Emergent Gravity Scenario Research Articles

Emergent Gravity Fails to Explain Color-dependent Galaxy–Galaxy Lensing Signal from SDSS DR7

Abstract We test Verlinde’s Emergent Gravity (EG) theory using a galaxy–galaxy lensing technique based on Sloan Digital Sky Survey DR7 data. In the EG scenario, we do not expect a color dependence of the galaxy sample in the apparent dark matter predicted by EG, which is exerted only by the baryonic mass. If the baryonic mass is similar, then the predicted lensing profiles from the baryonic mass should be similar according to EG, regardless of the color of the galaxy sample. We use the stellar mass of the galaxy as a proxy of its baryonic mass. We divide our galaxy sample into five stellar mass bins, and further classify them as red and blue subsamples in each stellar mass bin. If we set the halo mass and concentration as free parameters, ΛCDM is favored by our data in terms of the reduced χ 2, while EG fails to explain the color dependence of the excess surface density from the galaxy–galaxy lensing measurement.

Geometric torsion, four-form, Riemann duals and Quintessence

We revisit an emergent gravity scenario in [Formula: see text][Formula: see text][Formula: see text] dimensions underlying a propagating geometric torsion [Formula: see text] with a renewed interest. We show that a pair-symmetric [Formula: see text]th-order curvature tensor is sourced by Neveu–Schwarz (NS) two-form in a [Formula: see text] gauge theoretic formulation. Interestingly, the new spacetime curvature governs a torsion-free geometry sourced by an NS form and shares the properties of the Riemann tensor. On the other hand, a completely anti-symmetric [Formula: see text]th-order tensor in the formulation is shown to incorporate a dynamical geometric torsion and is argued to be identified with a nonperturbative correction. The four-form turns out to be [Formula: see text] gauge invariant underlying an onshell NS form. We show that an emergent gravity theory may elegantly be described with an axionic scalar presumably signifying a quintessence coupling to the Riemann-type geometries. The curvatures are appropriately worked out to obtain a [Formula: see text] emergent form theory. Investigation reveals that a pair of [Formula: see text]-brane is created across an event horizon. We show that an emergent [Formula: see text] theory in a decoupling limit identifies with the bosonic sector of [Formula: see text], Supergravity in [Formula: see text].

K -essence emergent spacetime as a generalized Vaidya geometry

We establish a formal connection between the {\bf K}-essence emergent gravity scenario and generalizations of Vaidya spacetime. Choosing the {\bf K}-essence action to be of the Dirac-Born-Infeld variety, the physical spacetime to be a general static spherically symmetric black hole and restricting the {\bf K}-essence scalar field to be a function solely of the advanced or the retarded time, we show that the emergent gravity metric resembles closely the generalized Vaidya metrics for null fluid collapse proposed by Husain. Imposing null energy conditions on the emergent energy-momentum tensor derived from the emergent Einstein equation, restrictions are obtained on the functions characterizing the emergent metric for consistent identification with generalized Vaidya spacetimes. We discuss the possibility of dynamical horizons in the {\bf K-}essence emergent Vaidya spacetime. Admissible explicit black hole background metrics are discussed as examples.

Time-like geodesic structure for the emergent Barriola–Vilenkin type spacetime

For a particular type of k-essence scalar field, the k-essence emergent gravity metric is exactly mapped on to the Barriola–Vilenkin (BV) type metric for Schwarzschild background established by Gangopadhyay and Manna. Based on the Chandrasekhar, we report the exciting features of the time-like geodesic structure in the presence of dark energy in an emergent gravity scenario for this Barriola–Vilenkin type metric. We trace the different kinds of trajectories for time-like geodesic in the presence of dark energy for the k-essence emergent Barriola–Vilenkin spacetime, which is same as the Schwarzchild spacetime in view of the basic orientation, but the allowed ranges of the aphelion and perihelion distances are much more different. The bound and unbound orbits are plotted for a fixed value of the dark energy density.

Thermodynamics for the k-essence emergent Reissner–Nordstrom–de Sitter spacetime

The {\bf k-}essence emergent Reissner-Nordstrom-de Sitter spacetime has exactly mapped on to the Robinson-Trautman (RT) type spacetime with cosmological constant $\L$ for certain configuration of {\bf k-}essence scalar field. Theoretically, we evaluated that the thermodynamical quantities for the RT type emergent black hole is different from the usual one in the presence of kinetic energy of the {\bf k-}essence scalar field i.e., the dark energy density. We restrict ourselves into the fact that the dark energy density (K) is to be unity, then the effective temperature and pressure both are negative for the RT type emergent black hole which implies that the system is thermodynamically unstable when the charge $Q\neq 0$ and the emergent spacetime is only dark energy dominated and it does not radiate when $Q=0$. The thermodynamically unstable situation is physically plausible only when we consider spin degrees of freedom of a system. We have made this analysis in the context of dark energy in an emergent gravity scenario having {\bf k-}essence scalar fields $\phi$ with a Dirac-Born-Infeld type lagrangian. The scalar field also satisfies the emergent equation of motion at $r\rightarrow\infty$.

