Cosmological Data Prefer An Interacting Vacuum Energy

(From Left to Right) Najla Said, Valentina Salvatelli, Marco Bruni, David Wands.

Authors: Najla Said¹, Valentina Salvatelli^1,2, Marco Bruni³, David Wands³

Affiliation:
¹Physics Department and INFN, Universita di Roma "La Sapienza", Rome, Italy
²Aix-Marseille Universite, Centre de Physique Theorique UMR 7332, France 
³Institute of Cosmology and Gravitation, University of Portsmouth, UK.

Over recent years, a growing number of high precision observations of the primordial Universe and our local cosmic neighbourhood have established a standard cosmological model, called LCDM. In LCDM, the Universe is filled with a constant dark energy, or cosmological constant (Lambda) that is responsible for the present accelerated expansion. Besides dark energy, a similar density of cold dark matter (CDM) supports the growth of large-scale structures (galaxies and clusters of galaxies) in broad agreement with the observations.

A crucial dataset for cosmological research is the Cosmic Microwave Background (CMB). This faint thermal radiation, which fills the whole sky, is a relic of the primordial Universe; it has been fundamental in supporting the homogeneity and isotropy of the Universe and in determining its composition. At the beginning of this year the Planck satellite of the European Space Agency (ESA) released a new set of CMB observations at striking precision, tightly constraining the parameters of the standard model [1].

These primordial Universe observations can be combined with a series of local Universe observations, coming from a number of galaxy surveys (SDSS [2], WiggleZ [3] and others). With these measurements we can map the distribution of structures at different epochs and use these maps to study their evolution up until the present. In particular, spectroscopic galaxy surveys enable us to quantify the clustering of the large-scale structure by measuring the so-called Redshift-Space Distortions (RSD) [4].

Since the late 1990s, the LCDM model has provided a simple framework to consistently describe local and primordial observations, but the latest precise measurements suggest that we may need to review and possibly revise this standard model. The clustering of structures, appears to be weaker than the one inferred from CMB observations assuming the standard LCDM evolution [10]. A common extension to the LCDM model is to allow the dark energy to have a non-constant density that evolves with time, but this, on its own, does not appear to resolve the conflict between different datasets [11].

In our recent work we investigated how the growth of structure is affected by an interaction between dark energy and dark matter [5,6,7]. These two components are usually assumed to evolve independently, and indeed an interaction between the two is not favored if one looks only at CMB measurements, for which the simple LCDM model fits perfectly the observations. We used the formalism of the Interacting Vacuum models, as developed in [5].

Figure 1

Our idea was to study a time-dependent interaction, but without imposing a-priori a specific time dependence. We parametrized the interaction in a simple, linear way, and studied its value at different epochs using a binned function, as sketched in Figure 1. On the x-axis we show the redshift z (a measure of time, where z=0 is today and z>0 is the past), while on the y-axis we show the dimensionless coupling parameter q_v. The purple line represents the binned model. The datasets we used for the analysis were the current CMB measurements from Planck and the RSD measurements from a number of surveys (refer to our Letter [8] for details).

The results we found suggested that an interaction at primordial epochs is not favored, but a zero coupling at recent times is excluded at 99% confidence level.

Starting from here we determined a best-fit model in which an interaction in the dark sector began around redshift z=1 (6 billion years ago) finding that, from a Bayesian point of view, this model is favored with respect to LCDM. This best-fit model is represented with the blue line in Figure 1.

We also compared our model to a cosmology with massive neutrinos [9], which is another extension of LCDM that has been proposed to solve the tension between primordial and local observations. In this case too we found that the observational evidence favors an interaction in the dark sector.

Figure 2

In Figure 2 we show the RSD dataset used in our work. On the x-axis we show the redshift z and on the y-axis the fσ₈ value, a typical measure of the growth of structure. The grey line, representing the LCDM model, predicts a higher growth than that observed, while our late-time interaction model, the blue line, fits the data much better.

The latest observations thus not only confirm that dark matter decay into dark energy is allowed, but also suggests that this interaction is actually favored with respect to the standard LCDM model.

References:
[1] P. A. R. Ade et al., ``Planck 2013 results. XVI. Cosmological parameters'', Astronomy & Astrophysics, 571, A16 (2014). Abstract.
[2] Lauren Anderson et al., ``The clustering of galaxies in the SDSS-III Baryon Oscillation Spectroscopic Survey: Baryon Acoustic Oscillations in the Data Release 10 and 11 galaxy samples''. arXiv:1312.4877 [astro-ph.CO] (2014).
[3] Chris Blake, Sarah Brough, Matthew Colless, Carlos Contreras, Warrick Couch, Scott Croom, Tamara Davis, Michael J. Drinkwater, Karl Forster, David Gilbank, Mike Gladders, Karl Glazebrook, Ben Jelliffe, Russell J. Jurek, I-hui Li, Barry Madore, D. Christopher Martin, Kevin Pimbblet, Gregory B. Poole, Michael Pracy, Rob Sharp, Emily Wisnioski, David Woods, Ted K. Wyder, H. K. C. Yee, "The WiggleZ Dark Energy Survey: the growth rate of cosmic structure since redshift z=0.9", Monthly Notices of the  Royal Astronomical Society, 415, 2876 (2011). Full Article.
[4] Lado Samushia, Beth A. Reid, Martin White, Will J. Percival, Antonio J. Cuesta, Lucas Lombriser, Marc Manera, Robert C. Nichol, Donald P. Schneider, Dmitry Bizyaev, Howard Brewington, Elena Malanushenko, Viktor Malanushenko, Daniel Oravetz, Kaike Pan, Audrey Simmons, Alaina Shelden, Stephanie Snedden, Jeremy L. Tinker, Benjamin A. Weaver, Donald G. York, Gong-Bo Zhao, "The Clustering of Galaxies in the SDSS-III DR9 Baryon Oscillation Spectroscopic Survey: Testing Deviations from Lambda and General Relativity using anisotropic clustering of galaxies'', Monthly Notices of the  Royal Astronomical Society, 429, 1514 (2013). Abstract.
[5] David Wands, Josue De-Santiago, Yuting Wang, "Inhomogeneous vacuum energy", Classical and Quantum Gravity, 29, 145017 (2012). Abstract.
[6] Claudia Quercellini, Marco Bruni, Amedeo Balbi, Davide Pietrobon, "Late universe dynamics with scale-independent linear couplings in the dark sector". Physical Review D, 78, 063527 (2008). Abstract.
[7] Valentina Salvatelli, Andrea Marchini, Laura Lopez-Honorez, Olga Mena, "New constraints on Coupled Dark Energy from the Planck satellite experiment", Physical Review D, 88, 023531 (2013). Abstract.
[8] Valentina Salvatelli, Najla Said, Marco Bruni, Alessandro Melchiorri, David Wands, "Indications of a late-time interaction in the dark sector", Physical Review Letters, 113, 181301 (2014). Abstract.
[9] Richard A. Battye, Adam Moss, "Evidence for massive neutrinos from CMB and lensing observations'', Physical Review Letters, 112, 051303 (2014). Abstract.
[10] E. Macaulay, I. K. Wehus, H. K. Eriksen, "Lower growth rate from recent redshift space distortion measurements than expected from Planck". Physical Review Letters, 111, 161301 (2013). Abstract.
[11] Eduardo J. Ruiz, Dragan Huterer, "Banana Split: Testing the Dark Energy Consistency with Geometry and Growth". arXiv:1410.5832 [astro-ph.CO] (2014).

Building A Precision Model of the Universe from the Biggest Color 3-D Map

Shirley Ho

Since 2000, the three Sloan Digital Sky Surveys (SDSS I, II, III) have surveyed well over a quarter of the night sky and produced the biggest color map of the universe in three dimensions ever. Now scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) and their SDSS colleagues, working with DOE’s National Energy Research Scientific Computing Center (NERSC) based at Berkeley Lab, have used this visual information for the most accurate calculation yet of how matter clumps together – from a time when the universe was only half its present age until now.

