Tag Archives: #gravity

How Would Be Thick Branes In Mimetic Gravity? (Quantum)

Qun-Ying Xie and colleagues investigated thick branes generated by a scalar field in mimetic gravity theory, which is inspired by considering the conformal symmetry under the conformal transformation of an auxiliary metric. They obtained a series of analytical thick brane solutions and found that, perturbations of the brane system are stable and the effective potentials for the tensor and scalar perturbations are dual to each other. Findings of this study recently appeared in the Journal Symmetry.

Mimetic gravity is a Weyl-symmetric extension of General Relativity, related to the latter by a singular disformal transformation, wherein the appearance of a dust-like perfect fluid can mimic cold dark matter at a cosmological level. Within this framework, it is possible to provide an unified geometrical explanation for dark matter, the late-time acceleration, and inflation, making it a very attractive theory. This theory was also extended to Horava-like theory and applied to galactic rotation curves. It was also applied to other gravity theories such as f(R) gravity, Horndeski gravity and Gauss–Bonnet gravity.

On the other hand, Lisa Randall and Raman Sundrum proposed that our four-dimensional world could be a brane embedded in five-dimensional space-time, in order to solve gauge hierarchy problem and the cosmological constant problem. With the warped extra dimension, it was further found that the size of extra dimension can be infinitely large without conflicting with Newtonian gravitational law. This charming idea has attracted substantial researches in particle physics, cosmology, gravity theory, and other related fields. In the RS model, the brane is geometrically thin, therefore the space-time is singular at the brane. Although in the thin brane approximation many interesting results have been obtained, in some situations the effects of the brane thickness cannot be neglected.

“In five-dimensional problems, the thin brane approximation is valid as long as the brane thickness cannot be resolved, in other words, if the energy scale of the brane thickness is much higher than those in the bulk and on the brane. In contrast, when thickness becomes as large as the scale of interest, its effect is no longer negligible.”

Now, Qun-Ying Xie and colleagues investigated the super-potential method with which the second-order equations can be reduced to the first-order ones for thick brane models in modified gravity with Lagrange multiplier. The main step of this method is to introduce a pair of auxiliary super-potentials, i.e., W(φ) and Q(φ). With these two super-potentials, the field equations are rewritten as Equations (1)–(5).

Then, they used this method to find a series of analytical thick brane solutions via some polynomial super-potentials, period super-potentials, and mixed super-potentials.

Finally, they analyzed the tensor and scalar perturbations of the brane system. It was shown that both equations of motion of the perturbations can be transformed into Schrodinger-like equations. They added that, both perturbations are stable and the effective potentials for the tensor and scalar perturbations are dual to each other. Moreover, the tensor zero mode can be localized on the brane while the scalar zero mode cannot. Thus, the four-dimensional Newtonian potential can be recovered on the brane and there is no additional fifth force contradicting with the experiments.

Featured image: The effective potential VS(z(y)) for solution I. The parameter is set as n = 1 (red solid thick lines), n = 3 (blue dashed lines), and n = 5 (black solid thin lines). © Xie et al.

Reference: Xie, Q.-Y.; Fu, Q.-M.; Sui, T.-T.; Zhao, L.; Zhong, Y. First-Order Formalism and Thick Branes in Mimetic Gravity. Symmetry 2021, 13, https://doi.org/10.3390/sym13081345

Note for editors of other websites: To reuse this article fully or partially kindly give credit either to our author/editor S. Aman or provide a link of our article

Is Dark Matter Real, Or Have We Misunderstood Gravity? (Cosmology)

For many years now, astronomers and physicists have been in a conflict. Is the mysterious dark matter that we observe deep in the Universe real, or is what we see the result of subtle deviations from the laws of gravity as we know them? In 2016, Dutch physicist Erik Verlinde proposed a theory of the second kind: emergent gravity. New research, published in Astronomy & Astrophysics this week, pushes the limits of dark matter observations to the unknown outer regions of galaxies, and in doing so re-evaluates several dark matter models and alternative theories of gravity. Measurements of the gravity of 259,000 isolated galaxies show a very close relation between the contributions of dark matter and those of ordinary matter, as predicted in Verlinde’s theory of emergent gravity and an alternative model called Modified Newtonian Dynamics. However, the results also appear to agree with a computer simulation of the Universe that assumes that dark matter is ‘real stuff’.

The new research was carried out by an international team of astronomers, led by Margot Brouwer (RUG and UvA).  Further important roles were played by Kyle Oman (RUG and Durham University) and Edwin Valentijn (RUG). In 2016, Brouwer also performed a first test of Verlinde’s ideas; this time, Verlinde himself also joined the research team.

Matter or gravity?

So far, dark matter has never been observed directly – hence the name. What astronomers observe in the night sky are the consequences of matter that is potentially present: bending of starlight, stars that move faster than expected, and even effects on the motion of entire galaxies. Without a doubt all of these effects are caused by gravity, but the question is: are we truly observing additional gravity, caused by invisible matter, or are the laws of gravity themselves the thing that we haven’t fully understood yet?

