Neutrons method to test dark energy theories by sensitive measurements of gravity at small scales

Schema dello spettrometro di risonanza per lo studio degli effetti della gravità su scale molto piccole. Credit: TU

Non sempre occorre un acceleratore di particelle per fare esperimenti di fisica fondamentale. I primi risultati di un esperimento a bassa energia sulla gravità newtoniana, spinto fino ad un limite più piccolo di cinque ordini di grandezza, stringono il cerchio sulle proprietà potenziali che forze e particelle potrebbero assumere al di là di questo limite di sensibilità pari ad almeno qualche centinaia di migliaia di volte. Infatti, un nuovo metodo sviluppato da alcuni fisici della Vienna University of Technlogy, denominato spettroscopia di risonanza gravitazionale, si è rivelato così sensibile che ora potrà essere applicato per studiare le due componenti più enigmatiche dell’Universo, la materia scura e l’energia scura.

All the particles we know to exist make up only about five per cent of the mass and energy of the Universe. The rest, dark matter and dark Energy, remains mysterious. A European collaboration led by researchers from the Vienna University of Technology has now carried out extremely sensitive measurements of gravitational effects at very small distances at the Institut Laue-Langevin (ILL) in Grenoble. These experiments provide limits for possible new particles or fundamental forces, which are a hundred thousand times more restrictive than previous estimations.

Undiscovered Particles?
Dark matter is invisible, but it acts on matter by its gravitational pull, influencing the rotation of galaxies. Dark energy, on the other hand, is responsible for the accelerated expansion of the Universe. It can be described by introducing a new physical quantity, Albert Einstein’s cosmological constant. Alternatively, so-called quintessence theories have been put forward: “Perhaps empty space is not completely empty after all, but permeated by an unknown field, similar to the Higgs-field”, says Professor Hartmut Abele (TU Vienna), director of the Atominstitut and group leader of the research group. These theories are named after Aristotle’s “quintessence”, a hypothetical fifth element, in addition to the four classical elements of ancient Greek philosophy. 

If new kinds of particles or additional forces of nature exist, it should be possible to observe them here on Earth.

Tobias Jenke and Hartmut Abele from the Vienna University of Technology developed an extremely sensitive instrument, which they used together with their colleagues to study gravitational forces. Neutrons are perfectly suited for this kind of research. They do not carry electric charge and they are hardly polarizable. They are only influenced by gravity, and possibly by additional, yet unknown forces. 

Forces at Small Distances
The technique they developed takes very slow neutrons from the strongest continuous ultracold neutron source in the world, at the ILL in Grenoble and funnels them between two parallel plates. According to quantum theory, the neutrons can only occupy discrete quantum states with energies which depend on the force that gravity exerts on the particle. By mechanically oscillating the two plates, the quantum state of the neutron can be switched. That way, the difference between the energy levels can be measured. “This work is an important step towards modelling gravitational interactions at very short distances. The ultracold neutrons produced at ILL together with the measurement devices from Vienna are the best tool in the world for studying the predicted tiny deviations from pure Newtonian gravity”, says Peter Geltenbort (ILL Grenoble). Different parameters determine the level of precision required to find such tiny deviations, for instance the coupling strength between hypothetical new fields and the matter we know. Certain parameter ranges for the coupling strength of quintessence particles or forces have already been excluded following other high-precision measurements. But all previous experiments still left a large parameter space in which new physical non-Newtonian phenomena could be hidden.

A Hundred Thousand Times Better than Other Methods
The new neutron method can test theories in this parameter range: “We have not yet detected any deviations from the well-established Newtonian law of gravity”, says Hartmut Abele. “Therefore, we can exclude a broad range of parameters”. The measurements determine a new limit for the coupling strength, which is lower than the limits established by other methods by a factor of a hundred thousand. Even if the existence of certain hypothetical quintessence particles is disproved by these measurements, the search will continue as it is possible that new physics can still be found below this improved level of accuracy. Therefore, Gravity Resonance Spectroscopy will need to be improved further, and increasing the accuracy by another few orders of magnitude seems feasible to the Abele’s team.

However, if even that does not yield any evidence of deviations from known forces, Albert Einstein would win yet another victory: his cosmological constant would then appear more and more plausible.

TU: Searching for Dark Energy with Neutrons

arXiv: Gravity Resonance Spectroscopy Constrains Dark Energy and Dark Matter Scenarios

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Using the Universe as ‘tool’ to detect gravitons

La gravità è l’unica tra le quattro forze fondamentali per la quale gli scienziati non hanno ancora rivelato la sua unità fondamentale. Infatti, secondo il modello standard delle particelle elementari si ritiene che l’interazione gravitazionale venga trasmessa attraverso il gravitone, allo stesso modo con cui l’interazione elettromagnetica viene trasmessa dai fotoni. Oggi, nonostante esistano delle basi teoriche a favore dell’esistenza dei gravitoni rimane, però, il problema di rivelarli, almeno sulla Terra dove le possibilità sono estremamente basse se non quasi nulle.

