatial support for the electric vector associated with the spin-one photon affect its polarization sufficiently to allow us to gain deep insights and unprecedented constraints on the most important and fundamental aspect of quantum gravity — the fluctuating structure of space–time with a Planck scale three-dimensional web. We show that the survival of strong polarization of X-rays and gamma rays from the Crab Nebula rejects the conventional space–time foam at Planck length scale and constrains the microscopic scale of quantum gravitational fluctuations to below 2 × 10^-8l_P.

Gravity is a macroscopic manifestation of a microscopic quantum theory of space–time, just as the theories of elasticity and hydrodynamics are the macroscopic manifestations of the underlying quantum theory of atoms. The connection between gravitation and thermodynamics is long and deep. The observation that space–time has a temperature for accelerating observers and horizons is direct evidence that there are underlying microscopic degrees of freedom. The equipartition of energy, the meaning of temperature, in these modes leads one to anticipate that there is also an entropy associated. When this entropy is maximized on a volume of space–time, then one retrieves the metric of space–time (i.e. the equations of gravity, e.g. general relativity). Since the metric satisfies the extremum in entropy on the volume, then the volume integral of the entropy can readily be converted to surface integral, via Gauss's Theorem. This surface integral is simply an integral of the macroscopic entropy flow producing the mean entropy holographic principle. This approach also has the added value that it naturally dispenses with the cosmological constant/vacuum energy problem in gravity except perhaps for second-order quantum effects on the mean surface entropy.

In an effective field theory model with an ultraviolet momentum cutoff, there is a relation between the effective equation-of-state of dark energy and the ultraviolet cutoff scale. It implies that a measure of the equation of state of dark energy different from minus one, ω ≠ -1, does not rule out vacuum energy as dark energy. It also indicates an interesting possibility that precise measurements of the infrared properties of dark energy can be used to probe the ultraviolet cutoff scale of effective quantum field theory coupled to gravity. In a toy model with a vacuum energy-dominated universe with a Planck scale cutoff, the dark energy effective equation of state is w_eff ≈ -0.96.

There are various reasons to believe that quantum theory could be an emergent phenomenon. Trace dynamics is an underlying classical dynamics of noncommuting matrices, from which quantum theory and classical mechanics have been shown to emerge, in the thermodynamic approximation. However, the time that is used to describe evolution in quantum theory is an external classical time, and is in turn expected to be an emergent feature — a relic of an underlying theory of quantum gravity. In this essay we borrow ideas from trace dynamics to show that classical time is a thermodynamic approximation to an operator time in quantum gravitational physics. This prediction will be put to test by ongoing laboratory experiments attempting to construct superposed states of macroscopic objects.

Normally one thinks of the motion of the planets around the Sun as a highly classical phenomenon, so that one can neglect quantum gravity in the solar system. However, classical chaos in the planetary motion amplifies quantum uncertainties so that they become very large, giving huge quantum gravity effects. For example, evidence suggests that Uranus may eventually be ejected from the solar system, but quantum uncertainties would make the direction at which it leaves almost entirely uncertain, and the time of its exit uncertain by about one quadrillion years. For a time a quadrillion years from now, there are huge quantum uncertainties whether Uranus will be within the solar system, within the galaxy, or even within causal contact of the galaxy.

One can identify the number density of the microscopic space–time degrees of freedom in any diffeomorphism-invariant theory of gravity by using the principle of equipartition, applied to the area elements of a surface which are at the local Unruh temperature. The entropy associated with these degrees of freedom, which matches with the Wald entropy for the theory, can be used to obtain the field equations of the theory through an extremization priciple. When the microscopic degrees of freedom are in local thermal equilibrium, the entropy of a bulk region of space–time resides on its boundary. These facts support an emergent perspective of gravity.

