|The Ghost of Paley
Joined: Oct. 2005
|This is definitely a problem for the model, and for inflation more generally, but it most certainly is not a problem for cosmic expansion (which, as I said earlier, is one of two possibilities, the other being a collapse). |
Which, as we'll see, is a very damaging admission.
Yes. Recall that inflation theory accounts for disturbing observations that threaten the entire Big Clang superstructure, including:
1) The Horizon Problem:
|The uniformity of cosmic background radiation--varying by no more than one part in 10,000, where ever you look--posed a problem to Standard Big Bang cosmology. Suppose the universe began 14 billion years ago. We look to the west, we detect cosmic background radiation. We turn our radio antennas to the east, we detect cosmic background radiation--at exactly the same temperature. The radiation from the east and the radiation from the west are separated by 28 billion light years. Common sense tells us that the radiation from the east could not possibly be causally connected to that from the west, because information cannot travel faster than the speed of light. Nor could the regions they traveled from ever have been in communication. |
2) The Flatness Problem:
|Our universe is apparently flat. That is, it appears to have just the "right" density--or nearly so--to continue its slow expansion forever. Too much matter, and the universe eventually collapses in on itself under the influence of its own gravitational pull. This scenario, essentially the Big Bang in reverse, has been called the "Big Crunch". Too little matter, and gravity will never be able to halt the expansion of the universe. The universe eventually be populated only by gas, dust and the relics of stars, growing increasingly cold with its infinite expansion. This bleak scenario is called the Big Chill. |
An intermediate scenario happens if the average density of our universe is equal to the critical density--the average density of matter needed to arrest the expansion of the universe without bringing about a Big Crunch. Cosmologists express this relationship as the ratio of the average density to the critical density: Omega. Measurements of Omega today range from 0.1 to 1. Most scientists believe that the universe is not headed for a Big Crunch.
Both the average density of the universe and the critical density change with time. When the universe was very young, and very dense, these numbers changed very rapidly. If the average density of the universe were even slightly greater or smaller than the critical density in the instant following the Big Bang, Omega would have zoomed to infinity (a quick Big Chill) or crashed to zero (the Big Crunch). The fact that we are still around, approximately 15 billion years later, is evidence that the critical density must have been extremely close--equal within 1 part in 10^15--to one after the Big Bang.
Inflation Flattens the Universe
To make the Standard Big Bang theory correspond to reality, cosmologists had to make the assumption that the average density of the universe was equal to the density immediately following the Big Bang. But how? This assumption, like the isotropy assumption, isn't explained. Since an Omega of one corresponds to a flat universe, this is known as "The Flatness Problem."
3) The Lack of Magnetic Monopoles:
|In the early 1970s, the successes of quantum field theory and gauge theory in the development of electroweak and the strong nuclear force led many theorists to move on to attempt to combine them in a single theory known as a grand unified theory, or GUT. Several GUTs were proposed, most of which had the curious feature of suggesting the presence of a real magnetic monopole particle. More accurately, GUTs predicted a range of particles known as dyons, of which the most basic state is a monopole. The charge on magnetic monopoles predicted by GUTs is either 1 or 2gD, depending on the theory.|
The majority of particles appearing in any quantum field theory are unstable, and decay into other particles in a variety of reactions that have to conserve various values. Stable particles are stable because there are no lighter particles to decay into that still conserve these values. For instance, the electron has a lepton number of 1 and an electric charge of 1, and there are no lighter particles that conserve these values. On the other hand, the muon, essentially a heavy electron, can decay into the electron and is therefore not stable.
The dyons in these same theories are also stable, but for an entirely different reason. The dyons are expected to exist as a side effect of the "freezing out" of the conditions of the early universe, or symmetry breaking. In this model the dyons arise due to the vacuum configuration in a particular area of the universe, according to the original Dirac theory. They remain stable not because of a conservation condition, but because there is no simpler topological state for them to decay to.
The length scale over which this special vacuum configuration exists is called the correlation length of the system. A correlation length cannot be larger than causality would allow, therefore the correlation length for making magnetic monopoles must be at least as big as the horizon size determined by the metric of the expanding universe. According to that logic, there should be at least one magnetic monopole per horizon volume as it was when the symmetry breaking took place.