To answer this question, the new research uses a similar method to the one used in the original test in 2016. Brouwer and her colleagues make use of an ongoing series of photographic measurements that started ten years ago: the KiloDegree Survey (KiDS), performed using ESO’s VLT Survey Telescope in Chili. In these observations one measures how starlight from far away galaxies is bent by gravity on its way to our telescopes. Whereas in 2016 the measurements of such ‘lens effects’ only covered an area of about 180 square degrees on the night sky, in the mean time this has been extended to about 1000 square degrees – allowing the researchers to measure the distribution of gravity in around a million different galaxies.

Comparative testing

Brouwer and her colleagues selected over 259,000 isolated galaxies, for which they were able to measure the so-called ‘Radial Acceleration Relation’ (RAR). This RAR compares the amount of gravity expected based on the visible matter in the galaxy, to the amount of gravity that is actually present – in other words: the result shows how much ‘extra’ gravity there is, in addition to that due to normal matter. Until now, the amount of extra gravity had only been determined in the outer regions of galaxies by observing the motions of stars, and in a region about five times larger by measuring the rotational velocity of cold gas. Using the lensing effects of gravity, the researchers were now able to determine the RAR at gravitational strengths which were one hundred times smaller, allowing them to penetrate much deeper into the regions far outside the individual galaxies.

This made it possible to measure the extra gravity extremely precisely – but is this gravity the result of invisible dark matter, or do we need to improve our understanding of gravity itself? Author Kyle Oman indicates that the assumption of ‘real stuff’ at least partially appears to work: “In our research, we compare the measurements to four different theoretical models: two that assume the existence of dark matter and form the base of computer simulations of our universe, and two that modify the laws of gravity – Erik Verlinde’s model of emergent gravity and the so-called ‘Modified Newtonian Dynamics’ or MOND. One of the two dark matter simulations, MICE, makes predictions that match our measurements very nicely. It came as a surprise to us that the other simulation, BAHAMAS, led to very different predictions. That the predictions of the two models differed at all was already surprising, since the models are so similar. But moreover, we would have expected that if a difference would show up, BAHAMAS was going to perform best. BAHAMAS is a much more detailed model than MICE, approaching our current understanding of how galaxies form in a universe with dark matter much closer. Still, MICE performs better if we compare its predictions to our measurements. In the future, based on our findings, we want to further investigate what causes the differences between the simulations.”

Young and old galaxies

Thus it seems that, at least one dark matter model does appear to work. However, the alternative models of gravity also predict the measured RAR. A standoff, it seems – so how do we find out which model is correct? Margot Brouwer, who led the research team, continues: “Based on our tests, our original conclusion was that the two alternative gravity models and MICE matched the observations reasonably well. However, the most exciting part was yet to come: because we had access to over 259,000 galaxies, we could divide them into several types – relatively young, blue spiral galaxies versus relatively old, red elliptical galaxies.” Those two types of galaxies come about in very different ways: red elliptical galaxies form when different galaxies interact, for example when two blue spiral galaxies pass by each other closely, or even collide. As a result, the expectation within the particle theory of dark matter is that the ratio between regular and dark matter in the different types of galaxies can vary. Models such as Verlinde’s theory and MOND on the other hand do not make use of dark matter particles, and therefore predict a fixed ratio between the expected and measured gravity in the two types of galaxies – that is, independent of their type. Brouwer: “We discovered that the RARs for the two types of galaxies differed significantly. That would be a strong hint towards the existence of dark matter as a particle.”

A plot showing the Radial Acceleration Relation (RAR). The background is an image of the elliptical galaxy M87, showing the distance to the centre of the galaxy. The plot shows how the measurements range from high gravitational acceleration in the centre of the galaxy, to low gravitational acceleration in the far outer regions. Image: Chris Mihos (Case Western Reserve University) / ESO.

However, there is a caveat: gas. Many galaxies are probably surrounded by a diffuse cloud of hot gas, which is very difficult to observe. If it were the case that there is hardly any gas around young blue spiral galaxies, but that old red elliptical galaxies live in a large cloud of gas – of roughly the same mass as the stars themselves – then that could explain the difference in the RAR between the two types. To reach a final judgement on the measured difference, one would therefore also need to measure the amounts of diffuse gas – and this is exactly what is not possible using the KiDS telescopes. Other measurements have been done for a small group of around one hundred galaxies, and these measurements indeed found more gas around elliptical galaxies, but it is still unclear how representative those measurements are for the 259,000 galaxies that were studied in the current research.

Dark matter for the win?

If it turns out that extra gas cannot explain the difference between the two types of galaxies, then the results of the measurements are easier to understand in terms of dark matter particles than in terms of alternative models of gravity. But even then, the matter is not settled yet. While the measured differences are hard to explain using MOND, Erik Verlinde still sees a way out for his own model. Verlinde: “My current model only applies to static, isolated, spherical galaxies, so it cannot be expected to distinguish the different types of galaxies. I view these results as a challenge and inspiration to develop an asymmetric, dynamical version of my theory, in which galaxies with a different shape and history can have a different amount of ‘apparent dark matter’.”