“The way galaxies cluster together over vast expanses of the sky tells us how both ordinary visible matter and underlying invisible dark matter are distributed, across space and back in time,” says Shirley Ho, an astrophysicist at Berkeley Lab and Carnegie Mellon University, who led the work. “The distribution gives us cosmic rulers to measure how the universe has expanded, and a basis for calculating what’s in it: how much dark matter, how much dark energy, even the mass of the hard-to-see neutrinos it contains. What’s left over is the ordinary matter and energy we’re familiar with.”

The Sloan Digital Sky Survey III surveyed 14,000 square degrees of the sky, more than a third of its total area, and delivered over a trillion pixels of imaging data. This image shows over a million luminous galaxies at redshifts indicating times when the universe was between seven and eleven billion years old, from which the sample in the current studies was selected. Click on image for best resolution. To watch animated visualizations of the luminous galaxies in the SDSS-III dataset, click here.[Credit: David Kirkby of the University of California at Irvine and the SDSS collaboration]

For the present study Ho and her colleagues first selected 900,000 luminous galaxies from among over 1.5 million such galaxies gathered by the Baryon Oscillation Spectrographic Survey, or BOSS, the largest component of the still-ongoing SDSS III. Most of these are ancient red galaxies, which contain only red stars because all their faster-burning stars are long gone, and which are exceptionally bright and visible at great distances. The galaxies chosen for this study populate the largest volume of space ever used for galaxy clustering measurements. Their brightness was measured in five different colors, allowing the redshift of each to be estimated.

“By covering such a large area of sky and working at such large distances, these measurements are able to probe the clustering of galaxies on incredibly vast scales, giving us unprecedented constraints on the expansion history, contents, and evolution of the universe,” says Martin White of Berkeley Lab’s Physics Division, a professor of physics and astronomy at the University of California at Berkeley and chair of the BOSS science survey teams. “The clustering we’re now measuring on the largest scales also contains vital information about the origin of the structure we see in our maps, all the way back to the epoch of inflation, and it helps us to constrain – or rule out – models of the very early universe.”

After augmenting their study with information from other data sets, the team derived a number of such cosmological constraints, measurements of the universe’s contents based on different cosmological models. Among the results: in the most widely accepted model, the researchers found – to less than two percent uncertainty – that dark energy accounts for 73 percent of the density of the universe.

The team’s results are presented January 11 at the annual meeting of the American Astronomical Society in Austin, Texas, and have been submitted to the Astrophysical Journal. They are currently available online [1,2].

The power of the universe

“The way mass clusters on the largest scales is graphed in an angular power spectrum, which shows how matter statistically varies in density across the sky,” says Ho. “The power spectrum gives a wealth of information, much of which is yet to be exploited.” For example, information about inflation – how the universe rapidly expanded shortly after the big bang – can be derived from the power spectrum.

Closely related to the power spectrum are two “standard rulers,” which can be used to measure the history of the expansion of the universe. One ruler has only a single mark – the time when matter and radiation were exactly equal in density.

“In the very early universe, shortly after the big bang, the universe was hot and dominated by photons, the fundamental particles of radiation,” Ho explains. “But as it expanded, it began the transition to a universe dominated by matter. By about 50,000 years after the big bang, the density of matter and radiation were equal. Only when matter dominated could structure form.”

The other cosmic ruler is also big, but it has many more than one mark in the power spectrum; this ruler is called BAO, for baryon acoustic oscillations. (Here, baryon is shorthand for ordinary matter.) Baryon acoustic oscillations are relics of the sound waves that traveled through the early universe when it was a hot, liquid-like soup of matter and photons. After about 50,000 years the matter began to dominate, and by about 300,000 years after the big bang the soup was finally cool enough for matter and light to go their separate ways.

Differences in density that the sound waves had created in the hot soup, however, left their signatures as statistical variations in the distribution of light, detectable as temperature variations in the cosmic microwave background (CMB), and in the distribution of baryons. The CMB is a kind of snapshot that can still be read today, almost 14 billion years later. Baryon oscillations – variations in galactic density peaking every 450 million light-years or so – descend directly from these fluctuations in the density of the early universe.

BAO is the target of the Baryon Oscillation Spectroscopic Survey. By the time it’s completed, BOSS will have measured the individual spectra of 1.5 million galaxies, a highly precise way of measuring their redshifts. The first BOSS spectroscopic results are expected to be announced early in 2012.

Meanwhile the photometric study by Ho and her colleagues deliberately uses many of the same luminous galaxies but derives redshifts from their brightnesses in different colors, extending the BAO ruler back over a previously inaccessible redshift range, from z = 0.45 to z = 0.65 (z stands for redshift).

“As an oscillatory feature in the power spectrum, not many things can corrupt or confuse BAO, which is why it is considered one of the most trustworthy ways to measure dark energy,” says Hee-Jong Seo of the Berkeley Center for Cosmological Physics at Berkeley Lab and the UC Berkeley Department of Physics, who led BAO measurement for the project. “We call BAO a standard ruler for a good reason. As dark energy stretches the universe against the gravity of dark matter, more dark energy places galaxies at a larger distance from us, and the BAO imprinted in their distribution looks smaller. As a standard ruler the true size of BAO is fixed, however. Thus the apparent size of BAO gives us an estimate of the cosmological distance to our target galaxies – which in turn depends on the properties of dark energy.”

Says Ho, “Our study has produced the most precise photometric measurement of BAO. Using data from the newly accessible redshift range, we have traced these wiggles back to when the universe was about half its present age, all the way back to z = 0.54.”

Seo adds, “And that’s to an accuracy within 4.5 percent.”

Reining in the systematics

“With such a large volume of the universe forming the basis of our study, precision cosmology was only possible if we could control for large-scale systematics,” says Ho. Systematic errors are those with a physical basis, including differences in the brightness of the sky, or stars that mimic the colors of distant galaxies, or variations in weather affecting “seeing” at the SDSS’s Sloan Telescope – a dedicated 2.5 meter telescope at the Apache Point Observatory in southern New Mexico.

After applying individual corrections to these and other systematics, the team cross-correlated the effects on the data and developed a novel procedure for deriving the best angular power-spectrum of the universe with the lowest statistical and systematic errors.

With the help of 40,000 central-processing-unit (CPU) hours at NERSC and another 20,000 CPU hours on the Riemann computer cluster operated by Berkeley Lab’s High-Performance Computing Services group, these powerful computers and algorithms enabled the team to use all the information from galactic clustering in a huge volume of sky, including the full shape of the power spectrum and, independently, BAO, to get excellent cosmological constraints. The data as well as the analysis output are stored at NERSC.

“Our dataset is purely imaging data, but our results are competitive with the latest large-scale spectroscopic surveys,” Ho says. “What we lack in redshift precision, we make up in sheer volume. This is good news for future imaging surveys like the Dark Energy Survey and the Large Synoptic Survey Telescope, suggesting they can achieve significant cosmological constraints even compared to future spectroscopy surveys.”

