1. Inflation: Motivation for this Theory

The Horizon Problem

isotropy

homogeneity

Consider points A and point B from fig. 27.8

- at the opposite ``ends'' of our horizon

- always farther apart than time x speed of light

- how did they get to be ``the same'' if they couldn't ``communicate?''

The Flatness Problem

The Universe today is near critical density.

= density / critical density

is at least 0.1, perhaps 0.2 or 0.3 (dark matter), maybe even 1 (based on the scale of CMBR fluctuations)

Fact: Whatever is today, was closer to 1 ``yesterday.''

Claim: Since is so close to 1 today, must have been very close to 1 ``yesterday.'' It would be esthetically unpleasing if were not exactly 1.

The expansion rate of the Universe depends on .

Relative to = 1,

Larger values of result in a smaller, denser Universe today

Smaller values of result in a larger, less-dense Universe today

So if isn't 1, let's say to 1 part in 10¹⁵ at the time of nucleosynthesis (a few minutes after the Bang), it would much different than 1 today.

Hence, the ``flatness problem'' is an argument against ``fine tunning.''

However since we can't explain all the other constants in the Universe (masses and charges of particles, scaling constants of forces),

And then there is the issue of fine tuning the partice/anti-particle ratio . . .

So what's the big deal about ?

It boils down to an esthetic argument.
In my opinion, this is not science.

2. Inflation: What it is

Enter: Particle physics circa late 1970's early 1980's

Grand Unified Theories (GUTS)

- a triumph of modern science

- mathematically unify 3 of the 4 forces

electro-magnectic force

weak force

strong force

Consequence: This unification occurs in nature at very high energies

(i) in particle accelerators - we wish!

(ii) in strong gravitational fields - don't know!

(iii) in the very early Universe!

At 10^-34 sec and less after the Bang the temperature (energy) was high enough for the 3 forces to be unified, i.e. not differentiated, same type of interactions. T = 10²⁸ K.

At t = 10^-34 sec the Universe should go through a ``phase transition,'' where the forces differentiate.

Analogy: gas liquefying, or liquid freezing

Possibly:

Universe ``super-cooled,'' like HO remaining liquid below the freezing point.

That is, at t = 10^-34 sec, for some reason the forces hadn't differentiated.

(The reason is a technical detail of the particle physics of GUTS)

If true,

``super-cooling'' produced a tremendous ``vacuum pressure''

``exponential'' expansion

- Universe is predicted to have expanded by a factor of 2 every 10^-34 sec.

- Soon, the ``phase transition'' occurred, the ``vacuum pressure'' disappeared, and the ``exponential'' expansion stopped.

- ``Normal'' expansion took over once again, around

t = 10^-32 sec.

- But the Universe had grown immensely

(by a factor of 10³⁰!).

Meanwhile, the Horizon and Flatness problems are solved:

(i) What we see as the ``entire Universe'' within our Horizon is but a small piece of the whole Universe

(ii) Something that has expanded so much will be flat.

(Balloon analogy, figure 17.18).

3. The Planck Epoch

GUTS unifies 3 of 4 forces.

What's missing?

Gravity.

More precisely: There is no quantum theory of gravity.

But at very early times, the Universe was so dense, energies so high, and times so small, even gravity -- i.e. space-time itself -- must have behaved in a ``quantum mechanical'' way.

We don't know what this ``way'' is.

Hence, the door is wide open for speculation.

4. Wide open speculation

Is our ``Universe'' just a quantum-mechanical fluctuation in a tiny pary of a larger, 'meta'-universe ?

This fluctuation went wild,

super-cooled,

inflated,

and became causally disconnected.

What's beyond our event horizon?

Are there many other ``Universes ?'' Do we live in a multiverse?

Mathematically possible.

Physically knowable?

- how would we know?

5.   Verified conjecture:   Why Teach Astro 103?

(a) the Universe is homogeneous on scales larger that the light travel time.

(b) the Universe expanded exponentially early on.

(c) the Universe appears nearly flat today.

(d) the microwave background is from the surface of last scattering.

(e) Inflation wipes out information on large scales.

(a) The Universe becomes flat on very large scales.

(b) The 4 forces differentiate.

(c) Vacuum pressure wipes away any original differences in the Horizon.

(d) Inflation mixes the early Universe.

(e) A very small part of the original space-time is inflated, which fills our observable Universe.