One of the least known regions of the cosmic electromagnetic background is in the infrared (IR). This infrared background (IRB) is of considerable astrophysical interest. Early stellar activity, either pregalactic (Bond, Carr and Hogan, 1986), or galactic (Partridge and Peebles, 1967), may have contributed significantly to the IRB. More significant, perhaps, are the superimposed IR emissions from normal galaxies, Seyferts, quasars, and rich clusters of galaxies (e.g., Stecker, Puget and Fazio, 1977). There are only few reported measurements of the IRB, which in general yield energy densities comparable to that of the Cosmic Microwave Background (CMB), ρo ≈4 10−13 erg cm−3. An upper limit on the energy density at 2 μm is δ ≡ ρ/ρo≤1.2 (Hoffman and Lemke, 1978). Matsumoto, Akiba and Murakami (1984) have detected diffuse emission in the 2–5 μm band, with an estimated energy density δ = 1.2.

The spatial distribution of the 1–3 keV X-ray background (XRB) in five Einstein Observatory Imaging Proportional Counter fields has been analyzed. The autocorrelation function does not exceed 9% on scales ~5′. The observed count probability distribution is then used to check the source number-flux distribution at faint levels. Agreement with the Einstein Observatory deep survey is obtained. A cutoff in the number-flux distribution for a Euclidean population of sources at a flux approximately one-half of the deep survey limit, previously suggested by Hamilton and Helfand (1987), is also inferred.

The HEAO–1 A2 scanning survey has provided small field-of-view data characterized by a full width zero intensity of 3° along the scan direction (at approximately constant ecliptic longitude λ) and of 6° perpendicular to it. We have used independent fields taken from scanning circles separated by Δλ = 6° and whose centers are separated by Δβ = 3°. After having excluded all fields at | β | < 60° or at | b | ≤ 20°, only 905 of the total turned out to be not significantly contaminated by diffuse galactic emission or by known discrete sources. The flux intensity at each field's central position was evaluated by fitting a point source plus a constant background. The sample average C (θ) of the products dI (θ) x dI'(θ + Δθ) of positive and negative intensity fluctuations computed for angular separations of 3°, 6°, …, and 27° provides the X-ray background (XRB) angular correlation function.

It has been established observationally that at least 30% of the all-sky X-ray background (XRB) in the energy range of 1–3 keV comes from active galactic nuclei (AGN) in the redshift interval of 0.4 to 1.2 (Griffiths et al. 1983, 1988); it is also quite plausible that AGN contribute half of the XRB in the 1–3 keV range (Morisawa and Takahara 1989). However, analysis of spatial fluctuations in the Einstein deep survey counts (Hamilton and Helfand 1987; Barcons and Fabian 1989) has indicated the presence of a relatively smooth component of the XRB with a corresponding surface density of discrete sources of at least several thousand per square degree.

X-ray observations have demonstrated that the intergalactic medium in many clusters (cf. Coma, Perseus) contains a thin, hot plasma that may be produced by the accretion process in the gravitational potential of clusters with radiative cooling; this is usually called “cooling flows” (Fabian, Nulsen, and Canizares 1984; Sarazin 1986). On the other hand, the existence of radio halos in some clusters has been reported (Coma: Jaffe, Perola, and Valentijn 1976; A401: Roland et al. 1981). In addition, many elliptical galaxies in the center of clusters are also strong synchrotron radio sources. These radio emissions provide evidence for large amounts of relativistic electrons associated with the active phenomena in or around these galaxies and clusters. We can estimate the values or limits on the magnetic field in the cluster from the limits on the inverse Compton X-ray emission with the synchrotron radio emission (cf. Jaffe 1980). The intracluster field strength Bo is roughly 1 μG. It has been suggested that the influence of cosmic rays and magnetic fields is important for the properties and dynamics of the intercluster medium (Böhringer and Morfill 1988; Soker and Sarazin 1989). If cooling flows are real, this inward flow can impede the escape of the cosmic rays from the central galaxies in clusters and enhance the magnetic field. The confinement of the cosmic rays and the magnetic field in the center of clusters affects the gas of the intracluster medium.

