One of the most severe challenges for inflation arises from the need for fine tuning in inflationary theories. In new inflation, the slow-roll conditions must be satisfied for inflation to occur. The slow-roll conditions say that the inflaton potential must be flat (compared to the large vacuum energy) and that the inflaton particles must have a small mass.[57] In order for the new inflation theory of Linde, Albrecht and Steinhardt to be successful, therefore, it seemed that the universe must have a scalar field with an especially flat potential and special initial conditions.

Skeptics like to say that fine tuning cannot be proven by science, since we have only one universe to study. However, the discovery and quantification of dark energy has puzzled a number of scientists, who realize that its extremely small value requires that the initial conditions of the universe must have been extremely fine tuned in order that even matter would exist in our universe. By chance, our universe would have been expected to consist of merely some thermal radiation.

On theoretical grounds, astronomers think that the very early universe experienced a time of ultra-fast expansion (called inflation). The inflation probably took place from about 10-38 to 10-36 seconds after the Big Bang, but astronomers are not sure of the cause of inflation so they cannot pinpoint the time it would have occurred. The size of the fluctuations in the cosmic microwave background indicate that the inflation could not have occurred before 10-38 seconds after the Big Bang. The leading theory for the cause of the inflation says that it occured when there was a break in the fundamental forces of nature. Before the time of 10-38 seconds after the Big Bang, the fundamental forces of the strong nuclear force, the weak nuclear force, and electromagnetic force behaved in the same way under the extreme temperatures. They were part of the same fundamental unified force. Theories that describe the conditions when the forces were unified are called Grand Unified Theories (GUTs for short). At about 10-38 seconds after the Big Bang, the universe had cooled down to "only" 1029 K and the strong nuclear force broke away from the weak nuclear and electromagnetic forces. This breaking apart of the forces from each other somehow produced the huge expansion that expanded the universe by about 1050 times in about 10-36 seconds.

In addressing audiences about the fine-tuning of the cosmic expansion rate, I have used the illustration that adding or subtracting a single dime to the mass of the observable universe would be enough of a change to make physical life impossible. This word picture helps to demonstrate a number used to quantify that fine-tuning, namely 1 part in 1060. Compared to the total mass of the observable universe, 1 part in 1060 works out to about a tenth part of a dime.

Let's consider a universe that contains only matter. If the matter density is sufficiently large, gravity will overcome the expansion and cause the universe to collapse on itself. If the density is sufficiently small, the cosmos will continue to expand forever with negligible slowing. If the density is just right, the universe will expand forever, but continually slow down its expansion rate until it becomes static at an infinite time into the future. In a universe that contains only matter, this corresponds to a "flat" geometry for the universe. Life and flatness are related because only a flat universe meets two life-essential requirements. First, a flat universe survives long enough for an adequate number of generations of stars to form that will make the heavy elements and long-lived radiometric isotopes that advanced life requires. Second, a flat universe expands slowly enough for the matter to clump together to form galaxies, stars, and planets, but not so slowly as to form only black holes and neutron stars.

Until the mid-1990s, astrophysicists found it remarkable that the universe was so close to a flat geometry because such flatness is unstable with respect to time. Even though they could detect only about 4 percent of the mass required to make the universe flat, this required the early universe to be exquisitely close to "flat" to within one part in 1060. The previous statement holds true even given the uncertainties that existed twenty years ago (and to a lesser extent still do) in measurements of the cosmic mass density. Thus, in the absence of dark energy, the expansion rate would have changed so dramatically that the galaxies, stars, and planets necessary for physical life would never have formed.

Over the past fifteen years the picture has changed significantly. First, measurements of the radiation left over from the cosmic creation event, also known as the cosmic microwave background radiation, confirmed (with an error bar of about 3 percent1) that the universe is geometrically flat. Second, the concept of an extremely early epoch of cosmic inflation (a brief period of hyperexpansion of the universe when it was less than a quadrillionth of a quadrillionth of a second old) was developed into a scientifically testable hypothesis that later measurements partially confirmed.2 Third, astronomers discovered another density parameter for the universe, namely space energy density or what is now known as dark energy. For most astronomers and physicists an early epoch of cosmic inflation solves the one part in 1060 fine-tuning problem because such inflation in the early universe drives it exquisitely close to a flat geometry regardless of the universe's initial mass density.

