Archive for the ‘DARK ENERGY’ Category

Is Dark Matter a Glimpse of a Deeper Level of Reality?

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(……) String theorists and other would-be unifiers of physics face a basic problem. The theories they seek to unify, quantum field theory and Einstein’s general theory of relativity, are well-grounded and well-tested, yet mutually incompatible. Reconciling them will demand that some deeply held intuition must give way. One such intuition is that the world exists within space and time. Participants at the Kavli workshop were inclined to think that space and time are not fundamental, but emergent. The universe we seeing playing out in space and time may be just the surface level, where we float like little boats while leviathans stir in the deep.

Black holes provide the strongest argument for this point of view. The laws of gravity predict that these cosmic vacuum cleaners obey versions of the laws of thermodynamics, which is strange, because thermodynamics is the branch of physics that describes composite systems, such as gases made up of molecules. A black hole sure doesn’t look like a composite system. It just looks like a warped region of space that you would do well to stay away from. For it to be composite, space itself must be.

In that case, black holes represent a new phase of matter. Outside the hole, the universe’s “degrees of freedom”—all that its most fundamental building blocks are capable of—are in a low-energy state, forming what you might think of as a crystal, with a fixed, regular arrangement we perceive as the spacetime continuum. But inside the hole, conditions become so extreme that the continuum breaks apart. “You can make spacetime melt,” Verlinde told me. “This is really where spacetime ends. To understand what goes on, you need to use these underlying degrees of freedom.” Those degrees of freedom cannot be thought of as existing in one place or another. They transcend space. Their true venue is a ginormous abstract realm of possibilities—in the jargon, a “phase space” commensurate with their almost unimaginably rich repertoire of behaviors.(……)

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June 11, 2012 at 9:30 pm

Physicists hunt for dark forces

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Cheap colliders probe debris for hint of ‘heavy’ photon.

The Jefferson Lab’s Free-Electron Laser is a low-cost option in the bid to discover dark-sector forces.

Eric Hand
n tunnels beneath the Thomas Jefferson National Accelerator Facility in Newport News, Virginia, an accelerator whips a beam of electrons around a racetrack. Their energies are modest, but the beam is tightly packed with them — for it takes a very bright beam to detect a photon that doesn’t shine.

In a three-week experiment due to start on 24 April, the electrons will crash into a thin tungsten target at 500 million times a second, creating a cascade of short-lived particles. Amid the debris, physicists with the Heavy Photon Search (HPS) are hoping that they will find signs of something exceedingly rare: a ‘heavy’ or ‘dark’ photon. The discovery would open the door to an unseen world of dark forces and dark atoms that theorists have long speculated about — and could help to pin down the dark matter that is thought to comprise 85% of the matter in the Universe.

The HPS researchers at the Jefferson Lab are quick to concede that the experiment, like two others at the lab probing this dark sector, is a long shot that is likely to achieve little more than null results. But the reasonable price tags for such projects — about US$3 million to build and run the HPS detector — have prompted more physicists to try. “It’s always a great question in physics to go around wondering if there are more fundamental forces,” says physicist John Jaros, co-spokesman for the HPS experiment.
The dark photon, unlike conventional photons, would have mass and would be detectable only indirectly — after the dark photons have decayed into electrons and positrons (the antimatter counterparts of electrons). Yet, like the familiar photon, which carries the electromagnetic force, the dark photon would carry a force — a new fundamental force in addition to the four that we already know about. It would be the first sign of a hidden sector, which could include entire zoos of new particles, including dark matter. “It would be like when Galileo saw moons orbiting Jupiter,” says Nima Arkani-Hamed, a theorist at the Institute for Advanced Study in Princeton, New Jersey.

Theorists had hoped that the Large Hadron Collider — the world’s highest-energy (and most expensive) particle accelerator at CERN, Europe’s high-energy physics lab near Geneva, Switzerland — would open the door to new concepts such as supersymmetry, a set of theories that would resolve some of the problems in the standard model of particle physics. But, so far, it has yielded no clues, such as the dark-matter particles predicted by some supersymmetry models. “The null results are not making people happy,” says Philip Schuster, a theorist at Canada’s Perimeter Institute for Theoretical Physics in Waterloo, Ontario. “People are wondering what other possibilities are out there.”