The Hawking temperature in the context of dark energy for Kerr\u2013Newman and Kerr\u2013Newman\u2013AdS backgrounds

We show that the Hawking temperature is modified in the presence of dark energy in an emergent gravity scenario for Kerr–Newman(KN) and Kerr–Newman–AdS(KNAdS) background metrics. The emergent gravity metric is not conformally equivalent to the gravitational metric. We calculate the Hawking temperatures for these emergent gravity metrics along theta =0. Also we show that the emergent black hole metrics are satisfying Einstein’s equations for large r and theta =0. Our analysis is done in the context of dark energy in an emergent gravity scenario having k-essence scalar fields phi with a Dirac–Born–Infeld type Lagrangian. In KN and KNAdS background, the scalar field phi (r,t)=phi _{1}(r)+phi _{2}(t) satisfies the emergent gravity equations of motion at rrightarrow infty for theta =0.

Analogue gravity models of emergent gravity: lessons and pitfalls

Analogue models of gravity have provided a test bed for many classical and quantum field theory effects in curve spacetime. Here we present a review of some relevant results towards their extension as toy models of emergent gravity scenarios. From these models we shall try to draw general lessons about the emergent gravity tackle on the cosmological constant problem as well as about the characteristic phenomenological signatures they suggest. Finally, we shall discuss current constraints on these signatures and the field’s future perspectives.

A note on graviton exchange in the emergent gravity scenario

It is well-known that there exists a close relation between a large $N$ matrix model and noncommutative (NC) field theory: The latter can be naturally obtained from the former by expanding it around a specific background. Because the matrix model can be a constructive formulation of string theory, this relation suggests that NC field theory can also include quantum gravity. In particular, the NC $U(1)$ gauge theory attracts much attention because its low-energy effective action partially contains gravity where the metric is determined by the $U(1)$ gauge field. Thus, the NC $U(1)$ gauge theory could be a quantum theory of gravity. In this paper, we investigate the scenario by calculating the scattering amplitude of massless test particles, and find that the NC $U(1)$ gauge theory correctly reproduces the amplitude of the usual graviton exchange if the noncommutativity that corresponds to the background of the matrix model is appropriately averaged. Although this result partially supports the relation between the NC $U(1)$ gauge theory and gravity, it is desirable to find a mechanism by which such an average is naturally realized.

Quantum effects in topological and Schwarzschild de Sitter brane: Aspects of torsion on $(D\!{\bar D})_4$-brane

We investigate an effective torsion curvature in a second-order formalism underlying a two-form world-volume dynamics in a D5-brane. In particular, we consider the two form in presence of a background (open string) metric in a U(1) gauge theory. Interestingly the formalism may be viewed via a noncoincident pair of [Formula: see text]-brane with a global Nereu–Schwarz (NS) two form on an anti-brane and a local two form on a brane. The energy–momentum tensor is computed in the six-dimensional (6D) conformal field theory (CFT). It is shown to source a metric fluctuation on a vacuum created pair of [Formula: see text]-brane at a cosmological horizon by the two-form quanta in the gauge theory. The emergent gravity scenario is shown to describe a low-energy (perturbative) string vacuum in 6D with a nonperturbative (NP) quantum correction by a lower (p < 5) dimensional Dp-brane or an anti-brane in the formalism. A closed string exchange between a pair of [Formula: see text]-brane, underlying a closed/open string duality, is argued to describe the Einstein vacuum in a low-energy limit. We obtain topological de Sitter (TdS) and Schwarzschild brane universe in six dimensions. The brane/anti-brane geometries are analyzed to explore some of their characteristic and thermal behaviors in presence of the quantum effects. They reveal an underlying nine-dimensional type IIA and IIB superstring theories on S1.

Cosmological Constant: A Lesson from Bose-Einstein Condensates

The cosmological constant is one of the most pressing problems in modern physics. We address this issue from an emergent gravity standpoint, by using an analogue gravity model. Indeed, the dynamics of the emergent metric in a Bose-Einstein condensate can be described by a Poisson-like equation with a vacuum source term reminiscent of a cosmological constant. The direct computation of this term shows that in emergent gravity scenarios this constant may be naturally much smaller than the naive ground-state energy of the emergent effective field theory. This suggests that a proper computation of the cosmological constant would require a detailed understanding about how Einstein equations emerge from the full microscopic quantum theory. In this light, the cosmological constant appears as a decisive test bench for any quantum or emergent gravity scenario.

Routes towards emergent gravity

We discuss a toy model for an emergent non-relativistic gravitational theory. Within a certain class of Bose–Einstein condensates, it is possible to show that, in a suitable regime, a modified version of non-relativistic Newtonian gravity does effectively describes the low energy dynamics of the coupled system condensate/quasi-particles. Furthermore, we study the role of Lorentz and diffeomorphism invariance in emergent gravity scenarios by developing a toy model showing an emergent Lorentzian signature from a Euclidean setting and simultaneously and emergent gravitational dynamics of the Nördstrom type (scalar gravity). Some lessons about the crucial challenges awaiting in the future the emergent gravity proposal are finally drawn.