Archivi tag: cosmological constant

Extended Theories of Gravity

The cosmological constant and the physics behind dark energy that accelerates the expansion of the universe remain among the biggest mysteries in theoretical physics. An intriguing possibility is that these problems stem from extrapolating Einstein’s General relativity from the Solar system to the far infrared cosmological scales. In the other extreme, at the ultraviolet regime Einstein’s theory encounters notorious infinities resulting in spacetime singularities and obstacles to quantisation, which suggest new gravitational physics with possible repercussions to early universe physics. Continua a leggere Extended Theories of Gravity


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

‘Mirage’ quintessence and phantom dark energy

Quintessenza e campi “fantasma” sono due tra le varie ipotesi formulate in seguito ai dati ottenuti dai satelliti, come WMAP e Planck, che tentano di spiegare la natura dell’enigmatica energia scura. Oggi, alcuni ricercatori di Barcellona e Atene suggeriscono che entrambe le possibilità sono una sorta di “miraggio” nelle osservazioni e potrebbe essere in definitiva l’energia del vuoto quantistico la principale e l’unica responsabile a celarsi dietro tutto ciò che muove il cosmo.

Cosmologists believe that some three quarters of the Universe are made up of a mysterious dark energy which would explain its accelerated expansion. The truth is that they do not know what it could be, therefore they put forward possible solutions. One is the existence of quintessence, an invisible gravitating agent that instead of attracting, repels and accelerates the expansion of the cosmos. From the Classical World until the Middle Ages, this term has referred to the ether or fifth element of nature, together with earth, fire, water and air. Another possibility is the presence of an energy or phantom field whose density increases with time, causing an exponential cosmic acceleration.

This would reach such speed that it could break the nuclear forces in the atoms and end the Universe in some 20,000 million years, in what is called the Big Rip.

The experimental data that underlie these two hypotheses comes from satellites such as Planck of the European Space Agency (ESA) and Wilkinson Microwave Anisotropy Probe (WMAP) of NASA. Observations from the two probes are essential for solving the so-called equation of the state of dark energy, a characterising mathematical formula, the same as that possessed by solid, liquid and gaseous states. Now researchers from the University of Barcelona (Spain) and the Academy of Athens (Greece) have used the same satellite data to demonstrate that the behaviour of dark energy does not need to resort to either quintessence or phantom energy in order to be explained. The details have been published in the ‘Monthly Notices of the Royal Astronomical Society’ journal. “Our theoretical study demonstrates that the equation of the state of dark energy can simulate a quintessence field, or even a phantom field, without being one in reality, thus when we see these effects in the observations from WMAP, Planck and other instruments, what we are seeing is an mirage”, told SINC Joan Solà, one of the authors from University of Barcelona. “What we think is happening is a dynamic effect of the quantum vacuum, a parameter that we can calculate”, explained the researcher. The concept of the quantum vacuum has nothing to do with the classic notion of absolute nothingness. “Nothing is more ‘full’ than the quantum vacuum since it is full of fluctuations that contribute fundamentally to the values that we observe and measure”, Solà pointed out.

These scientists propose that dark energy is a type of dynamical quantum vacuum energy that acts in the accelerated expansion of our Universe.

This is in contrast to the traditional static vacuum energy or cosmological constant. The drawback with this strange vacuum is that it is the source of problems such as the cosmological constant, a discrepancy between the theoretical data and the predictions of the quantum theory that drives physicists mad. “However, quintessence and phantom fields are still more problematic, therefore the explanation based on the dynamic quantum vacuum could be the more simple and natural one”, concluded Solà.

FECYT/Sinc: Dark energy hides behind phantom fields
arXiv: Effective equation of state for running vacuum: "mirage" quintessence and phantom dark energy
arXiv: Dark energy from a quintessence (phantom) field rolling near potential minimum (maximum)
arXiv: Cosmological constant and vacuum energy: old and new ideas
arXiv: Vacuum energy and cosmological evolution

Per approfondire questo ed altri argomenti: Idee sull’Universo