Archive for the ‘DARK ENERGY’ Category

First result from the AMS experiment

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The Alpha Magnetic Spectrometer (AMS) Collaboration announces the publication of its first physics result in Physical Review Letters. The AMS Experiment is the most powerful and sensitive particle physics spectrometer ever deployed in space. As seen in Figure 1, AMS is located on the exterior of the International Space Station (ISS) and since its installation on 19 May 2011 it has measured over 30 billion cosmic rays at energies up to trillions of electron volts. Its permanent magnet and array of precision particle detectors collect and identify charged cosmic rays passing through AMS from the far reaches of space. Over its long duration mission on the ISS, AMS will record signals from 16 billion cosmic rays every year and transmit them to Earth for analysis by the AMS Collaboration. This is the first of many physics results to be reported.

In the initial 18 month period of space operations, from 19 May 2011 to 10 December 2012, AMS analyzed 25 billion primary cosmic ray events. Of these, an unprecedented number, 6.8 million, were unambiguously identified as electrons and their antimatter counterpart, positrons. The 6.8 million particles observed in the energy range 0.5 to 350 GeV are the subject of the precision study reported in this first paper.

Electrons and positrons are identified by the accurate and redundant measurements provided by the various AMS instruments against a large background of protons. Positrons are clearly distinguished from this background through the robust rejection power of AMS of more than one in one million.

Currently, the total number of positrons identified by AMS, in excess of 400,000, is the largest number of energetic antimatter particles directly measured and analyzed from space. The present paper can be summarized as follows:

AMS has measured the positron fraction (ratio of the positron flux to the combined flux of positrons and electrons) in the energy range 0.5 to 350 GeV. We have observed that from 0.5 to 10 GeV, the fraction decreases with increasing energy. The fraction then increases steadily between 10 GeV to ~250 GeV. Yet the slope (rate of growth) of the positron fraction decreases by an order of magnitude from 20 to 250 GeV. At energies above 250 GeV, the spectrum appears to flatten but to study the behavior above 250 GeV requires more statistics – the data reported represents ~10% of the total expected. The positron fraction spectrum exhibits no structure nor time dependence. The positron to electron ratio shows no anisotropy indicating the energetic positrons are not coming from a preferred direction in space. Together, these features show evidence of a new physics phenomena.
Figure 1 illustrates the AMS data presented in the first publication.

Figure 1: The positron fraction measured by AMS demonstrates excellent agreement with the model described below. Even with the high statistics, 6.8 million events, and accuracy of AMS, the fraction shows no fine structure.

Figure 1: The positron fraction measured by AMS demonstrates excellent agreement with the model described below. Even with the high statistics, 6.8 million events, and accuracy of AMS, the fraction shows no fine structure.

The exact shape of the spectrum, as shown in Figure 2, extended to higher energies, will ultimately determine whether this spectrum originates from the collision of dark matter particles or from pulsars in the galaxy. The high level of accuracy of this data shows that AMS will soon resolve this issue.

Over the last few decades there has been much interest on the positron fraction from primary cosmic rays by both particle physicists and astrophysicists. The underlying reason for this interest is that by measuring the ratio between positrons and electrons and by studying the behavior of any excess across the energy spectrum, a better understanding of the origin of dark matter and other physics phenomena can be obtained.

The first AMS result has been analyzed using several phenomenological models, one of which is described in the paper and included in Figure 1. This model, with diffuse electron and positron components and a common source component, fits the AMS data surprisingly well. This agreement indicates that the positron fraction spectrum is consistent with electron positron fluxes each of which is the sum of its diffuse spectrum and a single energetic common source. In other words, a significant portion of the high‐energy electrons and positrons originate from a common source.

AMS is a magnetic spectrometer with the ability to explore new physics because of its precision, statistics, energy range, capability to identify different particles and nuclei and its long duration in space. As shown in Figure 2, the accuracy of AMS and the high statistics available distinguish the reported positron fraction spectrum from earlier experiments.

It is expected that hundreds of billions of cosmic rays will be measured by AMS throughout the lifetime of the Space Station. The volume of raw data requires a massive analysis effort. The parameters of each signal collected are meticulously reconstructed, characterized and archived before they undergo analysis by multiple independent groups of AMS physicists thus ensuring the accuracy of the physics results.

Figure 2: A comparison of AMS results with recent published measurements.

Figure 2: A comparison of AMS results with recent published measurements.

With its magnet and precision particle detectors, high accuracy and statistics, the first result of AMS, based on only ~10% of the total data expected, is clearly distinguished from earlier experiments (see References).