This leads to a direct prediction of the amount of monopoles in the universe today, which is about 1011 times the critical density of our universe. The universe appears to be close to critical density, so monopoles should be fairly common.
Non-inflationary Big Bang cosmology suggests that monopoles should be plentiful, and the failure to find magnetic monopoles is one of the main problems that led to the creation of cosmic inflation theory.
As a response, Alan Guth created the Inflation Model. This model purported to explain these obsevations, by positing:
|that the nascent universe passed through a phase of exponential expansion (the inflationary epoch) that was driven by a negative pressure vacuum energy density.|
This expansion is similar to a de Sitter universe with positive cosmological constant. As a direct consequence of this expansion, all of the observable universe originated in a small causally-connected region. Quantum fluctuations in this microscopic region, magnified to cosmic size, then became the seeds for the growth of structure in the universe (see galaxy formation and evolution). The particle responsible for inflation is generally called the inflaton.
The original model of inflation, proposed by Alan Guth, had the universe in a false vacuum. The universe was in an exactly de Sitter phase. In this model, regions of non-inflating universe are created through the nucleation of bubbles of true vacuum, while the rest of the universe continues inflating. When two such bubbles collide, the vast energy of the bubble walls is converted into the particles seen at the early universe. This process is called reheating. Alan Guth has described the inflationary universe as the ultimate "free lunch": new universes, similar to our own, are continuously produced in a vast inflating background. Gravitational interactions, in this case, circumvent (but do not violate) both the first law of thermodynamics or energy conservation and the second law of thermodynamics or the arrow of time problem.
However, the original model of Guth fails because, in order to guarantee a sufficient amount of inflation to solve the standard problems, the bubble nucleation rate must be too low for bubble walls to collide and for the reheating process to actually work, because the space between bubbles - which is still in the inflating phase - expands so fast that the separation between bubbles grows faster than the bubbles themselves. The energy that is released in the decay of the false vacuum is deposited entirely in the kinetic energy of the bubble walls, and none is liberated by the collision needed for the hot big bang. This is called the "graceful exit problem" and Guth's original model is now called "old inflation." Andrei Linde and, independently, Andreas Albrecht and Paul Steinhardt proposed a "new inflation" or "slow-roll inflation" in which the inflaton is modelled by a scalar field slowly rolling down a nearly flat potential. In this model, the expansion of the universe is only approximately de Sitter, and the Hubble parameter is actually decreasing: the expansion is slowing. While the spectrum of fluctuations generated in the false vacuum de Sitter universe of old inflation is exactly scale-invariant, new inflation produces only a nearly scale invariant spectrum. This means that information about the potential during inflation can be extracted, in principle, from the cosmic microwave background by measuring the spectral index. In "slow-roll inflation", inflation terminates when the inflaton potential reaches the end of its nearly-flat part, where its slope starts to increase and the roll speeds up. This is when reheating occurs in this scenario, as particles are created via ineractions with the inflaton, on the expense of the potential's energy density.
New inflation is generally eternal: that is, the process continues eternally. Although the scalar field is classically rolling down the potential, quantum fluctuations occasionally bring it back up the potential. These regions expand much faster than regions in which the inflaton has a lower potential energy. Thus, while inflation ends in some regions, the regions in which it continues are growing exponentially, and thus continue to dominate. This steady state, which was first described by Andrei Linde, in which inflation ends in some regions while quantum mechanical fluctuations keep it going in the majority of the universe, is called "eternal inflation". It is widely believed that eternal inflation, however, cannot be eternal in the past (although Andrei Linde disputes this) and so does not solve the problem of initial conditions for the universe.
Additional observations, such as COBE and WMAP satellite measurements, seemed to support the theory. You can get anything you want in Alan Guth's restaurant!
Unfortunately, recent observations question the model, and ditching the model reopens many old wounds. One simply can't discard inflation and maintain an atheistic POV. More later.
Dey can't 'andle my riddim.