References:
[1] “Clustering of Sloan Digital Sky Survey III photometric luminous galaxies: The measurement, systematics, and cosmological implications,” by Shirley Ho, Antonio Cuesta, Hee-Jong Seo, Roland de Putter, Ashley J. Ross, Martin White, Nikhil Padmanabhan, Shun Saito, David J. Schlegel, Eddie Schlafly, Uroŝ Seljak, Carlos Hernández-Monteagudo, Ariel G. Sánchez, Will J. Percival, Michael Blanton, Ramin Skibba, Don Schneider, Beth Reid, Olga Mena, Matteo Viel, Daniel J. Eisenstein, Francisco Prada, Benjamin Weaver, Neta Bahcall, Dimitry Bizyaev, Howard Brewinton, Jon Brinkman, Luiz Nicolaci da Costa, John R. Gott, Elena Malanushenko, Viktor Malanushenko, Bob Nichol, Daniel Oravetz, Kaike Pan, Nathalie Palanque-Delabrouille, Nicholas P. Ross, Audrey Simmons, Fernando de Simoni, Stephanie Snedden,and Christophe Yeche (submitted to Astrophysical Journal). arXiv:1201.2137.
[2] “Acoustic scale from the angular power spectra of SDSS-III DR8 photometric luminous galaxies,” by Hee-Jong Seo, Shirley Ho, Martin White, Antonio J. Cuesta, Ashley J. Ross, Shun Saito, Beth Reid, Nikhil Padmanabhan, Will J. Percival, Roland de Putter, David J. Schlegel, Daniel J. Eisenstein, Xiaoying Xu, Donald P. Schneider, Ramin Skibba, Licia Verde, Robert C. Nichol, Dmitry Bizyaev, Howard Brewington, J. Brinkmann, Luiz Alberto Nicolai da Costa, J. Richard Gott III, Elena Malanushenko, Viktor Malanushenko, Dan Oravetz, Nathalie Palanque-Delabrouille, Kaike Pan, Francisco Prada, Nicholas P. Ross, Audrey Simmons, Fernando Simoni, Alaina Shelden, Stephanie Snedden, and Idit Zehavi (submitted to Astrophysical Journal). arXiv:1201.2172 .

[This report is written by Paul Preuss of Lawrence Berkeley National Laboratory]

Looking for a Dark Matter Signature in the Sun’s Interior

Ilídio Lopes

[This is an invited article based on the author's work in collaboration with Joseph Silk of the University of Oxford -- 2Physics.com]

Author: Ilídio Lopes
Affiliation: Centro Multidisciplinar de Astrofísica, Instituto Superior Técnico, Lisboa, Portugal;
Departamento de Física, Universidade de Évora, Évora, Portugal.

The standard concordance cosmological model of the Universe firmly established that 85% of its mass is constituted by cold, non-baryonic particles which are almost collisionless. During its evolution, the Universe formed a complex network of dark matter haloes, where baryons are gravitationally trapped, leading to the formation of galaxies and stars, including our own Galaxy and our Sun. There are many particle physics candidates for dark matter, for which their specific mass and other properties are still unknown. Among these candidates, the neutralino, a fundamental particle proposed by supersymmetric particle physics models, seems to be the more suitable candidate. The neutralino is a weak interacting massive particle with a present day relic thermal abundance determined by the annihilating dark matter freeze-out in the primordial universe.

Among other celestial’s bodies, the Sun is a privileged place to look for dark matter particles, due to its proximity to the Earth. More significantly, its large mass – which constitutes 99% of the mass of the solar system - creates a natural local trap for the capture of dark matter particles. Present day simulations show that dark matter particles in our local dark matter halo, depending on their mass and other intrinsic properties, can be gravitationally captured by the Sun and accumulate in significant amounts in its core. By means of helioseismology and solar neutrinos we are able to probe the physics in the Sun’s interior, and by doing so, we can look for a dark matter signature.

Neutrinos, once produced in the nuclear reactions of the solar core, will leave the Sun travelling to Earth in less than 8 minutes. These neutrinos stream freely to Earth, subject only to interactions with baryons in a weak scale with a typical scattering cross section of the order of 10^-44 cm², and hence are natural “messengers” of the physical processes occurring in the Sun’s deepest layers. In a paper to be published in the scientific journal “Science” [1], Ilidio Lopes (from Évora University and Instituto Superior Técnico) and Joseph Silk (from Oxford University) suggest that the presence of dark matter particles in the Sun’s interior, depending upon their mass among other properties, can cause a significant drop in its central temperature, leading to a decrease in the neutrino fluxes being produced in the Sun’s core. The calculations have shown that, in some dark matter scenarios, an isothermal solar core is formed. In another paper published in “The Astrophysical Journal Letters” [2], the same authors suggest that, through the detection of gravity waves in the Sun’s interior, Helioseismology can also independently test the presence of dark matter in the Sun’s core.

The new generation of solar neutrino experiments will be able to measure the neutrino fluxes produced in different locations of the Sun’s core. The Borexino and SNO experiments are starting to measure the neutrino fluxes produced at different depths of the Sun’s interior by means of the nuclear reactions of the proton-proton chain. Namely these are pp-ν, ⁷Be-ν and ⁸B-ν electronic neutrinos, among others. The high precision measurements expected to be obtained by such neutrino experiments will provide an excellent tool for testing the existence of dark matter in the Sun’s core. In the near future, it is expected that the measurements of pep-ν neutrino fluxes and neutrinos from the CNO cycle will also be measured by the Borexino detector or by the upcoming experiments SNO+ or LENA.

This work is supported in part by Fundação para a Ciência e a Tecnologia and Fundação Calouste Gulbenkian.

References:
[1] Ilídio Lopes, Joseph Silk, ''Neutrino Spectroscopy Can Probe the Dark Matter Content in the Sun'', Science, DOI: 10.1126/science.1196564, in press. Abstract.
[2] Ilídio Lopes, Joseph Silk, ''Probing the Existence of a Dark Matter Isothermal Core Using Gravity Modes'', The Astrophysical Journal Letters, Volume 722, Issue 1, pp. L95-L99 (2010), DOI:10.1088/2041-8205/722/1/L95. Abstract.

General Relativity Is Valid On Cosmic Scale

Uros Seljak [photo courtesy: University of California, Berkeley]

An analysis of more than 70,000 galaxies by University of California, Berkeley, University of Zurich and Princeton University physicists demonstrates that the universe – at least up to a distance of 3.5 billion light years from Earth – plays by the rules set out 95 years ago by Albert Einstein in his General Theory of Relativity.

By calculating the clustering of these galaxies, which stretch nearly one-third of the way to the edge of the universe, and analyzing their velocities and distortion from intervening material, the researchers have shown that Einstein's theory explains the nearby universe better than alternative theories of gravity.

One major implication of the new study is that the existence of dark matter is the most likely explanation for the observation that galaxies and galaxy clusters move as if under the influence of some unseen mass, in addition to the stars astronomers observe.

A partial map of the distribution of galaxies in the Sloan Digital Sky Survey, going out to a distance of 7 billion light years. The amount of galaxy clustering that we observe today is a signature of how gravity acted over cosmic time, and allows as to test whether general relativity holds over these scales. (M. Blanton, Sloan Digital Sky Survey)

"The nice thing about going to the cosmological scale is that we can test any full, alternative theory of gravity, because it should predict the things we observe," said co-author Uros Seljak, a professor of physics and of astronomy at UC Berkeley and a faculty scientist at Lawrence Berkeley National Laboratory who is currently on leave at the Institute of Theoretical Physics at the University of Zurich. "Those alternative theories that do not require dark matter fail these tests."

In particular, the tensor-vector-scalar gravity (TeVeS) theory, which tweaks general relativity to avoid resorting to the existence of dark matter, fails the test.

The result conflicts with a report late last year that the very early universe, between 8 and 11 billion years ago, did deviate from the general relativistic description of gravity.

Seljak and his current and former students, including first authors Reinabelle Reyes, a Princeton University graduate student, and Rachel Mandelbaum, a recent Princeton Ph.D. recipient, report their findings in the March 11 issue of the journal Nature [1]. The other co-authors are Tobias Baldauf, Lucas Lombriser and Robert E. Smith of the University of Zurich, and James E. Gunn, professor of physics at Princeton and father of the Sloan Digital Sky Survey.