Background of AMS
The first publication from the AMS Experiment is a major milestone for the AMS international collaboration. Hundreds of scientists, engineers, technicians and students from all over the world have worked together for over 18 years to make AMS a reality. The collaboration represents 16 countries from Europe, Asia and North America (Finland, France, Germany, Italy, the Netherlands, Portugal, Spain, Switzerland, Romania, Russia, Turkey, China, Korea, Taiwan, Mexico and the United States) under the leadership of Nobel Laureate Samuel Ting of M.I.T. The collaboration continues to work closely with the excellent NASA AMS Project Management team from Johnson Space Center as it has throughout the entire process.

AMS is a U.S. Department of Energy sponsored particle physics experiment on the ISS under a DOE‐NASA Implementing Arrangement. AMS was constructed at universities and research institutes around the world and assembled at the European Organization for Nuclear Research, CERN, Geneva, Switzerland. It was transported to the Kennedy Space Center in August 2010 onboard a special C‐5M transport aircraft of the U.S. Air Force Air Mobility Command. AMS was launched by NASA to the ISS as the primary payload onboard the final mission of space shuttle Endeavour (STS‐134) on 16 May 2011. The crew of STS‐134, Greg Johnson, Mike Fincke, Greg Chamitoff, Drew Feustel, Roberto Vittori under the command of Mark Kelly, successfully deployed AMS as an external attachment on the U.S. ISS National Laboratory on 19 May 2011. Once installed, AMS was powered up and immediately began collecting data from primary sources in space and these were transmitted to the AMS Payload Operations Control Center (POCC). The POCC is located at CERN, Geneva, Switzerland.

Once AMS became operational, the first task for the AMS Collaboration was to ensure that all instruments and systems performed as designed and as tested on the ground. The AMS detector, with its multiple redundancies, has proven to perform flawlessly in space. Over the last 22 months in flight, AMS collaborators have gained invaluable operational experience in running a precision spectrometer in space and mitigating the hazardous conditions to which AMS is exposed as it orbits the Earth every 90 minutes. These are conditions that are not encountered by ground‐based accelerator experiments or satellite‐based experiments and require constant vigilance in order to avoid irreparable damage. These include the extreme thermal variations caused by solar effects and the re‐positioning of ISS onboard radiators and solar arrays. In addition, the AMS operators regularly transmit software updates from the AMS POCC at CERN to the AMS computers in space in order to match the regular upgrades of the ISS software and hardware.

With the wealth of data emitted by primary cosmic rays passing through AMS, the Collaboration will also explore other topics such as the precision measurements of the boron to carbon ratio, nuclei and antimatter nuclei, and antiprotons, precision measurements of the helium flux, proton flux and photons, as well as the search for new physics and astrophysics phenomena such as strangelets.

The AMS Collaboration will provide new, accurate information over the lifetime of the Space Station as the AMS detector continues its mission to explore new physics phenomena in the cosmos…
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April 3, 2013 at 4:44 pm

Dark energy from entanglement entropy

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Salvatore Capozziello, Orlando Luongo
We show that quantum decoherence, in the context of observational cosmology, can be connected to the cosmic dark energy.
The decoherence signature could be characterized by the existence of quantum entanglement between cosmological eras.
As a consequence, the Von Neumann entropy related to the entanglement process, can be compared to the thermodynamical entropy in a homogeneous and isotropic universe.
The corresponding cosmological models are compatible with the current observational bounds being able to reproduce viable equations of state without introducing {\it a priori} any cosmological constant.
In doing so, we investigate two cases, corresponding to two suitable cosmic volumes, $V\propto a^3$ and $V\propto H^{-3}$, and find two models which fairly well approximate the current cosmic speed up.
The existence of dark energy can be therefore reinterpreted as a quantum signature of entanglement, showing that the cosmological constant represents a limiting case of a more complicated model derived from the quantum decoherence….
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March 9, 2013 at 2:14 pm

Ground-Based Instruments Could Detect Cosmic Wall Structures

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Passing through. A handful of magnetometers on Earth (stars) could detect the passage of cosmic domain walls, if those walls exist and are abundant enough to affect the balance of dark matter or dark energy in the Universe.

Passing through. A handful of magnetometers on Earth (stars) could detect the passage of cosmic domain walls, if those walls exist and are abundant enough to affect the balance of dark matter or dark energy in the Universe.

Invisible sheetlike structures, which might pervade space and contribute to dark matter or dark energy, could be revealed as they pass by Earth-based detectors.

