High energy 7-rays combine three characteristics that make these energetic photons ideal carriers of information about nonthermal relativistic processes in astrophysical settings: (i) copious production in many galactic and extragalactic objects due to effective acceleration of charged particles and their subsequent interactions with the ambient gas, low frequency radiation, and magnetic fields; (ii) free propagation in space without deflection in the interstellar and intergalactic magnetic fields: (iii) effective detection by space-borne and ground based instruments. Therefore it is commonly believed that very high energy gamma-ray astronomy is destined to play a crucial role in exploration of nonthermal phenomena in the Universe in their most extreme and violent forms. The major driving motivations of this field conditionally can be grouped in three topical areas: (i) Origin of Cosmic Rays, (ii) Physics and Astrophysics of Relativistic Outflows, and (iii) Observational Gamma Ray Cosmology.
Origin of Cosmic Rays
Galactic Cosmic Rays. For more than 40 years, ideas have circulated about the crucial role of gamma-ray astronomy in solving the problem of origin of galactic cosmic rays (CRs). The realization of this seminal prediction recognised by pioneers of the field in the 1950′s and 1960′s is still considered as one of the major goals of 7-ray astronomy. The basic idea is simple and concerns both the acceleration and propagation aspects of CRs. Namely, while the localised 7-ray sources exhibit the sites of production/acceleration of CRs, the angular and spectral distributions of the diffuse galactic 7-ray emission provide unique information about the character of propagation of CRs in galactic magnetic fields. The prime objective of this activity is the decisive test of the hypothesis that supernova remnants (SNRs) are responsible for the bulk of observed CRs up to 1015 eV. Detection of TeV 7-rays from shell-type SNRs would be the first straightforward proof of the widely believed model of diffusive shock acceleration of CR protons in these objects. Conservative phenomenological estimates show that a certain number of 103 to 104 yr old SNRs should be visible in 7-rays. Although 3 shell type SNRs already have been reported as TeV emitters, the limited information about both the spectral and spatial distributions of detected signals does not allow definite conclusions concerning the nature of TeV emission, also because the latter could be substantially "contaminated" by Y-rays of leptonic (inverse Compton) origin.
The failure to detect positive Y-ray signals from several selected SNRs would impose a strong constraint on the total energy in the accelerated protons,
Contrary to current belief this would indicate the inability of SNRs to explain the bulk of the observed cosmic ray fluxes. SNRs are still only one of the plausible sites of CR acceleration. It is quite possible that different galactic source populations, e.g. pulsars and microquasars, contribute comparably to the observed cosmic ray flux. This makes the problem rather complicated.
Fig. 1.5 Gamma-rays radiated by dense molecular clouds located in the vicinity of a young
proton accelerator, where the density of relativistic particles significantly exceeds the average level of the "sea" of galactic
determined by the mixture of contributions from all individual sources during the CR propagation in the Galactic Disk on timescales
The particle accelerator itself is a source of Y-rays, but its intensity could be quite low due to the lack of sufficiently dense target material inside compared to the average density of the interstellar medium
Also, the Y-ray spectrum could be suppressed at very high energies due to the energy-dependent escape of particles from the accelerator.
Only direct identification of these sources as particle accelerators through their characteristic Y-ray emission can help to elucidate the origin of galactic CRs. The existence of a particle accelerator by itself is not enough for efficient Y-ray production; one needs the second component – the target. The so called giant molecular clouds (GMCs) with diffuse masses ~ 104 to 106 M0, seem to be ideal objects to play that role (see Fig. 1.5). These objects are intimately connected with star formation regions that are strongly believed to be the most probable locations (with or without SNRs) of cosmic ray production in our Galaxy. The search for TeV 7-rays from GMCs is important to ascertain the possible existence of nearby high energy proton accelerators.