Einstein's General Theory of Relativity holds that gravity warps space and time, which means that light bends as it passes near a massive object, such as the core of a galaxy. The theory has been validated numerous times on the scale of the solar system, but tests on a galactic or cosmic scale have been inconclusive.

"There are some crude and imprecise tests of general relativity at galaxy scales, but we don't have good predictions for those tests from competing theories," Seljak said.

An image of a galaxy cluster in the Sloan Digital Sky Survey, showing some of the 70,000 bright elliptical galaxies that were analyzed to test general relativity on cosmic scales. (Sloan Digital Sky Survey)

Such tests have become important in recent decades because the idea that some unseen mass permeates the universe disturbs some theorists and has spurred them to tweak general relativity to get rid of dark matter. TeVeS, for example, says that acceleration caused by the gravitational force from a body depends not only on the mass of that body, but also on the value of the acceleration caused by gravity.

The discovery of dark energy, an enigmatic force that is causing the expansion of the universe to accelerate, has led to other theories, such as one dubbed f(R), to explain the expansion without resorting to dark energy.

Tests to distinguish between competing theories are not easy, Seljak said. A theoretical cosmologist, he noted that cosmological experiments, such as detections of the cosmic microwave background, typically involve measurements of fluctuations in space, while gravity theories predict relationships between density and velocity, or between density and gravitational potential.

"The problem is that the size of the fluctuation, by itself, is not telling us anything about underlying cosmological theories. It is essentially a nuisance we would like to get rid of," Seljak said. "The novelty of this technique is that it looks at a particular combination of observations that does not depend on the magnitude of the fluctuations. The quantity is a smoking gun for deviations from general relativity."

Three years ago, a team of astrophysicists led by Pengjie Zhang of Shanghai Observatory suggested using a quantity dubbed EG to test cosmological models. EG reflects the amount of clustering in observed galaxies and the amount of distortion of galaxies caused by light bending as it passes through intervening matter, a process known as weak lensing. Weak lensing can make a round galaxy look elliptical, for example.

"Put simply, EG is proportional to the mean density of the universe and inversely proportional to the rate of growth of structure in the universe," he said. "This particular combination gets rid of the amplitude fluctuations and therefore focuses directly on the particular combination that is sensitive to modifications of general relativity."

Using data on more than 70,000 bright, and therefore distant, red galaxies from the Sloan Digital Sky Survey, Seljak and his colleagues calculated EG and compared it to the predictions of TeVeS, f(R) and the cold dark matter model of general relativity enhanced with a cosmological constant to account for dark energy.

The predictions of TeVeS were outside the observational error limits, while general relativity fit nicely within the experimental error. The EG predicted by f(R) was somewhat lower than that observed, but within the margin of error.

In an effort to reduce the error and thus test theories that obviate dark energy, Seljak hopes to expand his analysis to perhaps a million galaxies when SDSS-III's Baryon Oscillation Spectroscopic Survey (BOSS), led by a team at LBNL and UC Berkeley, is completed in about five years. To reduce the error even further, by perhaps as much as a factor of 10, requires an even more ambitious survey called BigBOSS, which has been proposed by physicists at LBNL and UC Berkeley, among other places.

Future space missions, such as NASA's Joint Dark Energy Mission (JDEM) and the European Space Agency's Euclid mission, will also provide data for a better analysis, though perhaps 10-15 years from now.

Seljak noted that these tests do not tell astronomers the actual identity of dark matter or dark energy. That can only be determined by other types of observations, such as direct detection experiments.

Reference
[1] Reinabelle Reyes, Rachel Mandelbaum, Uros Seljak, Tobias Baldauf, James E. Gunn, Lucas Lombriser, Robert E. Smith, "Confirmation of general relativity on large scales from weak lensing and galaxy velocities", Nature, 464, 256-258 (2010). Abstract.

[This report is written by Robert Sanders of University of California, Berkeley]

The Shadows of Gravity

Jose A. R. Cembranos

[This is an invited article based on the author's recently published work -- 2Physics.com]

Author: Jose A. R. Cembranos
Affiliation: William I. Fine Theoretical Physics Institute, University of Minnesota in Minneapolis, USA

Many authors have tried to explain the dark sectors of the cosmological model as modifications of Einstein’s gravity (EG). Dark Matter (DM) and Dark Energy (DE) are the main sources of the cosmological evolution at late times. They dominate the dynamics of the Universe at low densities or low curvatures. Therefore it is reasonable to expect that an infrared (IR) modification of EG can lead to a possible solution of these puzzles. However, it is in the opposite limit, at high energies (HE), where EG needs corrections from a quantum approach. These natural ultraviolet (UV) modifications of gravity are usually thought to be related to inflation or to the Big Bang singularity. In a recent work, I have shown that DM can be explained with HE modifications of EG. I have used an explicit model: R² gravity and study its possible experimental signatures [1].

Einstein’s General Relativity describes the classical gravitational interaction in a very successful way by the metric tensor of the space-time through the Einstein-Hilbert action (EHA). This theory is particularly beautiful and the action particularly simple, since it contains only one term proportional to the scalar curvature. The proportionality parameter which multiplies this term, defines the Newton’s constant of gravitation and the typical scale of gravity. This magnitude is known as the Planck scale and its approximated energy value is 10¹⁹ Giga-electronvolts, which is equivalent to a distance of 10^-35 meters.

However, the inconsistency of quantum computations within the gravitational theory described by the EHA demands its modification at HE. Quantum radiative corrections produced by standard matter provide divergent terms that are constant, linear, and quadratic in the Riemann curvature tensor of the space-time. The constant divergence can be regularized by the renormalization of the cosmological constant, which may explain the Dark Energy. The linear term is absorbed in the renormalization of the Planck scale itself. On the contrary, the quadratic terms are not included in the standard gravitational action. If these quantum corrections are not cancelled by invoking new symmetries, these terms need to be taken into account for the study of gravity at HE [2]. Indeed, these terms are also produced by radiative corrections coming from the own EG. Unfortunately, the gravitational corrections do not stop at this order as the associated with the matter content. There are cubic terms, quartic terms, etc. All these local quantum corrections are divergent and the fact that there is a non finite number of them implies that the theory is non-renormalizable. We know how to deal with gravity as an effective field theory, working order by order, but we cannot access higher energies than the Planck scale by using this effective approach [2]. In any case, the Planck scale is very high, and unreachable experimentally so far.

Inspired by this effective field theory point of view, which identifies higher energy corrections with higher curvature terms, I have studied the viability of a solution to the missing matter problem from the UV completion of gravity. As I have explained above, the first HE modification to EG is provided by the inclusion of quadratic terms in the curvature of the space-time geometry. The most general quadratic action supports, in addition to the usual massless spin-two graviton, a massive spin-two and a massive scalar mode, with a total of eight degrees of freedom (in the physical gauge [3]). In fact, this gravitational theory is renormalizable [3]. However, the massive spin-two gravitons are ghost-like particles that generate new unitarity violations, breaking of causality, and important instabilities.

In any case, there is a non-trivial quadratic extension of EG that is free of ghosts and phenomenologically viable. It is the so called R² gravity since it is defined by the only addition of a term proportional to the square of the scalar curvature to the EHA. This term by itself does not improve the UV behaviour of EG but illustrates the idea in a minimal way. This particular HE modification of EG introduces a new scalar graviton that can provide the solution to the DM problem.

In this model, the new scalar graviton has a well defined coupling to the standard matter content and it is possible to study its phenomenology and experimental signatures [1] [3][4]. Indeed, this DM candidate could be considered as a superweakly interacting massive particle (superWIMP [5]) since its interactions are gravitational, i.e. it couples universally to the energy-momentum tensor with Planck suppressed couplings. It means that the new scalar graviton mediates an attractive Yukawa force between two non-relativistic particles with strength similar to Newton’s gravity. Among other differences, this new component of the gravitational force has a finite range, shorter than 0.1 millimeters, since the new scalar graviton is massive.

