Inflation of universe ,Astronomy

Inflation was invented in the 1970s to solve two key problems in cosmology.
The horizon problem--everywhere the Universe looks the same, there is uniformity of the background radiation. There has not been enough time since the Big Bang for light to travel across the Universe and back. So how do the opposite horizons "know" how to keep in step with each other?

The flatness problem-- this is the puzzle that the space-time of the Universe is very nearly flat, which means that the Universe sits just on the dividing line between eternal expansion and eventual recollapse.

The flatness problem can be understood in terms of mass. The density parameter is a measure of the amount of gravitating material in the Universe, usually signified by the Greek letter omega (W). It is defined in such a way that if space-time is exactly flat then W = 1. One of the great difficulties in cosmology was the fact that the actual density of the Universe today is very close to this critical. This is extraordinary because as the Universe expands the density parameter should move away from the critical value.

If the Universe starts out with the parameter less than one, W gets smaller as the Universe ages, while if it starts out bigger than one W gets bigger as the Universe ages. The fact that W is between 0.1 and 1 today means that in the first second of the Big Bang it was precisely 1 to within 1 part in 10⁶⁰. This makes the value of the density parameter in the beginning one of the most precisely determined numbers in all of science, and the instinctive deduction is that the value is exactly 1. One important feature of this is that there is a large amount of dark matter or energy in the Universe. Another is that the Universe was made flat by inflation.

Inflation, in a short period of exponential expansion, caused the very early Universe, to blow-up to the size of what is now. The observable Universe was once a region about the size of a grapefruit. This process would flatten out space-time to make the Universe smooth, and would also resolve the horizon problem.

Another reason why theorists came up with the idea of inflation was to get rid of magnetic monopoles -- particles carrying isolated north or south magnetic fields, predicted by Grand Unified Theory (GUTs) but never found in nature. Standard models of inflation solve the "monopole problem" by arguing that the Universe grew from a quantum fluctuation so small that it only contained one monopole. That monopole is still out there, somewhere in the Universe, but it is highly unlikely that it will ever pass our way.

Inflation became established as the standard model in the 1980s. It resolved many puzzles about the nature of the Universe, and it did so using the grand unified theories and knowledge of quantum theory developed by particle physicists completely independently of any cosmological studies. These theories of the particle world had been developed with no thought that they might be applied in cosmology, and their success in this area suggested to many people that they must be telling us something of fundamental importance about the Universe.

The marriage of particle physics (the micro) and cosmology (the macro) seems to provide an explanation of how the Universe began, and how it got to be the way it is. Inflation is therefore regarded as the most important development in cosmological thinking since the discovery that the Universe is expanding first suggested that it began in a Big Bang.

Taken at face value, the observed expansion of the Universe implies that it was born out of a singularity, a point of infinite density, some 15 billion years ago. Quantum physics tells us that it is meaningless to talk in quite such extreme terms, and that instead we should consider the expansion as having started from a region no bigger across than the so-called Planck length (10^-35m), when the density was not infinite but "only" 10⁹⁴ grams per cubic centimetre. These are the absolute limits on size and density allowed by quantum physics.

The first puzzle is how anything that dense could ever expand -- it would have an enormously strong gravitational field, turning it into a black hole and snuffing it out of existence (back into the singularity) as soon as it was born. But it turns out that inflation can prevent this happening, while quantum physics allows the entire Universe to appear, in this super compact form, out of nothing at all, as a cosmic free lunch. The idea that the Universe may have appeared out of nothing at all, and contains zero energy overall, was developed by Edward Tryon, New York City University, who suggested in the 1970s, that it might have appeared out of nothing as a so-called vacuum fluctuation, allowed by quantum theory.

Quantum uncertainty allows the temporary creation of bubbles of energy, or pairs of particles (such as electron-positron pairs) out of nothing, provided that they disappear in a short time. The more mass created, the shorter the virtual particle can exist. The energy in a (space-time) gravitational field is negative, while the energy locked up in matter is positive. If the Universe is exactly flat, then as Tryon pointed out the two numbers cancel out, and the overall energy of the Universe is precisely zero. It is also expected that the rotation, and charge, of the Universe is also zero. . George Gamow in a conversation with Albert Einstein casually mentioned that one of his colleagues had pointed out to him that according to Einstein's equations a star could be created out of nothing at all, because its negative gravitational energy precisely cancels out its positive mass energy. "Einstein stopped in his tracks, and, since we were crossing a street, several cars had to stop to avoid running us down".

Unfortunately, if a quantum bubble (about as big as the Planck length) containing all the mass-energy of the Universe (or even a star) did appear out of nothing at all, its intense gravitational field would immediately crush it into a singularity.

