Probing the very early universe (part IV)


Catch up with the saga: part I, part II, part III

Negative Pressure, Exotic Particles, and the Emergence of Something out of Nothing

So, lets quickly recap. We have evidence that the Universe is expanding, and that the distribution of galaxies is both homogeneous (smooth) and isotropic (the same in all directions). What’s more, our theories of universal evolution tell us that the universe appears to be made up of 50,000 regions that are causally disconnected; they are independent. But since the universe is also homogeneous, this means that for some mystic reason these 50,000 regions managed to evolve in practically the same way as each other.

Physicists do not like mysticism and so they treated these phenomena as paradoxes, mysteries that need to be solved, and a neat solution was proposed in the late 70s/early 80s dubbed inflation. This is the very rapid expansion of the universe at early times, which allowed the universe to be much smaller than we thought it could be, and expanding to the size necessary to account for the subsequent evolution, without upsetting the status quo of the standard Big Bang theory. All this was discussed in part III of this series, and we left off at the point where we asked “How does inflation solve the origin of structure puzzle”, and we shall pick up there.

To be able to answer this question, we need to understand the process and nature of inflation a bit better. The equations that `govern’ or `describe’ the expansion of the universe are known as the acceleration (or Raychaudhari) equation, and the Friedman Equation. These equations relate the rate at which the universe is expanding to the content of the universe. So, if we know that the universe is composed mainly of radiation, the Friedman equation would then tell us the rate at which the universe is expanding, and the Raychaudhari equation would tell how that rate was changing.

acc

Combined these equations tell us that if we want the expansion to accelerate, as is necessary for inflation, then we need something which has negative pressure or negative energy. Since negative energy means negative mass that makes it too weird to make sense (even for cosmologists) so focus was placed on finding something with negative pressure.

Negative pressure:

So what is negative pressure? To get an intuitive understanding of this, imagine that you are blowing air into a glass bottle which cannot expand. As you blow into the bottle you are forcing the air molecules closer together, this is known as positive pressure. Negative pressure (you guessed it) is like sucking air out of the bottle, you are forcing the air molecules away from each other.

So negative pressure forces things away from each other, it acts like anti-gravity, which means that negative pressure could have caused the original expansion of the universe! So the question is what has negative pressure?

We know that everything we have encountered in the universe (be it in the form of matter or radiation) has normal positive pressure, so we really are looking for something as yet unencountered… something exotic.

Exotic particles:

I struggled with this bit. I wonder if people are aware of what is known as the particle-wave duality? Sometime in the 1800s Thomas Young showed that light behaved as a wave, it spreads out like ripples on a pond. In the late 1800s Thomson showed that light also behaves like a particle a.k.a a photon. Both theories are correct, light behaves as both a wave and a particle.

To get the universe to expand very very rapidly the average pressure must be negative, which means that you would need to fill it with a particle that pushes everything away from it. Such a particle has never been observed in nature (hence is exotic), but is a necessary component of fundemental theory, it is known as a scalar particle (wave) and in the context of inflation we call it the inflaton.

Initially introduced to solve the paradoxes of Big Bang cosmology, it was found that the inflaton has another trick up its sleeve … the ability to create the ‘wells’ into which matter will eventually fall and form structure (like galaxies). To achieve this we need to speak a bit of quantum field theory (dont stop reading I shall try to simplify this as best I can!).

The basic idea is that on very small length scales things don’t act normally … on a tiny level everything around us is fluctuating, an electric field may have a specific amplitude (power/strength/value) to the naken eye, but look close enough (hypothetically) this amplitude is changing, it is sometimes less and sometimes greater than the average value that we measure. This happens so rapidly however that you would never notice. Does this make sense? Check out the next figure …

In the classical view (i.e. visable to the naked eye) we would see the wave change with time. But invisible to us there are tiny fluctuations and if we magnigfied this we would get a picture like the one in the next figure.

Having magnified the circled region in the previous figure we see that our first impression of the wave was not complete. To get a better idea of what is going on you have to imagine that the classical wave is frozen and the small fluctuations are wiggling rapidly.

So if the fluctuations are wiggling rapidly, and dont have an effect, how can it lead to ‘fixed’ tangible wells into which matter will eventually fall into… read on.

The Quantum to Classical Transition (the Emergence of Something out of Nothing):

Remember that the universe is expanding extremely rapidly? Ok, well since the inflaton is also a wave, it stretches out across the universe  and gets stretched out or magnified by the rapid expansion. So pretty much as soon as a little wiggle appears on the background (the average value of the wave or the classical picture above) it gets blown up to ‘proper’ proportions. These ‘proper’ proportions are what we call classical, their value becomes fixed and the wiggle stops wiggling.

Since the wiggles can be any length when they get transformed to classical size we end up with a range of different sized peaks and troughs across the universe, and because they are classical they survive through the end  of inflation, and effectively determine the pattern of galaxies we see across the sky.

Questions?

Next up: how do we test this idea we call ‘inflation’?