A cosmic fine-tuning problem remains, however. The total cosmic mass density measured through several independent methods falls short by a little more than a factor of three from that required to make a flat-geometry universe,3 which measurements of the cosmic microwave background radiation have established. Dark energy comes to the rescue to make up the deficit, but not without a price. By any accounting, the source or sources of dark energy are at least 120 orders of magnitude larger than the amount detected. This implies that somehow the source(s) must cancel so as to leave just one part in 10120 in order to match the small amount of dark energy detected by astronomers. Therefore, while inflation and dark energy can "eliminate" the one part in 1060 fine-tuning in the mass density of the universe, they can only do so by introducing the far more exquisite one part in 10120 fine-tuning in the dark energy density.

The Inflation Theory proposes a period of extremely rapid (exponential) expansion of the universe during its first few moments. It was developed around 1980 to explain several puzzles with the standard Big Bang theory, in which the universe expands relatively gradually throughout its history.

The Inflation Theory, developed by Alan Guth, Andrei Linde, Paul Steinhardt, and Andy Albrecht, offers solutions to these problems and several other open questions in cosmology. It proposes a period of extremely rapid (exponential) expansion of the universe prior to the more gradual Big Bang expansion, during which time the energy density of the universe was dominated by a cosmological constant-type of vacuum energy that later decayed to produce the matter and radiation that fill the universe today.

Inflation was both rapid, and strong. It increased the linear size of the universe by more than 60 "e-folds", or a factor of ~10^26 in only a small fraction of a second! Inflation is now considered an extension of the Big Bang theory since it explains the above puzzles so well, while retaining the basic paradigm of a homogeneous expanding universe. Moreover, Inflation Theory links important ideas in modern physics, such as symmetry breaking and phase transitions, to cosmology.

Fine-tuning is also evident in the "initial conditions" or the beginning state of the universe. The initial conditions of the universe include such information as the expansion energy of the Big Bang, the overall amount of matter that was present, the ratio of matter to antimatter, the initial rate of the universe’s expansion and even the degree of its entropy.

Consider the expansion rate of the Big Bang. If it was greater, so the early universe expanded faster, the matter in the universe would have become so diffuse that gravity could never have gathered it into stars and galaxies. If it was less, so the early universe expanded more slowly, gravity could have overwhelmed the expansion and pulled all the matter back into a black hole. The expansion rate was just right, so that the universe could have stars in it.

Another interesting example of a finely-tuned initial condition is the critical density of the universe. In order to evolve in a life-sustaining manner, the universe must have maintained an extremely precise overall density. The precision of density must have been so great that a change of one part in 1015 (i.e. 0.0000000000001%) would have resulted in a collapse, or big crunch, occurring far too early for life to have developed, or there would have been an expansion so rapid that no stars, galaxies or life could have formed.9 This degree of precision would be like a blindfolded man choosing a single lucky penny in a pile large enough to pay off the United States’ national debt.

Based on Einstein's cosmological equation, if at any moment our universe's density is slightly greater than the critical density, it will become increasingly greater than the critical density. Eventually, the gravitational attraction will reverse the universe's expansion, leading to a big crunch. On the other hand, if the density is slightly less than the critical density, it will become increasingly less than the critical density. Eventually, our universe will fly apart and become essentially an empty space. Thus, our universe appears to be an unstable system, like a pencil balanced on its point. Because the universe's density cannot afford for a slight deviation from the critical density, cosmologists believe that it must be "exactly" (to the accuracy of 1 part in 1060) equal to the critical density when our universe was created. Otherwise, we would not be here today.

