THE STRUCTURE OF THE UNIVERSE
 

The cosmic background radiation
 

The theory of the Big Bang remained hypothetical for many years, in consideration of the lack of evidence to its validity, until the Cosmic Microwave Background Radiation, (CMBR) was discovered.
In 1965 two technicians of Bell Telephone realized that, what they thought the background noise of the radio antenna, was actually a weak radio signal coming from space, with the same intensity in all directions.
It was soon discovered that it could be the remains of the radiation produced after the explosion that originated the Universe, according to the Big Bang theory. The background radiation was therefore studied in order to verify the various cosmological models within the Big Bang theory.
In 1990, the COBE satellite (COsmic Background Explorer) launched by NASA, discovered that the CMBR had exactly the same intensity profile predicted by the theory (see figure below).

The CMBR is the radiation produced after the Big Bang, shifted to low frequencies due to the expansion and cooling down of the Universe.
The radiation is characterized, in astrophysics, by a typical temperature, called "black body temperature".  The higher the frequency of the radiation, the higher its black body temperature. The CMBR is a very low frequency radiation, with a wavelength of approximately 0.2 cm and a characteristic temperature of approximately 3 degrees Kelvin (-270 °C).
 

Intensity of the cosmic background radiation as a function of frequency, measured by the satellite COBE. (NASA Goddard Space Flight Center)

 

The large scale structure of the Universe
 

In 1992, COBE made another important discovery: the CMBR has very low intensity variations at the various frequencies (that is temperature variations ) in the various directions of space. Such discovery was very important, because it confirmed the theory. In fact, since the background radiation was produced by annihilation of matter and anti-matter, the inhomogeneity of the background radiation reflects that of the distribution of matter in space.
The primordial matter was not distributed homogeneously, but there were regions with larger or smaller density compared to the average.

The anisotropy of the temperature of the CMBR, as measured by the satellite COBE. The zones with higher temperatures are shown in red, those with the lower temperatures in blue and black. (COBE, NASA)

False colour image of the CMBR anisotropy between 2.724 K and 2.732 K. The plane of our Galaxy lies along the major axis of the ellipse. The difference in temperature between the red (hotter) and blue regions is 0.0002 K. (COBE,NASA)

Although the Universe, if considered on a large scale, is almost homogeneous, the measurement of the positions of thousands of galaxies, carried out by the astronomers in the last years, showed that their distribution is not uniform. Galaxies and clusters are gathered in huge flat aggregates, like gigantic "sheets", and long ones, called "filaments" , separated by immense empty regions, the so-called "voids". The whole structure of the Universe resembles a "sponge".
Moreover, many clusters of galaxies are involved in motions towards other gigantic clusters, called "attractors" due to their gravitational force.

How can these structures be explained? According to the cosmologists, they are due to the amplification of very small inhomogeneities in the initial distribution of matter, the same revealed by the anisotropy of the cosmic background radiation. After the Big Bang, on the scale of billions of years, the gravitational forces "condensed" the matter more and more. So, first the galaxies were formed, then the clusters and superclusters, and finally the larger structures like the attractors.

The large scale structure of the Universe, with its clusters of galaxies and its "voids", can be seen in this map that comprises 11 thousand galaxies. Our Galaxy is situated in the centre, and the external radius at a distance of approximately 450 million light years. The region along the plane of the Galaxy is the one about which we have little information, because the stars of the Milky Way obscure the external galaxies. (Smithsonian Astrophysical Observatory)

 

The dark matter
 

The way the large scale structures of the Universe were formed depends on the existing gravitational field, that is on the total amount of matter present. It seems that a particular component of matter, more than others, guided this condensation process: the so-called "dark matter", which, according to the modern theories, dominates the Universe.
In the last decades, the astronomers gathered a lot of evidence of the existence of a kind of invisible matter that binds galaxies and clusters of galaxies thanks to its gravitational attraction, but its nature is yet unknown. Its presence is revealed by some indirect evidence:

1) the rotation of the spiral galaxies.
The spiral galaxies have a differential rotation, in other words they have not a rigid rotation around the central axis, but each star rotates around the axis with a speed that varies according to its distance from the centre. Now, the rotation speed depends on the gravitational field, therefore on the distribution of matter in the same galaxy. The rotation speed of the spiral galaxies varies with the distance from the rotation axis, as if a large part of the matter they contain was distributed in the external regions. Actually, the luminous mass of the galaxies, (the stars and the gas) is concentrated towards the nucleus and its density decreases towards the outside. We deduce that these galaxies must be surrounded by a large halo of invisible matter, that contributes to their gravitational field but not to their luminous emission: the dark matter.

2) the speed distribution in the clusters of galaxies.
Within a group of galaxies there are two counteracting forces: the gravitational force of the whole, which tends to keep the cluster together, and the gravitational push of one galaxy on the others, which tends to increase their relative speed and to scatter the cluster.
Starting from an evaluation of the total mass of the galaxies of a cluster, it is possible to calculate the approximate maximum speed that they can have inside to preserve its stability. It was noticed that in many clusters the galaxies reach much higher speeds, even a hundred times higher, but they are still bound in a stable configuration. This means that the gravitational field that binds them is very intense, but it is not due to the observable matter.
The cluster must therefore be held together by a type of matter very abundant, but not visible.

Formation of multiple images in a gravitational lens. (Drawing by D. Berry, STScI)

3) the gravitational lenses
the gravitational lenses are agglomerates of matter, with a gravitational field so intense that it causes a deviation of the course of the nearby light beams, just like a lens deviates the light beams and conveys them to a focal point. As a result, if one of those "lenses" (typically, a very massive galaxy or a cluster of galaxies) is interposed between us and a far source of light (a galaxy or a quasar), it produces several images of the same source.
 

These phenomena, together with the predictions of the Big Bang theory, lead us to think that the dark matter constitutes approximately 90% of the matter present in the Universe, and therefore the vast majority of it escapes our observations.

Gravitational lenses in the Abell 2218 cluster of galaxies. The arches visible in the figure are the multiple images of the sources at the back. (HST)

Other images of gravitational lenses. Around the central object you can see multiple images (coloured in light blue). (HST)

 

But what is dark matter made of? Since we can only have indirect information regarding it, its nature is yet uncertain. It could be ordinary matter, that is the same stars and planets are made of, but not in the condition of emitting radiation.
For example, it could be made of planets or "brown dwarfs", not massive enough to produce energy by nuclear fusion. Nevertheless, it is thought that the number of these objects is lower than what would be necessary to explain the effects of dark matter observed.

According to a more likely hypothesis, it could be "exotic" matter, in other words different from the common protons, neutrons and electrons. For example, it could be made of massive neutrinos . In fact, it is thought that the neutrinos are particles without a mass, but some recent experiments suggest that they could have a mass, even though very small (1/5000 of the mass of the electron). Since the neutrinos are very common and fill the Universe like the radiation, they alone would account for the effects of dark matter observed.
Another kind of dark matter could be that composed of even "stranger" and yet unknown particles, the existence of which is predicted by theoretical physics but has not yet been demonstrated. We are talking about the so-called (weakly interacting massive particles": the axions, photinos, gravitinos, squarks , ... )

Whatever its composition is, dark matter dominates the Universe, represents its main source of gravitational force and is responsible for a large part of its structure. We can therefore say that we do not know yet what most of our Universe is made of!