In two previous publications (Mészáros and Mészáros 1988 “Paper I”; Bagoly, Mészáros, and Mészáros 1988 “Paper II”), we have studied the fluctuations of the X-ray background (XRB) expected if the XRB is produced by discrete sources distributed as galaxies. The distribution of matter was assumed to follow the large-scale structure in the form of spherical clusters (superclusters) or spherical voids. In Paper I the density contrast of XRB sources inside and outside structures was taken to be a step function of constant height, whereas in Paper II we introduced arbitrary density contrasts (independent of redshift) and allowed for a redshift evolution of the luminosity of the sources. This led to predicted angular fluctuations of the XRB, which, when we compared them with the HEAO–1 observational limit (Shafer 1983), allowed us to set limits on the type and density of structures.

An upper limit to the amplitude of the overall density fluctuation has been found by means of the gravitational lensing effect of the density inhomogeneity on luminosity of quasars with large redshifts. The observed differences of luminosities of quasars located at different directions are partially given by the lensing effect; therefore, a useful upper limit to the inhomogeneity can be derived.

One of the most important cosmological problems is the question of the properties of systems of particles in a regular field of homogeneous and isotropically expanding two-component Metagalactic substratum. Within the framework of the Newtonian cosmology, we consider the behaviour of the ensemble of point masses embedded in a homogeneous expanding medium having two different equations of the state. Dissipation phenomena caused by the chaotic motion of the background matter are taken into account.

The controversy about the position of the matter accumulation that causes our peculiar motion with respect to the cosmic background radiation (CBR) has very recently acquired great interest. So, the very local attractor suggested by Lynden-Bell et al. (1988) and Dressler (1988) begins to be questioned. The histogram done by Melnick and Moles (1987) shows that the dominant structure may be at ~140 h−1 Mpc. Similar results have been obtained by Scaramella et al. (1989). On the other hand, the analysis of the very local candidate in the X-ray band shows that this does not have rich clusters of galaxies (Jahoda and Mushotzky 1989). Moreover, Lahav et al. (1989) have studied a sample of 53 clusters emitting X-rays (E ≈ 2–20 keV, LX ≈ 5 × 1042–7 × 1044h−2 erg s−1); they have observed a great concentration of clusters in the direction of the attractor suggested by Lynden-Bell et al., but at a distance of ~100 h−1–150 h−1 Mpc. The detections of nonlocal structures (further than 300 h−1 Mpc) are very few. However, the study done by Shaver (1987) using catalogs of quasars showed that a superstructure with a radius of ~200 h−1 Mpc and with a density in excess of δ ≈ 1 may exist at a distance of ~800 h−1 Mpc.

The equilibrium distribution for a low-density infinite system of electromagnetically (EM) interacting particles placed in euclidean space in equilibrium with the Lorentz-invariant (LI) zero-point field (ZPF) is also LI with an energy density ρ(E)dE Ã E−1dE. This result follows semiclassically as well as quantum theoretically (QT). Such an idea is exploited to propose a means for energizing the intergalactic medium (IGM) and thus explain the X-ray background (XRB) as thermal bremsstrahlung (TB) of a 3 × 108 K thermally excited electron gas confirming observation (see, e.g., Marshall et al. 1980; Boldt 1987).

Although designed as a pointed infrared observatory for detailed studies of selected objects, the ISO satellite has remarkable capabilities for studying faint surface brightness sources and for helping to disentangle the overlaying background components. The 1993 launch of ISO is timely as it follows the COBE mission and offers a large guest observer program for which preparations can now be made.

A small, cryogenically cooled infrared telescope named IRTS (Infrared Telescope in Space) is under development. The telescope is equipped with spectrometers and a photometer with medium spatial and spectral resolution covering the wavelength range from 1 to 1000 μm. The instrument is specially designed for observations of the cosmic background radiation and galactic infrared radiation. It will be flown on board a Japanese reusable free-flyer called SFU (space flyer unit) to be launched in 1994.

