Article

# When Clusters Collide: Constraints On Antimatter On The Largest Scales

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## Abstract

Observations have ruled out the presence of significant amounts of antimatter in the Universe on scales ranging from the solar system, to the Galaxy, to groups and clusters of galaxies, and even to distances comparable to the scale of the present horizon. Except for the model-dependent constraints on the largest scales, the most significant upper limits to diffuse antimatter in the Universe are those on the Mpc scale of clusters of galaxies provided by the EGRET upper bounds to annihilation gamma-rays from galaxy clusters whose intra-cluster gas is revealed through its x-ray emission. On the scale of individual clusters of galaxies the upper bounds to the fraction of mixed matter and antimatter for the 55 clusters from a flux-limited x-ray survey range from < 5 x 10^(-9) to < 1 x 10^(-6), strongly suggesting that individual clusters of galaxies are made entirely of matter or, of antimatter. X-ray and gamma-ray observations of colliding clusters of galaxies, such as the Bullet Cluster, permit these constraints to be extended to even larger scales. If the observations of the Bullet Cluster, where the upper bound to the antimatter fraction is found to be < 3 x 10^-6, can be generalized to other colliding clusters of galaxies, cosmologically significant amounts of antimatter will be excluded on scales of order 20 Mpc (5 x 10^(15)M_Sun).

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... These bounds are expected to tighten after data from the AMS 02 experiment, currently mounted on the International Space Station (ISS), is released. Furthermore, in[18]it was argued that the fraction f of antimatter in the ISM cannot be larger than f < 10 −15 because the lifetime of antinuclei in the ISM due to annihilations is only 300 years[16]. Upper bounds on the presence of antimatter in other parts of the universe can be imposed by indirect detection methods. ...
... If the universe is a patchwork of regions that are strongly dominated by either matter or antimatter, the question arises as to what is the typical size of such regions. The possibility of individual antimatter stellar systems has been discussed in[16,18], see also[17]. The absence of annihilation signals from such stars passing through the ISM allows us to conclude that their fraction in the galaxy is <10 −4. ...
... However, no definite conclusions that hint towards the opposite could be drawn. In[18]it was furthermore pointed out that the authors of[20]may have underestimated the annihilation cross sections at low energies. This still leaves open the possibility that the universe is a patchwork of huge distinct regions of matter and antimatter, in the most extreme case with vanishing baryon number B = 0 when averaged over large volumes. ...
Article
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We review observational evidence for a matter-antimatter asymmetry in the early universe, which leads to the remnant matter density we observe today. We also discuss observational bounds on the presence of antimatter in the present day universe, including the possibility of a large lepton asymmetry in the cosmic neutrino background. We briefly review the theoretical framework within which baryogenesis, the dynamical generation of a matter-antimatter asymmetry, can occur. As an example, we discuss a testable minimal model that simultaneously explains the baryon asymmetry of the universe, neutrino oscillations and dark matter.
... Thus, 13.7 billion years ago elementary particles and their antiparticles can be expected to have annihilated, resulting in a universe with only radiation. Yet, we are very much here and are made of baryonic matter, and on larger scales up to tens of Mpc galaxies are thought to consist of matter rather than antimatter [5]. One endeavor that is expected to settle the issue is the ongoing AMS-02 experiment that is designed to look for indications of large antimatter domains. ...
... The fact that neutrino flavor states do not coincide with the mass eigenstates alters the charged current interaction (3.20) if we wish to write it entirely in terms of mass eigenstates Neutrino experiments have determined limits for the mixing angles θ ij and squared mass differences ∆m 2 ij ≡ m 2 i − m 2 j . They have been found to be [88,23] (sin 2 θ 23 ) best fit = 0. 5 Existing data does not restrict the sign of ∆m 31 (2) , and the two sign alternatives imply different hierarchies for the neutrino mass spectrum. With normal ordering m 1 < m 2 < m 3 the mass differences ∆m 2 31 > 0 and ∆m 2 21 > 0 whereas with inverted ordering m 3 < m 1 < m 2 the atmospheric mass difference is negative ∆m 2 32 < 0. ...
... The commonly accepted baryogenesis mechanisms fix the value as well as the sign of the BA universally in the whole Universe. In addition, the observations rule out the existence of significant amount of antimatter on scales ranging from the solar system to galaxy and galaxy clusters, and even at very large scales comparable to the present horizon [107,108]. However, more exotic baryogenesis mechanisms (for a review see [109]) can in principle allow the existence of small domains at well-tempered scales in which antimatter could survive in the form of anistars [110][111][112][113]. ...