Therefore, even after the new measurements, the dispute between dark matter and alternative gravity theories is not settled yet. Still, the new results are a major step forward: if the measured difference in gravity between the two types of galaxies is correct, then the ultimate model, whichever one that is, will have to be precise enough to explain this difference. This means in particular that many existing models can be discarded, which considerably thins out the landscape of possible explanations. On top of that, the new research shows that systematic measurements of the hot gas around galaxies are necessary. Edwin Valentijn formulates is as follows: “As observational astronomers, we have reached the point where we are able to measure the extra gravity around galaxies more precisely than we can measure the amount of visible matter. The counterintuitive conclusion is that we must first measure the presence of ordinary matter in the form of hot gas around galaxies, before future telescopes such as Euclid can finally solve the mystery of dark matter.”

Featured image: In the centre of the image the elliptical galaxy NGC5982, and to the right the spiral galaxy NGC5985. These two types of galaxies turn out to behave very differently when it comes to the extra gravity – and therefore possibly the dark matter – in their outer regions. Images: Bart Delsaert (www.delsaert.com).


The weak lensing radial acceleration relation: Constraining modified gravity and cold dark matter theories with KiDS-1000”, M. Brouwer et al., Astronomy & Astrophysics 2021.

Provided by University of Amsterdam

How Marine Flatworms Learn to Sense Gravity? (Biology)

Zoologists explore the mechanism and development of gravity-sensing ability in marine acoel flatworms

All living organisms are equipped with sensory organs to detect changes in their surrounding environment. It may not immediately strike us as obvious but, similar to how we can sense heat, cold, light, and darkness, we are also extremely adept at sensing gravity. In our case, it is our inner ear that does this job, helping us maintain balance, posture, and orientation in space. But, what about other organisms, for instance invertebrates that lack a backbone?

The gravity sensing organ in some aquatic invertebrates, known as a “statocyst,” is, in fact, rather fascinating. The statocyst is essentially a fluid-filled sac with sensory cells lining its inner wall and a small, mineralized mass called “statolith” contained inside. During any body movement, the statolith moves and consequently comes in contact with sensory cells in the inner wall, deflecting them. The deflections, in turn, activate the neurons (nerve cells), which then relay signals to the brain about changes in body orientation.

However, exactly how the sensory cells stimulate the neurons is not particularly clear for acoel flatworms–soft-bodied, marine animals with a simple anatomy, which represent one of the earliest extant life forms with bilateral (left-right) symmetry. What zoologists know so far, based on the finding that juvenile acoel flatworms occasionally fail to sense gravity, is that the ability is acquired sometime after hatching from the eggs.

In a new study published in Zoomorphology, scientists from Okayama University, Japan led by Prof. Motonori Ando have now taken a stab at understanding these curious creatures better. But what exactly is so attractive about acoel flatworms? Prof. Ando explains, “Understanding the stimulus response mechanism of Acoela can uncover a fundamental biological control mechanism that dates back to the origin of bilaterian animals, including humans. These organisms, therefore, are key to unravelling the process of evolution.”

For their study, the scientists used an acoel species called Praesagittifera naikaiensis or P. naikaiensis that is endemic to the Seto Island Sea coasts at Okayama. “The mysterious body plan of P. naikaiensis could be key to connecting Okayama and the world’s natural environment,” says Prof. Ando.

To examine the relationship between the statocyst and nervous system of P. naikaiensis, the scientists had to make them both visible, a task usually accomplished by a “marker” or a “label.” However, due to a lack of any suitable label for the statocyst, they adopted a different strategy in which they labeled instead the basal lamina, the layer on which the sensory cells sit. As for the nervous system, they labeled the nerve terminals using a well-known marker. Finally, they studied the specimen using confocal microscopy, a technique in which light is focused on to a defined spot at a specific depth to stimulate only local markers.

The results were illuminating. The scientists found that the acoel flatworm developed a gravity-sensing ability within 0 to 7 days after hatching, with the statolith forming after hatching. The statocyst comprised longitudinal and transverse nerve cords, forming what is called a “commissural brain” and a “statocyst-associated-commissure” (stc) characterized by transverse fibers. They hypothesized that a gravity-sensing ability developed when: 1) the statolith acquired a sufficient concentration of calcium salts, 2) stc functioned as the signal-relaying neurons, and 3) the sensory cells were present outside the sac and stimulated indirectly by the statolith through the basal lamina and stc.

Inspired by these findings, Prof. Ando has envisioned future research directions and even practical applications of their study. “It has been reported that closely related species of this organism inhabit the North Sea coast, the Mediterranean coast, and the east coast of North America. Since there is great interest about the commonality of their habitats, we can extend our research to a more global level, using these animals as a novel bioassay system for the environment they live in, especially in the face of the accelerated pace of climate change and anthropogenic habitat degradation. Furthermore, acoel flatworms could be an excellent biological model for studying diseases caused in humans due to abnormalities of sensory hair cells,” says an excited Prof. Ando.

It seems modern science is just warming up to the myriad mysteries of this minute worm!