The development of the inflation showed how to remove this difficulty and allow such a quantum fluctuation to expand exponentially up to macroscopic size before gravity could crush it out of existence. Something was needed to give the Universe an outward push (acting like antigravity) while it was a Planck length in size. Such a small region of space would be too small to contain irregularities, so it would start off isotropic and homogeneous. There would have been enough time for signals (travelling at the speed of light) to have crossed the tiny volume, so there is no horizon problem. And the expansion flattens space-time itself, in much the same way that a balloon becomes smooth, as it is blown-up. If we blow-up the balloon big enough, say the size of the earth, the surface will appear flat.

The reason why the GUTs created such a sensation when they were applied to cosmology is that they predict the existence of exactly the right kind of mechanisms to do this trick. They are called scalar fields, and they are associated with the splitting apart of the original grand unified force into the fundamental forces we know today, as the Universe began to expand and cool.

At the Planck time, 10^-43 of a second, gravity would be created as the super-symmetry was broken (this provided the energy for inflation), and by about 10^-35 of a second the strong nuclear force. Within about 10^-32 of a second, the scalar fields would have doubled the size of the Universe at least once every 10^-34 of a second (some versions of inflation suggest even more rapid expansion than this).

It would mean that in 10³² of a second there were 100 doublings. This rapid expansion is enough to take a quantum fluctuation 10²⁰ times smaller than a proton and inflate it to a sphere about 10 cm across in about 15 x 10³³ seconds. At that point, the scalar field had crystallized leaving the Universe rapidly expanding so that the influence of gravity would not pull everything back into a Big Crunch.

Curiously, this kind of exponential expansion of space-time is exactly described in 1917, by one of the first cosmological models developed using the general theory of relativity, by Willem de Sitter. For more than half a century, this de Sitter model seemed to be only a mathematical curiosity, but it is now one of the cornerstones of inflationary cosmology.

When the general theory of relativity was published in 1916, de Sitter wrote a series of three papers, which he sent to the Royal Astronomical Society in London. The third of these papers included discussion of an expanding universe and an oscillating universe.

De Sitter's solution to Einstein's equations seemed to describe an empty, static Universe (empty space-time). But in the early 1920s it was realised that if a tiny amount of matter were added to the model (in the form of particles scattered throughout the space-time), they would recede from each other exponentially fast as the space-time expanded.

In the expanding Universe as we see it now, the distances between galaxies increase steadily. In the 1980s, however, when the theory of inflation suggested that the Universe really did undergo a stage of exponential expansion during the first split-second after its birth, this inflationary exponential expansion turned out to be exactly described by the de Sitter model.

Inflation seems (wrongly!) to violate the faster-than-light rule. Even light takes 30 billionths of a second (3 x 10^-10 sec) to cross a single centimetre, and yet inflation expands the Universe from a size much smaller than a proton to 10 cm across in only 15 x 10^-33 sec. This is possible because it is space-time itself that is expanding, carrying matter along for the ride; nothing is moving through space-time faster than light. Indeed, it is just because the expansion takes place so quickly that matter has no time to move, and the process captured the original uniformity of the primordial quantum bubble.

The inflationary scenario has already gone through several stages of development during its short history. The first inflationary model was developed by Alexei Starobinsky, at the L. D. Landau Institute of Theoretical Physics in Moscow, at the end of the 1970s. It was a model based on a quantum theory of gravity, it became known as the "Starobinsky model" of the Universe.

In 1981, Alan Guth, then at MIT, published a different version of the inflationary scenario, not knowing anything of Starobinsky's work. Guth came up with the name "inflation" for the process he was describing. There were obvious flaws with the specific details of Guth's original model. In particular, Guth's model left the Universe after inflation filled with a mess of bubbles, all expanding in their own way and colliding with one another. We see no evidence for these bubbles in the real Universe, so obviously the simplest model of inflation couldn't be right.

In October 1981, there was an international meeting in Moscow, where inflation was a major talking point. Stephen Hawking presented a paper claiming that inflation could not be made to work at all, but the Russian cosmologist Andrei Linde presented an improved version, called "new inflation", which got around the difficulties with Guth's model.

Within a few months, Andreas Albrecht and Paul Steinhardt, of the University of Pennsylvania also published the new inflationary scenario, and by the end of 1982 inflation was well established. Linde has been involved in most of the significant developments with the theory since then. The next step forward came with the realization that there need not be anything special about the Planck- sized region of space-time that expanded to become our Universe. If that was part of some larger region of space-time in which all kinds of scalar fields were at work, then only the regions in which those fields produced inflation could lead to the emergence of a large universe like our own. Linde called this "chaotic inflation", because the scalar fields can have any value at different places in the early super-universe; it is the standard version of inflation today, and can be regarded as an example of the kind of reasoning associated with the anthropic principle (this has nothing to do with the mathematical subject known as "chaos theory").