Article
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The oscillation of the neutron n into mirror neutron n′, its partner from the dark mirror sector, can gradually transform an ordinary neutron star into a mixed star consisting in part of mirror dark matter. The implications of the reverse process taking place in the mirror neutron stars depend on the sign of baryon asymmetry in the mirror sector. Namely, if it is negative, as predicted by certain baryogenesis scenarios, then n′¯−n¯ transitions create a core of our antimatter gravitationally trapped in the mirror star interior. The annihilation of accreted gas on such antimatter cores could explain the origin of γ-source candidates with an unusual spectrum compatible with baryon–antibaryon annihilation, recently identified in the Fermi LAT catalog. In addition, some part of this antimatter escaping after the mergers of mirror neutron stars can produce the flux of cosmic antihelium and also heavier antinuclei which are hunted in the AMS-02 experiment.
... In the past decade, galaxy cluster mergers have been recognized as particularly fruitful laboratories because of the immense gravitational potential energy released into astrophysical interactions (∼10 57 erg). Cluster mergers have been used to study the nature of DM Clowe et al. 2006;Randall et al. 2008) and matter/antimatter asymmetry (Steigman 2008), as well as long-standing astronomical questions such as the origin of cosmic rays (e.g., Bell 1978;Giler et al. 1980;Gabici & Blasi 2003;Brunetti & Jones 2014) and of the red sequence, i.e., do mergers quench, stimulate, or have little effect on star formation and active galactic nucleus (AGN) activity (e.g., Miller & Owen 2003;Poggianti et al. 2004;Chung et al. 2009;Mansheim et al. 2017;Sobral et al. 2015;Stroe et al. 2017)? Yet most observed mergers are still poorly understood even in terms of basic dynamical properties such as time since first pericenter (which we will refer to simply as age), relative velocity at first pericenter, and viewing angle. ...
... As no such signals have yet been observed, the local abundance of antimatter can be tightly constrained on a multitude of length scales, ranging from our solar system, to galaxies and clusters of galaxies. X-and gamma-ray observations of the Bullet Cluster, a system of two colliding galaxy clusters, put for instance an upper bound of 3 × 10 −6 on the local antimatter fraction, thus ruling out serious amounts of antimatter on scales of O(20) Mpc, which are the largest scales directly probed so far [86]. Furthermore, assuming that matter and antimatter are present in equal shares on cosmological scales, one can show that the matter domain we inhabit virtually has to cover the entire visible universe [87]. ...
Book
We investigate the possibility that the hot thermal phase of the early universe is ignited in consequence of the B-L phase transition, which represents the cosmological realization of the spontaneous breaking of the Abelian gauge symmetry associated with B-L, the difference between baryon number B and lepton number L. Prior to the B-L phase transition, the universe experiences a stage of hybrid inflation. Towards the end of inflation, the false vacuum of unbroken B-L decays, which entails tachyonic preheating as well as the production of cosmic strings. The dynamics of the B-L breaking Higgs field and the B-L gauge degrees of freedom, in combination with thermal processes, generate an abundance of heavy (s)neutrinos. These (s)neutrinos decay into radiation, thereby reheating the universe, generating the baryon asymmetry of the universe and setting the stage for the thermal production of gravitinos. We study the B-L phase transition in the full supersymmetric Abelian Higgs model, for which we derive and discuss the Lagrangian in arbitrary and unitary gauge. As for the subsequent reheating process, we formulate the complete set of Boltzmann equations, the solutions of which enable us to give a detailed and time-resolved description of reheating. Assuming the gravitino to be the lightest superparticle (LSP), the requirement of consistency between hybrid inflation, leptogenesis and gravitino dark matter implies relations between neutrino and superparticle masses, in particular a lower bound on the gravitino mass of 10 GeV. Similarly, in the case of very heavy gravitinos, the nonthermal production of pure wino or higgsino LSPs in heavy gravitino decays can account for the observed amount of dark matter, while simultaneously fulfilling the constraints imposed by primordial nucleosynthesis and leptogenesis.
... The absence of excessive gamma radiation allows one to conclude that the nearest anti-galaxy could not be closer than ∼10 Mpc [10]. The mass fraction of antimatter in two observed colliding galaxies in the Bullet cluster cannot be larger than 10 −6 [11]. As for our Galaxy, it is shown in ref. [12], that the amount of antistars is bounded by N * /N * < 4 · 10 −5 within 150 pc from the Sun. ...
Article
Full-text available
Possible existence of baryo-dense (BD) stars and antistars, which might be created in the very early Universe in the frameworks of a slightly modified Affleck-Dine scenario of baryogenesis is studied. We discuss phenomenology, observational manifestations, and bounds on such antistars and show that they are allowed to be abundant in the Galaxy. New constraints on the possible number of compact antimatter objects are derived. We point out that explosion of an antistar as a type Ia supernova produces a remnant with relatively low annihilation signal. Another important conclusion is the strong reduction of the annihilation flux from antistars in the gaseous disk of the Galaxy due to the high velocities of BD stars in general, and antistars in particular. The contemporary observational data do not exclude significant amount of antimatter in the Galaxy (and in other galaxies) in the form of the BD stars with relatively low mass.