Featured image: Sensory organ of the acoel flatworm Praesagittifera naikaiensis. Shown here is a light micrograph of the whole body (left), and its enlarged anterior regions (right), including the ciliated epidermis with sensory cilia, statocyst, and eyes that constitute the sensory organ. © 2021 Okayama University

Reference: Sakagami, T., Watanabe, K., Ikeda, R. et al. Structural analysis of the statocyst and nervous system of Praesagittifera naikaiensis, an acoel flatworm, during development after hatching. Zoomorphology (2021). https://doi.org/10.1007/s00435-021-00521-9

Provided by Okayama University

Cosmologists Found First Evidence That Non-metricity f(Q) Gravity Can Challenge Lambda-CDM model (Cosmology / Astronomy / Maths)

Although General Relativity (GR) is the well-established theory for the description of the gravitational interaction, there are two main motivations that justify the large amount of research devoted to its modification and extension. The first arises from cosmological grounds, since modified gravity is very efficient in describing the universe’s two phases of accelerated expansion, and moreover it can alleviate the two possible tensions of ΛCDM cosmology, namely the H0 and the σ8 ones. The second, and chronologically older, motivation is purely theoretical, and aims towards the improvement of the renormalizability of General Relativity with the further goal to finally reach to a quantum gravitational theory. Hence, the goal is to construct gravitational theories that possess General Relativity as a particular limit, but which in general include extra degree(s) of freedom that are able to fulfill the above requirements.

Now, Anagnostopoulos and colleagues proposed a specific model in the framework of the recently constructed f(Q) modified gravity, which they showed is very efficient in fitting the cosmological data. In this class of modification, one starts from the so-called symmetric teleparallel theories, which is an equivalent description of gravity using the non-metricity scalar Q, and extends it to an arbitrary function f(Q). f(Q) gravity leads to interesting applications, and trivially passes the constraints arising from gravitational wave observations. By confronting their new model with data from Supernovae type Ia (SNIa), Baryonic Acoustic Oscillations (BAO), Hubble parameter cosmic chronometers (CC) and Redshift Space Distortions (RSD) fσ8 observations, they deduced that the scenario at hand is, in some cases, statistically better than ΛCDM, although it does not include it as a particular limit.

“Nevertheless, confrontation with observations at both background and perturbation levels, namely with Supernovae type Ia (SNIa), Baryonic Acoustic Oscillations (BAO), cosmic chronometers (CC), and Redshift Space Distortion (RSD) data, reveals that the scenario, according to Akaike Information Criterion (AIC), the Bayesian Information Criterion (BIC), and the Deviance Information Criterion (DIC), is at some cases statistically preferred comparing to ΛCDM cosmology.”

For SNIa + CC datasets, they found that the two models are statistically compatible. However, for SNIa + BAOs + CC datasets, their f(Q) model is slightly more preferred (by the data) compared to ΛCDM one. In contrast, for SNIa + BAOs + RSD datasets, the f(Q) model is deemed inferior by the data, however still statistically indistinguishable from ΛCDM.

The information criteria AIC, BIC and DIC for the examined cosmological models, alongside the corresponding differences ∆IC ≡ IC – ICmin. © Anagnostopoulos et al.

They also showed that, in the large redshift limit (i.e at large E²(z) ≡ H²(z)/H²0) the proposed f(Q) tends to Q and thus the scenario at hand tends to GR, hence it trivially passes the early universe constraints and in particular the Big Bang Nucleosynthesis (BBN) ones. Additionally, knowing the observational bounds of E²(z) throughout the evolution, and using parameter ‘λ’, they got the effective Newton’s constant,

from which they deduced that throughout the evolution | (Geff/G) − 1| remains smaller than 0.1 and therefore it satisfies the observational constraints.

“In simple terms, the model doesnt exhibit early dark energy features and thus it immediately passes Big Bang Nucleosynthesis constraints, while the variation of the effective Newtons constant lies well inside the observational bounds”

Finally, they concluded that, this result could be used as motivation for further study of the present model, as well as f(Q) gravity in general, as it constitutes one of the first alternatives to the concordance model that apart from being preferred by the data (at least by some datasets), it additionally possesses a Lagrangian description. Further studies on this model, using the full CMB and LSS spectra, weak lensing data and other datasets, could enlighten their findings and verify whether the present f(Q) model outperforms the concordance one or not.

Featured image: The 1σ and 2σ iso-likelihood contours for the f(Q) model, as well as for the ΛCDM scenario, for the 2D subsets of the parameter space (Ωm0, h,M, rd), using graphic package getdist. We have used the joint analysis of SNIa+CC+BAOs datasets. © Anagnostopoulos et al.

Reference: Fotios K. Anagnostopoulos, Spyros Basilakos, Emmanuel N. Saridakis, “First evidence that non-metricity f(Q) gravity can challenge ΛCDM”, Arxiv, pp. 1-4, 2021. https://arxiv.org/abs/2104.15123

Note for editors of other websites: To reuse this article partially or fully kindly give credit to our author S. Aman or either, provide a link of our article

New Light on Baryonic Matter and Gravity on Cosmic Scales (Cosmology / Astronomy)

Scientists estimate that dark matter and dark energy together are some 95% of the gravitational material in the universe while the remaining 5% is baryonic matter, which is the “normal” matter composing stars, planets, and living beings. However for decades almost one half of this matter has not been found either. Now, using a new technique, a team in which the Instituto de Astrofísica de Canarias (IAC) has participated, has shown that this “missing” baryonic matter is found filling the space between the galaxies as hot, low density gas. The same technique also gives a new tool that shows that the gravitational attraction experienced by galaxies is compatible with the theory of General Relativity. This research is published today in three articles in the journal Monthly Notices of the Royal Astronomical Society (MNRAS).

