The Expanding Universe: A Scientist's Perspective on Infinite vs Finite Space

[PeterDonis]

You make it sound like there were two periods, one called inflation, then one called the big bang. I think most people would just call the whole thing "the big bang".

"Most people" is not the appropriate criterion here. The appropriate criterion is, how is the term "big bang" used in actual textbooks and peer-reviewed papers in the field of cosmology? I think you will find that they use the term as I defined it. That is the actual scientific usage, which is what we are concerned with here at PF; what is said in pop science books and articles and websites is irrelevant.

what was inflating prior to the big bang?

The universe.

Now you seem to be saying no, the big go was actually contained in something else.

No, that's not what our current best model says.

 

[PeterDonis]

I do think it's very possible for them to miss the forest for the trees if they don't sometimes step out from behind the equations and consider things from a different perspective.

I think you should not be making such criticisms unless and until you thoroughly understand what the current best model physicists are using actually says. It does not seem like you do.

And I do think you can't always hide behind "gee that's pop science so go away".

Why not?

 

[Ken Ucarp]

I think most cosmologists, as best I understand them, who accept the theory of inflation, which are most, would say that the universe was cold and dark during the epoch of inflation and that the potential energy of the inflaton field was converted into heat, radiation, and matter and that is the beginning of what most people refer to as the big bang. I think inflation is a theory which set out to explain various problems areas in the current cosmological model of its day based on what they observed. A lot of physics proceeds by creating the mathematics to describe the observations instead of being derived from prior theories. Think of the example of the ultraviolet catastrophe where the mathematics was constructed to explain the observations. Then you take the physics you have just developed and test them to see if they hold true in all experiments and can be thus generalized. Cosmology is a little different because you can't rerun the experiment.

I see. So I think what I said originally is still correct, just poorly worded. According to cosmologists who accept the theory of inflation, there was actually an "era" prior to the big bang that was as you described. And you said "the universe was cold and dark...". The important piece is, it was the universe that went through this inflationary era, not some entirely different thing that then became the universe that we know and love during the big bang. The thing that was inflating was our universe itself including the actual "rulers and clocks" themselves. The thing that was inflating was not itself contained in a higher dimensional set of rules and clocks. Do I have that description right?

 

[PeterDonis]

The thing that was inflating was our universe itself including the actual "rulers and clocks" themselves. The thing that was inflating was not itself contained in a higher dimensional set of rules and clocks. Do I have that description right?

As far as our best current model is concerned, yes. If you talk to string theorists, they will start talking your ear off about "a higher dimensional set of rules and clocks". But all of that is speculative at this point.

 

[Ken Ucarp]

I think you should not be making such criticisms unless and until you thoroughly understand what the current best model physicists are using actually says. It does not seem like you do.
Why not?

Criticisms? Please. No criticism intended. A race car driver can provide input to the mechanics even though he may not understand the engine like they do. And yes, I do think it's very easy to simply write a non-expert off under the guise of "pop science". I'm a computer programmer. Sometimes the users describe what an application is doing in some pretty funky terms. If I merely wrote them off because they don't know "modern programming techniques" I'd probably end up losing my job.

 

[Ken Ucarp]

As far as our best current model is concerned, yes. If you talk to string theorists, they will start talking your ear off about "a higher dimensional set of rules and clocks". But all of that is speculative at this point.

Ok thank you. I get the part about string theorists. SO...the end point of my whole post was that I was watching a show where the cosmologist said at like a trillionth of a second after the big bang, the universe was approximately one centimeter across. I don't know if he meant the big bang era or the inflationary era. What struck me as odd was that as I mentioned, I thought the "rulers" themselves were expanding. So in what way can something be a centimeter across. Centimeter is only a centimeter in reference to a larger measurement. Or was THIS and example of a cosmologist just trying to explain something about relative size in terms the layman might relate to?

 

[PeterDonis]

No criticism intended.

I meant "criticism" in the sense of presenting a counter-argument, not in the sense of expressing a judgment of value.

