Discussion/essay for uni on the Inflation proscess

[JohnnyTheFox]

Hey, I'm doing a discussion/essay for uni on the Inflation proscess during the big bang. I'm not looking for facts/figures, that's what the uni wants me to do but I'd like to hear a little chat about other peoples views other than reading books. So who thinks its true/false or any better ideas?

Personally it seems to solve a lot of problems with the big bang model when its used but I've never been sure how this extra energy suddenly comes about? "Quantum Vacumn"? Or a change in the breaking of the 4 forces. Where does the engery come from in these?

 

[Garth]

Hi JohnnyTheFox!
Inflation has certainly been accepted now as an incontrovertible part of mainstream cosmology. It does as you say resolve several problems of GR cosmology:

1. The smoothness problem - why is the universe, a product of the big bang, so homogeneous yet not completely homogeneous so that large scale structures, galaxies, stars, planets and eventually us, could form within it?

2. The density problem - why is the observed density of the universe close to the critical value that separates the open and closed Friedmann models of GR cosmology? It is so close to call that even after 70 years of research no one is sure whether it is open or closed. Yet the deceleration of the universe's expansion should have driven the actual density away from the critical density by around 60 orders of magnitude!

3. The horizon problem - Why is the universe so isotropic that observations of one part of the sky look very similar to the opposite part? The Cosmic Microwave Background (CMB) is isotropic to one part in 10⁵, yet in the earliest stages of the BB these separated regions were too far away from each other for light, and therefore any information, to get from one to the other!

Inflation not only resolves these problems by saying that on top of the normal expansion there was a phase change in the "Higgs field" at
10^-33sec. after BB causing the universe to expand explosively and exponentially by a factor of about 10⁶⁰. This smeared out any inhomogeneities, forced the density down onto the critical value to a very high precision and threw apart causally connected regions to opposite ends of our present sky!

Inflation also produced most of the mass of the universe 'out of a hat' - not quite a free lunch, but certainly a very cheap one!

A further benefit if the hypothesis is that it also separates out primordial magnetic monopoles that are predicted by GUT's, thus explaining their non-observance.

Inflation predicts the present density of the universe should be very close to the critical value and that space should be 'flat'. The COBE/WMAP data seems to verify this prediction, although as I have constantly said the data actually only verify that the universe is spatially conformally flat.

However what are the problems with Inflation?

First there is debate as to whether it actually does solve the initial conditions problem completely. It certainly alleviates the problem but still rather special initial conditions are required to produce the homogeneous and isotropic universe we see today. http://arxiv.org/PS_cache/hep-ph/pdf/9506/9506283.pdf

A criticism that is often levelled at models of vacuum energy driven inflation is that they require unnatural fine-tuning [2]. There are two parts to this problem. First of all, such inflationary models necessarily contain small parameters in order to generate an inflationary potential significantly lower than the Planck scale. To provide a physical justification for this, one should perhaps associate the inflationary scale with one of the scales needed in viable unified field theories of the fundamental interactions. The second aspect of the problem concerns the initial conditions, specifically the question of how probable are the initial field configurations necessary to ensure an inflationary era.

The major problem of the theory is that it predicts the existence of a Higgs Boson, a very heavy fundamental particle that endows other particles with their inertial 'rest' mass. This has been sought after for more than thirty years without success, does it really exist? And if not, can the theory stand up without it?

Furthermore, mainstream Big Bang cosmology having resolved its smoothness/density/horizon/ problems with Inflation then required the further speculative additions of non-baryonic Dark Matter and then Dark Energy. Together this heady mix does produce a theory, the LambdaCDM model, which is concordant with observations but at the price of introducing new physics that has not been verified in the laboratory.

Given these caveats are there any other alternatives?

The major alternative to Inflation is the “Freely Coasting” Cosmology model. (FCM) The smoothness/density/horizon/ problems of the decelerating Friedmann models do not exist in the first place in a strictly linearly expanding model, which does not require exotic Dark Matter or Dark Energy either!

