more detail for Arch

[Martin]

Arch2008 has gotten confused about the standard mainstream cosmology model, and has taken to repeating over and over again a faulty argument. I've explained briefly why it's wrong and will do so here at somewhat greater length.

 

I don't want other people to be misled by Arch and get into a similar confusion, so I want to be really clear about this.

 

In cosmology all we have is models, which fit the data more or less well. They represent the most we know about the universe as a whole, at large scale.

 

The current consensus model is called LCDM (sorry about the technical alphabet soup, I'll try to keep such abbreviations to a minimum). It comes in two versions spatial infinite and spatial finite.

 

In the infinite universe version of LCDM, expansion starts with infinite volume.

There is a special case of this called flat LCDM. It starts with space infinite and with zero spatial curvature. As distances expand, space remains essentially flat in the sense familiar to all of us---triangles add up to 180 degrees etc. the usual vanilla geometry and trig. Cosmologists like the flat LCDM case. It is easy to calculate with and there are some plausible arguments that it might be the right version.

 

But they still don't commit to the infinite case. In professional research papers they tend to leave open both possibilities, work with both assumptions. This leads to extra columns in tables, two or more sets of the same numbers.

 

In the finite space version of LCDM, expansion starts with a finite volume (if you like that idealization, you can say a zero volume, but it's controversial---in any case not infinite!)

 

You can read more about this at the Einstein-Online website or at Ned Wright's Cosmology FAQ.

 

For exampe Wright's FAQ says the infinite space case, the flat LCDM is the best fit (and infinite space version is indeed preferred by a majority)

but as he indicates, neither case is ruled out. Both finite and infinite space are possible. As Wright says, we don't know---all we know is space is really big, it might be infinite or it might be finite but in any case really big.

 

I should say that either way the universe as a whole is much bigger than the chunk we are currently able to see, that observed chunk is of course finite but that's not what we are talking about.

 

==========================

 

Now where does Arch get into trouble? It seems like this is pretty simple and there is not much room to go wrong. How he gets hung up is by failing to recognize that in the flat LCDM model, expansion begins with an infinite spatial volume.

 

He implicitly assumes that in all cosmology models expansion has to begin with a finite spatial volume. Apparently he assumes this, I'm not sure he ever states it explicitly. And there is no evidence to support that assumption. Indeed the professionals' current preference is against it!

 

So Arch has gotten into a rut of repeating this argument: "space can't be infinite (because it started finite and) because it has only expanded a finite time at a finite rate."

 

The thing in parentheses is what is not founded on observation. I don't know if I really needed to post this thread. Maybe it is obvious to everybody that Arch's argument is wrong. But I wanted to make sure no one got fooled by it.

[Arch2008]

Martin, the LCDM is a model of the universe. It predicts a flat, infinite universe because it is a model, and one of the parameters of the model is the infinite spatial volume start point. It just states that the parameters create a model universe that remains flat even all the way out to an infinite point. Wright says that the model is infinite, but that what we know about the universe is that it is really big. These are Wright’s words. I’m not putting words in his mouth. You need to set aside the model and consider the real universe. The real universe is literally really big, but not infinite. Wright’s site states that he answers questions over the weekend, so I may have his answer for you by Monday.

[Martin]

The real universe is literally really big, but not infinite.

 

I don't want to spend a lot of time talking about this. I hope you get an answer from Wright, and post it word for word.

 

There are two issues

1. You apparently misunderstood what Wright says in his FAQ and are misrepresenting what he says. "Really big" includes the infinite space case as one possibility. Space can be really big infinite or really big finite. Nowhere does he say that being "really big" rules out the possibility of space being infinite.

 

2. More seriously, you are claiming to know that space is finite--and yet you have not presented any observation or valid logic to back that up. You don't have evidence to support your assertion that space is finite.

 

That's not how we should be operating.

[Arch2008]

Got an answer back from Ned Wright:

 

"This statement is correct:

> Martin-No, he says we don't know whether it is finite or infinite, but

> we do know that in either case it is really big."

 

I guess he really should change his FAQ to be more exact.

[Martin]

Got an answer back from Ned Wright:

 

"This statement is correct:

> Martin-No, he says we don't know whether it is finite or infinite, but

> we do know that in either case it is really big."

 

I guess he really should change his FAQ to be more exact.

 

It was nice of him to answer! Thanks for reporting back on your exchange of email with Ned Wright.