The idea of chaotic inflation led to the ultimate development of the inflationary scenario. The great-unanswered question in standard Big Bang cosmology is what came "before" the singularity. It is often said that the question is meaningless, since time itself began at the singularity. But chaotic inflation suggests that our Universe grew out of a quantum fluctuation in some pre-existing region of space-time, and that exactly equivalent processes can create regions of inflation within our own Universe. New universes could bud off from our Universe, and our Universe may itself have budded off from another universe, in a process, which had no beginning and will have no end. A twist on this theory suggests that the process takes place through black holes, and that every time a singularity is formed it expands out into another set of space-time dimensions, creating a new inflationary universe - this is called the baby universe scenario.

Even Darwinian principals can be applied to this process. As new Universes are formed they (probability) take on the physics of the parent Universe. If the initial conditions are exactly right then the baby Universe will collapse back. This explains why our Universe is so finely tuned.

There are similarities between the idea of eternal inflation and a self-reproducing universe and the version of the Steady State hypothesis developed by Fred Hoyle and Jayant Narlikar, with their C-field playing the part of the scalar field that drives inflation. As Hoyle wryly pointed out at a meeting of the Royal Astronomical Society in London in December 1994, the relevant equations in inflation theory are exactly the same as in his version of the Steady State idea, but with the letter "C" replaced by the Greek "Ø".

Modern proponents of the inflationary scenario arrived at these equations entirely independently of Hoyle's approach, and are reluctant to accept this analogy, having cut their cosmological teeth on the Big Bang model.

One of the first worries about the idea of inflation (long ago in 1981) was that the process was so efficient at smoothing out the Universe, how could irregularities as large as galaxies, clusters of galaxies and so on ever have arisen?

Quantum fluctuations could produce tiny ripples in the structure of the Universe even when our Universe was only 10^-25 of a centimetre across -- a hundred million times bigger than the Planck length.

Observations of the background radiation by a satellite called COBE showed exactly the pattern of tiny irregularities that the inflationary scenario predicts.

The theory said that inflation should have left behind an expanded version of these fluctuations, in the form of irregularities in the distribution of matter and energy in the Universe. These density perturbations would have left an imprint on the background radiation at the time matter and radiation decoupled (about 300,000 years after the Big Bang), producing exactly the kind of nonuniformity in the background radiation that has now been seen, initially by COBE and later by other instruments.

After decoupling, the density fluctuations grew to become the large-scale structure of the Universe revealed today by the distribution of galaxies. This means that the COBE observations are actually giving us information about what was happening in the Universe when it was less than 10^-20 of a second old.

No other theory can explain both why the Universe is so uniform overall, and yet contains exactly the kind of "ripples" represented by the distribution of galaxies. This of course does not prove that the theory is correct. The theory also makes another prediction, that the primordial perturbations may have left a trace in the form of gravitational radiation with particular characteristics, and it is hoped that detectors sensitive enough to identify this characteristic radiation may be developed within the next ten or twenty years.

The clean simplicity of this simple picture of inflation has now, however, begun to be obscured by refinements. Linde has pushed the theory of inflation to extremes, and brought new insights into how the Universe might be constructed. For example, could our Universe exist on the inside of a single magnetic monopole produced by cosmic inflation? .

But Linde has discovered that, according to theory, the conditions that create inflation persist inside a magnetic monopole even after inflation has halted in the Universe at large. Such a monopole would look like a magnetically charged black hole, connecting our Universe through a wormhole in space-time to another region of inflating space-time. Within this region of inflation, quantum processes can produce monopole-antimonopole pairs, which then separate exponentially rapidly as a result of the inflation. Inflation then stops, leaving an expanding Universe rather like our own which may contain one or two monopoles, within each of which there are more regions of inflating space-time.

The result is a never-ending fractal structure, with inflating universes embedded inside each other and connected through the magnetic monopole wormholes. Our Universe may be inside a monopole, which is inside another universe, which is inside another monopole, and so on indefinitely. What Linde calls "the continuous creation of exponentially expanding space" means that "monopoles by themselves can solve the monopole problem". Although it seems bizarre, the idea is, he stresses, "so simple that it certainly deserves further investigation".

That variation on the theme really is just for fun, and it is hard to see how it could ever be compared with observations of the real Universe. But most of the modifications to inflation now being made are in response to new observations, and in particular to the suggestion that space-time may not be quite "flat" after all. In the mid-1990s, many studies began to suggest that there might not be quite enough matter in the Universe to make it perfectly flat -- most of the observations suggest that there is only 20 per cent or 30 per cent as much matter around as the simplest versions of inflation require. The shortfall is embarrassing, because one of the most widely publicised predictions of simple inflation was the firm requirement of exactly 100 per cent of this critical density of matter. But there are ways around the difficulty; and here are two of them to be going on with.