If the FCM should pan out then Inflation/exotic DM and DE will be remembered as examples of a modern day set of epicycles invoked to 'save the appearances' of the Mainstream model!

Garth

 

[hellfire]

Inflation does another, very important thing: it generates density perturbations that are frozen at superhorizon scales and reenter the horizon afterwards, generating coherent oscillations in the matter-radiation fluid before recombination. This leads to a series of peaks in the anisotropies of the cosmic microwave background, which are actually observed. Within standard physics I am not aware of any other mechanism than inflation that is able to generate coherent oscillations.

 

[Garth]

Inflation does another, very important thing: it generates density perturbations that are frozen at superhorizon scales and reenter the horizon afterwards, generating coherent oscillations in the matter-radiation fluid before recombination. This leads to a series of peaks in the anisotropies of the cosmic microwave background, which are actually observed. Within standard physics I am not aware of any other mechanism than inflation that is able to generate coherent oscillations.

From my link above A Concordant “Freely Coasting” Cosmology:

The main point we make in this article is that in spite of a significantly different evolution, the recombination history of a linearly coasting cosmology can be expected to give the location of the primary acoustic peaks (in the CMB anisotropy data) in the same range of angles as that given in Standard Cosmology.

(Italics my insert)

I hope this helps.

Garth

 

[JohnnyTheFox]

Thats some reply! Very helpful thanks. Sums up all the mains points nicley. I'm understanding its uses and how works with all our observations. It's what exactly happens at this phase change I don't quite get just yet.

You meantioned a change in the Higgs Field? At the moment I see it as a liquid supercooling so leaving an unstable state with enegry to spare. I've also read that the expansion is from a "false vacum", a field of constant anti particle and particle collisions giving it a net 0 energy. Is this the same as the Higgs Field? How does this result in an expansion force? I've got some books on loan but they go from a nice little intro in some rather heavy maths I can't follow yet with little description of what's acctuly going on.

 

[Garth]

First one aspect of GR cosmology that is conter-intuitive: adding pressure to the universe makes its expansion slow down! This is because pressure is a form of energy (actually stress-energy) that has a mass equivalent and produces even more gravitational force than without it, as a consequence a universe dominated by radiation decelerates faster than dominated by dust.
The radiation dominated universe expands according to
$$R(t) = t^\frac{1}{2}$$
whereas the dust dominated (no pressure) universe expands according to
$$R(t) = t^\frac{2}{3}$$.

The Higgs field is measured by the Higgs potential, which has a valley-like slope. The universe begins with the Higgs potential high up on one side of the valley and a short time after BB the potential rolls down to the bottom of the valley releasing potential energy. However this is a negative pressure vacuum energy density, and the field deposits a large, temporary negative pressure, a tension into cosmological space-time.

This negative pressure causes the universe to expand exponentially and explosively for a very short time, (between 10^-35 and 10^-33 second after BB,) during which the universe expands by a factor ~ 10⁶⁰.

Note: the way the Higgs field 'rolls down the valley' and exits from the inflationary period has to be fine tuned to reproduce the observed features of the universe.

I hope this helps!

Garth

 

[SpaceTiger]

Inflation not only resolves these problems by saying that on top of the normal expansion there was a phase change in the "Higgs field" at
10^-33sec. after BB causing the universe to expand explosively and exponentially by a factor of about 10⁶⁰.

Perhaps someone more knowledgeable in particle physics can explain this in detail, but my understanding was that the "Higgs field" and inflaton field are quite separate things. They're both scalar fields, but are predicted for entirely different reasons. The Higgs field (and the accompanying Higgs Boson) are the source of mass in the standard model of particle physics, while the inflaton field is the scalar field responsible for the exponential expansion of inflation. I've seen theories in which they're coupled, such as some hybrid (multi-field) inflationary models, but I don't think it's generally true that they're the same thing.