... The fraction of antimatter in Bullet Cluster should be below < 3 × 10 −6 [22] based on the upper bounds to annihilation gamma-rays from galaxy clusters. Some limits on the fraction of antimatter at large scales can be obtained from the CMB data which excludes large isocurvature fluctuations at d > 10 Mpc, and from Big Bang Nucleosynthesis which does not allow large chemistry fluctuations at d > 1 Mpc. ...
Preprint
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Astronomical data of the several recent years, which present an evidence in favour of abundant antimatter population in our Galaxy, Milky Way, are analysed. The data include: registration of gamma-rays with energy 0.511 MeV, which surely originate from electron-positron annihilation at rest, very large flux of anti-helium nuclei, discovered at AMS, and 14 stars which produce excessive gamma-rays with energies of several hundred MeV which may be interpreted as indication that these stars consist of antimatter. Theoretical predictions of these phenomena, made much earlier ago are described
... vations is O(10 −6 -10 −10 ) [19,20]. In order to dynamically produce a matterantimatter asymmetry, three conditions need to be satisfied, which were formulated by A. Sakharov [21]: the conservation of baryon number must be violated, the C and CP symmetry must be violated, and there must be interactions out of the thermal equilibrium. ...
Article
Diese Arbeit beschreibt die Anwendung von statistischen Methoden auf Schluesselmessungen des LHCb Experiments. Die dafür nötige Software wurde im Rahmen dieser Arbeit entwickelt und auf die Messung des CKM Winkels und die Suche der seltenen Zerfälle B0s->µ+µ-und B0->µ+µ- angewendet. Im Verlauf der Arbeit wurde auf einem pp Kollisionsdatensatz, der einer integrierten Luminosität von 1 fb-1 bei ps = 7TeV Schwerpunktsenergie entspricht, der Winkel zum ersten Mal in B0s->Ds K Zerfällen gemessen. Das Ergebnis von gamma=(115+27-43)Grad wurde anschliessend mit Messungen aus B->Dh Zerfällen kombiniert. Die erreichte Präzision von unter 8Grad verbessert die Messungen der B Fabriken um mehr als einen Faktor zwei. Die Raten von B0s->µ+µ- und B0->µ+µ- Zerfällen wurden auf einem Datensatz von pp Kollisionen gemessen, der einer integrierten Luminosität von 3 fb-1 bei einer Schwerpunktsenergie von ps = 7 and 8TeV entspricht. Das Ergebnis ist kompatibel mit den Vorhersagen des Standardmodells. In der Kombination der in der ersten Datennahmeperiode des LHCs aufgenommenen Datensätze der LHCb und CMS-Experimente, ist es gelungen, den Zerfall B0s->µ+µ zum ersten Mal mit einer statistischen Signifikanz von sechs Standardabweichungen nachzuweisen. Mit 3sigma-Signifikanz über dem Untergrund wurde der erste Hinweis auf den Zerfall B0->µ+µ gefunden.
... Observational constraints exist on the presence of the antimatter in our local vicinity (within tens of Mpc), mainly based on Cosmic Ray data [5][6][7][8][9] and Gamma Ray data [10][11][12][13][14]. No antimatter in astronomically considerable amounts has been observed/detected (still, domains of antimatter are not absolutely ruled out [15][16][17]). ...
Article
Full-text available
We have numerically explored the Scalar Field Condensate baryogenesis model for numerous sets of model's parameters, within their natural range of values. We have investigated the evolution of the baryon charge carrying field, the evolution of the baryon charge contained in the scalar field condensate and the final value of the generated baryon charge on the model's parameters: the gauge coupling constant $\alpha$, the Hubble constant at the inflationary stage $H_I$, the mass $m$, the self-coupling constants $\lambda_i$.
... For example, the separation between DM and gas in the Bullet Cluster (1E 0657−558) provides empirical evidence that favors cold DM over theories of modified gravity (Clowe et al. 2006). Analyses of mergers can also constrain the DM self-interaction cross-section (e.g., Markevitch et al. 2004;Harvey et al. 2015) and the large-scale matter-antimatter ratio (Steigman 2008). ...
Article
2020. The American Astronomical Society. All rights reserved. We present an analysis of the mass distribution inferred from strong lensing by SPT-CL J0356-5337, a cluster of galaxies at redshift z=1.0359 revealed in the follow-up of the SPT-SZ clusters. The cluster has an Einstein radius of θE14″ for a source at z = 3 and a mass within 500 kpc of {equation presented}. Our spectroscopic identification of three multiply imaged systems (z=2.363, z=2.364 and z=3.048), combined with HSTF606W-band imaging allows us to build a strong lensing model for this cluster with an rms of ≤0."3. Our modeling reveals a two-component mass distribution in the cluster. One mass component is dominated by the brightest cluster Galaxy and the other component, separated by ∼170 kpc, contains a group of eight red elliptical galaxies confined in a ∼9″ (∼70 kpc) diameter circle. We estimate the mass ratio between the two components to be between 1:1.25 and 1:1.58. In addition, spectroscopic data reveal that these two near-equal mass cores have only a small velocity difference of ∼300 km s-1 between the two components. This small radial velocity difference suggests that most of the relative velocity takes place in the plane of the sky, and implies that SPT-CL J0356-5337 is a major merger with a small impact parameter seen face-on. We also assess the relative contributions of Galaxy-scale halos to the overall mass of the core of the cluster and find that within 800 kpc from the brightest cluster Galaxy about 27% of the total mass can be attributed to visible and dark matter associated with galaxies, whereas only 73% of the total mass in the core comes from cluster-scale dark matter halos.