One of the potential explanations for the Universe’s mysterious dark matter and dark energy is a cosmic latticework of energetic “domain walls.” In Physical Review Letters, a team proposes the first method to directly detect these structures as the Earth passes through them. They find that if such walls exist and are abundant enough, they may be detectable by a relatively cheap set of sensitive magnetic-field detectors at several locations on Earth.

Dark matter affects the motions of galaxies, and dark energy is accelerating the expansion of the Universe. Together, these two components are estimated to make up some 95 percent of the mass-energy of the Universe, but their nature is a mystery.

To look for the missing stuff, experimenters have searched deep underground for dark matter particles streaming through the Earth. At the same time, theorists have been imagining other exotic phenomena, beyond new particles, that could tie up large amounts of energy almost invisibly. One possibility theorists have explored is a hypothetical field pervading the Universe that has one of several possible “ground state” values at every location in space. The idea is that in the hot, early Universe, the values were uncorrelated from place to place, but as the Universe cooled, large regions settled on a single value. However, between any two regions with different field values, there would be a “domain wall,” and the disruption of the field in the wall would require extra energy. These domains are analogous to those in a magnetic material like iron, where a large slab usually consists of many regions, each having a different magnetic field alignment.

Maxim Pospelov of the University of Victoria and the Perimeter Institute in Waterloo, Canada, and his colleagues wanted to see if cosmic domain walls could be detected. To account for dark matter or dark energy, Pospelov imagines domain walls that form a foamlike network whose arrangement has been more or less frozen since the early Universe cooled. For a particular choice of wall energy, the known density of dark matter and dark energy allowed Pospelov to estimate the possible size of the domains, or “bubbles,” in this foam.

If the Earth moves through this network at typical galactic speeds of a thousandth of the speed of light, then over the course of several years we could pass through many domain walls, Pospelov found. His collaborators on the new research, who build extraordinarily sensitive magnetic field detectors, considered whether these passages could be detected with current technology. The field is expected to exert a torque on atomic spins, just as magnetic fields do, so the team expressed their predictions in terms of an “effective” magnetic field. Using assumptions from other theories for the coupling of their hypothetical field to ordinary matter, the collaboration calculated that changes equivalent to perhaps a billionth of the earth’s magnetic field over a millisecond might be expected. The best modern magnetometers could indeed detect such a signal, the team reports, as well as the signals they expect for a range of other values for the theory’s parameters.

But any single event would be hard to distinguish from a random glitch in the electronics or a passing truck. To be confident in their conclusions, the team proposes looking for near-simultaneous events at perhaps five different locations around the Earth. A true domain-wall passage should show a characteristic time signature as it passes each location, they say. The experimenters have already used two existing magnetometers to show that they could detect such correlated events, and they hope to secure the modest funds needed to scale up the effort.

“It’s certainly somewhat outside of the mainstream,” says Pierre Sikivie of the University of Florida in Gainesville, who has explored related models, but he notes that experimenters need to cast a wide net in looking for dark matter and dark energy. He says a successful detection “would be very tantalizing,” but he wonders how researchers would deal with a one-time detection if it didn’t recur.
–Don Monroe
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January 14, 2013 at 8:12 pm

Theoretical Models of Dark Energy

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There is strong evidence for the existence of dark energy. Plotted are Ωm - ΩΛ (left panel) and Ω m - w (right panel) confidence regions constrained from the observations of SN Ia, CMB and BAO

There is strong evidence for the existence of dark energy. Plotted are Ωm – ΩΛ (left panel) and Ωm – w (right panel) confidence regions constrained from the observations of SN Ia, CMB and BAO

Jaewon Yoo, Yuki Watanabe
Mounting observational data confirm that about 73% of the energy density consists of dark energy which is responsible for the current accelerated expansion of the Universe. We present observational evidences and dark energy projects. We then review various theoretical ideas that have been proposed to explain the origin of dark energy; they contain the cosmological constant, modified matter models, modified gravity models and the inhomogeneous model. The cosmological constant suffers from two major problems: one regarding fine-tuning and the other regarding coincidence. To solve them there arose modified matter models such as quintessence, k-essence, coupled dark energy, and unified dark energy. We compare those models by presenting attractive aspects, new rising problems and possible solutions. Furthermore we review modified gravity models that lead to late-time accelerated expansion without invoking a new form of dark energy; they contain f(R) gravity and the Dvali-Gabadadze-Porrati model. We also discuss observational constraints on those models and on future modified gravity theories. Finally we review the inhomogeneous Lemaitre-Tolman-Bondi model that drops an assumption of the spatial homogeneity of the Universe. We also present basics of cosmology and scalar field theory, which are useful especially for students and novices to understand dark energy models.
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December 20, 2012 at 9:24 am