The major problem of the theory is that it predicts the existence of a Higgs Boson, a very heavy fundamental particle that endows other particles with their inertial 'rest' mass.

If what I said above is correct, then this would not be a prediction of inflation. There are inflationary models that require the existence of a Higgs boson, but I don't think it's generally necessary to solve the cosmological problems listed at the beginning of your post.

 

[Physics Monkey]

I think Space Tiger is correct. Generally, single field inflation models require what is thought to be unrealistic fine tuning of the inflaton parameters but who knows. I know some hybrid models are very popular amongst the supersymmetry people and in these models a coupling between the inflaton and the Higgs can arise naturally. The supersymmetry provides for the flat false vacuum which is important for slow roll inflation. Also, these models contain various mechanisms for giving the Higgs a vacuum expectation value at the end of inflation. It's a nice picture, now if only we can find some supersymmetric particles ...

 

[Garth]

You have to think of a potential field that can 'hold' the energy, Higgs was Guth's original suggestion, which leads to the fine tuning and the 'graceful exit' problems as I said above; but although we have not discovered the Higgs Boson at least we know about inertial mass!

You can replace the Higgs potential field with anything you care to suggest - try hard enough and you can multiply the entities to explain anything - and the generalised name for the particle required is the inflaton, but until something is discovered in laboratory physics you are just guessing! The prediction by Inflation of the Higgs Boson also includes any other 'Inflaton' - they all continue to be undiscovered by laboratory science.

Garth

 

[Garth]

BTW ST - congratulations on your Guru award, you deserve it!
Garth

 

[SpaceTiger]

BTW ST - congratulations on your Guru award, you deserve it!

Thank you Garth, and thank you for your contributions as well.

You can replace the Higgs potential field with anything you care to suggest - try hard enough and you can multiply the entities to explain anything - and the generalised name for the particle required is the inflaton, but until something is discovered in laboratory physics you are just guessing! The prediction by Inflation of the Higgs Boson also includes any other 'Inflaton' - they all continue to be undiscovered by laboratory science.

It's a trivial matter to concoct a theory of inflation that is not testable by any forseeable laboratory experiment, so I don't think that's a useful means of falsifying it. I think Dr. Steinhardt would say that our best bet is to look for the signature of gravity waves produced by inflation. These could theoretically be detected directly by a gravity wave detector (obviously) or indirectly by the signature of B-mode polarization in the CMB. The former is probably at least 20 years off and the latter is maybe 10 years.

Unfortunately, it is possible (though somewhat ad hoc) to create a theory of inflation that produces an undetectable gravity wave signature. It's an extremely hard theory to kill, but then it's also hard to come up with a viable alternative (please don't link to your theory again).

 

[Garth]

it's also hard to come up with a viable alternative (please don't link to your theory again).

As if I would! Other competing models to inflation are the ekpyrotic cosmology and variable speed of light (VSL) cosmologies.

There are however problems with the ekpyrotic theory such as those discussed in Inflationary Theory versus Ekpyrotic/Cyclic Scenario.

And once the speed of light is allowed to vary there are problems in defining the process of cosmological measurement, clocks and rulers have to be carefully redefined. The question becomes: "What is it exactly that we are measuring?"

Garth

 

[SpaceTiger]

There are however problems with the ekpyrotic theory such as those discussed in Inflationary Theory versus Ekpyrotic/Cyclic Scenario.

There have been many papers criticizing both the Ekpyrotic and inflationary scenarios, but to my knowledge, none of them has been able to put either to rest. I find it a bit fishy that, in the article you link, Dr. Linde finds the theory to be effectively equivalent to his own. I'll give it a read if I get the chance.

