6. Ever Expanding Universe versus Oscillating Universe
Since there is acceleration in expansion, the universe should expand forever and ever expanding universe is favoured than oscillating universe. The density required to close the universe, i.e., so called critical density is some twenty five times larger than the baryon density i.e. if the matter were made up of only baryons, the universe would be an open universe, which expand forever, the combined gravity of all the matter not being strong enough to slow down the expansion.
An oscillating universe where universe expands and contracts alternately is a possibility. In this model oscillations occur but with very long periods. However from the thermodynamic point of view, such oscillating models run into problems. This has to do with production of entropy, i.e. increase in the amount of unavailable energy. Formation of black holes (from collapse of massive stars) considerably increases the entropy. So the oscillations will not be identical and the probability of getting the same properties of the universe in each cycle is incredibly small. So currently such models are not favoured.
7. Questions and Answers
The picture of a universe that started off very hot and cooled as it expanded is in agreement with all the observational evidence that we have today. Nevertheless, it leaves a number of important questions unanswered  :
1) Why was the early universe so hot?
Answer: Once we accept the observational fact that the universe is expanding at a definite rate given by the Hubble constant, it follows that the scale factor would have been much smaller at early times, i.e. in the analogy of an expanding balloon, the surface of the balloon would have been much smaller so that the densities of the radiation and matter would have been much higher. If the density is at a radius R1 and at a radius R2 then , as mass is conserved. For radiation, it follows that the temperature and radius scale as RT = constant, T is the temperature.
Radiation would be black body radiation (radiation in complete equilibrium with matter) as it is expanding with the universe. So if the radius of the universe is reduced to one-third, the temperature of the radiation increases 3 times, hundred times smaller, radiation would be hundred times hotter. So as the universe expands, it cools. The relation between temperature T and time since expansion began, t is given by a well known formula (First derived by George Gamow, more than 60 year ago):
So, one second after the universe started expanding Temperature is ten thousand million degrees. One picosecond after expansion began (t = 10−12 sec.) T = 1016 degrees. This is the TeV energy range which the LHC accelerator at CERN is now probing. It will mimic the conditions in the early universe, to see what new particles would be produced and various new possible phenomena to understand what happened in the beginning period of the universe. So the above formula explains why the early universe was so hot.
It is just an application of thermodynamic of black body radiation to the expanding space-time and predicts that now (14 billion years after expansion) we should be surrounded by radiation of a few degrees Kelvin, which is seen as background microwave cosmic radiation.
2) Why is the universe so uniform on a large scale? Why does it look the same at all points of space and in all directions? In particular, why is the temperature of the microwave background radiation so nearly the same when we look in different directions?
Answer: This is the so called horizon problem. A solution has been provided by one so called INFLATION MODEL. This postulates that the very early universe was dominated by the quantum vacuum energy which has negative pressure. Heisenberg’s uncertainty principle says that if you squeeze particles closer and closer together their velocities (momenta) would increase, the product position x momentum = h = Planck’s constant. The higher momenta would mean that they exert more pressure (like gas molecules hitting a wall), but as the total energy is conserved this pressure would be negative.
ρ is the density, C the speed of light. This ?ve pressure would make the universe expand exponentially fast, i.e. R = R0exp(tct), where t = time, tc = constant and R0 = initial radius.
This briefly implies that a single region in causal contact (i.e. within light horizon, ct) would blow up rapidly, giving it the same uniformity all over.
This rapid expansion would imply “memory loss” of initial conditions, i.e. something like terminal velocity in a fluid. Suppose a body is falling in a viscous medium at high velocity. After a long time it will reach a uniform speed called terminal speed irrespective of its initial speed or position. So a collection of particles at all different velocities, would end up after a sufficiently long time with the same uniform speed. Similarly here different parts or patches of the expanding region would tend to have the same uniform properties.
3) Why did the universe start out with so nearly the critical rate of expansion that separates models that recollapse from those that go on expanding forever, so that even now, ten thousand million years later, it is still expanding at nearly the critical rate? If the rate of expansion one second after the Big Bang had been smaller by even one part in a hundred thousand million, the universe would have recollapsed before it ever reached its present size.
Answer: This is the so called FLATNESS PROBLEM, i.e. the universe being now close to the critical density, would have been arbitrarily close to the critical density at early epochs. Thus one second after the expansion began, it would have been close to the critical density to within one part in 1020. This is also explained by the so called INFLATION MODEL as given above in Answer 2.
The exponential expansion due to the dominance of the quantum vacuum, would result in a universe expanding at exactly the critical density. This implies that the kinetic energy and potential energy are exactly equal at all epochs and that the total energy of the universe is ZERO and has been ZERO right from the beginning.
In Inflationary Picture, the universe was created from quantum vacuum, with total energy zero. Pressure just cancels the energy density , drives exponential expansion. As volume V increases, pressure energy (−PV) becomes more and more negative and to balance this particles must be created with positive energy, i.e. so that total . This is also called creation ex-nihilo, i.e. creation from nothing and as A GUTH, one of the proponent of inflation model noted “The universe is a FREE LUNCH”. So this seems to explain three things:
a) Why the universe started expanding;
b) How was all the matter created in the beginning;
c) Why it is so uniform on a large scale.
This prediction of expansion at exact critical density seems to be consistent with many observations recently.
4) Despite the fact that the universe is so uniform and homogenous on a large scale, it contains local irregularities, such as stars and galaxies. These are thought to have developed from small differences in the density of the early universe from one region to another. What was the origin of these density fluctuations?
Answer: The structures in the universe, are supposed to have grown from density fluctuations. Again the inflation model predicts that quantum fluctuations in a scalar field would grow in an expanding universe and these fluctuations even if just one part in 105 would be stretched with expansion and cause anisotropies in the microwave background. These anisotropies have now been seen in COBE and WMAP satellites. However to form the largest structures we need vast amounts of Dark Matter. We still have the singularity problem. It is supposed that classical general relativity may not be valid around , so quantum effects could avoid such infinite densities, etc. So there would be lower limits to time (quantified space-time) and upper limits to physical quantities like temperature etc. This is still a problem which specialists are trying to resolve.
8. Discussion and Findings
The entire matter (and energy) with its fields (i.e. universe) is not inside anything. Universe means-just fabric of scattered fragments of matter with energy and their fields. The fields are given very specifically-the electromagnetic field being subject to the Maxwell equations and the gravitational field to the Einstein equation  . The universe is finite. It is ever expanding. It expands but not into anything. Since there is acceleration in expansion, the universe should expand forever and ever expanding universe is favoured than oscillating universe.
There is no separate “space” without the presence of matter. Space-time is just a tool for recording or book keeping of events. In recent approaches, space-time is a derived concept and it is not a basic entity. So events create and distort space-time and without events (involving particles), one cannot construct a space-time. The universe consists of matter and energy only.
So far, various models and concepts of the universe have been proposed. But they have not given the ultimate physical reality of the universe and the answer for “What the universe expands into?”―An unexplained puzzling question. This concept makes the question meaningless by showing that universe expands, but not into anything. This concept shows the ultimate physical reality of the universe and the universe is finite and it is ever expanding. We have given answers for important unanswered questions so far.