For example, the conventional way of measuring gravitational forces, by bouncing light off a set of mirrors to measure tiny shifts in their separation, would be impossible in the case of gravitons. According to physicist Freeman Dyson, the sensitivity required to detect such a miniscule distance change caused by a graviton requires the mirrors to be so massive and heavy that they’d collapse and form a black hole. Because of this, some have claimed that measuring a single graviton is hopeless. But what if you used the largest entity you know of, in this case the Universe, to search for the telltale effects of gravitons. That is what two physicists are proposing. In the paper, “Using cosmology to establish the quantization of gravity”, published in Physical Review D (Feb. 20, 2014), Lawrence Krauss, a cosmologist at Arizona State University, and Frank Wilczek, a Nobel-prize winning physicist with MIT and ASU, have proposed that measuring minute changes in the cosmic background radiation of the Universe could be a pathway of detecting the telltale effects of gravitons.

Krauss and Wilczek suggest that the existence of gravitons, and the quantum nature of gravity, could be proved through some yet-to-be-detected feature of the early Universe.

“This may provide, if Freeman Dyson is correct about the fact that terrestrial detectors cannot detect gravitons, the only direct empirical verification of the existence of gravitons”, Krauss said. “Moreover, what we find most remarkable is that the Universe acts like a detector that is precisely the type that is impossible or impractical to build on Earth”. It is generally believed that in the first fraction of a second after the Big Bang, the Universe underwent rapid and dramatic growth during a period called “inflation.” If gravitons exist, they would be generated as “quantum fluctuations” during inflation. Ultimately, these would evolve, as the Universe expanded, into classically observable gravitational waves, which stretch space-time along one direction while contracting it along the other direction. This would affect how electromagnetic radiation in the cosmic microwave background (CMB) radiation left behind by the Big Bang is produced, causing it to become polarized. Researchers analyzing results from the European Space Agency’s Planck satellite are searching for this “imprint” of inflation in the polarization of the CMB.

Krauss said his and Wilczek’s paper combines what already is known with some new wrinkles.

“While the realization that gravitational waves are produced by inflation is not new, and the fact that we can calculate their intensity and that this background might be measured in future polarization measurements of the microwave background is not new, an explicit argument that such a measurement will provide, in principle, an unambiguous and direct confirmation that the gravitational field is quantized is new”, he said. “Indeed, it is perhaps the only empirical verification of this very important assumption that we might get in the foreseeable future”. Using a standard analytical tool called dimensional analysis, Wilczek and Krauss show how the generation of gravitational waves during inflation is proportional to the square of Planck’s constant, a numerical factor that only arises in quantum theory. That means that the gravitational process that results in the production of these waves is an inherently quantum-mechanical phenomenon. This implies that finding the fingerprint of gravitational waves in the polarization of CMB will provide evidence that gravitons exist, and it is just a matter of time (and instrument sensitivity) to finding their imprint. “I’m delighted that dimensional analysis, a simple but profound technique whose virtues I preach to students, supplies clear, clean insight into a subject notorious for its difficulty and obscurity”, said Wilczek. “It is quite possible that the next generation of experiments, in the coming decade or maybe even the Planck satellite, may see this background”, Krauss added.

ASU: Researchers propose a new way to detect the elusive graviton

arXiv: Using Cosmology to Establish the Quantization of Gravity

Information, weather forecast and black holes

In un recente articolo pubblicato su arXiv, Stephen Hawking si ricrede sui buchi neri. Si tratta, forse, di immaginazione collettiva? Probabilmente no, ma lo scienziato inglese porta alla ribalta un paradosso complesso della fisica che ha tenuto col fiato sospeso i teorici negli ultimi 18 mesi.

Most physicists foolhardy enough to write a paper claiming that “there are no black holes”, at least not in the sense we usually imagine, would probably be dismissed as cranks. But when the call to redefine these cosmic crunchers comes from Stephen Hawking, it’s worth taking notice. In a paper posted online, the physicist, based at the University of Cambridge, UK, and one of the creators of modern black-hole theory, does away with the notion of an event horizon, the invisible boundary thought to shroud every black hole, beyond which nothing, not even light, can escape. In its stead, Hawking’s radical proposal is a much more benign “apparent horizon”, which only temporarily holds matter and energy prisoner before eventually releasing them, albeit in a more garbled form. “There is no escape from a black hole in classical theory”, Hawking told Nature. Quantum theory, however, “enables energy and information to escape from a black hole”. A full explanation of the process, the physicist admits, would require a theory that successfully merges gravity with the other fundamental forces of nature. But that is a goal that has eluded physicists for nearly a century. “The correct treatment”, Hawking says, “remains a mystery”. Hawking posted his paper on the arXiv preprint server on 22 January. He titled it, whimsically, ‘Information preservation and weather forecasting for black holes’, and it has yet to pass peer review. The paper was based on a talk he gave via Skype at a meeting at the Kavli Institute for Theoretical Physics in Santa Barbara, California, in August 2013 (watch video of the talk).

