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?

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.

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.

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.

I’m reading your post while I’m working in a paper for a conference, which I should deliver tomorrow. I found this kind of posting very inspirational because remembers me why I decided, after a successful career in the industry, dedicate all my energies in basic research.

I think that is that indescribable excitement of being closer and closer to a good idea that keeps you going on, even when things don’t follow a clear path from the initial “what if” to the final paper (if they are published at all!)..

Maybe next time you could also talk about the unsuccessful papers or about those ideas that make sense only after been in the background of your mind for many years, which in my experience are two common cases.

Now, Im going back to my paper . . .

Pablo

cheers and thanks

How to find such problems, and when to give up, etc., are all perfectly reasonable questions that don’t have straightforward algorithmic answers. My motivation behind these posts was to provide a close-up look at one specific example of a successful paper. It’s only by considering examples that anyone can hope to develop judgment about how to choose good problems. Unfortunately, every problem is different, and at some point you have to take the plunge and hope for the best.

recently, i have been thinking a lot about this process, and it is useful to hear about it from outside my normal realm at PI. Specifically, I think one of the challenges for theory students is that all of the guide books about how to do research really target people who are grappling with data as well as ideas. When all you’ve got are ideas and the attempt to stretch them, things get … murky. so, i think we can take all the advice we can get about how to unmurk as much as possible.

Having said all that, I have a set of questions that have been coming up a lot for me recently: How do you evaluate whether a problem is worth pursuing? When do you decide to give up? As a graduate student with the attending pressures, how are these evaluations made? How do you adjust this decision making as you move up the career ladder?

I feel as though it is hard to get thoughtful answers to these questions, but I know we can rely on your willingness to “waste” time on pedagogy to give them some real thought if you have a moment to do so.

If there is something to say, you should write a paper! Or we should write one together. (See how the wise elder colleague tries to coast on the ideas of their [former] students?)