... Baryonic antimatter in our Solar and Galactic neighborhood can be constrained by the observation of high-energy gamma rays [1]: when coming into contact with normal matter, it would produce annihilation radiation featuring a characteristic spectrum peaking around half the mass of the neutral pion at ∼70 MeV, and with a cutoff around the mass of the proton at 938 MeV [2]. The non detection of this annihilation fea-ture in gamma rays has virtually excluded the existence of substantial amounts of baryonic antimatter in the solar system, the solar neighborhood, the Milky Way, and up to the scale of galaxy clusters [1,3,4]. When combined with observations of the largely isotropic Cosmic Microwave Background, the lack of an "MeV-bump" in gamma-rays has led to the presently accepted paradigm in which a matter-antimatter symmetric Universe can be ruled out [5]. ...
Article
It is generally taken for granted that our Universe is free of antimatter objects and domains. This certitude has recently been challenged by the possible detection of antihelium nuclei by AMS-02. Should the observation be confirmed, the existence of nearby antistars would make a plausible hypothesis to explain the origin of the antinuclei. In this paper, we use the 10-year Fermi Large Area Telescope (LAT) gamma-ray source catalog to set constraints on the abundance of antistars around the Sun. We identify in the catalog 14 antistar candidates not associated with any objects belonging to established gamma-ray source classes and with a spectrum compatible with baryon-antibaryon annihilation. We use them along with an estimate of the LAT sensitivity to antistars to set upper limits on the local antistar fraction f*¯ with respect to normal stars. We provide parametric limits as a function of the closest antistar mass, velocity, and surrounding matter density. We also employ a novel Monte Carlo method to set limits for a few hypotheses about the antistar population. For a population with properties equivalent to those of regular stars concentrated in the Galactic disk, we obtain f*¯<2.5×10−6 at 95% confidence level, which is 20 times more constraining than limits previously available. For a primordial population of antistars distributed in the Galactic halo, we obtain new local upper limits which decrease as a function of antistar mass M from f*¯<0.2 at 95% confidence level for M=1 M⊙ to f*¯<1.6×10−4 at 95% confidence level for M=10 M⊙. By combining these limits with existing microlensing constraints for lighter objects in the Magellanic Clouds, we infer that a primordial population of halo antistars must have a density lower than O(10−5 pc−3) to O(10−2 pc−3) depending on their masses. Our limits can constrain models for the origin and propagation of antinuclei in cosmic rays.
... ture in gamma rays has virtually excluded the existence of substantial amounts of baryonic antimatter in the solar system, the solar neighborhood, the Milky Way, and up to the scale of galaxy clusters [1,3,4]. When combined with observations of the largely isotropic Cosmic Microwave Background, the lack of an "MeV-bump" in gamma-rays has led to the presently accepted paradigm in which a matter-antimatter symmetric Universe can be ruled out [5]. ...
Preprint
It is generally taken for granted that our Universe is free of antimatter objects and domains. This certitude has recently been challenged by the possible detection of anti-helium nuclei by AMS-02. Should the observation be confirmed, the existence of nearby antistars would make a plausible hypothesis to explain the origin of the antinuclei. In this paper we use the 10-years \F Large Area Telescope (LAT) gamma-ray source catalog to set constraints on the abundance of antistars around the Sun. We identify in the catalog 14 antistar candidates not associated with any objects belonging to established gamma-ray source classes and with a spectrum compatible with baryon-antibaryon annihilation. We use them along with an estimate of the LAT sensitivity to antistars to set upper limits on the local antistar fraction $f_{\bar{\ast}}$ with respect to normal stars. We provide parametric limits as a function of the closest antistar mass, velocity, and surrounding matter density. We also employ a novel Monte~Carlo method to set limits for a few hypotheses about the antistar population. For a population with properties equivalent to those of regular stars concentrated in the Galactic disk we obtain $f_{\bar{\ast}} < 2.5 \times 10^{-6}$ at 95\% confidence level, which is 20 times more constraining than limits previously available. For a primordial population of antistars distributed in the Galactic halo we obtain new local upper limits which decrease as a function of antistar mass $M$ from $f_{\bar{\ast}} < 0.2$ at 95\% confidence level for $M = 1 \; M_\odot$ to $f_{\bar{\ast}} < 1.6 \times 10^{-4}$ at 95\% confidence level for $M = 10 \; M_\odot$. By combining these limits with existing microlensing constraints for lighter objects in the Magellanic clouds, we infer that a primordial population of halo antistars must have a density lower than $\mathcal{O}(10^{-5}\;\text{pc}^{-3})$ to $\mathcal{O}(10^{-2}\;\text{pc}^{-3})$ depending on their masses. Our limits can constrain models for the origin and propagation of antinuclei in cosmic rays.
