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Strong Gravity and High-Energy Physics

Periodic Reporting for period 2 - StronGrHEP (Strong Gravity and High-Energy Physics)

Reporting period: 2018-01-01 to 2019-12-31

"Theoretical physics is the effort to create mathematical models that describe phenomena in nature and to make predictions with these models that can be tested through experiment and observation. The standard model (SM) of particle physics and Einstein's theory of general relativity (GR) are the two main pillars of modern day theoretical physics that provide us with a magnificent framework to understand much of what we see in the universe, from particle collisions to the expansion of the cosmos.

And yet, there are gaps in this exquisite picture and indications that something is not quite right or, at least, incomplete in our understanding of the world of physics. Galactic rotation curves, the accelerated expansion of the universe and precision measurements of the cosmic microwave background cannot be explained in terms of the visible matter. Either we accept the existence of an unknown form of ""dark matter"" and ""dark energy"", whose form we can not satisfactorily explain within the standard model of particle physics, or we are prepared to modify the laws of gravity beyond Einstein's relativity. These questions are deeply related to the nature of gravity and the quest for answers forms the core of our RISE project.

More specifically, the goals of our project can be summarized as follows.
(1) What can we learn about the enigmatic dark matter from observing black holes?
(2) Are black holes really the specific class of objects described by general relativity or is there yet another twist in their saga?
(3) Can we directly observe the spacetime distortion created by black holes in the form of the shadows they cast?
(4) How can we test whether Einstein's theory is also correct in the regime of extremely strong gravity? Do we need to generalize the theory?
(5) Does our world have more than four dimensions which would explain the weakness of gravity?

From a practically minded viewpoint, one might wonder, of course, why we should pursue questions like this when immediate technical benefit is not obvious. History, however, thunders a warning against such a viewpoint: Fundamental research may at times be glacially slow in providing practical benefits, but in the end it always does and does so with the overwhelming power of a glacier. Quantum mechanics, for instance, forms the foundation for all modern electronics. Number theory, pursued as early as about 3000 years ago in antique Greece, became the foundation of modern day encryption in the 20th century. General relativity itself found its way into the multi-billion Euro business of global positioning."
"Before we go into more details on how we explore ""beyond SM+GR"" physics in our project, we note that such a situation is not new; we have been in it twice in the 19th century when astronomers observed anomalies in the orbits of Uranus and Mercury. The former found an explanation in the form of ""dark matter"": a new planet, Neptune, was found based on theoretical predictions derived from Uranus' anomalies. Similarly, a planet ""Vulcan"" was conjectured to explain Mercury's orbital peculiarities, but Vulcan was never found. Instead, a new theory of gravity, namely Einstein's general relativity, provided the answer. Bearing in mind this lesson of history, we have explored both possibilities, dark matter and modified gravity. We find:

* Contrary to previous belief, boson fields accumulating around stars, do not trigger collapse to a black hole, but settle down into breathing configurations.

* Binary systems composed of compact, self gravitating boson fields, ""boson stars"", emit characteristic gravitational wave signals distinguishing them from black hole or neutron star binaries.

* Rotating boson stars made of scalar fields are not stable. In contrast, rotating boson stars composed of vector or ""Proca"" fields are stable.

* The presence of a background scalar field leaves characteristic imprints on the gravitational wave signal generated in the inspiral and merger of compact binaries.

* Black holes act as gravitational lenses, creating a characteristic, self-similar lensed image of objects behind them.

* Extended measurements of black-hole rotation rates will be able to identify or constrain the presence of dark-matter candidate fields such as dark photons or axion-like particles.

* The presence of a boson field around astrophysical X-ray sources leads to a shift in the quasi-periodic oscillations of the X-ray signal which can be detected with future missions such as LOFT.

* So-called black-hole mimickers generate waveform signals that may be detected with future space interferometers such as the ESA L-class mission LISA.

* We have found a long-lived (years or centuries) gravitational wave pattern arising in massive scalar-tensor theory of gravity. By directing gravitational wave searches at historic supernovae such as Kepler 1604, we can test these theories.

* The spontaneous scalarization of compact objects is a widespread phenomenon that occurs for neutron stars and black holes in various alternative theories of gravity.

* Contrary to our expectations, grazing collisions of black holes in more than four spacetime dimension can form dumbbell shaped configurations that ultimately lead to formation of a naked singularity. This behavior is not present in four spacetime dimensions suggesting the special nature of the four-dimensional world of our daily lives."
The progress of our project is best summarized by answering the 5 questions raised above -- bypassing here the technical details we include in our deliverables and publications.

(1) Black holes have enormous potential to inform us about the presence or absence of dark matter candidates. From upper limits of black-hole rotation rates caused by superradiance to the finger prints that dark matter fields may leave on the gravitational-wave emission of black-hole binaries, we have identified a wealth of observational phenomena that can be or are tested.

(2) Observational tests of the nature of black holes are so far compatible with the predictions of vacuum general relativity. Our project has identified numerous tests for future, more precise, observations.

(3) We can observe black holes and their shadows. Over time, these observations will become much more accurate and enable us to compare the observed images with the predictions made by our project for black holes in general relativity and in other theories of gravity.

(4) Our project has uncovered a wealth of signatures exhibited in the gravitational-wave emission from compact objects in alternative theories of gravity. These range from tidal signatureseffects to the gravitational afterglow of historical supernovae.

(5) Extra spatial dimensions are theoretically possible and may help in explaining the weakness of gravity but work from our project shows that extra dimensions create difficulties. Collisions of black holes in higher dimensions may form naked singularities. This is in marked contrast to the case of four dimensions where we do not see this happening.

All our findings have advanced the state-of-the-art. Besides dissemination in expert journals, we have also presented our results in a wide range of public venues, ranging from outreach talks to radio and TV programs. Furthermore, our project has funded or supported nine conferences or summerschools and resulted in substantial training of young researchers.
Snapshot of a black hole binary during inspiral. Colors show the gravitational waves emitted
A dumbbell-shaped configuration formed in the collision of 2 black holes in 6 dimensions.
Century long gravitational wave signal by Kepler's 1604 supernova in massive scalar tensor theory.
End state of a fast roating black hole in 6 dimensions. The thin connections are naked singularities