We analyze how the presence of closed timelike curves (CTCs) characterizing a time machine can be discerned by placing a local particle detector in a region of spacetime which is causally disconnected from the CTCs. Our study shows that not only can the detector tell if there are CTCs, but also that the detector can separate topological from geometrical information and distinguish periodic spacetimes without CTCs (like the Einstein cylinder), curvature, and spacetimes with topological identifications that enable time-machines.

Germany

Ana Alonso-Serrano

Canada

Erickson Tjoa

Spain

Luis Garay

Eduardo Martín-Martínez

Particle detectors under chronological hazard

We implement a recent model proposed by Serrano–Liska (SL) (Alonso-Serrano and Liška, JHEP, 12:196, 2020) to study the ultra-compact star properties. The matter in the interior star is modeled by the quark model, deducted from QCD theory equipped with anisotropic pressure. Anisotropy is used to increase the compactness of the stars. We intend to see the signature of the “quantum gravity” effect through the SL model in ultra-compact stars. The SL model was motivated by quantum correction appearing in the black hole by adding a logarithmic term in gravity entropy used to derive the effective Einstein field equation. We expect this correction term in the SL model also affects ultra-compact star properties. The SL model with coupling constant  $${\tilde{c}}$$  <math> <mover> <mi>c</mi> <mo>~</mo> </mover> </math>   in logarithmic term equipped with spherically symmetric metric yields correction terms  $$\mathscr {O}({\tilde{c}})$$  <math> <mrow> <mi>O</mi> <mo>(</mo> <mover> <mi>c</mi> <mo>~</mo> </mover> <mo>)</mo> </mrow> </math>   that can be expressed by a function  $$\varXi (r)$$  <math> <mrow> <mi>Ξ</mi> <mo>(</mo> <mi>r</mi> <mo>)</mo> </mrow> </math>  . The  $$\varXi (r)$$  <math> <mrow> <mi>Ξ</mi> <mo>(</mo> <mi>r</mi> <mo>)</mo> </mrow> </math>   function vanishes at the star’s exterior. We found that the mass-radius relation prediction by the SL model with anisotropic matter deviates from the one predicted by the standard Tolman–Oppenheimer–Volkoff (TOV) equation for  $${\tilde{c}}\ge 10^7$$  <math> <mrow> <mover> <mi>c</mi> <mo>~</mo> </mover> <mo>≥</mo> <msup> <mn>10</mn> <mn>7</mn> </msup> </mrow> </math>   m $$^2$$  <math> <msup> <mrow></mrow> <mn>2</mn> </msup> </math>  . We also have a sufficiently deep enough effective potential to produce a quasi-normal mode. We obtain the echo frequency of 15.2 kHz using maximum anisotropic pressure contribution and  $${\tilde{c}}= 10^7$$  <math> <mrow> <mover> <mi>c</mi> <mo>~</mo> </mover> <mo>=</mo> <msup> <mn>10</mn> <mn>7</mn> </msup> </mrow> </math>   m $$^2$$  <math> <msup> <mrow></mrow> <mn>2</mn> </msup> </math>  . Because the corresponding effective potential is almost indistinguishable from that of GR, this echo frequency value can be indistinguishable to one of GR, but not comparable to the result from GW170817 data analysis, i.e., 72 Hz. To circumvent this problem, we can decrease the value of echo frequency by increasing the magnitude of  $${\tilde{c}}$$  <math> <mover> <mi>c</mi> <mo>~</mo> </mover> </math>   to orders of magnitude than  $${\tilde{c}}= 10^7$$  <math> <mrow> <mover> <mi>c</mi> <mo>~</mo> </mover> <mo>=</mo> <msup> <mn>10</mn> <mn>7</mn> </msup> </mrow> </math>   m $$^2$$  <math> <msup> <mrow></mrow> <mn>2</mn> </msup> </math>  . On the other hand, too large a strength from the logarithmic correction term is not physically favored because we learn from the black hole case that the logarithmic term is expected to be smaller than that of the Bekenstein term. Therefore, more precise gravitation echo measurements are crucial to understand this issue.

Indonesia

I. Prasetyo

I. Belfaqih

A. Suroso

A. Sulaksono

Anisotropic ultra-compact object in Serrano–Liska gravity model

In this work we present a derivation of modified Raychaudhuri and Friedmann equations from a phenomenological model of quantum gravity based on the thermodynamics of spacetime. Starting from general gravitational equations of motion which encode low-energy quantum gravity effects, we found its particular solution for homogeneous and isotropic universes with standard matter content, obtaining a modified Raychaudhuri equation. Then, we imposed local energy conservation and used a perturbative treatment to derive a modified Friedmann equation. The modified evolution in the early universe we obtained suggests a replacement of the Big Bang singularity by a regular bounce. Lastly, we also briefly discuss the range of validity of the perturbative approach and its results.

Czechia

Marek Liška

France

Antonio Vicente-Becerril

Friedmann equations and cosmic bounce in a modified cosmological scenario

We study the relationship between the quantization of a massless scalar field on the two-dimensional Einstein cylinder and in a spacetime with a time machine. We find that the latter picks out a unique prescription for the state of the zero mode in the Einstein cylinder. We show how this choice arises from the computation of the vacuum Wightman function and the vacuum renormalized stress-energy tensor in the time-machine geometry. Finally, we relate the previously proposed regularization of the zero mode state as a squeezed state with the time-machine warp parameter, thus demonstrating that the quantization in the latter regularizes the quantization in an Einstein cylinder.

The time traveler’s guide to the quantization of zero modes

Blackbody radiation contains (on average) an entropy of <math><mn>3.9</mn><mo>±</mo><mn>2.5</mn></math> bits per photon. If the emission process is unitary, then this entropy is exactly compensated by “hidden information” in the correlations. We extend this argument to the Hawking radiation from GR black holes, demonstrating that the assumption of unitarity leads to a perfectly reasonable entropy/information budget. The key technical aspect of our calculation is a variant of the “average subsystem” approach developed by Page, which we extend beyond bipartite pure systems, to a tripartite pure system that considers the influence of the environment.

New Zealand

Matt Visser

Entropy/information flux in Hawking radiation

Burning something, ( e.g. the proverbial lump of coal, or an encyclopaedia for that matter), in a blackbody furnace leads to an approximately Planck emission spectrum with an average entropy/information transfer of approximately 3.9±2.5 bits per emitted photon. This quantitative and qualitative result depends only on the underlying unitarity of the quantum physics of burning, combined with the statistical mechanics of blackbody radiation. The fact that the utterly standard and unitarity preserving process of burning something (in fact, burning anything) nevertheless has an associated entropy/information budget, and the quantitative size of that entropy/information budget, is a severely under-appreciated feature of standard quantum statistical physics.

On burning a lump of coal