Extragalactic Cosmic Rays. Although there is little doubt that the highest energy particles observed in cosmic rays, with E ~ 1020 eV, are produced outside of our Galaxy, the sites and relevant acceleration mechanisms continue to be a mystery. Powerful extragalactic objects like radio-galaxies, AGN, clusters of galaxies, the enigmatic gamma-ray burst sources, have all been (phenomenologically) suggested as possible acceleration sites. However boosting particles to such energies is a serious theoretical challenge. Even if the electrodynamical system accelerating particles had an infinite lifetime and the acceleration proceeds at the maximum possible rate of about ~ qBc, the attainable energy is limited by two conditions: (1) by the confinement – particles can stay in the acceleration region as long as their gyroradius remains smaller of the characteristic linear size of the accelerator; (2) by synchrotron energy losses. Even assuming that the particles are moving along smooth field lines, there is still curvature radiation which limits the maximum attainable energy. These two conditions allow us to derive a robust lower limit for the total electromagnetic energy, W, stored in the acceleration region, and thus estimate the size, l, and the magnetic field strength, B, that are optimal with respect to minimisation of the electromagnetic energy.
In large scale (1 kpc) structures the "confinement condition" is more critical. In this case, the radio lobes in radiogalaxies, hot spots in AGN jets, and clusters of galaxies seem the most likely sites of particle acceleration (see Fig. 1.6a). In many cases these objects may contain enough target material in the form of gas and photon fields to convert a substantial fraction of these particles into detectable VHE radiation. Moreover, even if these particles do not spend much time in their production region, but leave the source, they unavoidably collide with 2.7 CMBR photons at relatively small distance from the source (especially for objects at large redshifts). The secondary electrons and photons – the products of photohadronic interactions – initiate two-stage pair cascades in the 2.7 K CMBR and optical/infrared extragalactic radiation fields. The typical angular size of the active region of cascade development does not exceed 1 degree for a source located at a distance Gpc. The resulting Y-radiation at energies below 100 GeV can be detectable by GLAST and the new generation of low-threshold IACT arrays.
The relativistic bulk motion reduces the energy requirements significantly, but requires compact objects and large magnetic fields (see Fig. 1.6b). For example, for a bulk motion Lorentz factor r ~ 10, the optimal size and magnetic field are R ~ 1014 eV and B ~ 300 G, respectively. This corresponds to a quite reasonable total electromagnetic energy ~ 3 x 1047 erg for the inner jets of blazars. A reasonable combination of model parameters is obtained also for GRBs for which a bulk motion Lorenz factor of about 300 is more typical.
Energy losses, due to either synchrotron or curvature radiation, play an increasing role in the energy balance of accelerated particles, with decreasing size (and accordingly increasing magnetic field) of the accelerator. In compact objects like small scale AGN jets or GRBs, the radiative losses become the dominant factor that limits the maximum attainable energy. This implies that proton acceleration in such objects should be always accompanied by hard synchrotron (or curvature) radiation extending to 
(in the observer’s frame). The search for such a characteristic radiation can be used to probe the potential accelerators of highest energy cosmic rays. The protons can interact effectively also with ambient dense photon fields through photomeson production. Due to the hadronic and electromagnetic cascades initiated by these interactions, a substantial part of the primary energy can be effectively transported through neutrons and Y-rays to very large distances from the central engine. Moreover, the multiple conversions of nucleons from charged to neutral state and back may significantly increase the acceleration rate compared to the standard diffusive shock acceleration mechanism, and thus make more effective the production of 1020 eV protons. This mechanism provides a very effective conversion of the kinetic energy of bulk relativistic flows in GRBs and inner jets of blazars to the accompanying high energy Y-ray and neutrino emission.
Top-Down scenarios. The above constraints on the parameter space of 1020 eV particle accelerators set by classical electrodynamics obviously cannot be applied to the so-called "top-down" models of cosmic rays which make use of quantum effects and non-electromagnetic interactions. The hypothesis of the non-acceleration or "top-down" scenario as an alternative to the ordinary acceleration ("bottom-up") scenario is motivated by difficulties of current theoretical models to provide adequate acceleration rates that could boost the particles to energies
In the "top-down" models the cosmic rays are the result of decays of the so-called topologi-cal defects or relic super-heavy particles. These models, however, have a major problem.
