A white dwarf, also called a degenerate dwarf, is a  composed mostly of electron degenerated matter . A white dwarf is very dense,  its mass is comparable to that of the sun, and its volume is comparable to that of earth . A white dwarf’s faint light  comes from the emission of stored thermal energy . The nearest known white dwarf is Sirius B , at 8.6 light years, the smaller component of the Sirius BINARY STAR. (Takweer) The Sun will be smaller , denser .

For a long time, the big question was whether there was enough matter in the universe to make it recollapse, or whether it would expand forever. But in the late 1990’s, astronomical observations began to suggest that the expansion of the universe is actually speeding up!

Let’s suppose this is true, and let’s assume the most popular explanation for it: namely, that there is a nonzero CC ( with the right sign makes the energy density of the vacuum positive, but makes its pressure negative – and 3 times as big. This makes the universe tend to expand. Normal matter makes the universe tend to recollapse. If the effect of the cosmological constant ever beats out the effect of normal matter, the universe will keep expanding, making the density of normal matter less… so the cosmological constant will ultimately win hands down, and the universe will eventually expand at an almost exponential rate.

Let’s suppose this happens. What will be the ultimate fate of the universe?

First, galaxies will keep colliding. These collisions seem to destroy spiral galaxies – they fuse into bigger elliptical galaxies. We can already see this happening here and there, and our own Milky Way may collide with Andromeda in only 3 billion years or so. If this happens, a bunch of new stars will be born from the shock waves due to colliding interstellar gas, but eventually we will inhabit a large elliptical galaxy. Unfortunately, elliptical galaxies lack spiral arms, which seem to be a crucial part of the star formation process, so star formation may cease even before the raw materials run out.

Of course, even if this doesn’t happen, the birth of new stars must eventually cease, since there’s a limited amount of hydrogen, helium, and other smaller particles that can undergo fusion.

This means that all the stars will eventually burn out. They’ll either become black dwarfs, neutron stars, or black holes. Stars become white dwarfs – and eventually black dwarfs when they cool – if they have mass less than 1.4 solar masses. In this case they can be held up by the degeneracy pressure caused by the Pauli exclusion principle, which works even at zero temperature. If they are heavier than this, they collapse: they become neutron stars if they are between 1.4 and 2 solar masses, and they become black holes if they are more massive.

The black holes will suck up some of the other stars they encounter. Like a vacuum in the universe beside each other , vacuuming smaller black halls and every thing around it. This is especially true for the big black holes at the galactic centers, which power radio galaxies if they swallow stars at a sufficiently rapid rate. But most of the stars, as well as interstellar gas and dust, will eventually be hurled into intergalactic space. This happens to a star whenever it accidentally reaches escape velocity through its random encounters with other stars. It’s a slow process, but computer simulations show that about 90% of the mass of the galaxies will eventually “boil off” this way – while the rest becomes a big black hole.

(It may seem odd that first the galaxies form by gravitational attraction of matter and then fall apart again by “boiling off”, but the point is, intergalactic matter is less dense now than it was when galaxies first formed, thanks to the expansion of the universe. When the galaxies first formed, there was lots of gas around. Now the galaxies are essentially isolated – intergalactic space is almost a vacuum. And you can show in the really long run, any isolated system consisting of sufficiently many point particles interacting gravitationally – even an apparently “gravitationally bound” system – will “boil off” as individual particles randomly happen to acquire enough kinetic energy to reach escape velocity. Computer calculations already suggest that the solar system will fall apart this way, barring other interventions. With the galaxies it’s even more certain to happen, since there are more particles involved, so things are more chaotic.)

How long will all this take? Well, the white dwarfs will cool to black dwarfs with a temperature of at most 5 Kelvin in about 1017 years, and the galaxies will boil away by about 1019 years. Most planets will have already been knocked off their orbits by then, but any that are still orbiting stars will spiral in thanks to gravitational radiation in about 1020 years.

