Dark matter and the far future

Theories of the big bang and the existence of the ripples in the back ground radiation suggest that the matter in the universe is greatly the dark matter. With in the dark matter the universe may be only a thin froth on the ocean of the matter.

If the amount of matter in the universe consists only of what we can observe place the dark matter, then the gravitational force is not enough to reverse the expansion of the universe. The clusters of galaxies will recede ever further apart and fade from each other's view. Black holes at the centers of the galaxies will grow, swallowing some of the matter in each galaxy, while the rest will disperse into intergalactic space. The last stars will be born a trillion year from now.  After 100 trillion years, even the longest lived stars, the low mass red dwarfs, will have faded. After 10 ¹⁹   years, most dead stars and other matters in galaxies will have dispersed while the remainder will have been swallowed up by black holes. After 10 ¹¹⁷ years, even black holes will have evaporated into radiation, and the universe will consists of a featureless enormously diluted sea of light weight particles and low energy radiation

The big bang

Galaxies seems to be flying apart as a result of an enormous explosion that occurred 10-20 billion years ago. Cosmologists believe this “Big Bang” began with all matter and energy in the universe packed into a tiny space, smaller than an atom, with density and temperature at unimaginable levels. The known laws of physics cannot describe or account for the very beginning. But cosmologists can describe what happens from an incredible tiny fraction of a second after wards, At first, every possible particle existed, all of them colliding and changing into each other every moment. Many were particles unknown to us or only fleetingly glimpsed in experiments.

In today’s universe there are four fundamental forces; gravitation, electromagnetism, and two forces that act within the nucleus, a strong and weak. But at the colossal temperatures of the newly born universe, these were all merged into one super force felt equally by all particles. However, 10^-43   seconds after the beginning, gravity became a separate force. Then, at 10^-35 seconds, there was an enormous “inflation” of the universe, in which it blew up from the size on an atom to that of a beach ball in a split second. Then the strong nuclear force separated out, and finally the weak force and electromagnetism split, all well within a billionth of a second.

As the universe expanded, it thinned and cooled. Short lived, exotic particles ‘decayed’ into longer lived, more familiar particles that we recognize today.  Quarks were built up into the protons and neutrons that were to be the building blocks of atoms. But they were still too hot to stick to each other or to join up with electrons to form atoms.

All this took place within the first hundredth of a second. When the universe was 25 seconds old it consisted mostly of radiation at a temperature of several billion degrees and a thousand times denser than water is today, out weighing matter a hundred thousand times. Matter was knocked around by energetic photons, “bullets” if radiation

 During the first three minutes, neutrons stuck to proton’ to form nuclei of helium, with traces of one or two other kinds of light atom. But helium nuclei were outnumbered 12 to one by single protons, or hydrogen nuclei. The temperature had fallen below one billion degree centigrade.

 For 300,000 years the universe continued to expand.  Then, as the temperature fell to about 3000C; electrons joined with atomic nuclei to form uncharged atoms.  Radiation no longer interacted strongly with matter. After another few hundred million years, the thinning gas broke up into giant lumps the pro galaxies. The first stars may have begun to shine at this time.

Galactic black holes

The source of energy for all these different types of active galaxy is almost certainly a black hole. When a typical galaxy was formed, some of the stars near the center fell together and merged to form a massive black hole. For perhaps 50 million years this black hole swallowed stars, gas and dust, emitting vast quantities of radiation as it did so. Then the matter at the core of the galaxy was used up and the quasar faded. Quasar activity flared up and died away in galaxies over a two billion year period.

Similar activity continued on a smaller scale thereafter. The various sorts of active galaxy that we see today may be similar objects but they are viewed from different angle

Although the galaxies move randomly within a cluster, the clusters are rushing apart from each other. This is revealed by their red shifts. The galaxies motion has no center; viewed from any galaxy, all other galaxies seems to be rushing away. The further away the galaxy, the faster it recedes. The speed of recession is 17 to 30 Km/ Sec for every billion light years of distance.

Super Clusters and Active Galaxies

Counts of galaxies show that the clusters are themselves grouped into” super clusters” chains and sheets separated by apparently empty voids, and measuring hundreds and thousands of millions of light years across. Whether this frothy texture of the universe was present in the gas from which the proto galaxies condensed, or whether the galaxies gathered into these structures after their formation, is still not understood.

At the heart of any nearby galaxies, and of the Milky Way, there seems to be great activity, hidden behind dense gas clouds. Stars are being whirled around by intense gravitations fields, and high energy X ray radiation is being emitted. In some galaxies there is even greater activity, and these can be detected at greater distances.

