Cosmos

The big bang theory provides a model for the origin and evolution of the Universe. Approximately fifteen billion years ago, all matter in the Universe was in a very small, point-like region of extremely high density and temperature, which then exploded and vastly expanded to form the Universe we observe today. The general scenario of the big bang is now broadly accepted and supported by many observations, such as the Cosmic Microwave Background Radiation or the continuing expansion of the Universe. However, theoretical models for the origin and cause of the initial explosion remain highly speculative.

From all directions, we receive a nearly uniform flux of microwave radiation, the so-called cosmic microwave background radiation. This thermal radiation field originated in the hot big bang. Due to the expansion of the Universe, the temperature of the radiation has dropped to 2.73 degrees Kelvin at the present time. The cosmic microwave background was first discovered by Penzias and Wilson in 1965. Later in 1992, the COBE satellite detected tiny anisotropies in the microwave flux, providing evidence for small density fluctuations in the early Universe, which seeded structure formation.

The Cosmic Background Explorer (COBE) satellite was launched by NASA in 1989. COBE measured the temperature of the microwave background radiation very precisely in every direction. It detected very small fluctuations around the mean temperature of 2.73 degrees Kelvin, providing a fossil record of density perturbations in the early universe. These density perturbations have been amplified by gravity, and eventually formed the structure we observe today, ranging from planets to huge clusters of galaxies. Two future missions, the MAP and PLANCK satellites, will provide high-precision measurements of the temperature anisotropies, which will in turn lead to accurate measurements of cosmological properties of the Universe, such as its current density and expansion rate.

In many astrophysical systems, the mass inferred from the luminous stars is not sufficient to explain the dynamics of the object. There must be additional material, whose presence is only revealed by its gravitational effects, as seen for example in the orbital motions of galaxies in clusters, or in the rotation of spiral galaxies. Astrophysicists thus postulate the existence of some form of "dark matter". In fact, most of the mass in the Universe consists of this dark material, but the physical nature of the dark matter is still one of the greatest mysteries of cosmology. Dark matter candidates include as of yet unknown elementary particles, primordial black holes, and stellar remnants.

Spiral galaxies are disk-like stellar systems that rotate around their center. Our Milky Way is such a galaxy, with our sun located towards the outskirts of the disk. The number and prominence of spiral arms can vary strongly among disk galaxies. Often, spiral galaxies also contain a central luminous bulge, and sometimes they have a distinct stellar bar. The Andromeda galaxy is the nearest spiral galaxy from us. In about 6 billion years, Andromeda is going to merge with the Milky Way, presumably forming an elliptical galaxy.

Elliptical galaxies are stellar systems of spheriodal shape lacking prominent features in the distribution of their stars. The three-dimensional shape of elliptical galaxies is rarely exactly spherical, but can be well described by a triaxial ellipsoid. Ellipticals are preferentially found in clusters of galaxies, while spirals are rarer in this environment. Elliptical galaxies can be formed when two spiral galaxies collide and merge. However, it is an open question whether all of the ellipticals, or only a fraction of them, formed in this way.

Some 15 billion years ago, space and time came into existance. Immediately after the Big Bang, the Universe was extremely hot. All matter and energy was very uniformly distributed, and no structure existed. Afters its launch in 1990, the COBE satellite provided information about the first structure in the Universe. At a time just 300000 years after the Big Bang, COBE detected tiny fluctuations in the Cosmic Microwave Background Radiation. On the map, red and blue spots show regions that are slightly warmer or colder than the rest of the Universe. These differences in temperature are caused by minute variations of the density of matter. Gravity amplifies these perturbations. This growth can be followed in large computer-simulations, like in the one shown here. The simulation has finished at the present time. The region displayed is 600 million lightyears on a side. A three-dimensional visualization gives a better idea of the structure that has formed. This sphere has been carved out from a simulation that models our local cosmic neighbourhood. The diameter of the sphere is 500 million lightyears, and the Milky way is at the center. The large structure now visible on the left hand side is known as the Great Attractor. Another view of cosmic structure is obtained by flying through a simulation. All the yellow lumps are concentrations of so-called dark matter. At the centers of these lumps, the luminous galaxies form, but most of the mass remains dark. This is the simulation we have seen before. Here, a massive cluster of galaxies has formed at the center. The cluster is surrounded by filaments of dark matter. We are now using another simulation to zoom in onto the central object. All the bright spots visible in the cluster are galaxies. Because we here show a snapshot of the cluster, the galaxies seem to rest at fixed locations. However, in reality they move around each other like bees in a swarm. In this way they prevent the collapse of the whole structure under its own weight. This is a picture of the Coma cluster of galaxies. It contains around one thousand galaxies. Many of the galaxies in clusters are elliptical galaxies. This is the bright elliptical galaxy Messier-87. Outside clusters, beautiful spiral galaxies are much more common. This is the spiral M-101. And here we have M-83. Sometimes galaxies collide and merge into a single object. In this simulation, two ordinary spiral galaxies fall towards each other under their mutual gravitational attraction. During the collision, stars and gas are ejected out of the disks by gravitational tidal forces. This material forms long arms which are continously stretched and fade quickly. Finally, the galaxies coalesce and form a spheroidal merger remnant. It is believed, that this remnant corresponds to a newly formed elliptical galaxy. This is the Andromeda galaxy, a close companion of our Milky Way. In fact, our own galaxy will collide and merge with Andromeda in about 6 billion years.