Fire fighting

Hawking’s new work is an attempt to solve what is known as the black-hole firewall paradox, which has been vexing physicists for almost two years, after it was discovered by theoretical physicist Joseph Polchinski of the Kavli Institute and his colleagues (see ‘Astrophysics: Fire in the hole!‘). In a thought experiment, the researchers asked what would happen to an astronaut unlucky enough to fall into a black hole. Event horizons are mathematically simple consequences of Einstein’s general theory of relativity that were first pointed out by the German astronomer Karl Schwarzschild in a letter he wrote to Einstein in late 1915, less than a month after the publication of the theory. In that picture, physicists had long assumed, the astronaut would happily pass through the event horizon, unaware of his or her impending doom, before gradually being pulled inwards, stretched out along the way, like spaghetti, and eventually crushed at the ‘singularity’, the black hole’s hypothetical infinitely dense core. But on analysing the situation in detail, Polchinski’s team came to the startling realization that the laws of quantum mechanics, which govern particles on small scales, change the situation completely. Quantum theory, they said, dictates that the event horizon must actually be transformed into a highly energetic region, or ‘firewall’, that would burn the astronaut to a crisp. This was alarming because, although the firewall obeyed quantum rules, it flouted Einstein’s general theory of relativity. According to that theory, someone in free fall should perceive the laws of physics as being identical everywhere in the Universe, whether they are falling into a black hole or floating in empty intergalactic space. As far as Einstein is concerned, the event horizon should be an unremarkable place.

Beyond the horizon

Now Hawking proposes a third, tantalizingly simple, option. Quantum mechanics and general relativity remain intact, but black holes simply do not have an event horizon to catch fire. The key to his claim is that quantum effects around the black hole cause space-time to fluctuate too wildly for a sharp boundary surface to exist.

In place of the event horizon, Hawking invokes an “apparent horizon”, a surface along which light rays attempting to rush away from the black hole’s core will be suspended.

In general relativity, for an unchanging black hole, these two horizons are identical, because light trying to escape from inside a black hole can reach only as far as the event horizon and will be held there, as though stuck on a treadmill. However, the two horizons can, in principle, be distinguished. If more matter gets swallowed by the black hole, its event horizon will swell and grow larger than the apparent horizon. Conversely, in the 1970s, Hawking also showed that black holes can slowly shrink, spewing out ‘Hawking radiation‘. In that case, the event horizon would, in theory, become smaller than the apparent horizon.

Hawking’s new suggestion is that the apparent horizon is the real boundary.

“The absence of event horizons means that there are no black holes, in the sense of regimes from which light can’t escape to infinity”, Hawking writes. “The picture Hawking gives sounds reasonable”, says Don Page, a physicist and expert on black holes at the University of Alberta in Edmonton, Canada, who collaborated with Hawking in the 1970s. “You could say that it is radical to propose there’s no event horizon. But these are highly quantum conditions, and there’s ambiguity about what space-time even is, let alone whether there is a definite region that can be marked as an event horizon“. Although Page accepts Hawking’s proposal that a black hole could exist without an event horizon, he questions whether that alone is enough to get past the firewall paradox. The presence of even an ephemeral apparent horizon, he cautions, could well cause the same problems as does an event horizon. Unlike the event horizon, the apparent horizon can eventually dissolve. Page notes that Hawking is opening the door to a scenario so extreme “that anything in principle can get out of a black hole”. Although Hawking does not specify in his paper exactly how an apparent horizon would disappear, Page speculates that when it has shrunk to a certain size, at which the effects of both quantum mechanics and gravity combine, it is plausible that it could vanish. At that point, whatever was once trapped within the black hole would be released (although not in good shape).

If Hawking is correct, there could even be no singularity at the core of the black hole. Instead, matter would be only temporarily held behind the apparent horizon, which would gradually move inward owing to the pull of the black hole, but would never quite crunch down to the centre.

