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.

Evolution of Galaxy

 The solar system is located in a spiral galaxy known as the Milky Way. The galaxy formed from an ill defined globe of hydrogen and helium gas approximately 12 to 14 billion years ago. This proto galaxy gradually shrank under its own gravitational attraction. As it did so, smaller concentrations of gas condensed within it. These were the basis of globular clusters of stars, each orbiting the centre of what was to become the galaxy. Each in turn broke onto hundreds of thousands of smaller knots of gas, which evolved into stars.

 The bulk of the proto galaxy collapsed into the centre without forming part of a globular cluster. As the gas collapsed, it whirled faster round the centre, as water whirls down a plughole. Most of the matter formed a huge flattened globe – the galaxy’s central bulge. Most of this gas broke up into stars, but some was spun out into a great rotating disc, 100,000 light years across. The rotation of the disc slowed the condensing of gas into stars, which continues today.

Multiple Stars

   

Most stars have companions. Two, three or more stars, born from the same gas cloud may orbit each other. Some giant stars are in contact as they orbit each other. The members of other multiple systems are so far apart, they may not belong to one system.

The more massive star of a pair will go through its life cycle more rapidly, and its death may be affected by its companion. In some binary systems, one star has aged until it has become a white dwarf. When the second star at last ages and swells, gas from its out layers can be sucked onto the surface of the white dwarf, which flares up as a bright , seemingly “new” star,  a nova, over 100,000 times brighter than the Sun. the white dwarf blows away the excess matter and subsides.

But if the mass of the white dwarf is above a certain limit, there can be a much more massive explosion a super nova but caused in a different way from those discussed before.

Supernovae

A supernova flares up until it is briefly brighter than the hundred billion fellow stars of its galaxy. The heavy atomic nuclei that it has synthesized at the end of its life are flung across space, mixing with the interstellar nebulae and forming raw materials for new planetary systems around new stars, whose formation may be triggered by the shock waves emanating from the supernova.

Remnants of supernova explosions can be seen as enormous rings and globe of gas. At the centre there may remain a fragment of the original star, with a mass similar to the Sun’s enormously compressed into a sphere perhaps 20Km across. Here electrons and atomic nuclei are crushed together to form a ball of neutrons (the uncharged particles found in the heart of all atoms). One teaspoon full of this neutron star, or pulsar, has a mass of 100 million tones. Beams of radiation jet from its poles generated by gas falling on to it and focused by the neutron star’s stars intense magnetic field. The star spins a few tens or hundreds of times a second, and if one of the radiation beams sweeps across the direction of the Earth, we observe it as pulsating radio source or pulsar.

When a star of around ten solar masses a supergiant dies, the remnant it leaves may be even denser than a neutron star, and becomes a black hole. The gravitation of such a body is so intense that it begins a collapse that continues for ever towards a geometric point ; the more it shrinks the stronger its gravity becomes Neither radiation nor matter can escape from within a sphere only a few kilometers across, the boundary of which is called the even horizon. Such matter has effectively left the universe.

But paradoxically black holes can be at the heart of intensely bright objects. Gas falling into a black hole- perhaps from a companion star- releases enormous amount of energy, much of it at very short X ray wave length.

Types of Stars

The great majority of stars, including the Sun, are “dwarfs”. Only minority that is approaching death have swollen to become ‘giants’ or ‘super giants’. Stars vary in color according to their temperature, from the cooler red stars to the hottest blue or white stars. By comparing magnitude and color it is possible to classify stars.

If the core of the star forming gas cloud is less than a twelfth as massive as the Sun, nuclear reactions can never begin, and the object glows dimly as a reddish ball of gas, a so called “brown dwarf”. The search is on for brown dwarfs.

Star only slightly less massive than the Sun reddish, with surface temperatures of a few thousand degree C. they burn so slowly that they will live for hundreds of billions of years before fading away.

Stars of mass similar to the sun are called ‘yellow dwarfs’ because they give out most of their radiations as yellow light. The surface temperature of such star is similar to that of the Sun, about 6000 degree C, and its life time as a normal hydrogen burning star is about ten billion years. After this it will briefly extend its life by ‘burning’  helium into heavier nuclei in the core, swelling to hundreds of times its original diameter as red giant, with a cool surface,  but still intensely bright because of its huge size. As the helium runs low, a red giant grows unstable, and becomes a variable star, swelling and shrinking and becoming brighter and fainter as it does so. It puffs off shells of gas and finally a collapsed core is left behind as an extremely dense white dwarf, with the mass of a star squeezed into the volume of the Earth. Though hot, it is small and faint, and it continues to cool and fade.

 The most massive stars are about a hundred times as massive as the Sun. The most massive stars are about a hundred times as massive as the Sun. They burn fuel so quickly that they are blue hot, with surfaces over 25,000 degree C, and diameters ten times greater than the 1,390,000 Km of the Sun. they use up their fuel in a few millions years, rather than ten billion. Then they swell into giant or supergiant stars such as Antares. In their intensely hot, dense cores, they burn first helium and then heavier nuclei. They built a range of nuclei. In the last seconds of the star’s life it builds nuclei as heavy as those of iron. These cannot be burned, and the star, its internal power supply cut off, collapses and then explodes as a supernova.

Stars and Nebulae

Great clouds of gas and dust, called nebulae, extend across space. Bight clouds are limited regions stimulated to shine by the light of nearby hot stars. An example is the faint patch of light just discernible in the sword of the constellation Orion. In reality this is a nebula about 15 light years across, the birthplace of thousands of new stars.

More extensive are the dark nebulae. The bulk of these is transparent gas, but the dust in them blocks the light from stars beyond. The dark “lanes” that we seen in the Milky Way are actually dust clouds blocking our view of the piled up banks of stars lying beyond and preventing us from seeing to the center of the galaxy, 30,000 light years away.

 Gas in the galaxy is pretty much the same as the primordial matter that emerged from the Big Bang, and from which the galaxy formed perhaps 12 billion years ago. Three quarters of its mass consists of hydrogen, nearly all the rest of helium, but some of the gas and all of the dust consists of new elements formed since then in stars. The hydrogen, though dark, can be mapped by ultra high frequency radio emissions.

Impacts of Heavenly bodies

In 1994 the comet Shoemaker Levy-9 demonstrated the power of a cometary impact with a planet. The ‘target’ was Jupiter, but there have undoubtedly been many collisions between the Earth and comets or asteroids in the past. Traces of some craters survive; one off the Yucatan peninsula in Mexico seems to mark the impact of a body 10Km wide 65 million years ago. Although the theory is contested, many scientists believe that the dust cloud generated by this impact resulted in significant climatic change. The closest known approach to the Earth by any asteroid in historical times was within 170,000 Km less than half the distance of Moon, in 1991.