Then what? Well, in about 1023 years the dead stars will actually boil off from the galactic clusters, not just the galaxies, so the clusters will disintegrate. At this point the cosmic background radiation will have cooled to about 10-13 Kelvin, and most things will be at about that temperature unless proton decay or some other such process keeps them warmer.

So now we have a bunch of isolated black dwarfs, neutron stars, and black holes together with atoms and molecules of gas, dust particles, and of course planets and other crud, all very close to absolute zero.

As the universe expands these things eventually spread out to the point where each one is completely alone in the vastness of space.

Well, everybody loves to talk about how all matter eventually turns to iron thanks to quantum tunneling, since iron is the nucleus with the least binding energy, , this one actually takes quite a while. About 101500 years, to be precise. (Well, not too precise!) So it’s quite likely that proton decay or something else will happen long before this gets a chance to occur.

For example, everything except the black holes will have a tendency to “sublimate” or “ionize”, gradually losing atoms or even electrons and protons, despite the low temperature. Just to be specific, let’s consider the ionization of hydrogen gas – although the argument is much more general. If you take a box of hydrogen and keep making the box bigger while keeping its temperature fixed, it will eventually ionize. This happens no matter how low the temperature is, as long as it’s not exactly absolute zero – which is forbidden by the 3rd law of thermodynamics. EVERY THING MUST RETURN TO THE ORIGIN , Smoke, Gas, Water, Radiation, Waves and particles.

However, there’s a complication: in the expanding universe, the temperature is not constant – it decreases! So the question is, which effect wins as the universe expands: the decreasing density (which makes matter want to ionize) or the decreasing temperature (which makes it want to stick together)?

In the short run this is a fairly complicated question, but in the long run, things may simplify: if the universe is expanding exponentially thanks to a nonzero cosmological constant, the density of matter obviously goes to zero. But the temperature does not go to zero. It approaches a particular nonzero value! So all forms of matter made from protons, neutrons and electrons will eventually ionize!

 This effect is very much like the horizon of a black hole – it’s called a “cosmological horizon”. And, like the horizon of a black hole, a cosmological horizon emits thermal radiation at a specific temperature. This radiation is called Hawking radiation. Its temperature depends on the value of the cosmological constant. If we make a rough guess at the cosmological constant, the temperature we get is about 10-30 Kelvin.

This is very cold, but given a low enough density of matter, this temperature is enough to eventually ionize all forms of matter made of protons, neutrons and electrons! Even something big like a neutron star should slowly, slowly dissipate. (The crust of a neutron star is not made of neutronium: it’s mainly made of iron.). So the Iron that descended to earth and the planets once before , will go back to form again some where else.

Well, they probably evaporate due to Hawking radiation: a solar-mass black hole should do so in 1067 years, and a really big one, comparable to the mass of a galaxy, should take about 1099 years.

Actually, a black hole only shrinks by evaporation when it’s in an environment cooler than the temperature of its Hawking radiation – otherwise, it grows by swallowing thermal radiation. The Hawking temperature of a solar-mass black hole is about 6 × 10-8 Kelvin, and in general, it’s inversely proportional to the black hole’s mass. The universe should cool down below 10-8 Kelvin very soon compared to the 1067 years it takes for a solar-mass black holes to evaporate. However, before that time, such a black hole would grow by absorbing background radiation – which makes its temperature decrease and help it grow more!

As black holes evaporate, they will emit photons and other particles in the process, so for a while there will be a bit of radiation like this running around. That livens things up a little bit – but this process will eventually cease.

What about the neutron stars? Well, if they don’t ionize first, ultimately they should quantum-tunnel into becoming black holes, which then Hawking-radiate away.

Similarly, if the black dwarfs and planets and the like don’t evaporate and their protons don’t decay, they may quantum-tunnel into becoming solid iron – as I already mentioned, this takes about 101500 years. And then, if this iron doesn’t evaporate and nothing else happens, these balls of iron will eventually quantum-tunnel into becoming black holes, which then Hawking-radiate away. This would take about 42 to 55 billion years.