* Astronomy

Members Login
Post Info TOPIC: Virtual Particles


L

Posts: 131433
Date:
Vacuum energy
Permalink  
 


Title: Vacuum energy as dark matter
Author: F. D. Albareti, J. A. R. Cembranos, A. L. Maroto

We consider the vacuum energy of massive quantum fields in an expanding universe. We define a conserved renormalised energy-momentum tensor by means of a comoving cutoff regularisation. Using exact solutions for de Sitter space-time, we show that in a certain range of mass and renormalisation scales there is a contribution to the vacuum energy density that scales as non-relativistic matter and that such a contribution becomes dominant at late times. By means of the WKB approximation, we find that these results can be extended to arbitrary Robertson-Walker geometries. We study the range of parameters in which the vacuum energy density would be compatible with current limits on dark matter abundance. Finally, by calculating the vacuum energy in a perturbed Robertson-Walker background, we obtain the speed of sound of density perturbations and show that the vacuum energy density contrast can grow on sub-Hubble scales as in standard cold dark matter scenarios.

Read more (572kb, PDF)



__________________


L

Posts: 131433
Date:
RE: Virtual Particles
Permalink  
 


Chalmers scientists create light from vacuum

Scientists at Chalmers University of Technology have succeeded in creating light from vacuum - observing an effect first predicted over 40 years ago. The results is published tomorrow (Wednesday) in the journal Nature. In an innovative experiment, the scientists have managed to capture some of the photons that are constantly appearing and disappearing in the vacuum.
The experiment is based on one of the most counterintuitive, yet, one of the most important principles in quantum mechanics: that vacuum is by no means empty nothingness. In fact, the vacuum is full of various particles that are continuously fluctuating in and out of existence. They appear, exist for a brief moment and then disappear again. Since their existence is so fleeting, they are usually referred to as virtual particles.

Read more 



__________________


L

Posts: 131433
Date:
Permalink  
 

Title: On the Quantum Creation of Matter in the Expanding Universe
Authors: Natalia Gorobey, Alexander Lukyanenko

Quantum Action Principle which has been used as a ground for a probabilistic interpretation of one-particle relativistic quantum mechanics {GLL} is applied to quantum cosmology. The quantum creation of matter in a minisuperspace model with one homogeneous scalar field is considered. The initial state of the universe is defined as a stationary ground state of the Hamiltonian with the Euclidean signature in which the mean value of the universe radius is equal to the Plank length and the number of the scalar field quanta is equal zero. We interpret the change of the signature as the universe "birth". From this moment of time the dynamics of the scale factor is considered as classical. The real phase of the amplitude of the creation process is taken as a quantum action. The balance between matter and gravitation energies in the creation process is fulfilled by the condition of the stationarity of the quantum action with respect to the internal time of the universe.

Read more  (7kb, PDF)



__________________


L

Posts: 131433
Date:
Permalink  
 

Lasers could make virtual particles real

Next-generation lasers will have the power to create matter by capturing ghostly particles that, according to quantum mechanics, permeate seemingly empty space.
The uncertainty principle of quantum mechanics implies that space can never be truly empty. Instead, random fluctuations give birth to a seething cauldron of particles, such as electrons, and their antimatter counterparts, called positrons.
These so-called "virtual particles" normally annihilate one another too quickly for us to notice them. But physicists predicted in the 1930s that a very strong electric field would transform virtual particles into real ones that we can observe. The field pushes them in opposite directions because they have opposite electric charges, separating them so that they cannot destroy one another.

Read more

__________________


L

Posts: 131433
Date:
Casimir force
Permalink  
 


Two metal plates, placed almost infinitesimally close together, spontaneously attract each other.
What seems like magic is known as the Casimir force, and it has been well-documented in experiments. The cause goes to the heart of quantum physics: Seemingly empty space is not actually empty but contains virtual particles associated with fluctuating electromagnetic fields. These particles push the plates from both the inside and the outside. However, only virtual particles of shorter wavelengths in the quantum world, particles exist simultaneously as waves can fit into the space between the plates, so that the outward pressure is slightly smaller than the inward pressure. The result is the plates are forced together.