In designing this new technique they have analyzed the changes in the electromagnetic spectrum, its shift to the red, caused by the reddening of the light from the galaxies as they speed away from us. In the Universe, the sources which move away show a redder spectrum, and those which approach us show a bluer spectrum. This effect has given essential data for the development of modern cosmology. Almost a century ago, Edwin Hubble discovered that the redshifts of galaxies are bigger the further away from us they are, and this was the initial evidence which eventually led to the Big Bang model of the universe. Since then these redshifts have been used to find the distances to the galaxies and to build three dimensional maps of their distribution in the Universe.

In the work we are reporting here a new method has been developed, which studies the statistics of the redshifts of galaxies, without converting them to distances. In their first article, the team shows that these maps are sensitive to the gravitational attraction between galaxies on cosmological scales. In a second article, the same team compare the maps with observations of the cosmic microwave background,, and they permit, for the first time, a complete census of the baryonic matter during 90% of the life of the Universe.

“Most of this ‘ordinary’ matter is invisible to us because it is not sufficiently hot to emit energy. However, by using maps of the redshifts of the galaxies we find that all of this matter fills the space between them”, explains Jonás Chaves-Montero, a researcher at the Donostia International Physics Center (DIPC) and first author of this article.

Finally, as found in a third article, the researchers have also used the redshift maps of the galaxies to study the nature of gravity. “In contrast to previous approaches, our new method is not based on any conversion of redshift to distance, and it is shown to be robust agains noise and data impurities. Thanks to that it allow us to conclude with high accuracy, that the observations are compatible with Einstein’s theory of gravity”, notes Carlos Hernández-Monteagudo, an IAC researcher who is the first author on this third article.

These studies have been performed by researchers Carlos Hernández-Monteagudo, Jonás Chaves-Montero, Raúl Angulo and Giovanni Aricò, who designed the research during their time at the Centre for Studies of Cosmic Physics of Aragón (CEFCA), even though now they are working at other Spanish research centres, such as the Instituto de Astrofísica de Canarias, and the Donostia International Physics Center. In one of the articles there was participation also by J. D. Emberson, a Canadian researcher at the Argonne National Laboratory, Illinois, USA.


1) Hernandez-Monteagudo, Carlos; Chaves-Montero, Jonas; Angulo, Raul E. “Angular Redshift Fluctuations: a New Cosmological Observable”. MNRAShttps://ui.adsabs.harvard.edu/abs/2019arXiv191112056H/abstract

2) Chaves-Montero, Jonas; Hernandez-Monteagudo, Carlos; Angulo, Raul E.; Emberson, J. D. “Measuring the evolution of intergalactic gas from z=0 to 5 using the kinematic Sunyaev-Zel’dovich effect”. MNRAShttps://ui.adsabs.harvard.edu/abs/2019arXiv191110690C/abstract

3) Hernández-Monteagudo, Carlos; Chaves-Montero, Jonás; Angulo, Raúl E.; Ariccò, Giovanni. “Tomographic Constraints on Gravity from Angular Redshift Fluctuations in the Late Universe”, MNRAS: https://ui.adsabs.harvard.edu/abs/2020arXiv200506568H/abstract

Featured image: The presence of ionized gas around galaxies with moves with them leaves a trace in the microwave background radiation which can be detected knowing the pattern of velocities of the galaxies provided by the map. Credit: Carlos Hernández-Monteagudo (IAC).

Provided by IAC

The Irresistible Pull of Gravity (Physics)

It is well known that gravity attracts and after reading the essay “The irresistible attraction of gravity” by astrophysicist Luciano Rezzolla – published by Rizzoli and selected as a finalist for the Cosmos Prize – it will also be clear why. Like it or not, we cannot escape gravity and its irresistible pull on our mind, so let us be carried away by the author on a long journey to explore the wonders of the universe.

Whether from the edge of an underground abyss or from the edge of a rock jump on the top of a mountain, from the highest rung of a ladder or from the window of a building, when we look out and look down we inevitably feel attracted. In fact, what attracts us is gravity . It attracts us not only with the body but also with the mind… in the sense that gravity is attractive, both instinctively and rationally.

Just a few seconds after birth, the Moro reflex reveals an important truth of our interaction with gravity: we know it instinctively, well before having conscious interactions with the rest of the physical universe. Then, over time, our knowledge of gravity changes as we develop the ability to observe the physical universe and understand its laws.

There is a person who knows these laws very well, even when they apply to the most extreme objects in the universe. A person who several times, in recent years, has guided us in the understanding of the most fascinating phenomena of nature , and who with patience and clarity has told us what happens in places where our imagination struggles to reach . It is Luciano Rezzolla , astrophysicist of Goethe University in Frankfurt , author of a recent essay – The irresistible attraction of gravity , published by Rizzoli and selected as a finalist for the Cosmos Prize – of which today we propose a review (with some spoilers ).

Cover of the book “The irresistible attraction of gravity: a journey to discover black holes” by Luciano Rezzolla, published by Rizzoli © INAF

From the early years of school, to explain gravity to children we start from the famous Newton’s apple and the fact that – detached from the branch – it does not float in space but falls to the ground. Already in the first chapters of this book you will discover, retracing the work of the fathers of gravity wisely narrated by the author, that this view is limited and even misleading, that gravity is not a force and that mass, alone, is not sufficient to describe it. The author recounts, in a compelling way, the astronomical observations that contributed to opening cracks in Newton’s theory of gravitation, leading the reader to discover – thanks to Einstein’s theory of general relativity – what gravity really is.