Instead, some physicists are turning to the ‘intensity frontier’ — creating many collisions and teasing rare events from the wreckage. The electron beams at the Jefferson Lab are not the most powerful, but they are extremely intense.

The idea for a dark sector was first proposed in 1986 (B. Holdom Phys. Lett. B 166,196–198; 1986), but remained largely unexplored until a group of theorists, including Arkani-Hamed, resurrected the theory a few years ago (N. Arkani-Hamed et al. Phys. Rev. D 79, 015014; 2009). The group embellished the idea in light of results from a 2006 satellite mission called PAMELA (Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics), which had observed a puzzling excess of positrons in space. Theorists suggested that they might be spawned by dark-matter particles annihilating each other. But the heavy particles most often suggested (WIMPs, weakly interacting massive particles) would have also decayed into protons and antiprotons, which weren’t seen by PAMELA. A dark-matter particle from the dark sector — “even darker matter”, quips Arkani-Hamed — would be seen only through a decay involving the force-carrying dark photon, which would make positrons but not antiprotons.

Another motivation came from an intriguing result reported in 2004 by physicists at Brookhaven National Laboratory in Upton, New York. They found that the magnetic moment created by the spin and charge of the muon, a short-lived particle similar to an electron, did not match the predictions of the standard model. This anomaly, called the muon g-2, could also be rectified by a dark-sector force, says Arkani-Hamed. He adds that the idea is not as crazy as it sounds. “The whole set-up is totally vanilla and conservative from a theorist’s point of view,” he says.

The predictions can be tested cheaply and relatively quickly. The main 6-giga­electronvolt electron beam at Jefferson Lab has the right energy to probe the most likely mass range for a heavy photon. After the HPS’s three-week test run, the beam will be shut down for an upgrade that will double its energy. This will allow the HPS and another project, the A Prime EXperiment (APEX), to explore other parts of the dark sector in 2015. A third proposal, called DarkLight, would use the beam that drives the lab’s free-electron laser to look for heavy photons at lower energies (see ‘Feeling in the dark’).

Arkani-Hamed says that he won’t be surprised if the future path of particle physics emerges from modest experiments such as those at the Jefferson Lab, rather than from work at CERN. “It could be that these much smaller, faster, cheaper, upstart, high-intensity, low-energy experiments might actually dig up evidence for new physics before the big monsters.”…..

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April 4, 2012 at 12:45 pm


The Cosmological Constant Problem, Dark Energy, and the Landscape of String Theory

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The spectrum of the cosmological constant (vacuum energy, dark energy) in the string landscape (schematic). Each blue line represents one three-dimensional vacuum. With 10^500 vacua, the spectrum will be very dense, and many vacua will have values of  compatible with observation (red/shaded region).

Raphael Bousso
In this colloquium-level account, I describe the cosmological constant problem: why is the energy of empty space at least 60 orders of magnitude smaller than several known contributions to it from the Standard Model of particle physics?
I explain why the “dark energy” responsible for the accelerated expansion of the universe is almost certainly vacuum energy.
The second half of the paper explores a more speculative subject.
The vacuum landscape of string theory leads to a multiverse in which many different three-dimensional vacua coexist, albeit in widely separated regions.
This can explain both the smallness of the observed vacuum energy and the coincidence that its magnitude is comparable to the present matter density….
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March 5, 2012 at 7:43 am

Is Dark Energy Falsifiable?

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Carl H. Gibson (University of California at San Diego), Rudolph E. Schild (Harvard University)
Is the accelerating expansion of the Universe true, inferred through observations of distant supernovae, and is the implied existence of an enormous amount of anti-gravitational dark energy material driving the accelerating expansion of the universe also true?
To be physically useful these propositions must be falsifiable; that is, subject to observational tests that could render them false, and both fail when viscous, diffusive, astro-biological and turbulence effects are included in the interpretation of observations.
A more plausible explanation of negative stresses producing the big bang is turbulence at Planck temperatures. Inflation results from gluon viscous stresses at the strong force transition. Anti-gravitational (dark energy) turbulence stresses are powerful but only temporary. No permanent dark energy is needed.
At the plasma-gas transition, viscous stresses cause fragmentation of plasma proto-galaxies into dark matter clumps of primordial gas planets, each of which falsifies dark-energy cold-dark-matter cosmologies.
Clumps of these planets form all stars, and explain the alleged accelerating expansion of the universe as a systematic dimming error of Supernovae Ia by light scattered in the hot turbulent atmospheres of evaporated planets surrounding central white dwarf stars…….
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December 14, 2011 at 6:28 am

An interpretation of the acceleration of the universe’s expansion

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Dong-Biao Kang
We have observed the acceleration of the expansion of the universe.