Anyway, here's one of Dr. Steinhardt's most recent articles on the theory:

http://xxx.lanl.gov/abs/astro-ph/0404480"

Also, we've been discussing some of the properties of the cyclic universe in this thread:

https://www.physicsforums.com/showthread.php?t=103646"

 

[fasterthanjoao]

You have to think of a potential field that can 'hold' the energy, Higgs was Guth's original suggestion, which leads to the fine tuning and the 'graceful exit' problems as I said above; but although we have not discovered the Higgs Boson at least we know about inertial mass!
You can replace the Higgs potential field with anything you care to suggest - try hard enough and you can multiply the entities to explain anything - and the generalised name for the particle required is the inflaton, but until something is discovered in laboratory physics you are just guessing! The prediction by Inflation of the Higgs Boson also includes any other 'Inflaton' - they all continue to be undiscovered by laboratory science.
Garth

so, let me see if I'm understanding correctly, you're saying that Guth proposed that the energy required for the inflation comes the Higgs potential field and am I correct in saying that this comes from the stationary point where Higgs potential is zero (false vacuum energy)?

 

[Garth]

so, let me see if I'm understanding correctly, you're saying that Guth proposed that the energy required for the inflation comes the Higgs potential field and am I correct in saying that this comes from the stationary point where Higgs potential is zero (false vacuum energy)?

Yes - The only quibble is that the Higgs potential is not zero at the stationary point, its just a minimum. The actual value of the potential at this, or any, point would depend on how you calibrate the measurement.

Garth

 

[Labguy]

This negative pressure causes the universe to expand exponentially and explosively for a very short time, (between 10^-35 and 10^-33 second after BB,)

Yes, that's what most of the sources say. But, I have never been able to find any explanation of the "starting" mechanism at 10^-35 seconds or "stopping" mechanism at 10^-33 seconds. Have you seen any source or theory at all that can answer that question?

 

[Labguy]

Yes, that's what most of the sources say. But, I have never been able to find any explanation of the "starting" mechanism at 10^-35 seconds or "stopping" mechanism at 10^-33 seconds. Have you seen any source or theory at all that can answer that question?

Anyone??...

 

[Garth]

Yes, that's what most of the sources say. But, I have never been able to find any explanation of the "starting" mechanism at 10^-35 seconds or "stopping" mechanism at 10^-33 seconds. Have you seen any source or theory at all that can answer that question?

The Higgs potential was in a state of unstable equilibrium. The universe rapidly cooled down during the radiation dominated early phase and already by t = 10^-35 seconds the temperature had dropped by a factor of 10⁵. At t = 10^-35 seconds the potential started to roll off its narrow 'ledge' and fell until it reached its minimum at t = 10^-33 seconds, it then overshot and oscillated around that minimum for a bit providing testable predictions about gravity waves and anisotropic density/temperature fluctuations (the graceful exit). The process stopped because it ran out of 'steam'.

I hope this helps.

Garth

 

[SpaceTiger]

Yes, that's what most of the sources say. But, I have never been able to find any explanation of the "starting" mechanism at 10^-35 seconds or "stopping" mechanism at 10^-33 seconds. Have you seen any source or theory at all that can answer that question?

Take a look at one of Guth's recent review papers on the subject:

http://lanl.arxiv.org/abs/astro-ph/0404546"

The basic idea is that inflation starts when a region of the universe reaches thermal equilibrium and a "false vacuum" state is achieved. For a simple choice of scalar field potentials, one can create a condition of negative pressure (which, in the FRW metric, will lead to expansion):

$$p=\frac{1}{2}\dot{\phi}^2-V(\phi)$$

Here I've neglected the spatial derivatives for simplicity. The left term is the kinetic energy of the scalar field and the right term is the potential. The evolution of this field is governed by the FRW metric. As long as the potential energy is dominant, you'll have negative pressure and the universe will continue to expand. This won't always be the case, however, and eventually the field will "roll" down its potential, rendering the pressure positive once again and stopping inflation.