You will be joined by the author on a long journey in which, in addition to patience, the effort to imagine a reality very far from the one you are used to is required. On the way you will hardly be left behind, although the physics described are challenging. The author is able to explain in a simple but rigorous way the more complicated aspects of physics, making even the less experienced reader understand how the laws that govern it work. The text is extremely fluent, accurate, full of references to everyday life, of analogies, of comparisons that keep – like gravity – grounded. You will understand the concept of spacetimeand of curvature, you will understand how gravity is nothing more than a manifestation of the latter and that the reason why the apple falls downwards, once detached from the tree, is linked to the sign of the curvature of spacetime in which it is located the tree.

Every now and then you will be called to do mental experiments – Gedankenexperiment , in German – which do not require any instrumental apparatus but only a good dose of imagination and an excellent knowledge of physics. You will have to think about the first, the author will think about the second. These experiments, conducted using only the mind, based on logical and physical considerations, will allow you to obtain “virtual” results that are very difficult, if not impossible, to obtain in the laboratory and will allow you to investigate the limits of gravity.

Understanding the nature of gravity – and how it is inextricably linked to the curvature of spacetime – you will face the other stages of the journey, exploring the limits of gravity itself. Also in this case, the author retraces the historical steps that led – in the late 1950s, early 1960s – to the discovery of two wonders of physics: neutron stars and black holes .

The author tells how, with the advent of astronomy X – in a climate of trust and “cognitive optimism”, when stellar evolution was thought to be understood in every aspect -, astronomers discovered that in a dark corner of the sky, where there is a very ordinary small star when observed with a normal telescope, something shone with an enormously greater brightness X than might be expected. Data in hand, the astronomers soon realized that this something could have nothing to do with nuclear fusion processes. The discovery of this object, called Scorpius X-1 – along with that of another apparently similar but actually quite different object, Cygnus X-1– demolished most of the certainties that astrophysics had taken about thirty years to build. This is how, retracing the stages of the evolution of stars of great mass, the author describes how these stars are able to produce elements heavy up to iron and, having reached that point, find themselves collapsing on themselves until everything intervenes. another kind of pressure to stop the contraction.

«It has always struck me» writes Rezzolla, «that the final act of the bright and frenetic life of a massive star – as well as the catastrophic process that reveals its death – also marks the birth of one of the most fascinating objects in physics: a star of neutrons “. Perfect spheres of unimaginable density, with very high rotation frequencies and extremely high temperatures and magnetic fields, which the reader will be able to deepen in terms of observational properties, structure and internal composition, in a chapter entirely dedicated to these wonders of physics.

After the neutron stars it is the turn of black holes, samples of curvature, and you will be amazed by the fact that – although daily experience suggests that the more complex an object, organism or physical phenomenon, the greater the amount of information needed. to describe it – black holes are the simplest macroscopic physical objects of all. In addition to describing the physics of these objects, a dedicated chapter retraces the steps that led to the first image of a black hole , in the heart of M87, by the Event Horizon Telescope , explaining in detail why this image appears as we see it.

Having almost reached the end of the long journey, aware of what the curvature of spacetime is and what it entails in terms of gravity, in the last chapter the author deals with another interesting aspect: the propagation of perturbations in the curvature of spacetime. We are talking about gravitational waves , the revelation of which in 2015 marked the birth of the so-called multi-messenger astronomy, which opened a new window on the universe.

In the journey traveled with the author, you will find the answers to the many questions that each of us has asked himself at least once in his life, in addition to those that inevitably arise when reading the book. “What we experience in the course of our life on Earth is nothing more than a drop in the ocean of the physically possible”, concludes Rezzolla, and it is important not to limit the imagination: “Before advanced mathematics, complex simulations and sophisticated experiments – all however indispensable – comes the agility of our minds and their journeys of imagination. It is they who, more than anything else, allow us to extend the limits of knowledge ».

That gravity attracts is obvious to everyone, and after reading this book, it will become clear why. Like it or not, as the author states, we cannot escape gravity and its irresistible pull on our mind.

Featured image: Luciano Rezzolla is full professor of Theoretical Astrophysics and director of the Institute of Theoretical Physics at the Goethe Universität in Frankfurt. A member of the scientific committee of the Event Horizon Telescope project, his research helped capture the first photographic images of a supermassive black hole in 2019. Credits: Jürgen Lecher, Goethe-Universität Frankfurt

Provided by INAF

Quantum Collaboration Gives New Gravity to the Mysteries of the Universe (Quantum)

Scientists have used cutting-edge research in quantum computation and quantum technology to pioneer a radical new approach to determining how our Universe works at its most fundamental level.

An international team of experts, led by the University of Nottingham, have demonstrated that only quantum and not classical gravity could be used to create a certain informatic ingredient that is needed for quantum computation. Their research “Non-Gaussianity as a signature of a quantum theory of gravity” has been published today in PRX Quantum.