To explain this phenomenon, we usually introduce the dark energy (DE) which has a negative pressure or we need to modify the Einstein’s equation to produce a term which is equivalent to the dark energy.

Are there other possibilities? Combining our previous works of statistical mechanics of self-gravitating system with the derivation of van der waals equation, we propose a different matter’s equation of state (EoS) in this paper.

Then we find that if the matter’s density is low enough, its pressure can be negative, which means that it is the matter that drives the expansion’s acceleration. So here we will not need to add the DE to the universe.

Our results also predict that the universe finally tends to be dominated by an approximate constant energy density, but its value can be smaller than DE.

The data of Supernova can not differentiate our model from the standard model, but they may indicate some deviations from LCDM……
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November 5, 2011 at 4:36 pm


Universe’s expansion may be understood without dark energy

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Least-time paths of light
by Arto Annila

Light disperses from a supernova explosion (yellow) to a site of detection (blue). As the universe expands, the light energy becomes diluted as it travels from its past, dense surroundings to its present, sparse surroundings. The light’s wavelength increases as a result of the decrease in surrounding energy density.

The variational principle in its original form á la Maupertuis is used to delineate paths of light through varying energy densities and to associate shifts in frequency and changes in momentum. The gravitational bending and Doppler shift are in this way found as mere manifestations of least-time energy dispersal. In particular, the general principle of least action due to Maupertuis accounts for the brightness of Type 1a supernovae versus redshift without introducing extraneous parameters or invoking conjectures such as dark energy. Likewise, the least-time principle explains the gravitational lensing without the involvement of additional ingredients such as dark matter. Moreover, time delays along curved geodesics relative to straight paths are obtained from the ratio of the local to global energy density. According to the principle of least action the Universe is expanding uniformly due to the irrevocable least-time consumption of diverse forms of bound energy to the lowest form of energy, i.e. the free electromagnetic radiation.