I'm a computer programmer. Sometimes the users describe what an application is doing in some pretty funky terms. If I merely wrote them off because they don't know "modern programming techniques" I'd probably end up losing my job.

I don't think your implied analogy here is valid. A computer programmer has to meet the requirements of his users, because the users are the reason he's writing the program in the first place. But science does not have to meet the requirements of people who don't understand science (except in the sense, irrelevant to this discussion, that scientific knowledge underlies our technology). If scientific theories that work happen to be expressed in terms that are hard for lay people to understand, that's just a fact about science that has to be accepted and dealt with; and if pop science presentations distort the science so that they don't provide a proper understanding of it, that's also a fact that just has to be accepted and dealt with.

I do think it's very easy to simply write a non-expert off under the guise of "pop science".

We're not "writing off" the non-expert; we're just pointing out that he's a non-expert who doesn't properly understand the science, and that reading pop science won't fix that condition. You have to look at the actual science--the textbooks and peer-reviewed papers that describe, in precise technical language and math, what the best current scientific theories actually say. Again, that's just a fact about science that has to be accepted and dealt with; complaining that a non-expert is being "written off" when this unpleasant fact is pointed out to him doesn't change the fact.

 

[phinds]

Ok thank you. I get the part about string theorists. SO...the end point of my whole post was that I was watching a show where the cosmologist said at like a trillionth of a second after the big bang, the universe was approximately one centimeter across. I don't know if he meant the big bang era or the inflationary era. What struck me as odd was that as I mentioned, I thought the "rulers" themselves were expanding. So in what way can something be a centimeter across. Centimeter is only a centimeter in reference to a larger measurement. Or was THIS and example of a cosmologist just trying to explain something about relative size in terms the layman might relate to?

That is NOT a description of the universe. It is, and he most likely did not know this, a description of the OBSERVABLE universe which is a totally different thing. This is the kind of crap you get in pop science.

 

[PeterDonis]

I was watching a show where the cosmologist said at like a trillionth of a second after the big bang, the universe was approximately one centimeter across. I don't know if he meant the big bang era or the inflationary era.

Strictly speaking, he meant neither. Those times quoted by cosmologists are not actually times in the current best model; they are times in an idealized model in which there is no inflation and the "big bang" is indeed an idealized "initial singularity" (which does not appear at all in the current best model). Basically, what the cosmologists are doing is taking the temperature of the universe at some point in time in the actual best current model, looking at the idealized model to see at what time after the "initial singularity" that temperature occurs, and then giving that time as though it was an actual time in the actual best current model.

(The comment by @phinds regarding what the "size" given actually refers to is also valid.)

Yes, this means cosmologists routinely use confusing and misleading language (at least it's misleading if you're trying to understand what's actually going on). And yes, these are actual cosmologists, actual experts in the field. This is a good illustration of why you cannot trust pop science sources, even when they are written by actual scientists.

 

[mfb]

What struck me as odd was that as I mentioned, I thought the "rulers" themselves were expanding.

It depends on the type of rulers. Are they made out of freely floating particles in space? Then they expand. Are they made out of a solid material (neglecting that there was nothing solid back then)? Then they do not.
The length of a second is defined based on fundamental quantities (here: in cesium atoms - they didn't exist either back then, but let's ignore practical details), and a centimeter is based on the length time travels in a defined length of time. That allows a measurement of a centimeter no matter where you are. The observable universe today had a diameter of something like a centimeter back then. The precise value depends on the model and the question which point in time exactly you consider.

Criticisms? Please. No criticism intended. A race car driver can provide input to the mechanics even though he may not understand the engine like they do. And yes, I do think it's very easy to simply write a non-expert off under the guise of "pop science". I'm a computer programmer. Sometimes the users describe what an application is doing in some pretty funky terms. If I merely wrote them off because they don't know "modern programming techniques" I'd probably end up losing my job.

In this analogy the race car drivers are professional astronomers.
You are someone who has seen descriptions of car races made for people who have never seen them.