Fig. 1.6 The minimum energy requirement to the potential accelerators of 1020 eV protons as a function of the characteristic linear size of the accelerator. The dashed lines corresponds to the condition of particle confinement in large scale magnetic field or by the difference in electric field potential (generalised Hillas criterion), the solid lines correspond to the limits set by radiative (synchrotron or curvature) energy losses: a (top panel) accelerators at rest; for two values of the acceleration efficiency n given by £acc = neBc; b (bottom panel) accelerators moving with relativistic speed; for n =1, and 3 different values of the bulk motion Lorentz factor r. The characteristic ranges of different source populations (as potential cosmic ray accelerators) on the (W,l) plane are also shown. For the inner AGN jets, the upper zone corresponds to hadronic models of Y-ray emission, the lower zone – to leptonic models.
The electromagnetic cascades initiated by secondary electrons and photons lead to fluxes of < 100 GeV 7-rays that exceed the observed flux of diffuse extragalactic 7-ray background shown in Fig. 1.3. This inconsistency can be avoided if one associates these exotic parents of cosmic rays to the halo of our Galaxy. Independent of the type of GUT particles, their decays lead to an excess of pions (and thus photons) over nucleons at production, and consequently to a high photon-to-proton ratio in cosmic rays at energies above 1019 eV. Thus the photon-to-proton ratio can be used as a diagnostic tool for the "top-down" model of highest energy cosmic rays. The detection of an unusually large content of 7-rays at E ~ 1020 eV may become the first astrophysically meaningful result in the EHE gamma-ray domain.
Physics and Astrophysics of Relativistic Flows
Relativistic flows in astrophysics in the forms of winds and jets are common in many astrophysical settings. Most nonthermal phenomena observed from pulsars, microquasars, AGN and Gamma Ray Bursts are linked in one way or another to relativistically moving plasmas. The relativistic outflows are tightly coupled with compact relativistic objects – neutron stars and black holes. The theory of relativistic collimated outflows is very complex and not yet properly understood. It deals with magnetohydrody-namics, electrodynamics, strong shock waves and related with them particle acceleration. Each of these aspects challenges many uncertainties and problems. For example, many fundamental questions do not have yet definite answers concerning the origin (electromagnetic or gas dynamical ?) and content (electron-positron or electron-proton ?) of jets in AGN, as well as the processes which determine the jet power and support its propagation over distances up to several hundred kpc. There is an unsolved fundamental problem also in the theory of pulsar winds. It is believed that the spin-down power of a pulsar is carried away by a MHD wind in which the energy originally (closer to the magnetosphere) is dominated by Poynting flux. On the other hand, observations of the Crab Nebula show that when the wind approaches the inner edge of the visible synchrotron nebula, which is believed to be the site of wind termination, most of the energy must be in the form of kinetic energy of ultrarelativistic flow with Lorentz factor r ~ 106. Apparently somewhere between the pulsar and the termination shock the Poynting flux is converted to kinetic energy. How and where the acceleration of the wind takes place, remains a theoretical challenge despite recent intensive efforts in this direction.
The distinct feature of relativistic outflows is the effective acceleration of particles at different stages of its development – close to the central engine, during the propagation on large scales, and at the termination shock fronts. Nonthermal radio emission observed from different regions associated with relativistic jets and winds, e.g. from inner jets of blazars, large scale structures (knots, hot spots) of jets of powerful radiogalaxies and quasars, from compact expanding plasmons in microquasars, from pulsar-driven nebulae (plerions), etc., carry information about relativistic electrons accelerated to relatively modest (GeV) energies. Remarkably, in many cases the synchrotron emission extends to the optical and X-ray bands. This implies that the electrons are accelerated to TeV energies. The inverse Comp-ton scattering of the same electrons as well as interactions of the hadronic component of accelerated particles with the the surrounding radiation and magnetic fields, result in very high energy Y-rays. Both the acceleration and radiation processes with participation of ultrarelativistic electrons and protons may proceed with very high efficiency. Therefore it is believed that high energy Y-ray emission should provides us with important, in some cases crucial, information about nonthermal processes in jets and winds.