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8 Responses

  1. This part:

    Since the wiggles can be any length when they get transformed to classical size we end up with a range of different sized peaks and troughs across the universe, and because they are classical they survive through the end of inflation, and effectively determine the pattern of galaxies we see across the sky.

    Still doesn’t explain why everything appears so homogenized around the universe. Or was it something I missed?

  2. that was quick… maybe you missed part III (link at the top of this post) if that doesnt answer your question, I will elaborate some more :)

  3. So these “wiggles” ended up becoming physical because they formed during inflation? This is the part that confuses me. How did something intangible such as variation in waves end up creating tangible physical objects. It’s similar to the part in quantum mechanics where things can materialize out of no where (My personal opinion on this is that if something can be shown in equations, doesn’t mean that it can possibly exist) Like for example, 1 Apple, and -1 Apple. It doesn’t even have a meaning.
    Or it could be a result (and possible indication) of a lacking equation that’s left unbalanced therefore predicting spontaneous materialization of matter out of nothing

    Love this series by the way… Ya3teeki el 3afyeh

    • We refer to the event horizon as the furthest distance we can ‘see’, this also defines the furhest objects which can ever have an effect on our future. During inflation we obviously did not exist, but we shall position ourselves at the point where we will eventually appear (we are not physically there we will just be defining a spatial location). Around us these fluctuations are appearing and disappearing rapidly, but because of the accelerated expansion, when one appears it is quickly stretched out beyond the event horizon. Once it exits the horizon it can no longer evolve, because it is bigger than the maximum distance that information can travel. This happens over and over, for different sizes of fluctuations, which gives us that spectrum of fluctuations. Once inflation ends, ‘stuff’ within the horizon starts to evolve normally, and the horizon itself starts to grow. This slower pace though allows the ‘banished’ fluctuations to reenter our world, but at ‘normal’ sizes.

      We refer to the event horizon as the furthest distance we can ‘see’, this also defines the furhest objects which can ever have an effect on our future. During inflation we obviously did not exist, but we shall position ourselves at the point where we will eventually appear (we are not physically there we will just be defining a spatial location). Around us these fluctuations are appearing and disappearing rapidly, but because of the accelerated expansion, when one appears it is quickly stretched out beyond the event horizon. Once it exits the horizon it can no longer evolve, because it is bigger than the maximum distance that information can travel. This happens over and over, for different sizes of fluctuations, which gives us that spectrum of fluctuations. Once inflation ends, ‘stuff’ within the horizon starts to evolve normally, and the horizon itself starts to grow. This slower pace though allows the ‘banished’ fluctuations to reenter our world, but at ‘normal’ sizes.

      The concept of quantum fluctuations is ‘real’ in the sense that they are physical. In a sense their houdini characteristics are due to the size of the objects which are governed by quantum mechanics. When we observe something we are actually measuring or recording the characteristics of the light that bounced off this something. So if the object was tiny, the light bouncing off it changes it, so observing it changes the object you are observing. Hence the heisenberg uncertainty principle, which dictates that there is a statistical uncertainty when measuring the properties of an object. For example, simply shining a light on a hydrogen atom can move the orbiting electron from one orbit to another. Some people interpreted this (like you did) as a lack of knowledge, people used to say that the electron can be in the ground state OR the 1st state OR etc… However, the alternative view is to recall the particle-wave duality (that electrons can behave as particles AND as waves), and then we can say that until we measure the system the electron is in the ground state AND in the 1st state. This can make more reasonable sense if we view the electron as a wave packet (http://vedantic-physics.com/web_images/wave_packet.jpg), and we can note that it is not as localized as a particle, the electron is made up of a sum of waves of different lengths and hence different energies. This is the same concept that goes into building quantum computers.

      Does that help? (I have a feeling I rambled a bit! :D )

      Glad you love the series, I am trying to keep it up, its great practice esp since I am a firm believer in Feynmans statement that we dont really understand something until we can teach it to a first year class. I am extending this to: I dont really understand something until I can explain it on the blog… turns out I didnt understand things as well as i thought I did :D

      And Allah 3ayfeek, shokran :)

  4. [...] This post was mentioned on Twitter by Jordan Blogs, Qwaider Planet. Qwaider Planet said: Probing the very early universe (part IV) #cosmology #theuniverse #originofstructure | Talking Virtually To Myself → http://bit.ly/bwX4hz [...]

  5. oh! very cool! btw, the inflation theory also was proposed to solve the problem of the very fine tuning that the initial conditions of the universe needs to reach its current state, the stable one!

    Good job =)

    • ahaaa, the fine tuning argument :) technically it reduces the fine tuning problem to one dimension/or one degree of freedom. We still need the inflaton to behave in a certain way, which means it’s evolution needs to be goverened by an equation with particular characteristics, that so far we need to engineer. But it does reduce the problem.

      and thanks :)

  6. take what you said back and delete everything you wrote before you get blown up by some jihadi warrior.

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