Generic cosmological models derived from higher-dimensional theories with warped extra-dimensions have a nonzero cosmological constant-like term induced on the 3 + 1 space–time, or a physical three-brane. In the scenario where this 3 + 1 space–time is an inflating de Sitter "bran" embedded in a higher-dimensional space–time, described by warped geometry, the four-dimensional cosmological term is determined in terms of two length scales: one is a scale associated with the size of extra-dimension(s) and the other is a scale associated with the warping of extra-space(s). The existence of this term in four dimensions provides a tantalizing possibility of explaining the observed accelerating expansion of the universe from fundamental theories of gravity, e.g. string theory.

It is possible that fundamental constants may not be constants at all. There is a generally accepted view that one can only talk about variations of dimensionless quantities, such as the fine structure constant α_e ≡ e²/4πϵ₀ℏc. However, constraints on the strength of gravity tend to focus on G itself, which is problematic. We stress that G needs to be multiplied by the square of a mass, and hence, for example, one should be constraining , where m_p is the proton mass. Failure to focus on such dimensionless quantities makes it difficult to interpret the physical dependence of constraints on the variation of G in many published studies. A thought-experiment involving talking to observers in another universe about the values of physical constants may be useful for distinguishing what is genuinely measurable from what is merely part of our particular system of units.

Coalescing binary systems (e.g. pulsars, neutron stars and black holes) are currently considered to be the most likely sources of gravitational radiation, yet to be detected on or near Earth, where the local gravitational field is negligible and the Poincaré symmetry rules. On the other hand, the general theory of gravitational waves emitted by axially symmetric rotating sources predicts the existence of a nonvanishing news function. The existence of such function implies that, for a distant observer, the asymptotic group of isometries, the BMS group, has a translational symmetry that depends on the orbit periodicity of the source, thus breaking the isotropy of the Poincaré translations. These results suggest that the asymptotic BMS-covariant wave equation should be applied to obtain a proper theoretical basis for the gravitational waves observations from those binary sources.

We consider that the universe is trapped in an excited vacuum state and the resulting excitation energy provides the observed dark energy. We explore the conditions under which this situation can arise from physics already known. Considering the example of how macroscopic quantum electrodynamic fields alter the vacuum structure, we find that the energy scale 1 meV–1 eV is particularly interesting. We discuss how dark energy of this form is accessible to laboratory experiments.

The arithmetic chaos of classical (super)gravity near a spacelike singularity is elevated to the quantum level via the construction of a cosmological quantum billiard system. Its precise formulation, together with its underlying algebraic structure, allows for a general analysis of the wavefunction of the universe near the singularity. We argue that the extension of these results beyond the billiard approximation may provide a concrete mechanism for emergent space as well as new perspectives on several long-standing issues in canonical quantum gravity. The exponentially growing complexity of the underlying symmetry structure could introduce an element of non-computability that effectively "screens" the cosmological singularity from a complete resolution.

We apply recent advances in quantum gravity to the problem of turbulence. Adopting the AdS/CFT approach we propose a string theory of turbulence that explains the Kolmogorov scaling in 3 + 1 dimensions and the Kraichnan and Kolmogorov scalings in 2 + 1 dimensions. In the gravitational context, turbulence is intimately related to the properties of space–time or quantum foam.

The dimensionless fine-structure constant α ≡ e²/ℏc ≃ 1/137.036 has fascinated many scientists since its introduction by Sommerfeld almost a century ago. Dirac and Feynman have conjectured that this important physical constant may be composed of fundamental mathematical quantities like π. In this essay we argue that, the interplay between gravity, quantum theory, and thermodynamics may shed much light on the origins of this mysterious constant. In particular, we show that a unified quantum theory of gravity may set a lower bound on the value of the fine-structure constant, α > ln 3/48π ≃ 1/137. 3.

We propose a class of simple dark energy models which predict a late-time dark radiation component and a distinctive time-dependent equation of state w(z) for redshift z < 3. The dark energy field can be coupled strongly enough to standard model particles to be detected in colliders, and the model requires only modest additional particle content and little or no fine-tuning other than a new energy scale of order milli-electron volts.