Small and Large scale AGN jets. Active Galactic Nuclei represent a large population of compact extragalactic objects characterised with extremely luminous electromagnetic radiation produced in very compact volumes. Although this source population consists of several classes of galaxies with substantially different characteristics, the prevailing concept of structure of AGN assumes that the differences are basically due to the strongly anisotropic radiation patterns. Consequently, the current classification schemes are dominated by random pointing directions rather than by intrinsic physical properties (Urry and Padovani, 1995). The presently most popular picture of physical structure of’AGN is illustrated in Fig. 1.7.
AGN with relativistic jets close to the line of sight (so-called blazars) are very effective TeV Y-ray emitters. The dramatically enhanced fluxes of the Doppler-boosted radiation
coupled with the fortuitous orientation of the jets towards the observer, make these objects ideal laboratories to study the underlying physics of AGN jets through multi-wavelength observations of temporal and spectral characteristics of radiation from radio to very high energy Y-rays. First of all this concerns the BL Lac objects, a sub-population of AGN of which several nearby representatives are already established as TeV Y-ray emitters. The TeV radiation not only tells us that particles in these objects are accelerated to very high energies, but also provides the strongest evidence in favour of the commonly accepted paradigm that the nonthermal radiation is produced in relativistic outflows (jets) with Doppler factors
Fig. 1.7 Schematic illustration of the current paradigm of radio-laud AGN. At the center of the galaxy there is a supermassive black hole (r -106 to ~1010 M0) the gravitational potential energy of which is the ultimate source of power of the system released in different forms — through the thermal emission of the accretion disk, as well as through nonthermal processes in the relativistic jets that emanate perpendicular to the plane of the accretion disc. Particle acceleration takes place throughout the entire jet extending up to 1024 cm, i.e. well beyond the host galaxy. These particles interact with the ambient photon and magnetic fields,and thus result in nonthermal (synchrotron and inverse Compton) emission components observed on different (sub-pc, kpc, and multi-hundred kpc) scales. Broad emission lines are produced in clouds orbiting above the accretion disc. They are located typically within the zone between 0.01 to 0.1 pc. The accretion disk and the broad-line region is surrounded by a thick dusty torus. Narrow emission lines are produced in clouds located much farther from the central engine, typically between 0.3 and 30 pc.
Presently, the leptonic (basically, inverse Compton) models of TeV emission represent the preferred concept for TeV blazars. These models have two attractive features: (i) capability of the relatively well developed model of shock waves to accelerate electrons to multi-TeV energies, (ii) effective production of tightly correlated X-ray and TeV emission components via synchrotron and inverse Compton channels. However, the very fact of strong X/TeV correlations does not yet exclude the hadronic models.
Effective acceleration of electrons to very high energies takes place also in large scale structures of AGN jets. Although there is no alternative to the nonthermal origin of large scale (up to several 100 kpc) jets of radiogalaxies, it remains a theoretical challenge to explain the variety of morphological and spectroscopic peculiarities observed from these objects. This concerns, first of all, the X-ray data. The recent exciting discoveries by the Chandra X-ray Observatory added much to our knowledge of X-ray structures of large scale jets in quasars and radiogalaxies. However, these results did not solve the old problems, and, in fact, brought new puzzles. The standard models that relate X-ray emission of distinct jet features to the synchrotron radiation or inverse Compton scattering of directly accelerated electrons face certain problems. The synchrotron mechanism is "over-efficient" in the sense that the TeV electrons, due to severe radiative losses, have very short propagation lengths, and thus hardly can form diffuse X-ray structures on kpc scales. The inverse Compton models require low energy 1 GeV) electrons, and, therefore, are free of this problem. On the other hand, in many cases this mechanism appears not sufficiently efficient to provide the observed X-ray fluxes.
