## Inflaton, Mother of the Universe 163

quantalm writes

*"Forget the god particle, we're talking about the universe's particle mother. The theory of supersymmetry has rolled out two new ideas about the particle that puffed spacetime up from smaller than a proton to bigger than a soccer ball: it could be the 'unified particle' of Grand Unified Theories or a smaller-scale version that could be tested at the Large Hadron Collider at CERN."*
## Re:Inflationary theory (Score:4, Informative)

I hear what you're saying. But the problem is, if the inflationary theory is false then we need some other mechanism to explain the cosmic background. Inflation solves the problem without breaking the speed of light or special relativity -- both of which are kind of important to keep around.

Inflation *could*, ultimately, be proven false. But if that happens it will topple a lot of important theories along with it. So you can understand why most physicists are assuming it's the correct model, and trying to figure out exactly how it happened.

## Brief(!) Explanation of Inflation (Score:5, Informative)

The "inflation" we're talking about here is the accelerated expansion of the early universe. So, first off why do we need it?

It turns out that parts of the observable cosmic microwave background are 'causally disconnected'. This means that you take two patches of sky as observed at the time the CMB formed (300k years after the big bang, we now think - approximately 15 billion years ago) and track their behavior back to the big bang. In the normal models where the universe is full of dust or radiation they never were in contact in the past: Light from one area could never reach another. Why is this a problem? Because they are remarkably similar. They appear to have come into thermal equilibrium (same temperature) yet this shouldn't be possible if they were never in contact. So we need to have a method by which the universe expanded faster before this period.

There are a few ways to do this - one is a cosmological constant. But the problem with a constant is that it's constant - we should still see it today, and we don't. The universe is not expanding that fast anymore - the bounds we can place on the cosmological constant today put it well below the effect we want from inflation. What we need is something that acts like a cosmological constant for a while and then drops away. This is what inflationary models are all about. The inflaton is a theoretical particle that starts off behaving like the comsmological constant, but eventually decays into the matter we see today. We model this by a particle moving in a potential - think of a ball rolling on the side of a hill. How the inflaton behaves is all about the ratio of its kinetic to potential energy - high potential energy looks like a cosmological constant, high kinetic energy looks more like normal matter. (I can explain this in more detail if anyone's interested). So the ball rolls down the hill, losing potential, gaining kinetic (there's also friction from the expansion of the universe so it loses 'energy' overall) and hence our inflaton does exactly what we need - slowly changing from looking like a cosmological constant to normal matter. In theory too, it decays once it reaches the bottom of the hill, but no-one provides much of a model for this.

This is old (20-30 years old is old in theory standards) stuff from Linde, Mukhanov etc. No-one would take it seriously, except that when you calculate things from it, it works incredibly well - it's the source of http://xkcd.com/54/ [xkcd.com] - it's still controversial. Some people love it, others think it's a fudge and doesn't do much for you. The new stuff here is that there is a method being proposed by which a multiplet of supersymmetric particles (again, I can say a bit more but it's not my field) is shown to be able to act like the inflaton. Ie a stable state of multiple particles bound together could act this way, and could be found at the LHC. Now, that's a lot of 'could' - the usual inflaton mass is set to around 10^12 GeV - way above what the LHC can reach, and this is the same across most inflationary models. But if the LHC can see evidence of supersymmetry (again, another discussion, but it is thought to be likely that if supersymmetry is real then the LHC will see it) it might be able to at least give some credibility to some of these models of inflation.

## Re:Brief(!) Explanation of Inflation (Score:3, Informative)

No-one would take [inflation] seriously, except that when you calculate things from it, it works incredibly well - it's the source of http://xkcd.com/54/ [xkcd.com]

Not quite. You don't need inflation to get the blackbody spectrum of the cosmic microwave background radiation (CMBR) observed by the COBE satellite, which is what the xkcd comic depcits. That's a prediction of plain vanilla Big Bang cosmology, with or without an early inflationary phase.

However, inflation does predict details in the CMBR angular power spectrum, the "acoustic peaks", which were observed by the later WMAP satellite. And it solves other "paradoxes", like the horizon problem you mention.

## Re:Inflationary theory (Score:5, Informative)

There are certainly alternatives to inflation that people do find attractive - ekpyrotic, cyclic or simply oscillatory universes for example can easily bring points into causal contact by extending the past of these points beyond where there would be a classical big bang. Various string models, and Loop Quantum Cosmology have methods for this (LQC has a really neat well understood bounce) and the idea goes back to Lemaitre's 'Phoenix Universe' ideas. However, inflation does more than just explain existing phenomena - it predicted a spectral index between 0.98 and 0.92, and COBE/WMAP bring it in at around 0.96. It also does a really good job of explaining structure formation. Now, that isn't to say that it's necessarily right, and that other theories couldn't do a similar thing, but inflation really does a good job. It's certainly far from perfect, numerous people have objections to it, but so far it fits the data we have.