This is the most constraining lower bound on the mass of the scalar mode and it is independent of any supposition about its abundance. On the contrary, depending on its contribution to the total amount of DM, its mass is constrained from above. I have shown that it cannot be much heavier than twice the mass of the electron. If that is the case, this graviton decays in an electron-positron pair. These positrons annihilate producing a flux of gamma rays that we should have observed. In fact, the SPI spectrometer on the INTEGRAL (International Gamma-ray Astrophysics Laboratory) satellite, has observed a flux of gamma rays coming from the galactic centre (GC), whose characteristics are fully consistent with electron-positron annihilation [6].

If the mass of the new graviton is tuned close to the electron-positron production threshold, this line could be the first observation of R² gravity. The same gravitational DM can explain this observation with a less tuned mass and a lower abundance. For heavier masses, the gamma ray spectrum originated by inflight annihilation of the positrons with interstellar electrons is even more constraining than the 511 keV photons [7].

On the contrary, for lighter masses, the only decay channel that may be observable is in two photons. It is difficult to detect these gravitational decays in the isotropic diffuse photon background (iDPB) [8]. A most promising analysis is associated with the search of gamma-ray lines from localized sources, as the GC. The iDPB is continuum since it suffers the cosmological redshift, but the mono-energetic photons originated by local sources may give a clear signal of R² gravity [1].

In conclusion, I have analyzed the possibility that the DM origin resides in UV modifications of gravity [1]. Although, strictly speaking, my results are particular of R² gravity, I think they are qualitatively general with a minimum set of assumptions about the gravitational sector. In any case, different approaches to try to link our ignorance about gravitation with the dark sectors of standard cosmology can be taken [9], and it is a very interesting subject which surely deserves further investigations.

This work is supported in part by DOE Grant No. DOE/DE-FG02-94ER40823, FPA 2005-02327 project (DGICYT, Spain), and CAM/UCM 910309 project.

References

[1] J. A. R. Cembranos, ‘Dark Matter from R² Gravity’ Phys. Rev. Lett. 102, 141301 (2009). Abstract

[2] N. D. Birrell and P. C. W. Davies, 'Quantum Fields In Curved Space’ (Cambridge Univ. Pr, 1982); J. F.Donoghue, ‘General Relativity As An Effective Field Theory: The Leading Quantum Corrections’ Phys. Rev. D 50, 3874 (1994) Abstract; A. Dobado, et al., ‘Effective lagrangians for the standard model’ (Springer-Verlag, 1997).

[3] K. S. Stelle, ‘Renormalization Of Higher Derivative Quantum Gravity’ Phys. Rev. D 16, 953 (1977) Abstract; K.S. Stelle, ‘Classical Gravity With Higher Derivatives’ Gen Rel. Grav. 9, 353 (1978) Abstract.

[4] A. A. Starobinsky, ‘A New Type of Isotropic Cosmological Models Without Singularity’ Phys. Lett. B 91, 99 (1980) Abstract; S. Kalara, N. Kaloper and K. A. Olive, ‘Theories of Inflation and Conformal Transformations’ Nucl. Phys. B 341, 252 (1990) Abstract; J. A. R. Cembranos, ‘The Newtonian Limit at Intermediate Energies’ Phys. Rev. D 73, 064029 (2006) Abstract.

[5] J. L. Feng, A. Rajaraman and F. Takayama, ‘Superweakly-Interacting Massive Particles’ Phys. Rev. Lett. 91, 011302 (2003) Abstract; J. A. R. Cembranos,Jonathan L. Feng, Arvind Rajaraman, and Fumihiro Takayama,‘SuperWIMP Solutions to Small Scale Structure Problems’ Phys. Rev. Lett. 95, 181301 (2005) Abstract.

[6] B. J. Teegarden et al., 'INTEGRAL/SPI Limits on Electron-Positron Annihilation Radiation from the Galactic Plane’ Astrophys. J. 621, 296 (2005) Article.

[7] J. F. Beacom and H. Yuksel, ‘Stringent Constraint on Galactic Positron Production’ Phys. Rev. Lett. 97, 071102 (2006) Abstract.

[8] J. A. R. Cembranos, J. L. Feng and L. E. Strigari, ‘Resolving Cosmic Gamma Ray Anomalies with Dark Matter Decaying Now’ Phys. Rev. Lett. 99, 191301 (2007) Abstract; J. A. R. Cembranos and L. E. Strigari, ‘Diffuse MeV Gamma-rays and Galactic 511 keV Line from Decaying WIMP Dark Matter’ Phys. Rev. D 77, 123519 (2008) Abstract.

[9] J. A. R. Cembranos, A. Dobado and A. L. Maroto, ‘Brane-World Dark Matter’ Phys. Rev. Lett. 90, 241301 (2003) Abstract; ‘Dark Geometry’ Int. J. Mod. Phys. D 13, 2275 (2004) arXiv:hep-ph/0405165.

Discovery of an Unexpected Surplus of Cosmic Ray Electrons at Very High Energy (300-800 GeV)

John Wefel [Photo courtesy: Louisianna State University]

In an article published in today's issue of 'Nature', a team of researchers from the Advanced Thin Ionization Calorimeter, or ATIC collaboration, have announced the discovery of an unexpected surplus of cosmic ray electrons at very high energy – 300-800 billion electron volts – which must come from a previously unidentified source near the Earth’s solar system.

John P. Wefel, principal investigator for the ATIC project and professor in the Department of Physics & Astronomy at Louisianna State University, said,“This electron excess cannot be explained by the standard model of cosmic ray origin, in which electrons are accelerated in sources such as supernova remnants and then propagate through the galaxy to us. Rather, there must be another source relatively near us that is producing these additional particles.”

According to the paper, this source would need to be within about 3,000 light years of the sun and could be an exotic object such as a pulsar, mini-quasar, supernova remnant or even an intermediate mass black hole. “The trouble with this explanation is that there are very few such objects close to the solar system and none have been observed with characteristics that could fit our results,” said Prof. Wefel. “However, we cannot rule out this possibility, and the ATIC results may be the first indication of a very interesting object near our solar system waiting to be studied by other instruments.”

An alternative explanation is that the surplus of high energy electrons might result from the annihilation of very exotic particles put forward to explain dark matter. Over the last several decades, scientists have learned that the kind of material making up the world around us only accounts for about 5% of the mass composition of the universe. Close to 70% of the universe is composed of dark energy – so called because its nature is unknown – which appears to be causing the universe’s expansion to accelerate. The remaining 25% acts gravitationally just like regular matter, but does nothing else so is normally not visible and consequently is referred to as dark matter.

T. Gregory Guzik [Photo courtesy: Louisianna State University]

The nature of dark matter is not understood, but several theories that attempt to describe how gravity works at very small, quantum distances predict exotic particles that could be good dark matter candidates and which, upon annihilation with each other, produce normal particles that scientists can observe. T. Gregory Guzik, ATIC co-investigator, said,“One such predicted particle has annihilation characteristics that would produce a very good fit for the ATIC results. If true, this would be a major advance in our understanding of dark matter and its role in the universe.”

However, the trouble with this model is that the ATIC result would require the dark matter annihilation rate to be 200 times larger than that calculated by some theoreticians. “This might be possible if dark matter is found in clumps, and one of these clumps is very close to our solar system", said Prof. Guzik.

It may take quite some time before having a full understanding of the nature and origin of these very high energy cosmic rays, but the current result certainly paves the way for a new exciting physics in the 100-500 GeV region of the cosmic-ray spectrum.

About ATIC

ATIC in external frame [Photo courtesy: ATIC Collaboration]

The 4,300-pound ATIC experiment was designed to be carried to an altitude of about 124,000 feet above Earth in order to study the cosmic rays that would otherwise be absorbed into the atmosphere. The original purpose of ATIC was to investigate where cosmic rays come from and how they are accelerated to such high speeds.