The synchrotron radiation of protons could be an alternative interpretation of X-ray emission. For a certain combination of parameters characterising the acceleration, propagation and radiation of very high energy protons, this model can provide effective cooling of protons via synchrotron radiation on quite comfortable timescales of about 107 — 108 yr. This allows effective propagation of protons in the jet over kpc scales, and thus production of extended X-ray structures. Yet, the model allows high radiation efficiencies, and demands quite reasonable proton acceleration rates to explain the observed X-ray fluxes from typical representatives of this source population. Although these rates are comparable with the electron acceleration rates required in the electron-synchrotron models, the proton-synchrotron model implies much higher energy densities in the form of nonthermal particles and magnetic fields. The success of the proton-synchrotron model largely relies on 3 principal assumptions: (i) acceleration of protons to energies of at least Ep = 1018 eV; (ii) a strong ambient magnetic field, B > 1 mG; (iii) slow propagation of protons in the knots. Any observational evidence in favour of the proton-synchrotron origin of the large-scale X-structures would imply acceleration of protons to extremely high energies. This may have a direct link to another puzzle of the high energy astrophysics – the origin of extremely high energy cosmic rays (EHECRs) observed up to 1020 eV. An interesting consequence of this model is that it predicts significant Y-ray emission from the surrounding cluster environments initiated by interactions of runaway protons with the 2.7 K CMBR . The spectral and angular characteristics of this radiation component strongly depend on the ambient magnetic field, thus they carry important information about both the total power of acceleration of the highest energy particles in the jet, and the strength and structure of intra-cluster magnetic fields.
An interesting alternative to the conventional interpretations of X-ray features of AGN jets is a scenario in which the jet is powered by external Y-rays through their interactions with local low-frequency radiation fields and by subsequent development of electromagnetic cascades penetrating through the regular and random fields in the MHD jet. Generally, the nonthermal Y-ray phenomena are associated with accelerated particles. In this scenario, we have exactly opposite picture when the external Y-radiation is the primary substance. Consequently, there is no need for particle acceleration immediately in the jet. At the same time, this hypothesis contains certain components of standard MHD jets. Because of its prime motivation to explain the diffuse X-ray structures of large scale jets by electron synchrotron radiation, this model can be considered "leptonic". On the other hand, this scenario might be called "hadronic", because the primary Y-rays of energy 1015 — 1019 eV can be produced only in hadronic interactions, most likely in the vicinity of the central engine. The high energy Y-ray beam provides a direct link between the large scale jet and the central engine, thus it can be considered as an alternative to the Poynting flux assumed in the standard AGN models for extraction of energy from rotating black holes. While the transformation of Poynting flux into kinetic energy in the outflow, and eventually (through termination shocks) into relativistic particles remains an unsolved theoretical problem, the transformation of Y-rays to relativistic electrons can be realized effectively through the photon-photon pair production. The ultrahigh energy Y-rays may have broader implications. In particular, the Y-ray beams can power the surrounding intergalactic medium resulting in diffuse synchrotron X- and Y-ray emission from clusters of galaxies.
Pulsar winds and nebulae. High energy Y-rays emitted by rotation powered pulsars can be produced in three physically distinct regions: the pulsar magnetosphere, the unshocked relativistic wind, and the synchrotron nebula (Fig. 1.8). High energy Y-ray observations have a great potential to test current theoretical concepts concerning the production of an ultra-relativistic pulsar wind in the vicinity of the star and its interactions with the ambient medium.
The energy spectra and the structure of light curves in the region from several GeV to 30 GeV carry key information about the location of the Y-ray production region in the magnetosphere.
Fig. 1.8 Three regions of nonthermal radiation associated with a rotation powered pulsar: pulsar — magnetospheric pulsed Y-ray emission produced within the light cylinder due to the curvature, synchrotron, and inverse Compton processes; unshocked wind — gamma-radiation of the cold wind at GeV and TeV energies through the relativistic bulk-motion Comptonization; synchrotron nebula — broad-band, synchrotron and IC emission of the nonthermal nebulae.