## Re:Inflationary theory (Score:2, Informative)

Not quite. The Inflationary phase was anything but controlled. The current model predicts that between the initial Big Bang and the start of the inflationary phase (roughly one planck time), the universe expanded at some unknown rate. We can't observe the pre-inflationary phase, so there is no useful model for it. When the conditions for inflation were met, the Universe suddenly expanded at a truly fantastic rate (effectively faster than light). This inflationary phase not only generated an enormous amount of space very quickly, but also generated an enormous amount of matter very quickly. This is a consequence of quantum foam having a zero sum over a non-zero amount of space and non-zero amount of time but a non-zero sum at any given instant in space and time. (Hawking Radiation, likewise, results in something from an average of nothing for the same reason.)

The inflationary phase is extremely hard to model because, as Professor Hawking has noted, not only does space vary non-linearly in the universe, so does time. At whatever point the density of the universe was greatest, the rate of time was slowest. In some models of the very early universe, time follows a parabolic path. As you approach T=0, the rate of change of time also approaches zero. If this is correct, then there was no moment of Big Bang (and therefore no singularity) because there wasn't any point in time for it to occur. (Since Black Hole theory stems from Big Bang theory, and since the argument over time revolves around the density of matter bending time as well as space, this raises questions about whether models of Black Holes can be correct. A singularity cannot accumulate mass if delta-T is zero, for there is no point in time for the accumulation to occur in. However, that is another debate.)

Because mass bends time as well as space, we cannot accurately model the effects of inflation on the universe without knowing how mass changed due to the properties of quantum foam, because we cannot know the effect on time otherwise. All we know is that mass/energy was not a constant during this phase and that at no point in this phase did it equal the mass/energy of the universe today. We

thinkthe latter part of inflation will have tended to this value, but frankly there is no evidence for that. The universe dropped out of the inflationary phase, and it is assumed that the transition was relatively non-turbulent - or can at least be modeled as such - but most transitions we do know of are extremely turbulent and disruptive.Some of this can be solved experimentally. You need an extremely high energy density - about the same as the output of a hydrogen bomb packed into a cubic centimeter is how I've heard it described - but it's not an unachievable amount of energy (obviously) even if we're not sure quite how to get the density that high. It's perfectly safe, too. Well, so long as theory is correct, at least. It would form a universe attached to this one through a mini black hole. Essentially you'd form a blister on this universe, where the blister contained another universe. The black hole is a good thing - prevents this universe getting fried from the inflationary phase of the new one - but since the black hole exists in some form in both universes, its state must reveal something about that other universe.

## Re:Inflationary theory (Score:5, Informative)

I should be clear: My experience is with scalar field inflation with a quadratic potential - the simplest models that are most common. Hybrid inflation can do almost anything, it's true.

My references for that statement:

Tegmark: http://arxiv.org/abs/astro-ph/0410281 [arxiv.org]

Steinhardt: http://arxiv.org/abs/astro-ph/0507455 [arxiv.org]

I believe Mukhanov and Turok both talk about it too, though I can't find the references easily at the moment.

## Re:Inflationary theory (Score:3, Informative)

That's an interesting question to which an answer can take a few forms. One is that below describing the balloon. This would be a closed universe model (finite size, wraps around itself) such as a torus or sphere. However, suppose the universe is truly infinite - what does it mean to be expanding?

Well, suppose one can put a mark on two points in space, and watch them over time. To be expanding these two points will move apart, as will any two points in space. To do it a little more mathematically: Suppose I have a line - the real numbers - that goes on forever. And I pick two points on the line (arbitrarily again suppose we get -0.45 and 3.09 for purpose of example). Then I double the coordinate of each point (now at -0.9 and 6.18). This way the line is still infinite - it's as long as the real line - but it has 'expanded' as every point before is now further away than previous points.

In cosmology we do this in a very similar way - using what we call a 'fiducial cell'; a blob of space. We assume the universe is (to first order) homogeneous (same everywhere) and isotropic (same in every direction). Thus if we see the blob expand we're seeing the whole universe expand. Mathematically, we form a 'metric' a way of measuring space and time, the simplest homogeneous and isotropic version of which is:

ds^2 = -dt^2 +a(t)^2(dx^2+dy^2+dz^2)

You might recognize the last bit as being like pythagoras' theorem in 3D. (There are two other homogeneous isotropic examples, but I'm choosing the simplest one to make life easy). a(t) can only be a function of time, as if it were a function of space, this would break homogeneity. Likewise it must multiply all directions equally to retain isotropy. (Again for the pros I'm being fast and loose here to make life easier). Thus our universe can be infinite (x,y,z go on forever) but distances can change over time as a(t) changes. That way we truly can see expansion or contraction in the universe. This was what Friedmann first put forward as a solution to Einstein's equations, and Robertson+Walker later showed that it's true in general cases for homogeneity and isotropy. Finally Lemaitre worked out what this really meant physically.

Now a(t) has no meaning by itself - I could just have chosen a smaller piece of space on which to start measuring. But the rate of change 1/a * da/dt is very meaningful - the relative change rate known as the Hubble parameter. It is from measurements of the doppler effect on light (called redshift/blueshift for stars moving away from/towards us) that we can get a handle on this and see that a(t) is indeed increasing - the universe is expanding. The point of inflation is to understand a model in which a(t) was not only increasing, but accelerating, but that's a much longer discussion.

Hope that helps!