The far-ultraviolet wavelength region between 1216 and ~2000 Å offers unusually favorable conditions for observing a variety of known or predicted diffuse emissions of both interstellar and extragalactic origin. We describe here a proposed instrument for studying those emissions, the Hopkins Ultraviolet Background Experiment (HUBE). HUBE consists of two compact, fast components: a broadband camera (1350-2000 Å) with 2′ resolution and an imaging spectrograph (1230-1800 Å) with 5 Å resolution. These complementary components will be utilized for both a sky survey and for sensitive deep pointings. As either a free-flying Scout-class satellite or as an attached payload on the Space Station, HUBE will make possible an investigation of unprecedented scope into the sources of the far ultraviolet background.

The FAUST telescope is an ultraviolet survey instrument that features a wide 8° field of view, ~1′ angular resolution, and a photon counting detector. Operating in the 1400–1800 Å band, it will be sufficiently sensitive to detect blue mv =17 objects in a single 20 minute night. The instrument is part of the ATLAS-1 shuttle mission, presently scheduled for flight in May 1991. A substantial number of high galactic latitude fields will be investigated, with particular emphasis on studies of the origin of the diffuse far UV background.

The Israeli Space Agency approved funds for a Phase A study of a scientific satellite to be orbited by a national launcher. Our proposal of an imager in the UV with a 2° field of view (FOV) is one of the two approved for Phase A. We discuss properties of the orbital platform and requirements for the scientific payload. Some scientific programs for the mission are detailed.

We describe a proposed far-ultraviolet (FUV) photometric and spectroscopic survey covering the 1300–2000 Å band with a sensitivity comparable to that of the Palomar Sky Survey. Among the numerous scientific objectives of this mission, many have direct bearing on the nature of the FUV background.

The study of diffuse celestial sources in the ground-inaccessible ultraviolet spectral range is less advanced than UV studies of point and compact sources. The main reason is that the characteristics of instrumentation optimized for the two types of objects are quite different. Studies of diffuse objects are best made with fast focal ratio optics with wide fields of view, whereas studies of point and compact objects are best made with large telescope aperture and high angular resolution. As a result, most space ultraviolet instruments to date (such as the International Ultraviolet Explorer and the forthcoming Hubble Space Telescope) are not well suited to the study of faint, extended diffuse objects in the ultraviolet.

The measurement of faint extended surface brightness at a level of a few S10 is hampered by the disturbingly bright contributions of the Light of the Night Sky (LONS), consisting of, among others, the zodiacal light, airglow, and integrated starlight. Special observing techniques are necessary for investigations of the diffuse light in clusters of galaxies (DLCG) and for the determination of the extragalactic background light (EBL). We argue for the two-telescope method as an effective means to compensate for the bright components of the LONS, which will allow an accurate determination of faint extended surface brightnesses.

The ROSAT X-ray astronomy satellite, due to be launched in early 1990, will carry two separate and complementary grazing-incidence telescopes with co-aligned axes. The German X-ray telescope (XRT) will cover the soft X-ray region in the range 0.15–2 keV (6–80 Å), while the U.K. XUV Wide Field Camera (WFC) will extend coverage to beyond 200 Å. The WFC is a joint project of Leicester and Birmingham Universities, the Mullard Space Science Laboratory, and the authors' institutes. The primary objective of ROSAT is to perform an all-sky survey over a period of six months. This will be followed by a guest-observer, “pointed” phase. We briefly discuss the sensitivity of the WFC to the soft X-ray/XUV background (SXRB) and the problems and techniques associated with distinguishing the astronomical background from other sources of background.

The ROSAT X-ray satellite mission and its X-ray telescope (XRT) are described by Trümper (1984). The characteristics of the Wide Field Camera (WFC) on ROSAT and its potential for studies of the soft X-ray background (SXRB) are discussed by Harris, Sumner, and Walker (1989, this volume). The energy range covered by the WFC is 0.06 keV to 0.21 keV (60 Å to 200 Å), whilst the XRT covers the higher energy range from 0.2 keV to 2 keV. Observations performed to date in this field have given rise to conflicting evidence on the location and nature of the 106 K gas, which is presumed to be the origin of the observed emission (see references in Harris, Sumner, and Walker, 1989, this volume).