The energy interval from 10 GeV to 1 TeV is the most informative region to search for Y-ray emission from the unshocked pulsar wind. Detection and identification of a specific ("asymmetric-line") type emission would provide direct information about the Lorentz factor and the site of acceleration of the wind i.e. the sites where the transformation from a Poynting flux dominated state to a kinetic energy dominated state occur. Finally, combined with X-ray observations, the spectral properties and morphology of 10 GeV to TeV Y-rays provide unambiguous information about the distributions of electrons and the magnetic fields in pulsar nebulae produced by wind termination shocks.
Microquasars. X-ray binaries are traditionally treated as thermal sources that transform the gravitational energy of accretion onto a compact object (a neutron star or a black hole) into X-ray emission radiated by the hot accreting plasma. However, since the discovery of galactic sources with relativistic jets – called microquasars – the basic concepts on X-ray binaries have been significantly revised. It is established that the non-thermal power of synchrotron radio jets (in the form of accelerated electrons and kinetic energy of the relativistic outflow) during strong radio flares can be comparable to or even exceed the thermal radiation luminosity of these accretion-driven objects. The discovery of microquasars opened new possibilities to study the phenomenon of relativistic jets common elsewhere in AGN. Because of their proximity, microquasars offer an opportunity for monitoring jets on much smaller spatial and temporal timescales.
Synchrotron and infrared emission observed from microquasars implies the existence of electrons up to energies ~ 10 GeV. If the electron acceleration proceeds at a sufficiently high rate, the synchrotron spectrum can extend to hard X-rays. In addition, the high density photon fields produced by the jet itself, as well as coming from the accretion disk and the companion normal star, create favourable conditions for effective production of inverse Compton Y-radiation. Apart from this episodic component of radiation associated with strong radio flares, one may expect persistent X-and high energy Y-ray emissions components from extended regions caused by synchrotron and inverse Compton radiation of ultrarelativistic electrons accelerated at the interface between the relativistic jet and the interstellar medium. Termination of the jets in the interstellar medium may result also in effective acceleration of protons. In this regard, microquasars are potential sites for the production of galactic cosmic rays.
Observational Gamma-Ray Cosmology
The fact that the spectra of extragalactic sources extend beyond 100 GeV opens a unique path for realization of exciting cosmological aspects of very high energy (VHE) gamma-ray astronomy. The promise here is connected with the energy dependent absorption of Y-rays interacting with diffuse ex-tragalactic photon fields. The photon-photon absorption features, expected in the spectra of high energy Y-rays arriving from distant extragalactic objects depends on the spectrum and absolute flux of the diffuse extragalactic background at infrared and optical wavelengths. The detection and identification of these features should provide important information about the epochs of galaxy formation and their evolution in the past. Obviously this method of extracting information about the diffuse extragalactic background requires (i) TeV Y-ray beams emitted from extragalactic objects located at different distances between 100 and 1000 Mpc; (2) good gamma-ray spectrometry, and (3) good understanding of the intrinsic spectra of Y-rays (i.e. before their deformation in the intergalactic medium). Blazars do provide us with intense TeV beams, and the IACT arrays do allow an adequate Y-ray spectrometry based on an energy resolution as good as 10 per cent and large Y-ray photon statistics. A serious obstacle in practical realization of this interesting method is our poor knowledge about the primary Y-ray spectra produced in the source. The recent remarkable progress in well coordinated observations of several TeV blazars in different energy bands gives a certain optimism that eventually the gamma-ray astronomers will be able to identify the principal radiation mechanisms, fix/constrain the relevant model parameter space, and reconstruct robustly the intrinsic Y-ray spectra based on multiwavelength studies of the spectral and temporal characteristics of blazars. This should allow reliable estimates of the intergalactic absorption effect, and consequently derivation of the flux and spectrum of diffuse extragalactic background between 1 and 100 ^m.