... The generation of the baryon asymmetry of the Universe is one of the open cosmological issues. Both cosmic ray data and gamma ray data indicate that there are no significant quantities of antimatter in the local vicinity up to galaxy cluster scales of 10-20 Mpc [6][7][8][9][10]. It is most probable that our universe is made of matter. ...
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The review is devoted to the development and subsequent application of methods for studying the stability of solutions of higher-order differential equations arising in various models of modified gravity and baryosynthesis. Major attention is paid to F(R)-modified theories proposed to describe the accelerated expansion of the universe and to baryon asymmetry generation in the framework of the spontaneous and gravitational baryogenesis.
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The main intention of this thesis is to motivate and investigate the $$B$$–$$L$$ phase transition as the possible origin for the thermal phase of the hot early universe. Before we are ready to do so, we have to acquaint ourselves with the observational evidence for this phase and understand which physical processes have or may have taken place in it. For this reason we shall provide a brief review of early universe cosmology in this chapter, thereby compiling the background material for the further discussion. We will first discuss the present composition of the universe (cf. Sect. 2.1) and then some of the main events in the thermal history of the universe in reverse chronological order (cf. Sect. 2.2). We would like to emphasize that in this introductory chapter we will crudely restrict ourselves to aspects which are relevant for our purposes. More balanced and comprehensive presentations of the topic are for instance provided in standard textbooks [1–3] or dedicated review articles.
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Observations show that, at least in our astronomical neighborhood, the Universe is matter-dominated. The amount of antimatter is very small and it can be explained by its secondary production in high energy cosmic ray collisions or in catastrophic astrophysical phenomena. While there are many potentially possible scenarios proposed in the literature for the creation of the observed matter-antimatter asymmetry, we do not yet know the exact mechanism responsible for it. An initially tiny asymmetry seems to be excluded by the inflationary paradigm and therefore it is necessary a baryogenesis period, namely the creation of an asymmetry between baryons and antibaryons. Today we know that the Standard Model of particle physics cannot do it, and therefore the observed matter-antimatter asymmetry can be seen as an evidence of new physics. Baryogenesis models usually involve very high energy physics, which makes these scenarios extremely difficult to test with the available accelerator energies.
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This Chapter does not follow the main line of the book that is the theory of HF compounds but illustrates how the ideas of FC may be applicable to describe a very dissimilar system. Namely, here we consider a novel mechanism for explaining the matter-antimatter asymmetry of the universe. We assume that the universe starts from completely symmetric state and then, as it cools down, it undergoes a quantum phase transition, which in turn causes an asymmetry between matter and anti-matter. As we shall see the quantum phase transition is represented by FCQPT. The mechanism does not require the baryon number violating interactions or $${\textit{CP}}$$ violation at a microscopic level. The state FC emerging behind FCQPT can be viewed as the state possessing the supersymmetry that interchanges bosons and fermions eliminating the difference between them. Thus restoring one important symmetry, the FC state violates another essential symmetry destroying matter—anti-matter symmetry. Our analysis of the matter antimatter asymmetry is in the context of remarkable experimental results obtained in the condensed matter physics.
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Possible signatures which may indicate an existence of antimatter in the Galaxy and in the early universe are reviewed. A model which could give rise to abundant antimatter in the Galaxy is considered.
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Astronomical observations of recent years show that the universe at high redshifts (about ten) is densely populated by early formed objects: bright galaxies, quasars, gamma-bursters, and it contains a lot of metals and dust. Such a rich variety of early formed objects have not been expected in the standard model of formation of astrophysical objects. There is serious tension between the standard theory and the observations. We describe the model which relaxes this tension and nicely fits the data. The model naturally leads to the creation of cosmologically significant antimatter which may be abundant even in the Galaxy. Phenomenological consequences of our scenario and the possibility of distant registration of antimatter are discussed.
Article
Possible mechanisms of baryogenesis are reviewed. Special attention is payed to those which allow for creation of astronomically significant domains or objects consisting of antimatter. Observational manifestations of cosmological antimatter are discussed. Comment: Lectures presented at XIII Mexican School of Particles and Fields, San Carlos, October, 2008, 15pages
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I suggest that the dominance of matter over antimatter in the present universe is a consequence of baryon-number-nonconserving reactions in the very early fireball. Unified guage theories of weak, electromagnetic, and strong interactions provide a basis for such a conjecture and a computation in specific SU(5) models gives a small ratio of baryon- to photon-number density in rough agreement with observation.