The picture I've just painted is pretty simple, but there are a lot of inflationary models, many with different starting and stopping conditions. Fortunately, the majority of them (and the most popular ones) work on principles very similar to this.

 

[Labguy]

Take a look at one of Guth's recent review papers on the subject:
http://lanl.arxiv.org/abs/astro-ph/0404546"

Excellent paper, and the best summary I have seen yet. I actually read it..
For those who don't want to read the 22 pages, some quotes from it as:

shows that the time constant χ-1 of the exponential expansion would be about 10^-38 sec, and that the corresponding Hubble length would be about 10^{-28 cm}. For inflation to achieve its goals, this patch has to expand exponentially for at least 65 e-foldings, but the amount of inflation could be much larger than this. The exponential expansion dilutes away any particles that are present at the start of inflation, and also smooths out the metric.

10^-38 sec.? Damn; that's a shorter time than I had seen before. Also:

Eventually, however, the inflaton field at any given location will roll off the hill, ending inflation. When it does, the energy density that has been locked in the inflaton field is released. Because of the coupling of the inflaton to other fields, that energy becomes thermalized to produce a hot soup of particles, which is exactly what had always been taken as the starting point of the standard big bang theory before inflation was introduced.

was a good generic explanation. And:

The total energy remains constant at some very small value, and could in fact be exactly zero. There is nothing known that places any limit on the amount of inflation that can occur while the total energy remains exactly zero.

the "no limit" while zero energy I hadn't seen before. All of pages 12-15 warrant reading. Somewhere, thank god, the "magnetic monopole" issue was resolved.
But, I have a problem with the "infinite number of bubble universes" paragraph. No defined reason (by me), it just doesn't seem to fly with various ponderings I have had for several years about inflation.
By the credits, he should have just said that Linde wrote the paper...
This post really doesn't beg a response, it is just my thinking-out-loud.

 

[Spin_Network]

Take a look at one of Guth's recent review papers on the subject:
http://lanl.arxiv.org/abs/astro-ph/0404546"
The basic idea is that inflation starts when a region of the universe reaches thermal equilibrium and a "false vacuum" state is achieved. For a simple choice of scalar field potentials, one can create a condition of negative pressure (which, in the FRW metric, will lead to expansion):
$$p=\frac{1}{2}\dot{\phi}^2-V(\phi)$$
Here I've neglected the spatial derivatives for simplicity. The left term is the kinetic energy of the scalar field and the right term is the potential. The evolution of this field is governed by the FRW metric. As long as the potential energy is dominant, you'll have negative pressure and the universe will continue to expand. This won't always be the case, however, and eventually the field will "roll" down its potential, rendering the pressure positive once again and stopping inflation.
The picture I've just painted is pretty simple, but there are a lot of inflationary models, many with different starting and stopping conditions. Fortunately, the majority of them (and the most popular ones) work on principles very similar to this.

There is a new/newer?..Perspective (thanks S.T!) just appeared by T.Banks and M.Johnson:

http://arxiv.org/abs/hep-th/0512141

It appears very interesting, quite deep and thorough.

 

[SpaceTiger]

There is a new/newer?..theory just appeared by T.Banks and M.Johnson:
http://arxiv.org/abs/hep-th/0512141
It appears very interesting, quite deep and thorough.

That appears to be a new perspective on an old theory (eternal inflation) rather than an entirely new theory.

 

[Chronos]

I might be restating what has already been said. But, I see no meaningful role the Higgs boson plays in modern cosmology. I agree that particle models are necessary, but that is a big chasm to cross before we can connect the two. And I agree that eternal inflation is a non-player.

 

[SpaceTiger]

And I agree that eternal inflation is a non-player.

I wasn't trying to say that, I was just pointing out that the article Spin Network linked wasn't suggesting a new theory. It wasn't meant as a criticism, just an observation.

 

[Chronos]

I think that paper is just plain wrong. The assumptions rub me wrong. Too much math in the middle.