How Einstein Discovered Dark Energy

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Alex Harvey
In 1917 Einstein published his Cosmological Considerations Concerning the General Theory of Relativity. In it was the first use of the cosmological constant. Shortly thereafter Schröodinger presented a note providing a solution to these same equations with the cosmological constant term transposed to the right hand side thus making it part of the stress-energy tensor. Einstein commented that if Schröodinger had something more than a mere mathematical convenience in mind he should describe its properties. Then Einstein detailed what some of these properties might be. In so doing, he gave the first description of Dark Energy. We present a translation of Schrödinger’s paper and Einstein’s response….
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Read also: The Cosmological Constant

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December 1, 2012 at 6:21 pm


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Dark Energy and Dark Matter as Inertial Effects

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Serkan Zorba
A rigidly rotating model of the universe is postulated. It is shown that dark energy and dark matter are cosmic inertial effects resulting from such a cosmic rotation, corresponding to centrifugal and a combination of centrifugal and the Coriolis forces, respectively. The physics and the cosmological and galactic parameters obtained from the model closely match those attributed to dark energy and dark matter in the standard Λ-CDM model. (…)

(…) Conclusions
In summary, I have demonstrated that a rigidly rotating universe model of the universe––with an angular frequency equal to the Hubble’s constant––resolves some of most outstanding problems in cosmology today in a heartbeat.
Specifically, in the first part, I have shown that the rotary model naturally explains many seemingly mysterious and difficult issues surrounding dark energy such as the origin and extreme small value of the cosmological constant/dark energy, its
smoothness, its gravitationally repulsive nature and Hubble’s law.
The model also successfully predicts the recent onset of cosmic acceleration with a high degree of precision.
In the second part, I have shown that dark matter is also a cosmic inertial effect resulting from the rotation of the universe, specifically due to the cosmic Coriolis and centrifugal forces.
The hitherto mysterious universal acceleration parameter, and the global contributions in the local dynamics of galaxies––as encountered, but not at all understood, in observations and the observationally-backed nonstandard gravity theories of MOND, CG and MSTG––all find a natural explanation in the rotary model without the additional cost of forsaking the standard
Newton-Einstein gravity paradigm. Furthermore, the rotary model naturally produces the linear and quadratic potential terms recently deduced from galactic rotation curves as nothing but the effects of cosmic Coriolis and centrifugal forces.
My model thus has significant implications for the cosmological principle and the standard model of cosmology.
According to the rotary model, our universe appears to possess mysterious dark energy and dark matter because it is rotating, and the centrifugal and Coriolis forces due to rotation are perceived by us as dark energy and dark matter, respectively.
Furthermore, the logical antecedent of a spinning universe is a “Big Spin,” with a colossal initial angular momentum, instead of a giant spatial expansion called the Big Bang.
Spinning expands the universe with repulsive (centrifugal) forces, and naturally results in Hubble’s law.

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October 16, 2012 at 9:29 pm

Gravitational Field Equations and Theory of Dark Energy and Dark Matter

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Mathematicians offer unified theory of dark matter, dark energy, altering Einstein field equations

Sichuan University professor Tian Ma, left, and IU Department of Mathematics professor Shouhong Wang have developed a unified theory of dark matter and dark energy they believe could change our view of energy, gravitational interactions and the structure and formation of the universe

A pair of mathematicians—one from Indiana University and the other from Sichuan University in China—have proposed a unified theory of dark matter and dark energy that alters Einstein’s equations describing the fundamentals of gravity.

Shouhong Wang, a professor in the IU College of Arts and Sciences’ Department of Mathematics, and Tian Ma, a professor at Sichuan University, suggest the law of energy and momentum conservation in spacetime is valid only when normal matter, dark matter and dark energy are all taken into account. For normal matter alone, energy and momentum are no longer conserved, they argue.

While still employing the metric of curved spacetime that Einstein used in his field equations, the researchers argue the presence of dark matter and dark energy—which scientists believe accounts for at least 95 percent of the universe—requires a new set of gravitational field equations that take into account a new type of energy caused by the non-uniform distribution of matter in the universe. This new energy can be both positive and negative, and the total over spacetime is conserved, Wang said.

It is curved spacetime, along with a new scalar potential field representing the new energy density, and the interactions between the two that form the foundation for the new gravitational field equations.

“Many people have come up with different theories for dark energy,” Wang said. “Unfortunately, the mystery remains, and in fact, the nature of dark energy is now perhaps the most profound mystery in cosmology and astrophysics. It is considered the most outstanding problem in theoretical physics.