The first suggestion is almost heretical, in the light of the way astronomy has developed since the time of Copernicus. Is it possible that we are living near the centre of the Universe? The suggestion that the "ordinary" place to find observers like us may be in the middle of a bubble in a much greater volume of expanding space.

The conventional version of inflation says that our entire visible Universe is a bubble of inflation in an eternal chaotic inflation sea of many. The process of rapid expansion forces space-time in all the bubbles to be flat. A useful analogy is with the bubbles that form in a bottle of fizzy cola when the top is opened. But that suggestion, along with other cherished cosmological beliefs, has now been challenged by Linde, working with his son Dmitri Linde (CalTech) and Arthur Mezhlumian (Stanford).

Linde and his colleague’s point out that the Universe we live in is like a hole in a sea of superdense, exponentially expanding inflationary cosmic material, within which there are other holes. All kinds of bubble universes will exist, and it is possible to work out the statistical nature of their properties. In particular, the two Lindes and Mezhlumian have calculated the probability of finding yourself in a region of this super- Universe with a particular density -- for example, the density of "our" Universe.

Because very dense regions blow up exponentially quickly (doubling in size every fraction of a second), it turns out that the volume of all regions of the super-Universe with twice any chosen density is 10¹⁰ million times greater than the volume of the super- Universe with the chosen density. For any chosen density, most of the matter at that density is near the middle of an expanding bubble, with a concentration of more dense material round the edge of the bubble. But even though some of the higher density material is round the edges of low-density bubbles, there is even more (vastly more!) higher density material in the middle of higher density bubbles, and so on forever. The discovery of this variation on the theme of fractal structure surprised the researchers so much that they confirmed it by four independent methods before venturing to announce it to their colleagues. Because the density distribution is non-uniform on the appropriate distance scales, it means that not only may we be living near the middle of a bubble universe, but that the density of the region of space we can see may be less than the critical density, compensated for by extra density beyond our field of view.

This is convenient, since those observations by the Hubble Space Telescope have suggested that cosmological models, which require exactly the critical density of matter, may be in trouble. But there is more. Those Hubble observations assume that the parameter, which measures the rate at which the Universe is expanding, the Hubble Constant, really is a constant, the same everywhere in the observable Universe. If Linde's team is right, however, the measured value of the "constant" may be different for galaxies at different distances from us. We may seem to live in a low-density universe in which both the measured density and the value of the Hubble Constant will depend on where these properties are measured!

That would mean abandoning many cherished ideas about the Universe. But there is a simpler solution to the density puzzle, one that involves tweaking the model of inflation. By turning inflation on twice, they have found a way to have all the benefits of the inflationary scenario, while still leaving the Universe in an "open" state, so that it will expand forever.

In those simplest inflation models, remember, the big snag is that after inflation even the observable Universe is left like a mass of bubbles, each expanding in its own way. We see no sign of this structure, which has led to all the refinements of the basic model. Now, however, Martin Bucher and Neil Turok, of Princeton University, working with Alfred Goldhaber, of the State University of New York, have turned this difficulty to advantage.

They suggest that after the Universe had been homogenised by the original bout of inflation, a second burst of inflation could have occurred within one of the bubbles. As inflation begins (essentially at a point), the density is effectively "renormalized" to zero, and rises towards the critical density as inflation proceeds and energy from the inflation process is turned into mass. But because the Universe has already been homogenised, there is no need to require this bout of inflation to last until the density reaches the critical value. It can stop a little sooner, leaving an open bubble (what we see as our entire visible Universe) to carry on expanding at a more sedate rate. They actually use what looked like the fatal flaw in Guth's model as the basis for their scenario. According to Bucher and his colleagues, an end product looking very much like the Universe we live in can arise naturally in this way, with no "fine-tuning" of the inflationary parameters. All they have done is to use the very simplest possible version of inflation, going back to Alan Guth's work, but to apply it twice. And you don't have to stop there. Once any portion of expanding space-time has been smoothed out by inflation, new inflationary bubbles arising inside that volume of space-time will all be pre-smoothed and can end up with any amount of matter from zero to the critical density (but no more).

With all these Universes we may now need to clarify the actual vocabulary.

The term Universe, with the capital "U", is usually used to describe space-time and the matter it contains. This may seem like a fairly comprehensive definition, and in the past it has traditionally been regarded as synonymous with the entirety of everything that exists. But the development of ideas such as inflation suggests that there may be something else beyond the boundaries of the observable Universe -- regions of space and time that are unobservable in principle, not just because light from them has not yet had time to reach us, or because our telescopes are not sensitive enough to detect their light.

This has led to some ambiguity in the use of the term "Universe". Some people restrict it to the observable Universe, while others argue that it should be used to refer to all of space and time or other space-times. There may be an infinite number of other expanding bubbles of space-time, other universes with which we can never communicate.