Dark Energy: A Short Review

data combination
Michael J. Mortonson, David H. Weinberg, Martin White
The accelerating expansion of the universe is the most surprising cosmological discovery in many decades.
In this short review, we briefly summarize theories for the origin of cosmic acceleration and the observational methods being used to test these theories.
We then discuss the current observational state of the field, with constraints from the cosmic microwave background (CMB), baryon acoustic oscillations (BAO), Type Ia supernovae (SN), direct measurements of the Hubble constant (H0), and measurements of galaxy and matter clustering.
Assuming a flat universe and dark energy with a constant equation-of-state parameter w=P/ρ, the combination of Planck CMB temperature anisotropies, WMAP CMB polarization, the Union2.1 SN compilation, and a compilation of BAO measurements yields ….
… Read more at http://arxiv.org/abs/1401.0046http://arxiv.org/pdf/1401.0046v1.pdf

Leading Dark Energy Theory Incompatible with New Measurement

The latest observations of exploding stars could call into question the cosmological constant explanation of dark energy
By Clara Moskowitz
How much to read into the calculation depends on how uncertain it is, and whether systematic errors associated with the telescope and the analysis skewed the result. “It’s generally accepted that telescope calibration, supernova physics and galaxy properties are big sources of uncertainties, so everyone’s trying to figure these out in different ways,” says Daniel Scolnic of Johns Hopkins University, who led an accompanying paper estimating the data’s uncertainties. “I think that Dan did an excellent job characterizing their systematics,” says Alexander Conley of the University of Colorado at Boulder who worked on a different supernova study called the Supernova Legacy Survey that found similar results. “They still have a lot of work to do to improve the characterization for future papers, but they know that and are working on it.” However, another survey researcher, Julien Guy of University Pierre and Marie Curie in Paris, says the team may have underestimated their systematic error by ignoring an extra source of uncertainty from supernova light-curve models. He’s been in touch with the Pan-STARRS researchers, who are looking into that factor. Ultimately, most experts say the new results are tantalizing, but don’t prove the existence of new physics. “The Pan-STARRS paper presents a very thorough, careful analysis and a solid result, but it doesn’t qualitatively change our view of the cosmological parameters,” says Joshua Frieman, an astrophysicist at Fermilab in Batavia, Ill., who was not involved in the research….
Read more at www.scientificamerican.com and http://arxiv.org/abs/1310.3828

On the Trail of Dark Energy: Physicists Propose Higgs Boson ‘Portal’

One of the biggest mysteries in contemporary particle physics and cosmology is why dark energy, which is observed to dominate energy density of the universe, has a remarkably small (but not zero) value. This value is so small, it is perhaps 120 orders of magnitude less than would be expected based on fundamental physics.

Illustration of Standard Model particles. (Credit: Image courtesy of DOE/Fermi National Accelerator Laboratory)

Illustration of Standard Model particles. (Credit: Image courtesy of DOE/Fermi National Accelerator Laboratory)

Resolving this problem, often called the cosmological constant problem, has so far eluded theorists.
Now, two physicists — Lawrence Krauss of Arizona State University and James Dent of the University of Louisiana-Lafayette — suggest that the recently discovered Higgs boson could provide a possible “portal” to physics that could help explain some of the attributes of the enigmatic dark energy, and help resolve the cosmological constant problem.
In their paper, “Higgs Seesaw Mechanism as a Source for Dark Energy,” Krauss and Dent explore how a possible small coupling between the Higgs particle, and possible new particles likely to be associated with what is conventionally called the Grand Unified Scale — a scale perhaps 16 orders of magnitude smaller than the size of a proton, at which the three known non-gravitational forces in nature might converge into a single theory — could result in the existence of another background field in nature in addition to the Higgs field, which would contribute an energy density to empty space of precisely the correct scale to correspond to the observed energy density.
The paper was published online, Aug. 9, in Physical Review Letters.
Current observations of the universe show it is expanding at an accelerated rate. But this acceleration cannot be accounted for on the basis of matter alone. Putting energy in empty space produces a repulsive gravitational force opposing the attractive force produced by matter, including the dark matter that is inferred to dominate the mass of essentially all galaxies, but which doesn’t interact directly with light and, therefore, can only be estimated by its gravitational influence.
Because of this phenomenon and because of what is observed in the universe, it is thought that such ‘dark energy’ contributes up to 70 percent of the total energy density in the universe, while observable matter contributes only 2 to 5 percent, with the remaining 25 percent or so coming from dark matter.
The source of this dark energy and the reason its magnitude matches the inferred magnitude of the energy in empty space is not currently understood, making it one of the leading outstanding problems in particle physics today.
“Our paper makes progress in one aspect of this problem,” said Krauss, a Foundation Professor in ASU’s School of Earth and Space Exploration and Physics, and the director of the Origins Project at ASU. “Now that the Higgs boson has been discovered, it provides a possible ‘portal’ to physics at much higher energy scales through very small possible mixings and couplings to new scalar fields which may operate at these scales.”….

Read more at http://www.sciencedaily.com/releases/2013/08/130810063645.htm

Why is the Universe Accelerating?

sean_carrolSEAN M. CARROLL
The universe appears to be accelerating, but the reason why is a complete mystery.
The simplest explanation, a small vacuum energy (cosmological constant), raises three difficult issues: why the vacuum energy is so small, why it is not quite zero, and why it is comparable to the matter density today.
I discuss these mysteries, some of their possible resolutions, and some issues confronting future observations….
… Read more at http://www.astro.caltech.edu/~george/ay21/readings/carroll.pdf

Physics of the Dark Universe

Some Highlights of Volume 2:

Read more at http://www.journals.elsevier.com/physics-of-the-dark-universe/news/volume-2-now-online/

Can dark energy be gravitational waves?

Estimated distribution of matter and energy in the universe http://en.wikipedia.org/wiki/File:DMPie_2013.svg

Estimated distribution of matter and energy in the universe (en.wikipedia.org)

Peter L. Biermann, Benjamin C. Harms
The idea that dark energy is gravitational waves may explain its strength and its time-evolution.
A possible concept is that dark energy is the ensemble of coherent bursts (solitons) of gravitational waves originally produced when the first generation of super-massive black holes was formed.
These solitons get their initial energy as well as keep up their energy density throughout the evolution of the universe by stimulating emission from a background, a process which we model by working out this energy transfer in a Boltzmann equation approach.
New Planck data suggest that dark energy has increased in strength over cosmic time, supporting the concept here.
The transit of these gravitational wave solitons may be detectable. Key tests include pulsar timing, clock jitter and the radio background….
Read more at http://arxiv.org/pdf/1305.0498v1.pdf

First result from the AMS experiment

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…
Read more: http://press.web.cern.ch/backgrounders/first-result-ams-experiment