Dr Richard Howl led the research during his time at the University of Nottingham’s School of Mathematics, he said: “For more than a hundred years, physicists have struggled to determine how the two foundational theories of science, quantum theory and general relativity, which respectively describe microscopic and macroscopic phenomena, are unified into a single overarching theory of nature.

During this time, they have come up with two fundamentally contrasting approaches, called ‘quantum gravity’ and ‘classical gravity’. However, a complete lack of experimental evidence means that physicists do not know which approach the overarching theory actually takes, our research provides an experimental approach to solving this.”

This new research, which is a collaboration between experts in quantum computing, quantum gravity, and quantum experiments finds an unexpected connection between the fields of quantum computing and quantum gravity and uses this to propose a way to test experimentally that there is quantum not classical gravity. The suggested experiment would involve cooling billions of atoms in a millimetre-sized spherical trap to extremely low temperatures such that they enter a new phase of matter, called a Bose-Einstein condensate, and start to behave like a single large, quantum atom. A magnetic field is then applied to this “atom” so that it feels only its own gravitational pull. With this all in place, if the single gravitating atom demonstrates the key ingredient needed for quantum computation, which is curiously associated with “negative probability”, nature must take the quantum gravity approach.

This proposed experiment uses current technology, involves just a single quantum system, the gravitating “atom”, and does not rely on assumptions concerning the locality of the interaction, making it simpler than previous approaches and potentially expediating the delivery of the first experimental test of quantum gravity. Physicists would then, after more than a hundred years of research, finally have information on the true overarching, fundamental theory of nature.

Dr Marios Christodoulou, from the University of Hong Kong who was part of the collaboration, added: “This research is particularly exciting as the experiment proposed would also connect with the more philosophical idea that the universe is behaving as an immense quantum computer that is calculating itself, by demonstrating that quantum fluctuations of spacetime are a vast natural resource for quantum computation.”

The research brought together experimental and theoretical physicists from a range of disciplines and international research institutions. The other authors are: Richard Howl, Vlatko Vedral (Oxford and Singapore), Devang Naik (CNRS, Bordeaux), Marios Christodoulou (Hong Kong and Oxford), Carlo Rovelli (Marseille) and Aditya Iyer (Oxford).

Reference: Richard Howl, Vlatko Vedral, Devang Naik, Marios Christodoulou, Carlo Rovelli, and Aditya Iyer, “Non-Gaussianity as a Signature of a Quantum Theory of Gravity”, PRX Quantum 2, 010325 – Published 17 February 2021. https://journals.aps.org/prxquantum/abstract/10.1103/PRXQuantum.2.010325

Provided by University of Nottingham

RUDN University Physicists Analyzed The Role of Gravity in Elementary Particles Formation (Planetary Science)

Gravity might play a bigger role in the formation of elementary particles than scientists used to believe. A team of physicists from RUDN University obtained some solutions of semi-classical models that describe particle-like waves. They also calculated the ratio between the gravitational interaction of particles and the interaction of their charges. The results of the study were published in the Universe journal.

Due to their small size, the gravitational interaction between elementary particles (electrons, protons, and neutrons) is weak compared to Coulomb forces–attraction and repulsion determined by charge. For example, negatively charged electrons move around the atomic nucleus that contains positively charged protons. Therefore, the ratio of Newtonian attraction to Coulomb repulsion (or γ,) is negligible. However, on the Planck scale, i.e. at distances around 1.6?10?35 m, these forces become comparable. A team of physicists from RUDN University found solutions of existing models that correspond to particles in the Planck’s range.

“Gravity can potentially play an important role in the microworld, and this assumption is confirmed by certain data. γ is considered a ‘magical’ dimensionless number, and we are unaware of any serious attempts to theoretically obtain such a small value of γ — 10-40. We presented a simple model that allowed for obtaining this particular value in a natural way,” said Vladimir Kassandrov, PhD, and an Assistant Professor of the Institute of Gravitation and Cosmology, RUDN University.

The team used semi-classical models based on electromagnetic field equations. They have several solutions for particles as well as solitons (stable solitary waves). In equations like this, gravity is usually not taken into consideration and is replaced with a nonlinear correction that is chosen almost arbitrarily. This is where the main issue with these models lies. However, it can be solved by adding the equations of three fundamental fields to the system. Then, following the requirements of gauge invariance (that prevent physical values from changing simultaneously with the transformation of the fields), the form of nonlinearity becomes strictly defined. The team from RUDN University used this approach to find solutions that matched the characteristics of typical elementary particles. The existence of such solutions would confirm the fundamental role of gravity in the formation of particles.

The team failed to find solutions in which the charge and mass matched elementary particles at γ<0.9, and the very possibility of their existence remains questionable. However, the scientists managed to obtain solutions to the model for γ~1. They describe charged semi-quantum objects in the Planck range (i.e. with a mass around 10?5 g and size in the order of 10?33 cm). The physicists are still unsure what these solutions correspond to. Hypothetical particles with these parameters are called maximons or planckeons. The team from RUDN University was the first to obtain a discreet energy spectrum for objects with γ tending to infinity (i.e. with electric field excluded from the model). In this case, the solution describes objects with near-solar mass.

“Although our attempt to calculate probability characteristics at γ<0.9 was not successful, the model still could have such particle-like solutions. In the future, we would like to shed light on this problem that is intriguing for physicists by extremely complex from the point of view of mathematics. We want to find out if solutions for elementary particles really exist in the three-field model”, added Vladimir Kassandrov from RUDN University.