Large scale square ring laser gyros with a length of four meters on each side are approaching a sensitivity of . This is approximately the regime required to measure the gravito-magnetic effect (Lense–Thirring) of the Earth. For an ensemble of linearly independent gyros each measurement signal depends upon the orientation of each single axis gyro with respect to the rotational axis of the Earth. Therefore at least three gyros are necessary to reconstruct the complete angular orientation of the apparatus. In general, the setup consists of several laser gyroscopes (we would prefer more than three for sufficient redundancy), rigidly referenced to each other. Adding more gyros for one plane of observation provides a cross-check against intra-system biases and furthermore has the advantage of improving the signal-to-noise ratio by the square root of the number of gyros. In this paper we analyze a system of two pairs of identical gyros (twins) with a slightly different orientation with respect to the Earth's axis. The twin-gyro configuration has several interesting properties. The relative angle can be controlled and provides a useful null measurement. A quadruple twin system could reach a 1% sensitivity after 3.2 years of data taking, provided each square ring has 6 m length on a side, the system is limited by shot noise and there is no source for 1/f-noise.

Hawking–Bekenstein entropy formula seems to tell us that no quantum degrees of freedom can reside in the interior of a black hole. We suggest that this is a consequence of the fact that the volume of any interior sphere of finite surface area simply vanishes. Obviously, this is not the case in general relativity. However, we show that such a phenomenon does occur in various gravitational theories which admit a spontaneously induced general relativity. In such theories, due to a phase transition (one-parameter family degenerates) which takes place precisely at the would-have-been horizon, the recovered exterior Schwarzschild solution connects, by means of a self-similar transition profile, with a novel "hollow" interior exhibiting a vanishing spatial volume and a locally varying Newton constant. This constitutes the so-called "hollowgraphy" driven holography.

In this essay, we introduce a new approach to energy–momentum in general relativity. Space–time, as opposed to space, is recognized as the necessary arena for its examination, leading us to define new extended space–time energy and momentum constructs. From local and global considerations, we conclude that the Ricci tensor is the required element for a localized expression of energy–momentum to include the gravitational field. We present and rationalize a fully invariant extended expression for space–time energy, guided by Tolman's well-known energy integral for an arbitrary bounded stationary system. This raises fundamental issues which we discuss. The role of the observer emerges naturally and we are led to an extension of the uncertainty principle to general relativity, of particular relevance to ultra-strong gravity.

The averaging problem in cosmology is of considerable importance for the correct interpretation of cosmological data. In this essay an approach to averaging based on scalar curvature invariants is presented, which gives rise to significant effects on cosmological evolution.

Many inequivalent approaches to study black holes yield identical results. Any meaningful theory of gravity should explain the origin of this property. Here we show that the basic holomorphic modes characterizing the underlying two-dimensional conformal symmetry near the horizon bring about this universality. Moreover these modes lead to a law of equipartition of energy for black holes which suggests a statistical origin of gravity. This emergent nature of gravity is further bolstered by showing the equivalence of entropy with the action (S = -(i/ℏ), I) and expressing the generalized Smarr formula for mass as a thermodynamic relation, S = E/2T where S, E and T are the entropy, energy and temperature, respectively, of a black hole.

Hawking's prediction of thermal radiation by black holes has been shown by Unruh to be expected also in condensed matter systems. We show here that in a black hole-like configuration realized in a BEC this particle-creation does indeed take place and can be unambiguously identified via a characteristic pattern in the density–density correlations. This opens the concrete possibility of the experimental verification of this effect.

