Comments on: Anatomy of a Paper: Part I, Inspiration

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By: Ask a String Theorist! Or an Atomic Physicist. | Cosmic Variance

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By: Michael D

Michael D — Fri, 10 Aug 2007 10:32:57 +0000

Nice post Sean!

In my case rather timely in fact, as I’ve just learnt a little about density perturbations in my Cosmology class.

One question I have (that my lecturer couldn’t answer and indicated that no one really knows):

Why did inflation stop?

By: Jacques Distler

Jacques Distler — Fri, 10 Aug 2007 02:39:40 +0000

If we want to study the propagation of a temperature field (scalar) we solve a PDE. The scalar field propagation speed is clearly limited. … If this is a quantum field and obeys all the QM rules how can it travel faster than the speed of light?

Quantum, schmontum! The question is a purely classical one.

The evolution of the inflaton and metric are governed by a well-known set of coupled PDE’s. In fact, because we are interested in spatially homogeneous solutions, we can reduce these to coupled ODE’s.

I really don’t grasp what it is about those equations that makes you think their solutions should be acausal.

By: Cecil Kirksey

Cecil Kirksey — Thu, 09 Aug 2007 23:30:54 +0000

Jacques:
Thanks for the reply. OK let me try to explain why I have difficult time grasping this expanding scalar field during inflation. If we want to study the propagation of a temperature field (scalar) we solve a PDE. The scalar field propagation speed is clearly limited. What I had in mind were similar equations for the inflaton field. But it seems that the propagation speed of the inflaton field is directly tied to the expansion speed of space which is also tied to the value of the inflaton field.

If this is a quantum field and obeys all the QM rules how can it travel faster than the speed of light? I have read all of the layman’s books like Guth’s and others and this question never seems to get answered satisfactorily, at least to me.

Well if you have a readily available reference I will just refer to that and thank you for your time.

By: Jacques Distler

Jacques Distler — Thu, 09 Aug 2007 03:32:49 +0000

I have always wandered how the inflation field ( scalar field) is conveyed over space as space expands faster than the speed of light? A typical answer is: not to worry because you cannot communicate any information via this field. Hmmm. No information. Is not the expansion rate dictated by this field? Is this not information? This question has been a source of concern ever since I read about inflation back in the mid eighties. I have yet to read more than the info scenario as the reason for the acceptance of the expansion rate can be greater than the speed of light. Now you want to suggest some asymmetry in the expansion!! What exactly is the mechanism that transmits this inflation field over space as the space is expanding?

I am not sure I understand the question.

The evolution of the inflaton field (and the metric) is governed by causal local equations. So there’s clearly no issue of superluminal communication there.

There is a bit of a mystery about the initial conditions for inflation to start. They require that the inflaton field (and the metric) be fairly homogeneous over a region somewhat larger than horizon at the start of inflation. That’s hardly impossible, but it is a bit disappointing.

However, from the text of your message, it doesn’t seem to be what you are asking about. You don’t seem to be worried about the initial conditions, but rather about the subsequent evolution during inflation.

But I don’t see why.

By: Cecil Kirksey

Cecil Kirksey — Wed, 08 Aug 2007 23:02:22 +0000

Sean:
I have always wandered how the inflation field ( scalar field) is conveyed over space as space expands faster than the speed of light? A typical answer is: not to worry because you cannot communicate any information via this field. Hmmm. No information. Is not the expansion rate dictated by this field? Is this not information? This question has been a source of concern ever since I read about inflation back in the mid eighties. I have yet to read more than the info scenario as the reason for the acceptance of the expansion rate can be greater than the speed of light. Now you want to suggest some asymmetry in the expansion!! What exactly is the mechanism that transmits this inflation field over space as the space is expanding?