ATIC is an international collaboration of researchers from LSU, University of Maryland, Marshall Space Flight Center, Pruple Mountain Observatory in China, Moscow State University in Russia and Max-Planck Institute for Solar System Research in German. ATIC is supported in the United States by NASA and flights are conducted under the auspices of the Balloon Program Office at Wallops Flight Facility by the staff of the Columbia Scientific Balloon Facility.

Reference
"An excess of cosmic ray electrons at energies of 300–800 GeV"
J. Chang, J. H. Adams, H. S. Ahn, G. L. Bashindzhagyan, M. Christl, O. Ganel, T. G. Guzik, J. Isbert, K. C. Kim, E. N. Kuznetsov, M. I. Panasyuk, A. D. Panov, W. K. H. Schmidt, E. S. Seo, N. V. Sokolskaya, J. W. Watts, J. P. Wefel, J. Wu & V. I. Zatsepin,
Nature 456, 362-365 (20 November 2008), Abstract.

High Energy Physics: 5 Needed Breakthroughs -- Michael Dine

[Professor Michael Dine of the University of California at Santa Cruz is today's guest in our ongoing feature '5 Breakthroughs'. He is also a faculty member at the university's Santa Cruz Institute for Particle Physics (SCIPP).

In his long career spanning about 3 decades (He got his PhD from Yale University in 1978), Prof. Dine made major contributions in the areas of supersymmetry, string theory, and other efforts to develop a "new physics" beyond the standard model of particle physics.

He has been one of the principal contributors (with various collaborators) to the set of ideas associated with supersymmetry, and was among the first to propose that supersymmetry might well be broken at these energy scales. Prof. Dine developed some of the first potentially realistic models of supersymmetry phenomenology, and was among the first to explore the dynamics of supersymmetric theories, uncovering an array of surprising phenomena, some of potential relevance to experiments, and others of interest to mathematicians and more theoretically minded physicists. In recent years, he developed a proposal for the phenomenology of supersymmetry which has become a standard for both theoretical and experimental analyses. Currently, he is engaged in a number of projects exploring the experimental possibilities for the Large Hadron Collider (LHC).

Prof. Dine also made significant contributions in superstring theory. Most of his work has been motivated by the hope of making specific predictions from the theory for accelerators, but in the course of these efforts, he made several important contributions to the overall theoretical structure. Much of his current effort is involved with trying to understand whether one can make predictions from this theory relevant to the Large Hadron Collider (LHC). At the moment, he believes there is a promising (but not certain), approach, based on a popular set of ideas commonly referred to as the `landscape'.

In December, 2007 issue of 'Physics Today', Prof. Dine provided an excellent account of the relationship between string theory and particle experiments in an article entitled "String Theory in the era of the Large Hadron Collider" (p.33, Article Link).

He also authored a widely acclaimed book on this topic: "Supersymmetry and String Theory: Beyond the Standard Model" (Cambridge University, 2007).

In the field of cosmology, he made significant contributions to the theory of inflation, and to ideas about the dark energy and dark matter. Simultaneously with others, he proposed the axion as a dark matter candidate, which has remained, over the years, one of the two most plausible possibilities (the other arising in supersymmetric theories). He also proposed one of the most widely studied ideas for understanding the origin of the matter-antimatter asymmetry (known as the Affleck-Dine mechanism) explaining why there was not, initially, an equal amount of matter and antimatter, which could have simply annihilated each other.

It's our pleasure to present this list of 5 most important breakthroughs that Prof. Dine would like to see in the physics of elementary particles.
-- 2Physics.com ]

Five needed breakthroughs in elementary particle physics

1) Determination of the origin of electroweak symmetry breaking – the masses of the W and Z bosons, quarks and leptons. Is it a single Higgs field (particle), as in the simplest version of the standard model? Or is it associated with supersymmetry, large or warped extra dimensions, or something else? This question should be settled over the next three to five years by the Large Hadron Collider at CERN, due to be commissioned late this year.

2) Identifying the dark matter. There are several plausible, well-motivated candidates coming from particle physics: the lightest supersymmetric particle (LSP), the axion (a hypothetical particle seemingly required to understand features of the strong nuclear force), and others. There are ongoing, dedicated searches for both the LSP and the axion. If the LHC discovers supersymmetry, there is a good chance we will discover the dark matter particle in underground experiments, and we will be able to study in some detail how this particle was produced at the earliest stages of the big bang. The axion searches also have a real chance of finding something, if the axion is the dark matter, though detectors with a broader reach may be necessary.

3) Theoretically, one urgent question is: does string theory predict that supersymmetry, warping, or something else is responsible for electroweak symmetry breaking? Can we settle this question theoretically before the LHC? Can we make more detailed predictions? Recent developments associated with the string landscape suggest this might be possible, but the problem is challenging.

4) There are many problems of quark and lepton flavor (the occurrence of several types of quarks and leptons, and the puzzling features of their masses and couplings) which we would like to understand. What is the scale of baryon number violation? What can we understand, theoretically and experimentally, about the origin of neutrino mass? Can we develop a compelling theory, which explains the very different features of the charged fermion masses and those of the neutrinos? Can we establish experimentally the nature of the neutrino masses? Can we decide that leptogenesis, and not, say, coherent effects associated with supersymmetry, are responsible for the asymmetry between matter and antimatter in the universe?

5) Theoretically and experimentally, what more can we learn about inflation, the period of rapid expansion in the very early universe for which there is growing observational evidence, as well as strong theoretical arguments? At a microscopic level, we are far from understanding how inflation comes about. All existing models have troubling features. Can we get beyond this situation? Can supersymmetry or string theory help? If we have improved theories, they will be subject to some experimental tests; how far can we go?

High Energy Physics : 5 Needed Breakthroughs -- Mark Wise

[In the ongoing feature '5 Breakthroughs', our guest today is Mark Wise, the John A. McCone Professor of High Energy Physics at California Institute of Technology.

Prof. Wise is a fellow of the American Physical Society, and member of the American Academy of Arts and Sciences and the National Academy of Sciences. He was a fellow of the Alfred P. Sloan Foundation from 1984 to 1987.

Although Prof. Wise has done some research in cosmology and nuclear physics, his interests are primarily in theoretical elementary particle physics. Much of his research has focused on the nature and implications of the symmetries of the strong and weak interactions. He is best known for his role in the development of heavy quark effective theory (HQET), a mathematical formalism that has allowed physicists to make predictions about otherwise intractable problems in the theory of the strong nuclear interactions.

To provide a background of his current research activities, Prof. Wise said,"Currently we have a theory for the strong, weak and electromagnetic interactions of elementary particles that has been extensively tested in experiments. It is usually called the standard model. Even with this theory many features of the data are not explained. For example, the quark and lepton masses are free parameters in the standard model and are not predicted. Furthermore the theory has some unattractive aspects -- the most noteworthy of them being the extreme fine tuning needed to keep the Higgs mass small compared to the ultraviolet cutoff for the theory. This is sometimes called the hierarchy problem."

He explained,"My own research breaks into two parts. One part is using the standard model to predict experimental observables. Just because you have a theory doesn’t mean it’s straightforward to use it to compare with experiment. Usually such comparisons involve expansions in some small quantity. One area I have done considerable research on is the development of methods to make predictions for the properties of hadrons that contain a single heavy quark".

He elaborated,"The other part is research on physics that is beyond what is in the standard model. In particular I have worked on the development of several extensions of the standard model that solve the hierarchy problem: low energy supersymmetry, the Randall-Sundrum model and most recently the Lee-Wick standard model. This work is very speculative. It is possible that none of the extensions of the standard model discussed in the scientific literature are realized in nature."