Three of the four forces of Nature are described by quantum Yang–Mills theories with remarkable precision. The fourth force, gravity, is described classically by the Einstein–Hilbert theory. There appears to be an inherent incompatibility between quantum mechanics and the Einstein–Hilbert theory which prevents us from developing a consistent quantum theory of gravity. The Einstein–Hilbert theory is therefore believed to differ greatly from Yang–Mills theory (which does have a sensible quantum mechanical description). It is therefore very surprising that these two theories actually share close perturbative ties. This essay focuses on these ties between Yang–Mills theory and the Einstein–Hilbert theory. We discuss the origin of these ties and their implications for a quantum theory of gravity.

The literature on quantum-gravity-inspired scenarios for the quantization of space–time has so far focused on particle-physics-like studies. This is partly justified by the present limitations of our understanding of quantum gravity theories, but we here argue that valuable insight can be gained through semi-heuristic analyses of the implications for gravitational phenomena of some results obtained in the quantum space–time literature. In particular, we show that the types of description of particle propagation that emerged in certain quantum space–time frameworks have striking implications for gravitational collapse and for the behavior of gravity at large distances.

A relation between the photon mass, its frequency, ν, and the deflection parameter, γ, determined by experimentalists (that characterizes the contribution of space curvature to gravitational deflection) is found. This amazing result allows us to conclude that the knowledge of the parameters ν and γ is all we need to set up gravitational bounds on the photon mass. By considering as inputs the most recent measurements of the solar gravitational deflection of radio waves obtained via the Very Long Baseline Interferometry, upper bounds on the photon mass are estimated.

The present-day universe appears to be homogeneous on very-large scales. Yet when the casual structure of the early universe is considered, it becomes apparent that the early universe must have been highly inhomogeneous. The current paradigm attempts to answer this problem by postulating the inflation mechanism. However, inflation in order to start requires a homogeneous patch of at least the horizon size. This paper examines if dynamical processes of the early universe could lead to homogenization. In the past similar studies seem to imply that the set of initial conditions that leads to homogenization is of measure zero. This essay proves the contrary: a set of initial conditions for spontaneous homogenization of cosmological models can form a set of nonzero measure.

It is shown that torsion is required for a complete theory of graviation, and that without it, the equations of gravitation violate fundamental laws. In the first case, we are reminded that, in the absence of external forces, the correct conservation law of total angular momentum arises only if torsion, whose origin is intrinsic spin, is included into gravitation. The second case considers the "mass reversal" transformation. It has been known that under a global chiral transformation and "mass to negative mass" transformation, the Dirac equation is invariant. But global transformations violate special relativity, so this transformation must be made local. It is shown that the torsion is the gauge field for this local invariance.

While extreme black hole space–times with smooth horizons are known at the level of mathematics, we argue that the horizons of physical extreme black holes are effectively singular. Test particles encounter a singularity the moment they cross the horizon, and only objects with significant back-reaction can fall across a smooth (now non-extreme) horizon. As a result, classical interior solutions for extreme black holes are theoretical fictions that need not be reproduced by any quantum mechanical model. This observation suggests that significant quantum effects might be visible outside extreme or nearly extreme black holes. It also suggests that the microphysics of such black holes may be very different from that of their Schwarzschild cousins.

Are there any degrees of freedom on the black hole horizon? Using the "membrane paradigm" we can reproduce coarse-grained physics outside the hole by assuming a fictitious membrane just outside the horizon. But to solve the information puzzle we need "real" degrees of freedom at the horizon, which can modify Hawking's evolution of quantum modes. We argue that recent results on gravitational microstates imply a set of real degrees of freedom just outside the horizon; the state of the hole is a linear combination of rapidly oscillating gravitational solutions with support concentrated just outside the horizon radius. The collective behavior of these microstate solutions may give a realization of the membrane paradigm, with the fictitious membrane now replaced by real, explicit degrees of freedom.

In this essay, we argue that the emergence of classically connected space–times is intimately related to the quantum entanglement of degrees of freedom in a nonperturbative description of quantum gravity. Disentangling the degrees of freedom associated with two regions of space–time results in these regions pulling apart and pinching off from each other in a way that can be quantified by standard measures of entanglement.