the anthropic principle refers to a collection of scientific insights indicating that the possibility of the evolution of carbon-based life is dependent upon a very delicate balance among the basic forces of nature and also on very specific initial circumstances for the universe.
An example of one of these scientific insights is set out by Stephen Hawking in the following way: "Why is the universe so close to the dividing line between collapsing again and expanding indefinitely? In order to be as close as we are now, the rate of early expansion had to be chosen fantastically accurately. If the rate of expansion one second after the Big Bang had been less than one part in 10 to the 10th power, the universe would have collapsed after a few million years. If it had been greater by one part in 10 to the 10th power, the universe would have been essentially empty after a few million years. In neither case would it have lasted long enough for life to develop. Thus one either has to appeal to the anthropic principle or find some physical explanation of why the universe is the way it is." Hawking is saying that a difference of one part in ten billion in the rate of cosmic expansion would have been enough to preclude the emergence of life.
This is one of the scientific insights that make up the anthropic principle. There are others. The question remains: What is the best explanation of these anthropic phenomena?

If the temperature of the primal fireball that resulted from the Big Bang some fifteen to twenty billion years ago, which was the beginning of our universe, had been a trillionth of a degree colder or hotter, the carbon molecule that is the foundation of all organic life could never have developed. The number of possible universes is trillions of trillions; only one of them could support human life: this one. Sounds suspiciously like a plot.

http://www.faithhelper.com/astrophysics1.htm

a Big Bang does not necessarily result in a world like ours! If subatomic particles created at the Big Bang do not stick together, there will only be gas in the universe, thus no stars or galaxies and no Earth. If such nuclear force is too strong, all matter created at the Big Bang will be compressed, consumed and become black holes, and there will be no chance for life.

Scientists discovered that for the world as we know it to exist, many parameters have to fall into very narrow range. Just like listening to a radio station, you must turn the dial to a specific frequency. Not just any arbitrary numbers will do. In fact, there are more than 70 such parameters. Scientists call our universe a fine-tuned universe. The astronomic and cosmological evidences overwhelmingly point to the design of the universe.

The world is not an accident. It was created and fine-tuned to exist.

If the Big Bang theory is correct, then these finely tuned parameters affected the nature of the universe from its earliest moments. The setting of various constants determined if our universe contained any protons, atoms, molecules, or any life, period:
"the Big Bang cooled just quickly enough to allow neutrons to become bound to protons inside atoms. Here the presence of electrons and the Pauli principle discouraged their decay, but even that would not prevent it were the mass difference slightly greater. And were it smaller--one third of what it is--then neutrons outside atoms would not decay. All protons would thus change irreversibly into neutrons during the Bang, whose violence produced frequent proton-to-neutron conversions. There could be no atoms: the universe would be neutron stars and black holes ... The mass of the electron enters the picture like this. If the neutron mass failed to exceed the proton mass by a little more than the electron's mass, then atoms would collapse, their electrons combining with their protons to yield neutrons ... As things are, the neutron is just enough heavier to ensure that the Bang yielded only about one neutron to every seven protons. The excess protons were available for making hydrogen of long-lived stable stars, water, and carbohydrates."20