Until now, no significant or nontrivial corrections to the Casimir force due to boundary conditions have been observed experimentally - Steve Lamoreaux, Yale University.

Read more

__________________


L

Posts: 131433
Date:
Vacuum Energy
Permalink  
 


Title: Vacuum Energy, the Cosmological Constant and Compact Extra Dimensions: Constraints from Casimir Effect Experiments
Authors: Leandros Perivolaropoulos

We consider a universe with a compact extra dimension and a cosmological constant emerging from a suitable ultraviolet cutoff on the zero point energy of the vacuum. We derive the Casimir force between parallel conducting plates as a function of the following scales: plate separation, radius of the extra dimension and cutoff energy scale. We find that there are critical values of these scales where the Casimir force between the plates changes sign. For the cutoff energy scale required to reproduce the observed value of the cosmological constant, we find that the Casimir force changes sign and becomes repulsive for plate separations less than a critical separation d_0=0.6mm, assuming a zero radius of the extra dimension (no extra dimension). This prediction contradicts Casimir experiments which indicate an attractive force down to plate separations of 100nm. For a non-zero extra dimension radius, the critical separation d_0 gets even larger than 0.6mm and remains inconsistent with Casimir force experiments. We conclude that with or without the presence of a compact extra dimension, vacuum energy with any suitable cutoff can not play the role of the cosmological constant.

Read more (412kb, PDF)

__________________


L

Posts: 131433
Date:
RE: Virtual Particles
Permalink  
 


Casimir effect goes classical
Physicists in Germany have made the first direct measurements of the critical Casimir effect, a classical analogue of the strange quantum effect that draws two conducting surfaces together in a vacuum. They also say that the classical effect could be easily tuned to repel rather than attract for reducing undesirable friction in nanomachines.

Read more 

__________________


L

Posts: 131433
Date:
Permalink  
 

POP. What are the chances that an everyday object - a rock, a chair, you name it - could suddenly appear out of thin air? Not zero, surprisingly. In fact, given enough space and time, it is conceivable that a conscious being could arise, even if only for a microsecond.
OK, such an event would be incredibly unlikely, but not impossible - at least in theory. Physicists have dubbed such hypothetical beings "Boltzmann brains", after the 19th-century Austrian physicist Ludwig Boltzmann, a pioneer in thermodynamics and statistical mechanics. Boltzmann posed the question of whether the universe could have arisen from a thermal fluctuation; his work presaged the idea that a fluctuation could also give rise to a conscious entity that sees the universe. In this regard Boltzmann brains are not necessarily actual brains, but rather are a metaphor for observers of the universe that might appear spontaneously.

Read more 

__________________


L

Posts: 131433
Date:
Casimir force
Permalink  
 


Levitation has been elevated from being pure science fiction to science fact, according to a study reported today by physicists.
In earlier work the same team of theoretical physicists showed that invisibility cloaks are feasible.
Now, in another report that sounds like it comes out of the pages of a Harry Potter book, the University of St Andrews team has created an 'incredible levitation effects by engineering the force of nature which normally causes objects to stick together.
Professor Ulf Leonhardt and Dr Thomas Philbin, from the University of St Andrews in Scotland, have worked out a way of reversing this phenomenon, known as the Casimir force, so that it repels instead of attracts.

Read more

__________________


L

Posts: 131433
Date:
Permalink  
 

 For the first time, a group led by Nobel laureate Eric Cornell at the National Institute of Standards and Technology and the University of Colorado in Boulder has confirmed a 1955 prediction, by physicist Evgeny Lifschitz, that temperature affects the Casimir force, the attraction between two objects when they come to within 5 millionths of a meter  of each other or less. These efforts heighten the understanding of the force and enable future experiments to better account for its effects.
Tiny as it is, the Casimir effect causes parts in nano- and microelectromechanical systems (NEMS and MEMS) to stick together. It confounds tabletop experimental efforts to detect exotic new forces beyond those predicted by Newtonian gravity and the Standard Model of particle physics.

Read more

__________________
1 2  >  Last»  | Page of 2  sorted by
Quick Reply

Please log in to post quick replies.



Create your own FREE Forum
Report Abuse
Powered by ActiveBoard