Prof. Wise shared the 2001 Sakurai Prize for Theoretical Particle Physics with Nathan Isgur and Mikhail Voloshin. The citation mentioned his work on "the construction of the heavy quark mass expansion and the discovery of the heavy quark symmetry in quantum chromodynamics, which led to a quantitative theory of the decays of c and b flavored hadrons."

He obtained his PhD from Stanford University in 1980. While doing his thesis work, he also co-authored the book 'From Physical Concept to Mathematical Structure: an Introduction to Theoretical Physics' (U. Toronto Press, 1980) with Prof Lynn Trainor of the University of Toronto (where he did his B.S. in 1976 and M.S. in 1977). He also coauthored, with Aneesh Manohar, a monograph on 'Heavy Quark Physics' (Cambridge Univ Press, 2000).

We are pleased to present the list of 5 needed breakthroughs that Prof. Mark Wise would be happy to see in the field of high energy physics.
-- 2Physics.com]

"Here go five breakthroughs that would be great to see:

1) An understanding of the mechanism that breaks the weak interaction symmetry giving the W's and Z's mass. This we should know the answer to in my lifetime since it will be studied at the LHC (Large Hadron Collider) and I am trying to stay healthy.

2) Reconciling gravity with quantum mechanics. Currently the favored candidate for a quantum theory of gravity is String Theory. However, there is no evidence from experiment that this is the correct theory. Perhaps quantum mechanics itself gives way to a more fundamental theory at extremely short distances.

3) An answer to the question, why is the value of the cosmological constant so small? I am assuming here that dark energy is a cosmological constant. (Hey if it looks like a duck and quacks like a duck it's probably a duck.) A cosmological constant is a very simple term in the effective low energy Lagrangian for General Relativity. The weird thing about dark energy is not what it is but rather why it's so small.

4) An understanding of why the scale at which the weak symmetry is broken is so small compared to the scale at which quantum effects in gravity become strong. This is usually called the hierarchy problem. Breakthrough (1) might provide the solution to the hierarchy problem or it might not.

5) Discovery of the particle that makes up the dark matter of the universe and the measurement of its properties (e.g., spin, mass, ...).

There are other things I would love to know. For example, is there a way to explain the values of the quark and lepton masses? But you asked for five."

Particle Astrophysics: 5 Needed Breakthroughs -- James Hough

James Hough [Photo Courtesy: Institute for Gravitational Research, University of Glasgow]

[Today's guest in our ongoing feature '5-Breakthroughs' is James Hough, Director of the Institute for Gravitational Research, and Professor of Experimental Physics in the Department of Physics and Astronomy, University of Glasgow.

Prof. Hough is also the Chairperson of Gravitational Wave International Committee (GWIC) which was formed in 1997 by the directors and representatives of projects and research groups around the world whose research is aimed at the detection of gravitational radiation. The purpose of GWIC is to encourage coordination of research and development across the groups and collaboration in the scheduling of detector operation and data analysis. GWIC also advises on the location, timing and programme of the Edoardo Amaldi Conferences on Gravitational Waves which are held every 2 years, and presents a prize for the best Ph.D. thesis submitted each year (for details, visit 'GWIC Thesis Prize')

His current research interests are in the investigation of materials for test masses and mirror coatings, and in the development of suspension systems of ultra-low mechanical loss towards
a) second generation gravitational wave detectors, in particular Advanced LIGO – upgrade to the US LIGO gravitational wave detector systems (Advanced LIGO is now approved by the National Science Board in the USA and supported by a significant capital contribution from PPARC in the UK and MPG in Germany).
b) third generation long baseline gravitational wave detectors, in particular the proposed Einstein Telescope in Europe, and towards LISA the ESA/NASA space borne gravitational wave detector.

Prof. Hough is Fellow of the Royal Society of London (2003), the American Physical Society (2001), the Institute of Physics (1993) and the Royal Society of Edinburgh (1991). He received Duddell Prize and Medal of the Institute of Physics in 2004 and Max Planck Research Prize in 2001.

It's our pleasure to present the 5 most important breakthroughs that Prof. Hough would like to see in the field of Particle Astrophysics.
-- 2Physics.com Team]

1) The direct detection of gravitational radiation
It is very important to make a direct detection to verify one of the few unproven predictions of Einstein's General Relativity and even more importantly to lead to the birth of a new astronomy. Gravitational wave astronomy will let us look into the hearts of some of the most violent events in the Universe.

2) The quantisation of Gravity
The challenge of developing a quantum theory of gravity and unifying gravity with the other fundamental forces in nature will undoubtedly lead to new discoveries about our Universe

3) The understanding of Dark Energy
Dark Energy - the mysterious reason for our Universe expanding anomalously - is not understood. Solving this enigma may help with understanding quantum gravity and will certainly give us a new perspective on fundamental interactions.

4) The successful launching of LISA, the space-borne gravitational wave detector
LISA will allow the study of the birth and interaction of massive black holes in the Universe in a way that cannot be achieved by any other mission.

5) The identification of dark matter
Observations suggest that there is much more matter in the Universe than we observe by standard means. Finding out the nature of the unseen 'dark' matter is a challenging problem for experimental physicists.

Dwarf Galaxies and Dark Matter

Marla Geha

Today's issue of Astrophysical Journal contains a paper by Joshua Simon of Department of Astronomy, California Institute of Technology and Marla Geha of Hertzberg Institute of Astrophysics, Victoria , Canada (currently at Department of Astronomy, Yale University) reporting results of a new observation that have shed new light on the "Missing Dwarf Galaxy" puzzle--a discrepancy between the number of extremely small, faint galaxies that cosmological theories predict should exist near the Milky Way, and the number that have actually been observed.

The "Cold Dark Matter" model, which explains the growth and evolution of the universe, predicts that large galaxies like the Milky Way should be surrounded by a swarm of up to several hundred smaller galaxies, known as "dwarf galaxies" because of their diminutive size. But until recently, only 11 such companions were known to be orbiting the Milky Way. To explain why the missing dwarfs were not seen, theorists suggested that although hundreds of the galaxies indeed may exist near the Milky Way, most have few, if any, stars. If so, they would be comprised almost entirely of dark matter which does not interact with electromagnetic waves and thus cannot be directly observed but has gravitational effects on ordinary atoms.

Joshua Simon

In the past two years, researchers used images from the Sloan Digital Sky Survey to find out as many as 12 additional very faint dwarf galaxies near the Milky Way. The new systems are unusually small, even compared to other dwarf galaxies; the least massive among them contain only 1% as many stars as the most minuscule galaxies previously known. "These new dwarf galaxies are fascinating systems, not only because of their major contribution to the Missing Dwarf problem, but also as individual galaxies," says Joshua Simon, "We had no idea that such small galaxies could even exist until these objects were discovered last year."

Marla Geha added,"We thought some of them might simply be globular star clusters, or that they could be the shredded remnants of ancient galaxies torn apart by the Milky Way long ago. To test these possibilities, we needed to measure their masses." Joshua and Marla used the DEIMOS spectrograph on the 10-meter Keck II telescope at the W. M. Keck Observatory in Hawaii to study 8 of the new galaxies. The Doppler effect--a shift in the wavelength of the light coming from the galaxies caused by their motion with respect to the earth-- was closely observed to determine the speeds of stars of each dwarf galaxy, which are determined by the total mass of the galaxy.

They measured precise speeds of 18 to 214 stars in each galaxy, three times more stars per galaxy than any previous study. The speeds of the stars ranged between 4 to 7 km/s, which were much slower than the stellar velocities in any other known galaxy [For comparison, the sun orbits the center of the Milky Way at about 220 km/s]. When the speeds were coverted to masses, all these galaxies fell among the smallest ever measured, more than 10,000 times less massive than the Milky Way. Joshua and Marla conclude that the fierce ultraviolet radiation given off by the first stars, born just a few hundred million years after the Big Bang, may have blown away all of the hydrogen gas from dwarf galaxies also forming at that time. The loss of gas prevented the galaxies from creating new stars, leaving them very faint, or, in many cases, completely dark. When this effect is included in theoretical models, the number of expected dwarf galaxies agrees with the number of observed dwarf galaxies.