But the presence of matter isn't all that matters, and the fact of the matter is that the type of matter matters much in deciding whether life can even exist to ponder these matters. Physicist John Polkinghorne clarifies:
"In the first three minutes of cosmic history, the whole universe was the arena of nuclear reactions. When that era came to an end, through the cooling produced by expansion, the world was left, as it is today on the large scale, a mixture of three-quarters hydrogen and one-quarter helium. A little change in the balance between the strong and weak nuclear forces could have resulted in there being no hydrogen--and so ultimately no water, that fluid that seems so essential to life. A small increase (about 2 percent) in the strong nuclear force would bind two protons to form diprotons. There would then be no hydrogen-burning main-sequence stars, but only helium burners, which are far too fierce and rapid to be energy sources capable of sustaining the coming to be of planetary life. A decrease in the strong nuclear force by a similar amount would have unbound the deuteron and played havoc with fruitful nuclear physics."19
Though the Big Bang itself is said to have created mostly helium and hydrogen, nuclear physics says that other elements could have been produced in the nuclear reactions going on inside of stars. Carbon and oxygen, elements vital to life, are two such heavier elements which, due to their chemical bonding properties, appear to be vital for complex life-form metabolic chemistry. The only other element like carbon is silicon, but silicon is much heavier and has significantly different bonding properties (carbon bonds with many other elements to form mobile gas and liquid substances which are useful for allowing for complex organic chemical reactions. When silicon bonds, it typically forms solids, which makes it no surprise that it is the second most abundant element on earth--it comprises the bulk of rock!!). Oxygen is also useful in its bonding capabilities. However, if either carbon or oxygen are to be produced in stellar reactions, the resonance levels of atomic nuclei must match the levels of the processes which create them. Astrophysicist Hugh Ross notes that these levels are "fine-tuned":
"As you tune your radio, there are certain frequencies where the circuit has just the right resonance and you lock onto a station. The internal structure of an atomic nucleus is something like that, with specific energy or resonance levels. If two nuclear fragments collide with a resulting energy that just matches a resonance level, they will tend to stick and form a stable nucleus. Behold! Cosmic alchemy will occur! In the carbon atom, the resonance just happens to match the combined energy of the beryllium atom and a colliding helium nucleus. Without it, there would be relatively few carbon atoms. Similarly, the internal details of the oxygen nucleus play a critical role. Oxygen can be formed by combining helium and carbon nuclei, but the corresponding resonance level in the oxygen nucleus is half a percent too low for the combination to stay together easily. Had the resonance level in the carbon been 4 percent lower, there would be essentially no carbon. Had that level in the oxygen been only half a percent higher, virtually all the carbon would have been converted to oxygen. Without that carbon abundance, neither you nor I would be here."15
These observations led atheist Fred Hoyle to conclude that, "If you wanted to produce carbon and oxygen in roughly equal quantities by stellar nucleosynthesis ... your fixing would have to be just about where these [oxygen and carbon resonance] levels are actually found to be ... A common sense interpretation of the facts suggests that a superintellect has monkeyed with physics, as well as with chemistry and biology..."16

The Remarkable Requirements for Initial Conditions

http://www.genesispark.org/genpark/finetune/finetune.htm

The cosmos is hurtling outward at a remarkably balanced velocity. In his fascinating work Beside Still Waters: Searching for Meaning in an Age of Doubt, Gregg Easterbrook discusses the concept. If the expansion were slightly less, the universe would have collapsed back onto itself soon after its birth. If it were slightly more rapid, the universe would have dispersed into a thin soup with no aggregated matter. The ratio of matter and energy to the volume of space at the birth of the universe must have been within about one quadrillionth of one percent ideal! After reflecting upon this unlikely scenario, Dr. Bradley notes that it "has been the impetus for creative alternatives, most recently the new inflationary model of the big bang. However, inflation itself seems to require fine-tuning for it to occur at all and for it to yield irregularities neither too small nor too large for galaxies to form. ...Recently in Scientific American, the required accuracy was stated to be 1 part in 10123. Furthermore, the ratio of the gravitational energy to the kinetic energy must equal to 1.00000 with a variation of 1 part in 100,000. This is an active area of research and the values may change over time. However, it appears that the essential requirements of very highly specified boundary conditions will be present in whatever model is finally confirmed for the big bang origin of the universe."

Berkeley astronomer Mark Davis told me: "The universe is an amazingly fine-tuned environment, and physicists are very keen to understand how it came to be this way." As an example, he described the critical density of the universe that allowed it to expand at a rate just right for the formation of galaxies, rather than to result in an early collapse or a too-quick dispersion. Physicists agree that the expansion rate at the beginning had to be fine-tuned to one part in 10 to the 60th power – that’s 1 with 60 zeros after it – a level of precision that Davis calls "crazy." Stephen Hawking referred to this critical balance when he said: "The odds against a universe like ours emerging out of something like the big bang are enormous."