Inflationary Cosmology

Contents

Inflation Theory
A Simple Mathematical Model
Cosmic Evolution of Entropy
Magnetic Monopoles

Inflation Theory

inflation According to the latest cosmological model, the universe sprang into being about 14 billion years ago. At birth, the space was likely to have been curved and warped due to quantum effect within the tiny speck and time may be meaningless. After about 10-35 seconds, there began a brief period of exponentially fast expansion, known as inflation, that ironed out any curves or warps in space and made the universe flat (because it becomes so large). Inflation also predicts a much smaller initial region, which is required for smoothing out the distribution of matter and radiation, only leaving behind tiny quantum fluctuations that match the observed spatial variations in the cosmic microwave background radiation and provide the seeds for galaxy formation.

Figure 01 Cosmic Inflation [large image]
inflation The orange curve in Figure 01 shows the period of inflation from 10-35 sec to 10-32 sec after the initial expansion. Figure 02 shows the actual size of the universe after the inflation. Our observable unvierse is only part of the whole thing. The mechanism to drive the inflation is related to a "yet-to-be-discovered" inflaton field, which is thought to be similar to the Higgs fields responsible for the mass of the elementary particles. When the temperature fell below a certain value, a phase transition (similar to the transition of water to ice at 0oC with the release of latent heat) of the inflaton field occurred. The phase transition released energy, which was conversed to hot matter and radiation. It also developed repulsive force to drive the inflation. The inflation stopped when the inflaton field settled down into lower energy state.

Figure 02 Unobservable Universe
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Recent observations of the cosmic microwave background radiation in the post-inflationary universe support the inflation theory. But the precise inflation mechanism is still unclear. Researchers can concoct different inflationary theories by adjusting the shape of the energy density curve as long as there is a relatively stable false vacuum in the beginning. There are also issues with the nature of the inflaton field, the events that led to the onset of inflation and then the nagging question about the beginning (and before the beginning§) of the universe. It is hoped that advances in fundamental physics will eventually address these
Inflationary Theories issues. Figure 03 depicts two "energy landscapes" for the process (at a certain point in space). In the old theory, bubbles of true vacuum form by tunneling. It results in a haphazard pattern of bubbles that never merge, so the decay process is never complete leaving an empty universe. In the new theory, the downhill rolling is very slow such that the energy density is almost at a constant value to sustain the exponential expansion. When the field gets to the steeper part of the energy slope, it rolls down faster, and when it finally reaches the minimum, it oscillates and dumps its energy into a hot fireball of particles. At this point we have an enormous, hot, expanding universe, which is also homogeneous and nearly flat - it solves the graceful exit problem on how to stop the inflation sensibly.

Figure 03 Theories of Inflation [view large image]

 § There are at least four different scenarios about the pre-bang universe: 1. Brane collision, 2. Universe inside blackhole, 3. Multiverse, and 4. Tunneling from nothing.

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A Simple Mathematical Model

Repulsion vs Attraction The concept of cosmic inflation can be illustrated by simple mathematics using only elementary calculus. Suppose the universe is uniform and isotropic as demonstrated by observations. This means that every point in the universe is similar to every other point and can be considered as the "centre" (Figure 04). Now consider a "test particle" of mass m and at distance R from the centre. Since only mass inside the sphere has a net effect on the particle, and if the total mass M insider the sphere of radius R is constant, then the

Figure 04 Repulsion vs Attraction [view large image]

 potential energy of the particle is:


V(R) = -GMm/R ---------- (1)

where G is the gravitational constant.

The gravitational force on the particle is the negative derivative of V(R):

F(R) = -dV(R)/dR = -GMm/R2 ----------(2)

where the minus sign signifies attraction, which tends to pull the particle towards the centre.

Next consider the case where the total mass M insider the sphere of radius R is not constant, instead the matter density is fixed (see Figure 04). The the gravitational energy of the particle is:

V(R) = -Gm x (4/3) x R2 ---------- (3)

The force on the particle is now:

F(R) = -dV(R)/dR = 2Gm x (4/3) x R ---------- (4)

where the plus sign indicates that the force is repulsive.

The particles are now being pushed away, the whole universe will expand (inflate). Since the kinetic energy acquired by the test particle is equal to the sum over the distance with the force acting on it:

K.E. = Fdr = Gm x (4/3) x R2 ---------- (5)

The total energy = K.E. + V = 0 (for the test particle) is conserved like all other dynamical systems. The increase in K.E. is cancelled exactly by the decrease in the potential energy V cumulating in a huge explosion called the Big Bang. Meanwhile, the mass inside the sphere increases with R like:

M = (4/3) x x R3 ---------- (6)

Inflation Thus in this scenario, there is a repulsive force to drive the inflation (Eqs.(4) and (5)); matter and radiation are being created according to Eq.(6). Under normal circumstance, the matter density would decrease with the expansion. It is thought that the decay of the "inflaton field" to the lower energy state is responsible for keeping the matter density constant. Figure 05 shows the variation of the energy density since the Big Bang. It maintained a constant value during the inflationary era as described in the simple

Figure 05 Energy Density
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 mathematical model. The energy density of water (1021 ergs/cm3) and of an atomic nucleus (1036 ergs/cm3) are included in the graph for comparison. A mathematical treatment based on general relativity can be found in the appendix on "Relativity".