Featured image: Gravity might play a bigger role in the formation of elementary particles than scientists used to believe. A team of physicists from RUDN University obtained some solutions of semi-classical models that describe particle-like waves. They also calculated the ratio between the gravitational interaction of particles and the interaction of their charges. © RUDN University

Reference: Alharthy, Ahmed; Kassandrov, Vladimir V. 2020. “On a Crucial Role of Gravity in the Formation of Elementary Particles” Universe 6, no. 11: 193. https://doi.org/10.3390/universe6110193

Provided by RUDN University

Primordial Black Holes Can Be Produced By A Fifth Force, Which Is Stronger Than Gravity (Cosmology / Astronomy)

Amendola and colleagues presented a novel framework for primordial black hole (BH) formation which does not rely on a particular feature in the spectrum of primordial density fluctuations generated during inflation. They proposed that the primordial black holes can be produced by a long range attractive fifth force stronger than gravity, mediated by a light scalar field interacting with nonrelativistic “heavy” particles.

In spite of the many observations leading to the establishment of dark matter as an essential ingredient of modern cosmology, its fundamental nature remains an open question. Among the many dark matter candidates, primordial black holes (BH) are interesting since they could account for the gravitational wave signals observed by the Laser Interferometer Gravitational Wave Observatory (LIGO) and the VIRGO observatory or seed the formation of supermassive black holes. The existence of primordial BHs could be a natural consequence of inflation. In particular, if the inflationary potential contains a nontrivial feature along the inflaton trajectory, the spectrum of primordial perturbations might develop a peak at intermediate scales. If the amplitude of this peak is large enough, the nonlinear perturbations will collapse into BHs after horizon reentry. Alternatively, the formation of primordial BHs could take place at phase transitions or be associated with the fragmentation of a scalar condensate into Q-balls.

Now, Amendola and colleagues in their paper presented a novel framework for primordial BH formation which does not rely on a particular feature in the spectrum of primordial density fluctuations generated during inflation. The main assumption of their scenario is the presence in the early Universe of a long-range interaction stronger than gravity. They associate this fifth force to a light scalar field interacting with some heavy degrees of freedom beyond the Standard Model particle content. More precisely, they assume that during some epoch in cosmology the Hubble parameter, which they designate as ‘H’, is larger than the mass of a scalar field ‘φ’. If this scalar field couples to some “heavy particles” ψ with masses larger than Hubble parameter ‘H’, it mediates an attractive fifth force which is effectively long range, similar to gravity. This attraction can be, however, substantially stronger than the gravitational attraction. As a result, the fluctuations in the energy density of the heavy fields can grow rapidly and eventually become nonlinear. If the range and strength of the fifth force is large enough, it seems likely that a substantial part of the ψ fluid will collapse into BHs or similar screened objects.

Figure 1. Schematic evolution of the energy density of different species. The red and blue lines stand respectively for radiation and BH dark matter. Green lines denote ρψ. Larger values of β imply earlier BH formation and therefore smaller masses. For ΩBH(aeq) < 1/2 the radiation line moves upwards.

“As soon as the energy fraction of heavy particles reaches a threshold, the fluctuations rapidly become nonlinear. The overdensities collapse into black holes or similar screened objects, without the need for any particular feature in the spectrum of primordial density fluctuations generated during inflation.”, said Amendola.

So, the question is whether such primordial black holes can constitute the total dark matter component in the Universe?

Well friends, existence of long-range attractive forces stronger than gravity is a natural expectation in particle physics models containing scalar fields. The simplest example is the attractive interaction among Standard Model fermions via Higgs particle exchange. In early cosmology it is much stronger than the gravitational attraction. If not countered by electromagnetic interactions, even the Standard Model Higgs would have induced gravitational collapse at early times. Alternatively, the scalar field i.e. φ and heavy particle ‘ψ’ fields could be associated with grand unified frameworks involving, for instance, a scalar triplet interacting with heavy neutrinos. In this case, the BH formation process could occur very early in cosmology, for example nearly after the end of inflation. For different properties of the participating particles it could also take place rather late in the cosmological history, say after nucleosynthesis.

The heavy particles remaining outside primordial BHs might decay after the formation epoch and be unobservable today. The scalar field could relax after BH formation to a minimum of its effective potential with mass eventually exceeding the decreasing Hubble parameter. In this case, the scalar field φ would not be observable at the present time either. Alternatively, φ could be an additional dark matter candidate, or have a runaway behavior and be associated with dynamical dark energy. The BH formation process is not affected by what happens to the participating fields or particles at later times. Once BHs are formed, they behave as nonrelativistic matter. If the total energy density of BHs is large enough, they could constitute the dark matter component of our Universe.

Their work got support from the DFG through the project TRR33, “The Dark Universe”.

Reference: Luca Amendola, Javier Rubio, and Christof Wetterich, “Primordial black holes from fifth forces”, Phys. Rev. D, 97(8), 2018. https://journals.aps.org/prd/abstract/10.1103/PhysRevD.97.081302

Copyright of this article totally belongs to our author S. Aman. One is allowed to reuse it only by giving proper credit either to him or to us.