“The other great mystery concerning our universe is that it contains much more matter than can be accounted for in our visible stars. The missing mass is termed as dark matter, and despite many attempts at detecting dark matter, the mystery remains and even deepens.”

The researchers postulate that the energy-momentum tensor of normal matter is no longer conserved and that new gravitational field equations follow from Einstein’s principles of equivalence and general relativity, and the principle of Lagrangian dynamics, just as Einstein derived his field equations. Wang said the new equations were the unique outcome of the non-conservation of the energy-momentum tensor of normal matter.

When Einstein developed his theory, dark energy and dark matter had not yet been discovered, so it was natural for him to start his theory using the conservation law of energy and momentum of normal matter, Wang added.

“The difference between the new field equations and Einstein’s equations is the addition of a second-order covariant derivative of a scalar potential field,” he said. “Gravity theory is fundamentally changed and is now described by the metric of the curved spacetime, the new scalar potential field and their interactions.”

Tensors provide a concise framework for solving general relativity problems and the energy-momentum tensor quantifies the density and current of energy and momentum in spacetime. The second-order covariant derivative would be the geometric analog of a second order derivative in calculus which measures how the rate of change of a quantity is itself changing.

Associated with the scalar field is a scalar potential energy density consisting of positive and negative energies and representing a new type of energy caused by the non-uniform distribution of matter in the universe. The scalar potential energy density varies as the galaxies move and matter redistributes, affecting every part of the universe as a field.

Wang said negative energy produces attraction while the positive energy produces a repelling force fundamentally different from the four forces—gravity, electromagnetism, the weak interaction and the strong interaction—recognized in physics today.

“Most importantly, this new energy and the new field equations offer a unified theory for both dark energy and dark matter, which until now have been considered as two totally different beasts sharing only ‘dark’ in name,” he said. “Both dark matter and dark energy can now be represented by the sum of the new scalar potential energy density and the coupling energy between the energy-momentum tensor and the scalar potential field.”

The negative part of this sum represents the dark matter, which produces attraction, and the positive part represents the dark energy, which drives the acceleration of expanding galaxies, he said.

“In a nutshell, we believe that new gravity theory will change our view on energy, gravitational interactions, and the structure and formation of our universe,” Wang said.

Kevin Zumbrun, chair of the Department of Mathematics at IU Bloomington, said the new unified theory looked sound in principle.

“It is speculative at the cosmological level, since one must match with experiment, but the math is solid,” he said. “It’s a new and elegant angle on things, and if this does match experiment, it is a huge discovery. Quite exciting!”

Wang said the new field equations also lead to a modified Newtonian gravitational force formula, which shows that dark matter plays a more important role in a galactic scale at about 1,000 to 100,000 light years, but is less important in the larger scale, where dark energy will be significant (more than 10 million light years).

“This unified theory is consistent with general characterizations of dark energy and dark matter, and further tests of the theory up to measured precisions of cosmic observations are certainly crucial for an eventual validation of the theory,” Wang added.

The full research paper, “Gravitational Field Equations and Theory of Dark Energy and Dark Matter,” is available at the open access online preprint archive arXiv.

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September 7, 2012 at 4:53 pm

Euclid telescope to probe dark universe

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Euclid will conduct its surveys 1.5 million kilometres from Earth on its “night side”

Europe has given the final go-ahead to a space mission to investigate the “dark universe”.

The Euclid telescope will look deep into the cosmos for clues to the nature of dark matter and dark energy.

These phenomena dominate the Universe, and yet scientists concede they know virtually nothing about them.

European Space Agency (Esa) member states made their decision at a meeting in Paris. Euclid should be ready for launch in 2019.

Esa nations had already selected the telescope as a preferred venture in October last year, but Tuesday’s “adoption” by the Science Programme Committee (SPC) means the financing and the technical wherewithal is now in place to proceed.

The cost to Esa of building, launching and operating Euclid is expected to be just over 600m euros (£480m; $760m). Member states will provide Euclid’s visible wavelength camera and a near-infrared camera/spectrometer, taking the likely cost of the whole endeavour beyond 800m euros.

The US has been offered, and will accept, a junior role in the mission valued at around 5%. The American space agency (Nasa) will pay for this by picking up the tab for the infrared detectors needed on Euclid. A memorandum of understanding to this effect will be signed between the agencies in due course.

“We have negotiated a detailed text with Nasa, which both parties consider final, and it is ready for signature,” said Dr Fabio Favata, Esa’s head of science planning……

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June 20, 2012 at 12:59 pm