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The Chandra image of the merging, hot galaxy cluster 1E 0657-56 reveals a bow shock propagating in front of a bullet-like gas cloud just exiting the disrupted cluster core. This is the first clear example of a shock front in a cluster. From the jumps in the gas density and temperature at the shock, the Mach number of the bullet-like cloud is 2-3. This corresponds to a velocity of 3000-4000 km s-1 relative to the main cluster, which means that the cloud traversed the core just 0.1-0.2 Gyr ago. The 6-7 keV bullet'' appears to be a remnant of a dense cooling flow region once located at the center of a merging subcluster whose outer gas has been stripped by ram pressure. The bullet's shape indicates that it is near the final stage of being destroyed by ram pressure and gasdynamic instabilities, as the subcluster galaxies move well ahead of the cool gas. The unique simplicity of the shock front and bullet geometry in 1E 0657-56 may allow a number of interesting future measurements. The cluster's average temperature is 14-15 keV but shows large spatial variations. The hottest gas (T>20 keV) lies in the region of the radio halo enhancement and extensive merging activity involving subclusters other than the bullet.
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An X-ray flux-limited sample of clusters of galaxies is presented. Over 90 per cent of the clusters have been well-observed with both imaging and spectroscopic detectors. We have determined both the local luminosity function and the space density of clusters as a function of temperature. A significant deficit in the numbers of high-luminosity clusters is found in the redshift range z ~ 0.1-0.2 compared with numbers of nearby clusters. This indicates that the X-ray luminous clusters are undergoing strong evolution at the present time. A simple stochastic merging model which reproduces the rapid evolution seen in the data is investigated. The 'temperature function' is used to put limits on the large-scale mass spectrum of the Universe.
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The third catalog of high-energy gamma-ray sources detected by the EGRET telescope on the Compton Gamma Ray Observatory includes data from 1991 April 22 to 1995 October 3 (cycles 1, 2, 3, and 4 of the mission). In addition to including more data than the second EGRET catalog and its supplement, this catalog uses completely reprocessed data (to correct a number of mostly minimal errors and problems). The 271 sources (E > 100 MeV) in the catalog include the single 1991 solar flare bright enough to be detected as a source, the Large Magellanic Cloud, five pulsars, one probable radio galaxy detection (Cen A), and 66 high-confidence identifications of blazars (BL Lac objects, flat-spectrum radio quasars, or unidentified flat-spectrum radio sources). In addition, 27 lower confidence potential blazar identifications are noted. Finally, the catalog contains 170 sources not yet identified firmly with known objects, although potential identifications have been suggested for a number of those. A figure is presented that gives approximate upper limits for gamma-ray sources at any point in the sky, as well as information about sources listed in the second catalog and its supplement, that do not appear in this catalog.
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We constrain the physical nature of dark matter using the newly identified massive merging galaxy cluster MACS J0025.4–1222. As was previously shown by the example of the Bullet Cluster (1E 0657–56), such systems are ideal laboratories for detecting isolated dark matter and distinguishing between cold dark matter (CDM) and other scenarios (e.g., self-interacting dark matter, alternative gravity theories). MACS J0025.4–1222 consists of two merging subclusters of similar richness at z = 0.586. We measure the distribution of X-ray-emitting gas from Chandra X-ray data and find it to be clearly displaced from the distribution of galaxies. A strong (information from highly distorted arcs) and weak (using weakly distorted background galaxies) gravitational lensing analysis based on Hubble Space Telescope observations and Keck arc spectroscopy confirms that the subclusters have near-equal mass. The total mass distribution in each of the subclusters is clearly offset (at >4 σ significance) from the peak of the hot X-ray-emitting gas (the main baryonic component) but aligned with the distribution of galaxies. We measure the fractions of mass in hot gas (0.09+ 0.07−0.03) and stars (0.010+ 0.007−0.004), consistent with those of typical clusters, finding that dark matter is the dominant contributor to the gravitational field. Under the assumption that the subclusters experienced a head-on collision in the plane of the sky, we obtain an order-of-magnitude estimate of the dark matter self-interaction cross section of σ/m < 4 cm 2 g−1, reaffirming the results from the Bullet Cluster on the collisionless nature of dark matter.
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The rich cluster Abell 520 (z = 0.201) exhibits truly extreme and puzzling multiwavelength characteristics. It may best be described as a "cosmic train wreck." It is a major merger showing abundant evidence for ram pressure stripping, with a clear offset in the gas distribution compared to the galaxies (as in the Bullet Cluster, 1E 0657-558). However, the most striking feature is a massive dark core (721 h70 M☉/L☉B) in our weak-lensing mass reconstruction. The core coincides with the central X-ray emission peak, but is largely devoid of galaxies. An unusually low mass-to-light ratio region lies 500 kpc to the east, and coincides with a shock feature visible in radio observations of the cluster. Although a displacement between the X-ray gas and the galaxy/dark matter distributions may be expected in a merger, a mass peak without galaxies cannot be easily explained within the current collisionless dark matter paradigm. Interestingly, the integrated gas mass fraction (≈0.15), mass-to-light ratio (220 h70 M☉/L☉B), and position on the X-ray luminosity-temperature and mass-temperature relations are unremarkable. Thus, gross properties and scaling relations are not always useful indicators of the dynamical state of clusters.