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Cosmic Evolution of Entropy

Entropy is defined as the degree of randomness, which can be expressed alternatively as the degree of freedom in a system (the degree of freedom is the number of different parameters or arrangements needed to specify completely the state of a particle or system). The evolution of entropy in the universe as a whole can be separated into four phases:

Leptogenesis 1. The inflaton field is a coherent system changing gradually until the end of the inflationary era (Figure 06). Such system has very few degrees of freedom, so it has a very low entropy.
2. At the end of inflation the energy density of the inflaton field decays to zero (see Figure 03), thereby releasing lots of energy to produce particle anti-particle pairs, and to heat up the universe. It is this "reheating" that produce lots of degree of freedom, and thus lots of entropy.
3. The infusion of energy dU ceased once the inflaton energy density vanished, i.e., dU = 0. According to the thermodynamics relation dU = TdS - pdV (where p = pressure, V = volume,

Figure 06 Initial Entropy [view large image]

 T = temperature), the entropy now varies as dS = (p/T)dV. The universe was dominated by radiation up to 104 years after the Big Bang. During this era p T4; since in term of the size of the universe R, T 1/R, dV R2dR, thus dS dR/R and the entropy S log (R).
4. In a matter dominated universe p = 0, thus dS = 0; the entropy is conserved as a whole for the rest of the cosmic expansion.
5. If acceleration of the cosmic expansion is taken into account, then there is infusion of energy by an amount dU. The entropy dS dU/T will increase until space is nearly empty in attaining the highest entropy state.

Figure 06 also briefly portrays the leptogenesis, which is a theory to explain the asymmetry between matter and anti-matter.

Cosmic Entropy It is pointed out that the above model fails to explain what set up the initial low entropy state. An alternate scenario adds a period of prehistory (see Figure 07) in which space was nearly empty (thus avoiding the necessity of setting up the low entropy state), and inflation was brought about by fluctuation of quantum fields. It implies the existence of multiverse where the arrow of time may run backward. The appearance of stars and galaxies is a temporary deviation from the equilibrium of empty space.

Figure 07 Entropy Evolution
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Magnetic Monopoles

The inflation theory also offers an explanation for the absence of magnetic monopoles. It was back in the 19th century when scientists noticed the asymmetry between the electric and magnetic field sources. While electric field can be generated from monopole, dipole, and other multipoles, the magnetic monopole is missing in this world. In experiment, no one can isolate a "north" or a "south" magnetic pole - chopping up a magnet just produces smaller magnets, each with two poles. Correspondingly in theory, there is no magnetic charge in the electromagnetic field equations. Observationally, the magnetic field pervading the entire Milky Way implies that there is a lack of magnetic monopoles to cancel out (or short out) such field on the galactic scale. But the idea about monopoles was rather persistent. In 1931 Paul Dirac was trying to understand why electric charges were quantized. He devised an elegant explanation, which would work only if monopoles indeed existed. Monopoles are now perceived in a new guise as "knots" in the vacuum according to the Grand Unified Theories (GUT). It was realized that if GUT were correct, monopoles must have created only 10-36 seconds after the Big Bang, when the forces differentiated. These monopoles, would be very massive - about 1015 times heavier than ordinary particles - and would therefore be impossible to make in the lab. However, the number expected to have survived from the early universe seemed embarrassingly large: there would have been enough to short out the galactic magnetic field; even worse, their total mass would far exceed that of everything else in the universe (far too much, even, for the dark matter). For GUT physics, monopoles are extremely interesting objects: they have an onion-like structure, which contains the whole world of GUT (Figure 08):
Magnetic Monopole
  • Near the center ( about 10-29 cm ) there is a GUT symmetric vacuum.
  • At about 10-16 cm, its content is the electroweak gauge fields of the standard model.
  • At 10-15 cm, it is made up of photons and gluons.
  • At the edge to the distance of 10-13 cm, there are fermion-antifermion pairs.
  • Far beyond nuclear distances it behaves as a magnetically-charged pole of the Dirac type.

    • In effect, the sequence of events during the earliest moment of the universe had been fossilized inside the magnetic monopole.

Figure 08 Monopole Structure in GUT

One of the successes with the inflation theory was that it solved this so-called "monopole problem": monopoles would be exponentially diluted during the inflation, to such an extent that there would be little chance of even one in the Milky Way. By the way, skeptics about exotic physics might not be hugely impressed by a theoretical argument to explain the absence of particles that are themselves only hypothetic.

Meanwhile in 1995 the MACRO§ (Monopole, Astrophysics, and Cosmic Ray Observatory) detector located at the Gran Sasso National Laboratories in Italy has been designed to look for supermassive magnetic monopoles among other exotic particles. It had stopped collecting data by December 2000 and represents another failed attempt for the search of magnetic monopoles.

§A short introduction to MACRO can be found in: http://www.aas.org/publications/baas/v31n2/head99/246.htm