Many active galaxies are throwing out great jets of radiation together with charged particles which show up clearly at radio wavelengths the jets collide with the intergalactic gas and are halted to form puffs of radio emitting matter, called radio lobes, mainly consisting of fast moving charged particles but including a scattering of stars.

The most intense activity is showing by the quasars, the nearest of which is two billion light years from us. A quasar is a quasi-stellar object, that is it looks like a star, but radiates in other regions of the electromagnetic spectrum like a galaxy. Not only can we see more quasars at great distances, because they are so bright, but the really are more. Most are found so far away that their light left them about eleven billion years ago- only a few billion years after the Big Bang. The outpouring of radiation seems to come from a tiny region at the centre of a galaxy.

Groups of Galaxies

Galaxies are grouped into clusters, typically tens of millions of light years across, with hundreds or thousands of members. They are held together by their mutual gravitation, and each galaxy moves around the centre of gravity of the cluster. Their speed of movement shows that they experience a greater gravitational pull than can be accounted for by the observed galaxies. Some of this missing mass consists of dim galaxies too faint to be detectable and some in dark intergalactic gas which emits X ray that can be detected. But there must be much more “dark matter” than has so far been accounted for. Furthermore, current theories of the origin of the universe suggest that there must be a hundred times as much matter in the universe as astronomers have so far detected.

Interacting galaxies

 A typical galaxy will have had a collision or a near miss with another galaxy half a dozen times in its life so far. Pair of galaxies that are in the process of near collision can be observed now. One pair is called the Mice because of the streamers of stars, resembling mouse tails that they are pulling from each other; similar features in another pair have led to the galaxies being dubbed the Antennae.

One galaxy can score a direct hit on another and pass right through. The individual stars are much more widely spaced in relation to their diameters than galaxies are, and they do not collide. But gas clouds in the two galaxies can collide and generate intense radiation, thus spawning millions of new stars.  Such objects are called “starburst’ galaxies. When the intruding galaxy has departed,

The local group of Galaxy

Galaxies tent to cluster together. The Milky Way galaxy belongs to a small cluster, consisting of at least 25 galaxies. Four are visible to the naked eye. One is a small spiral that can just be seen under good conditions in Triangulum. The others are the Andromeda galaxy and the large and Small Magellanic Clouds.

 The Local Group of galaxies is dominated by two spirals, the Milky Way galaxy and the Andromeda Galaxy, and the two are often compared. Both are larger than the average spiral, but Andromeda is twice as broad as ours and the largest member of the Local Group. It is approaching our galaxy at a speed of 275 Km/Sec – negligible speed in comparison with its distance of over two million light years.

The Local group contains hundred trillion members. They contain little gas, and a few young stars, so they are predominantly reddish in color. There are no large elliptical galaxies in the Local Group, but some small ones are satellite galaxies of Andromeda.  The large and Small Magellanic clouds are satellite galaxies of the Milky Way. They are visible as two patches of light close to the Milky Way in the Southern Sky. The Large Magellanic Cloud is 170,000 light years distance and 30,000 light years. The Small Magellanic Cloud is 190,000 light years away and 16,000 light years diameter. Both are classified as spiral, though they have been heavily distorted by the Milky Way.

Structure of the galaxy

The globular clusters contain the galaxy’s oldest stars and consequently look red dish, because many of their stars have expanded to become giants or super giants. The clusters orbit the galaxy’s centre, and often pass through the disc, but there are no collisions among the stars, which are enormous distances apart, even within the relatively closely packed globular clusters. There is almost certainly a vast amount of undetected”dark mater” spread through the halo of every galaxy. It might consist of huge numbers of brown dwarfs – too dim to be seen – but many astronomers think this matter consists of undiscovered types of subatomic particles.

The stars near the centre of the galaxy are also old and reddish, and relatively little gas were left over here from the process of star formation. But there are gas clouds in a central region of ten light years or so in diameter that can be probed only with radio telescopes. The clouds are violently agitated by some object or objects at the centre with a mass of a few million Suns. That object may be a black hole, swallowing mass of the equivalent of ten suns a year, or the remains of super massive stars that may have exploded there within the last 100 million years or so.

The disc of the galaxy is 100,000 light years across. The sun lies about two thirds of the way out from the centre, and takes about 230 million years to orbit the galactic centre. Two or perhaps four tightly coiled spirals arms are marked out by bright bluish stars. The arms are regions where gravitational effects are believed to cause ripples of slightly increased density though this is poorly understood. Stars and gas clouds pass through the arms but their passage is slowed. Star birth is triggered here. Though all kinds of stars are born, the minority of short lived; fast burning blue hot massive stars are conspicuous, marking out the arms.