Strictly speaking the intergalactic absorption features contain information about the product of the diffuse extragalactic background radiation density ur and the Hubble constant H0. In principle, it would be possible to decouple ur and H0 by studying the spectral and angular characteristics of VHE Y radiation from hypothetical electron-positron Pair Halos surrounding powerful nonthermal extragalactic objects. These giant but light (electron-positron) structures which are unavoidably formed around any extragalactic VHE source due to development of pair cascades initiated by interactions of primary multi-TeV photons with the extragalactic background photon fields, may serve as unique cosmological candles. The formation and radiation of a Pair Halo is illustrated in Fig. 1.9.
Fig. 1.9 Formation and radiation of a Pair Halo.
The radiation of a Pair Halo can be recognised by its distinct variation in spectrum and intensity with angular distance from the halo centre. This variation depends weakly on the details of the central source, for example on the orientation and beaming/opening angle of a possible emitting jet, but depends on ur and H0. Thus detection of a halo would give us two observables – angular and spectral distributions of y-radiation – that might make it possible to disentangle ur and H0. Since the angular size of a halo around a source with known redshift z0 is determined by the density of the background radiation in the vicinity of the source (i.e. at the epoch z0), observing pair halo radiation from sources at different redshifts should provide an important probe of cosmological evolution of the background radiation.
The extended character of Y-ray emission emitted by isotropic Pair Ha-los (up to several degrees) makes their mapping and spectroscopy a difficult measurement that must wait future sensitive IACT arrays. But in any case, the existence of strong extragalactic TeV sources sustains the hope that the Pair Halos will be eventually discovered. For formation of isotropic Pair Ha-los the intergalactic field should be sufficiently strong (larger than 10-12 G) In fact, both the current observations and cosmological concepts do not exclude the possibility that in some regions with typical linear scales of about 100 Mpc, the intergalactic magnetic field could be arbitrarily small. If so, instead of detecting extended and persistent isotropic Pair Halos, we should expect cascade radiation penetrating almost rectilinearly from the source to the observer. As far as cosmological distances are concerned, even very small deflections of the cascade electrons in the intergalactic magnetic fields should lead to significant delays of arriving cascade radiation. If such delays can be distinguished from the intrinsic time structure of radiation of a Y-ray source, it would be possible to probe the primordial magnetic fields of the Universe at the level down to 10-18 G.
Independent of details concerning the structure and strength of inter-galactic magnetic fields, as well as the flux and spectrum of the extragalactic radiation fields, the ensemble of all VHE source in the Universe produce isotropic cascade Y-radiation which serves as a calorimetric measure for the integrated power of the Universe in the form of any phenomenon accompanied by radiation of VHE Y-rays. These Y-rays may have quite different natures. They are copiously produced by AGN jets, radiogalaxies and galaxy clusters – the most powerful extragalactic objects in the Universe. They are also contributed by less powerful but more populous sources like Pulsars and SNRs – the most active sites of Y-ray production in ordinary galaxies. Gamma-rays may appear during grandiose processes like the formation of large-scale structures in the Universe and at brief solitary events like Gamma Ray Bursts. We may also expect quite intense Y-ray production of "non-acceleration" origin related to decays of hypothetical cosmological relics like the topological defects as well as to annihilation of non-baryonic Dark Matter indexDark Matter. Independent of their origin, all high energy Y-rays have a common fate – due to interactions with the extragalactic radiation fields they inevitably terminate on Hubble (space and time) scales, and thus make the entire Universe an active scene of continuous creation and development of electromagnetic cascades in the intergalactic medium. The superposition of these cascades should not exceed the observed flux of the diffuse extragalactic Y-ray background. This provides robust constraints on the overall VHE Y-ray luminosity of the Universe.
Finally, the search for hypothetical Y-ray emission from Dark Matter Halos in our Galaxy and other nearby galaxies by GLAST and by forthcoming powerful ground-based instruments should provide a very deep and meaningful probe for the existence of non-baryonic Dark Matter in the Universe. The detection of positive signals would reveal the nature of Dark Matter with extremely important implications for both Cosmology and Particle Physics.