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We ask whether the universe can be a patchwork consisting of distinct regions of matter and antimatter. We demonstrate that, after recombination, it is impossible to avoid annihilation near regional boundaries. We study the dynamics of this process to estimate two of its signatures: a contribution to the cosmic diffuse gamma-ray background and a distortion of the cosmic microwave background. The former signal exceeds observational limits unless the matter domain we inhabit is virtually the entire visible universe. On general grounds, we conclude that a matter-antimatter symmetric universe is empirically excluded.
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We report EGRET upper limits on the high-energy gamma-ray emission from clusters of galaxies. EGRET observations between 1991 and 2000 were analyzed at positions of 58 individual clusters from a flux-limited sample of nearby X-ray bright galaxy clusters. Subsequently, a coadded image from individual galaxy clusters has been analyzed using an adequately adapted diffuse gamma-ray foreground model. The resulting 2 sigma upper limit for the average cluster is \~ 6 x 10^{-9} cm^{-2} s^{-1} for E > 100 MeV. Implications of the non--detection of prominent individual clusters and of the general inability to detect the X-ray brightest galaxy clusters as a class of gamma-ray emitters are discussed. We compare our results with model predictions on the high-energy gamma-ray emission from galaxy clusters as well as with recent claims of an association between unidentified or unresolved gamma-ray sources and Abell clusters of galaxies and find these contradictory.
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We present a comprehensive mass reconstruction of the rich galaxy cluster Cl 0024+17 at z~0.4 from ACS data, unifying both strong- and weak-lensing constraints. The weak-lensing signal from a dense distribution of background galaxies (~120 per square arcmin) across the cluster enables the derivation of a high-resolution parameter-free mass map. The strongly-lensed objects tightly constrain the mass structure of the cluster inner region on an absolute scale, breaking the mass-sheet degeneracy. The mass reconstruction of Cl 0024+17 obtained in such a way is remarkable. It reveals a ringlike dark matter substructure at r~75" surrounding a soft, dense core at r~50". We interpret this peculiar sub-structure as the result of a high-speed line-of-sight collision of two massive clusters 1-2 Gyr ago. Such an event is also indicated by the cluster velocity distribution. Our numerical simulation with purely collisionless particles demonstrates that such density ripples can arise by radially expanding, decelerating particles that originally comprised the pre-collision cores. Cl 0024+17 can be likened to the bullet cluster 1E0657-56, but viewed $along$ the collision axis at a much later epoch. In addition, we show that the long-standing mass discrepancy for Cl 0024+17 between X-ray and lensing can be resolved by treating the cluster X-ray emission as coming from a superposition of two X-ray systems. The cluster's unusual X-ray surface brightness profile that requires a two isothermal sphere description supports this hypothesis. Comment: To appear in the June 1 issue of The Astrophysical Journal
Article
Departures from thermal equilibrium which are likely to occur in an expanding universe allow the production of an appreciable net baryon density by processes which violate baryon-number conservation. It is shown that the resulting baryon to entropy ratio can be calculated in terms of purely microscopic quantities.
Article
We consider the possibility that the observed particle-antiparticle imbalance in the universe is due to baryon-number, C, and CP nonconservation. We make general observations and describe a framework for making quantitative estimates.
Article
We discuss several issues bearing on the observed asymmetry between matter and antimatter in the content of the universe, in particular, the possible role in this of Hawking radiation from black holes, with allowance for weak C- and T-violating interactions. We show that the radiation, species by species, can be asymmetric between baryons and antibaryons. However, if baryon number is microscopically conserved there cannot be a net flux of baryon number in the radiation. Black-hole absorption from a medium with net baryon number zero can drive the medium to an asymmetric state. On the other hand, if baryon conservation is violated, a net asymmetry can develop. This can arise through asymmetric gravitational interactions of the radiated particles, and conceivably, by radiation of long-lived particles which decay asymmetrically. In the absence of microscopic baryon conservation, asymmetries can also arise from collision processes generally, say in the early stages of the universe as a whole. However, no asymmetries can develop (indeed any "initial" ones are erased) insofar as the baryon-violating interactions are in thermal equilibrium, as they might well be in the dense, high-temperature stages of the very early universe. Thus particle collisions can generate asymmetries only when nonequilibrium effects driven by cosmological expansion come into play. A scenario for baryon-number generation suggested by superunified theories is discussed in some detail. Black-hole radiation is another highly nonequilibrium process which is very efficient in producing asymmetry, given microscopic C, T, and baryon-number violation.