A ray of light takes the path of least time. Thi well – known principle by Pierre de Fermat is a special form of the general principle of least action (De Maupertuis 1744; Tuisku, Pernu & Annila 2009).
Namely, light, as any other form of energy in motion, will naturally select the path of propagation that will maximize the dispersal of energy (Kaila & Annila 2008). The derivation of Snell’s law by the least-time principle is a familiar textbook example (Alonso & Finn 1983).
However, does the same variations principle govern also light’s passage through the expanding Universe from a high-density distant past to the low-density near-by present?
The answer will illuminate interpretation of supernovae data (Goldhaber & Perlmutter 1998; Garnavich et al., 1998) that seems to signal for a faster expansion than is expected on the basis of known forms of energy – possibly due to dark energy (Perlmutter 2003).
Moreover, light caught bending when passing by the Sun, is a famous proof of general relativity (Einstein 1911; Berry 2001).
However, does the least-time principle also govern light’s refraction when passing by all gravitating bodies?
The answer will explain galactic gravitational lensing (Blandford & Narayan 1992) that seems to be stronger than expected on the basis of luminous matter – possibly due to dark matter (Goldsmith 1991).
According to the principle of least action, light will follow the path where the integrand of variations is at a minimum (Feynman 1965).
Customarily the integrand is a Lagrangian which, as a conserved quantity, can be used to determine stationary paths of stationary-state systems. However, the expanding Universe is an evolutionary system where light must on its way adapt to changing circumstances. Likewise, light must adjust its energy to varying surroundings, when passing by a local variation in the universal energy density.
Enlightening light’s least-time paths through changing surroundings is the objective of this study. Therefore, rather than using the conserved Lagrangian form of the action principle (Kovner 1990), its original form á la Pierre Louis Moreau de Maupertuis will be used here. In the general form of the action principle kinetic energy is integrated over time, or quivalently momentum is integrated over the path.
This form has for long been shunned but recently it has been derived from the statistical physics of open systems (Kaila & Annila 2008; Sharma & Annila 2007; Annila 2010a). Subsequently it has been used to describe diverse evolutionary processes (Mäkelä & Annila 2010; Annila & Salthe 2009; Annila & Salthe 2010; Annila, 2010b).
The principle of least time is known to be a powerful way of analyzing propagation of light through a varying energy density. However, the variations in the evolutionary trajectory to be minimized are given by Maupertuis action, whereas the commonly used Lagrangian integrand qualifies only to elucidate paths within stationary surroundings.
For this reason the results by Fermat’s principle presented here differ from those obtained via general relativity. More generally any formulation that complies with any one group
of symmetry, such as that of Poincaré, cannot break its norm which would be necessary to delineate least-time paths through varying energy densities.
When the spontaneous symmetry breaking is not understood as a nonunitary process, but the invariant form of the space-time curvature is insisted, the discrepancy between observations of evolutionary processes and predictions will be inevitable.
It will prompt one to save the unitary theory by invoking ad hoc explanations, most notably dark energy and dark matter or to propose impromptu expansions, most notably modified
gravity. In short We cannot solve problems by using the same kind of thinking we used when we created them (Calaprice 2005).
Obviously energy density gradients affect not only rays of light but also paths of bodies. To this end the principle of least action á la Maupertuis accounts also for galactic rotational curves and anomalous accelerations as well as for advancing perihelion precession (Koskela & Annila 2010; Annila 2009).
Thus the universal principle provides a holistic and self-consistent worldview in an elementary mathematical form (Annila 2010). The Universe is irrevocably processing from high-symmetry states of bound energy to states of lower and lower symmetry, eventually
attaining the lowest group U(1).
This free form of energy is electromagnetic radiation. In thermodynamic terms it makes
it makes sense to measure all bound forms of energy via
E = mc2= m/μοεο
relative to the free space, the lowest state, characterized by permittivity εo and permeability μo. This is to say that the speed of light is dictated by the surrounding energy density of any kind, most notably, by that of free space. The cosmological principle, i.e., the high degree of
homogeneity at the largest scale is, according to the thermodynamic tenet, a mere consequence of maximal dispersal of energy. It is the combustion of bound forms of
energy to the free form of energy by stars, pulsars, black holes etc. that powers the expansion. This is to say, the Big Bang did not happen – it is still going on.

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October 24, 2011 at 8:49 pm

Lectures on Cosmology and Particle Physics

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by Sean Carroll

Lecture One: Introduction to Cosmology

Lecture Two: Dark Matter

Lecture Three: Dark Energy

Lecture Four: Thermodynamics and the Early Universe

Lecture Five: Inflation and Beyond

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October 20, 2011 at 4:39 pm


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Attraction-repulsion coupled and energy conserved universe

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Ti-Pei Li

A schematic depiction for expansion history of an attraction-repulsion coupled and energy conserved universe. Marked times: ti – inflation beginning, te – generation of electromagnetic, tm – matter creation, td – starting the deceleration phase, ta – starting the acceleration phase, t0 – the present time, t’d – deceleration again, t’a – acceleration again. For clarity proposes the figure is drawn not to scale.

The discovery of an accelerating cosmic expansion rate implies that, in addition to the attractive gravity of matter, there exist in our universe some other form of energy (dark energy or cosmological constant) producing a repulsive force.
The natural interpretation of dark energy is the vacuum energy.
However, the density of vacuum energy expected by the quantum field theory is 120 orders of magnitude larger than what allowed by cosmological observations, which is called the cosmological constant problem and remains one of the most significant unsolved problems in fundamental physics.
Here we show that the huge discrepancy between theoretical expectation and observational data can be resolved by assuming that our universe is an attraction-repulsion coupled system with energy conservation, and that the pre-inflation vacuum is in balance between attraction and repulsion (a flat Minkowski spacetime, not de Sitter or anti de Sitter).
The attraction-repulsion coupling picture can also easily explain why both kinds of energy in our universe have similar magnitude today, and avoid singularity problems in general relativity and cosmology…..
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October 16, 2011 at 2:26 pm