Information about this matter would not destroyed, but would be highly scrambled so that, as it is released through Hawking radiation, it would be in a vastly different form, making it almost impossible to work out what the swallowed objects once were. “It would be worse than trying to reconstruct a book that you burned from its ashes”, says Page. In his paper, Hawking compares it to trying to forecast the weather ahead of time: in theory it is possible, but in practice it is too difficult to do with much accuracy. Polchinski, however, is sceptical that black holes without an event horizon could exist in nature. The kind of violent fluctuations needed to erase it are too rare in the Universe, he says. “In Einstein’s gravity, the black-hole horizon is not so different from any other part of space”, says Polchinski. “We never see space-time fluctuate in our own neighbourhood: it is just too rare on large scales”. Raphael Bousso, a theoretical physicist at the University of California, Berkeley, and a former student of Hawking’s, says that this latest contribution highlights how “abhorrent” physicists find the potential existence of firewalls. However, he is also cautious about Hawking’s solution. “The idea that there are no points from which you cannot escape a black hole is in some ways an even more radical and problematic suggestion than the existence of firewalls”, he says. “But the fact that we’re still discussing such questions 40 years after Hawking’s first papers on black holes and information is testament to their enormous significance“.

Nature: Stephen Hawking: 'There are no black holes'

arXiv: Information Preservation and Weather Forecasting for Black Holes

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see also: A Black Hole Mystery Wrapped in a Firewall Paradox

Is our Universe a hologram?

E’ circolata di recente nei media la notizia pubblicata da Nature secondo la quale un gruppo di fisici giapponesi avrebbero formulato una teoria che “potrebbe essere considerata l’evidenza più chiara sul fatto che il nostro Universo sarebbe una gigantesca proiezione“. Nei loro articoli, Yoshifumi Hyakutake e colleghi della Ibaraki University in Giappone spiegano  come la loro idea suggerisca che la realtà fisica, così come noi la concepiamo, potrebbe essere in definitiva un ologramma appartenente ad un altro spazio bidimensionale.

In 1997, theoretical physicist Juan Maldacena proposed that an audacious model of the Universe in which gravity arises from infinitesimally thin, vibrating strings could be reinterpreted in terms of well-established physics. The mathematically intricate world of strings, which exist in nine dimensions of space plus one of time, would be merely a hologram: the real action would play out in a simpler, flatter cosmos where there is no gravity. Maldacena’s idea thrilled physicists because it offered a way to put the popular but still unproven theory of strings on solid footing, and because it solved apparent inconsistencies between quantum physics and Einstein’s theory of gravity. It provided physicists with a mathematical “Rosetta stone”, a ‘duality’, that allowed them to translate back and forth between the two languages, and solve problems in one model that seemed intractable in the other and vice versa (see ‘Collaborative physics: String theory finds a bench mate‘). But although the validity of Maldacena’s ideas has pretty much been taken for granted ever since, a rigorous proof has been elusive.

In two papers posted on the arXiv repository, Yoshifumi Hyakutake of Ibaraki University in Japan and his colleagues now provide, if not an actual proof, at least compelling evidence that Maldacena’s conjecture is true.

In one paper, Hyakutake computes the internal energy of a black hole, the position of its event horizon (the boundary between the black hole and the rest of the Universe), its entropy and other properties based on the predictions of string theory as well as the effects of so-called virtual particles that continuously pop into and out of existence (see ‘Astrophysics: Fire in the Hole!‘). In the other, he and his collaborators calculate the internal energy of the corresponding lower-dimensional cosmos with no gravity. The two computer calculations match. “It seems to be a correct computation”, says Maldacena, who is now at the Institute for Advanced Study in Princeton, New Jersey and who did not contribute to the team’s work.

The findings “are an interesting way to test many ideas in quantum gravity and string theory”, Maldacena adds.

The two papers, he notes, are the culmination of a series of articles contributed by the Japanese team over the past few years. “The whole sequence of papers is very nice because it tests the dual [nature of the universes] in regimes where there are no analytic tests. They have numerically confirmed, perhaps for the first time, something we were fairly sure had to be true, but was still a conjecture, namely that the thermodynamics of certain black holes can be reproduced from a lower-dimensional Universe”, says Leonard Susskind, a theoretical physicist at Stanford University in California who was among the first theoreticians to explore the idea of holographic universes. Neither of the model universes explored by the Japanese team resembles our own, Maldacena notes. The cosmos with a black hole has ten dimensions, with eight of them forming an eight-dimensional sphere. The lower-dimensional, gravity-free one has but a single dimension, and its menagerie of quantum particles resembles a group of idealized springs, or harmonic oscillators, attached to one another. Nevertheless, says Maldacena, the numerical proof that these two seemingly disparate worlds are actually identical gives hope that the gravitational properties of our Universe can one day be explained by a simpler cosmos purely in terms of quantum theory.

Nature: Simulations back up theory that Universe is a hologram

arXiv: Quantum Near Horizon Geometry of Black 0-Brane

arXiv: Holographic description of quantum black hole on a computer

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