Article
It is well known that there cannot be symmetry between matter and anti- matter on scales up to and including that of clusters of galaxies. It is often stated, however, that there could be such symmetry at the ultimate grouping scale - that of superclusters (SCs). In such a situation there would be equal masses of material in superclusters and in anti- superclusters in the Universe, but the overlap regions would be sufficiently weak that the ensuing annihilation gamma-rays would have a flux below that observed. A number of situations are examined here, including the SC, SC^bar^ hypothesis, and it is shown that even this last-mentioned situation is not allowed. There is thus apparently no geometry that would allow the matter, anti-matter symmetry hypothesis to be retained. It is pointed out that if protons are unstable, and if the lifetime of the Universe is much longer than the proton lifetime, then matter, anti- matter symmetry is in fact restored eventually - at least for nucleons and electrons.
Article
In approaching the problem of the amount and astrophysical role of antimatter in the Universe, it is valuable to distinguish between two separate questions. First, must the universe be symmetric. Does an application of the microscopic laws of physics to the macroscopic scale of the Universe require that there be exactly equal numbers of particles and antiparticles. In contrast, is the Universe symmetric. The extent to which these questions can be or have been answered is the subject of this review. 2 tables, 118 refs. (GHT)
Article
The Alpha Magnetic Spectrometer (AMS) was flown on the space shuttle Discovery during flight STS-91 in a 51.7° orbit at altitudes between 320 and 390 km. A total of 2.86×106 helium nuclei were observed in the rigidity range 1 to 140 GV. No antihelium nuclei were detected at any rigidity. An upper limit on the flux ratio of antihelium to helium of <1.1×10−6 is obtained.
Article
Observational signatures of existence of antimatter objects in the Galaxy are discussed. We focus on point-like sources of gamma radiation, diffuse galactic gamma ray background and anti-nuclei in cosmic rays.
Article
A model of inhomogeneous baryogenesis based on the Affleck and Dine mechanism is described. A simple coupling of the scalar baryon field to the inflaton allows for formation of astronomically significant bubbles with a large baryon (or antibaryon) asymmetry. During the farther evolution these domains form compact stellar-like objects, or lower density clouds, or primordial black holes of different size. According to the scenario, such high baryonic number objects occupy relatively small fraction of space but despite that they may significantly contribute to the cosmological mass density. For some values of parameters the model allows the possibility the whole dark matter in the universe to be baryonic. Furthermore, the model allows the existence of the antibaryonic B-bubbles, i.e. a significant fraction of the mass density in the universe can be in the form of the compact antimatter objects (e.g. antistars).
Article
In some baryogenesis scenarios, the universe acquires a non-vanishing average baryonic charge, but the baryon to photon ratio is not spatially constant and can be even negative in some space regions. This allows for existence of lumps of antimatter in our neighborhood and the possibility that very compact antimatter objects make a part of cosmological dark matter. Here I discuss the peculiar signatures which may be observed in a near future. One can conclude from simple considerations that there is much more matter than antimatter around us [1]. However, the origin of matter–antimatter asymmetry in the universe is unknown: the Standard Model of particle physics is certainly unable to explain it and new physics is necessary [2]. Assuming a homogeneous and isotropic universe, from the Big Bang Nucleosynthesis
Article
The PAMELA experiment is a satellite-borne apparatus designed to study charged particles in the cosmic radiation with a particular focus on antiparticles. PAMELA is mounted on the Resurs DK1 satellite that was launched from the Baikonur cosmodrome on June 15th 2006. The PAMELA apparatus comprises a time-of-flight system, a magnetic spectrometer, a silicon-tungsten electromagnetic calorimeter, an anticoincidence system, a shower tail catcher scintillator and a neutron detector. The combination of these devices allows antiparticles to be reliably identified from a large background of other charged particles. This paper reviews the design, space qualification and on-ground performance of PAMELA. The in-orbit performance will be discussed in future publications. Comment: Replaced some paragraphs in text and some pictures
Article
We present a simplified dynamical model of the ‘Bullet’ system of two colliding clusters. The model constrains the masses of the system by requiring that the orbits of the main and the subcomponents satisfy the cosmological initial conditions of vanishing physical separation a Hubble time ago. This is also known as the timing argument. The model considers a system embedded in an overdense region. We argue that a relative speed of 4500 km s−1 between the two components is consistent with cosmological conditions if the system is of a total mass of 2.8 × 1015h−1M⊙ and is embedded in a region of a (mild) overdensity of 10 times the cosmological background density. Combining this with the lensing measurements of the projected mass, the model yields a ratio of 3:1 for the mass of the main relative to that of the subcomponent. The effect of the background weakens as the relative speed between the two components is decreased. For relative speeds lower than ∼3700 km s−1, the timing argument yields masses which are too low to be consistent with lensing.
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