An image showing positions of these dwarf galaxies relative to Milky Way can be accessed here: http://www.keckobservatory.org/images/article_pictures/147_308.jpg

Although the Sloan Digital Sky Survey was successful in finding a dozen ultrafaint dwarfs, it covered only about 25% of the sky. Future surveys that scan the remainder of the sky are expected to discover as many as 50 additional dark matter-dominated dwarf galaxies orbiting the Milky Way. Telescopes for one such effort, the Pan-STARRS project on Maui, are now under construction.

"Explaining how stars form inside these remarkably tiny galaxies is difficult, and so it is hard to predict exactly how many star-containing dwarfs we should find near the Milky Way", says Joshua, "Our work narrows the gap between the Cold Dark Matter theory and observations by significantly increasing the number of Milky Way dwarf galaxies and telling us more about the properties of these galaxies."

Marla says,"One implication of our results is that up to a few hundred completely dark galaxies really should exist in the Milky Way's cosmic neighborhood. If the Cold Dark Matter model is correct they have to be out there, and the next challenge for astronomers will be finding a way to detect their presence."

Reference:
"The Kinematics of the Ultra-faint Milky Way Satellites: Solving the Missing Satellite Problem" ,
Joshua D. Simon and Marla Geha,
The Astrophysical Journal, v670, p313-331 (2007 November 20), Abstract

[We thank Caltech Media Relations for materials used in this posting]

Limit on Size of Dark Matter Clumps

Joseph Silk (photo courtesy: Oxford University)

Considering that the theory of gravitation is correct, when cosmologists analyze various observed data of our universe, they arrive at an intriguing observation -- that there's not nearly enough visible matter to hold the universe together. In fact, up to 95% appears to be missing.
The idea of the existence of dark matter originates from this. 'Dark matter' is supposed to be that illusive mass that remains invisible to modern day telescopes because it does not interact strongly with electromagnetic waves.

According to the model agreed upon by most physicists so far, dark matter could exist either as an accumulation of as-yet unseen 'weakly interacting massive particles' (WIMPs), or large clumps of 'massive compact objects' (MCOs) that do not emit any observable amount of radiation – or even as a mixture of both types.

Now Benton Metcalf from the Max Planck Institute for Astrophysics in Germany and Joseph Silk from the University of Oxford in the UK have attempted to see just how large these MCOs can be. They analysed the light from a supernovae five billion light years away. If an MCO had been there near the path of one of these light beams, the light would be undergo a measurable amount of dispersion by the MCO's gravitational field in an effect known as "gravitational lensing".

Because of the long path the light took to arrive the earth, the chances of a large MCO straying through would have been fairly high. But even after ploughing through data collected from almost 300 supernovae, the scientists could not find any dispersion caused by possible MCOs larger than one-hundredth the mass of the Sun. This implies that there is an 89% certainty they do not exist at all. Moreover, the physicists claim that MCOs larger than one-tenth the mass of the Earth can be confidently "eliminated" as the sole constituent of dark matter.

Until now many cosmologists believed in the existence of faint stars, neutron stars and black holes as significant constituents of dark matter. This result comes as a shock to those ideas. According to the recent Physical Review Letters paper by Metcalf and Silk, dark matter is more likely made of WIMPs.

Reference:
R. Benton Metcalf and Joseph Silk, "New Constraints on Macroscopic Compact Objects as Dark Matter Candidates from Gravitational Lensing of Type Ia Supernovae", Phys. Rev. Lett. 98, 099903 (E) (2007). Link to Abstract

"Existence of Axion" -- R.N. Mohapatra

Rabindra N. Mohapatra (photo courtsey: U. of Maryland)

[This is an invited article. In a recent publication in Physical Review Letters, R.N. Mohapatra (U. of Maryland) and Salah Nasri (U. of Florida) have put forward a theory that can reconcile conflicting results from two experiments that tried to test the existence of "axion" -- an ultralight particle that could make up dark matter. We thank Prof. Mohapatra for contributing this article on our request. -- 2Physics.com Team]

Axions were proposed by Roberto Peccei and Helen Quinn as a way to solve one of the fundamental mysteries of nuclear forces i.e. an inordinately large amount of CP violation in the otherwise successful theory of these forces, Quantum Chromodynamics (QCD) proposed by D. Gross, F. Wilczek and H. D. Politzer. Since their debut into the world of theoretical physics, axions have also been found useful in another context: being ultralight particles (believed to be a billion times or more lighter than the electron) they are capable of populating the Universe so abundantly that they could be candidates for the dark matter of the Universe and thereby resolve another fundamental mystery of cosmology.

Salah Nasri (photo courtsey: U. of Florida)

Because of these twin attributes (solving the problem of QCD and being a candidate for dark matter), considerable amount of research is being devoted to establishing the existence of axions. One of their key properties is that they couple to two photons (one being the magnetic and the other the electric component of light). Therefore interaction of laser beams with strong magnetic fields is considered to be an efficient way to search for them[1].

Two recent attempts that use this technique to search for axions are the CERN CAST (CERN Axion Solar Telescope) experiment[2] and PVLAS experiment at INFN-LNL[3]. The CAST experiment searched for axions produced by light-by-light collision at the center of the Sun and gave a negative result setting strong limits on the axion-photon coupling and its mass. The PVLAS experiment on the other hand looked for axions produced by laser-magnetic field interaction in the laboratory and seems to have a positive evidence for an axion like particle. Their observations can be understood only if the axion-photon coupling are considerably larger than the upper limits set by the CAST result. This has posed a major challenge for theory and in the very least implies that the axion solution to the problems of QCD may be much more complex than previously envisioned or the PVLAS experiment could be the result of completely new kind of phenomenon, not related to the axion.

In a recent Phys. Rev. Lett. Paper [4], we have proposed a new way to reconcile the CAST and the PVLAS results. We use the axion possibility in our approach except the theory has several new features compared to the conventional axion models. We use the phenomenon of phase transition so well known in the study of condensed matter physics (e.g. loss of magnetism of ferromagnets at high temperature). Our basic observation is that the axion photon coupling is not a primordial coupling but is induced by the formation of a vacuum condensate. Therefore the strength of the coupling depends on the environment temperature.

Note that the solar axions are produced at a very high temperature of about 10 million degrees in the core of the Sun whereas the PVLAS axions are produced at room temperature. Therefore if the vacuum condensate responsible for axion-photon coupling undergoes phase transition to zero value in the solar core due to its high temperature, there would be no axion production in the solar core explaining the CAST result. On the other hand, the PVLAS experiment is taking place at the room temperature and therefore the vacuum condensate has nonzero value and the axion-photon coupling is present giving rise to the PVLAS signal for the axion.

This idea is consistent with all known experimental observations in particle physics and astrophysics. We predict that the axion must be accompanied by a twin particle with mass about 100 times that of the electron which undergoes the condensation and is responsible for our effect. It can be produced in the decay of heavy quark bound states, which can provide a way to test our model.

[1] P.Sikivie, Phys. Rev. Lett. 51, 1415 (1983)
[2] S.Andriamonje [CAST Collaboration], arXiv:hep-ex/0702006.
[3] E.Zavattini et al. [PVLAS Collaboration], Nucl. Phys. Proc. Suppl. 164, 264 (2007).
[4] R. N. Mohapatra and S. Nasri, "Reconciling the CAST and PVLAS Results", Phys. Rev. Letters 98, 050402 (2007) Link to Abstract