A Neat Solution, Inflation
In the early 1980s, physicists started to take note of a theory that seemed to hold all the answers. The theory, cutely called ‘Inflation’ by MITs Alan Guth, offers a neat solutions to The Three Paradoxes of the Universe. In this post I will attempt to explain what inflation is, when it happened, and how it works.
What is Inflation?
Inflation is basically the very rapid expansion of the universe, where two points move away from each other at the speed of light(1). This does not contradict the special theory of relativity, since at the time this happened, the universe was classically empty, only energy existed at this time.
When did it happen?
Hard to quantify too precisely, but we can take on board two bits of information from two different sources in order to give a ball-park figure on this. Inflation must obey the physical laws of nature, there is no escape, however we are not entirely sure what these laws looked like such a long time ago.
[Extra Reading] The origin of the fundamental laws of physics:
One of the fundamental ideas of physics is that all the physical forces that we see as independent today (i.e. the force of gravity is seen as independent of say the theory of electromagnetism), were actually all ‘united’ many years ago, when the energy of the universe was ‘free’ (i.e. not bounded by structure, like atoms and such). That is, billions of years ago, physicists think only one law of nature existed, and that as the universe grew and cooled, this one law subdivided into a few sub-laws of nature. If you’re a bioligist or are more comfortable with the ideas of biology, think of this evolution as a top down process, as opposed to biology’s bottom up approach to evolution. So if a physicist had come up with a theory for the origin and diversity of the species she would have imagined one super-being (not divine, just super, as in ‘super duper’) that then spawned lots of other species, who then spawned even more species! But this spawning of mutants was already encoded in the first super-being, i.e. all the DNA any animal/plant needed existed within the chromosomes of that first super-being, and the genes ‘came to life’ as it were in response to the environment. That is the basic idea of the unification and subsequent diversification of the laws of physics, with the environment being the temperature/size of the universe.
Now, the physics world has managed to unite all the fundamental forces… except for one: Gravity. But that is not what we’re talking about, we assume that gravity did somehow unite with the other forces, and generally, gravity appears after what is known as the Planck epoch. The Planck epoch defines a time when the early universe was only a Planck length in diameter. This Planck length is special, in that it defines the smallest size that we know how to analyse physically i.e. we have a theory of how things behave when they are very small, or very very close together, but not if they measure less than or are closer than a Planck length. Since inflation deals with the expansion of space time, it needs general relativity (aka gravity) in order to operate, so Inflation takes place after the appearance of gravity.
So as a first guess Inflation takes place at least billion billion billion billionth of a second after the Big Bang. But to be brutally honest, since the theory of inflation is still work on progress, then inflationary cosmologists also look at scenarios when inflation took place at or before the Plank time, that is: less than 10 million billion billion billion billionth of a second after the Big Bang.
The second bit of information we need is when did the contents of the Universe become dominated by radiation? That occurred about a 10 millionth of a second after the Big Bang, and we need the universe to have ‘settled down’ by this time, because our theories of what happened after this time (the `standard’ Big Bang evolution) hold up pretty well under scrutiny, and we dont want to change things too much.
So inflation took place between a billion billion billion billionth of second after the big bang and 10 millionth of a second after the Big Bang. This would seem pretty precise for most people, but remember in the early universe aLOT of stuff could have happened within this time, after the universe became radiation dominated it took only 3 minutes for the temperature to drop 999,999,999,999,999,999,900,000,000 degrees Celsius. So we may have nailed the epoch of inflation to a few millionth of a second, but that still leaves alot of room for uncertainty.
How does it work? i.e. how does it solve the Three Paradoxes of Cosmology
How inflation explains the causality, homogeneity and isotropy of the universe?
Looking at the figure I have included (click on it for a better view), I have attempted to illustrate how the two different scenarios (Standard Big Bang vs. Inflation and Big Bang) expand. You need to imagine that our universe is contained within the two vertical black lines, so it grows as you follow the arrows. Now, what we know of the age of the universe, and what we know about how radiation and matter (regular stuff) effect the rate of expansion, then it turns out that there were 50,000 parts of the universe not in causal contact. That is, the universe could not have been small enough for these parts to communicate. That is, 14 billion(ish) years ago, according to standard lore, the universe was made up of 50,000 independent regions. So why did these regions all evolve in the same way?
This is where inflation comes in, because inflation stretches the universe out in such a tiny amount of time (see previous section), it means that the universe could have started out much smaller than was actually thought, and expanded very quickly, all this without effecting the evolution of the universe from light to atoms to galaxies to us (i.e. Big Bang Nucleosynthesis). Referring back to the figure, the red and blue circles represent two regions in the universe, in the inflationary picture they start off very close to each other, so information gets shared between them, or more to the point the universe mixes and homogenizes.
How does it solve the problem of the age of the universe? (or why isn’t it older?)
Since inflation causes the universe to grow to the size required by the theories which govern Big Bang Nucleosynthesis in a teeny fraction of a second, our theory of universal evolution now accounts for the age of the universe. We no longer need to add to the age of the universe to account for phenomena.
How does it solve the origin of structure? (or where did all this stuff come from?)
The answer to this one is quite involved, we need to look at how one gets inflation, i.e. what causes the universe to expand so rapidly? Once we answer this question, we discover that inflation also explains the origin of structure AND why it started expanding in the first place.. for next time though🙂
(1)this is one scenario, but the other one is too complicated to explain right now
Next up: Negative pressure, exotic particles, and the emergence of something out of nothing