Is it possible space was not always expanding but is oscillating?

[BigMoosie]

We know that the universe is expanding and the expansion is accelerating, it is also commonly believed that it has been acellerating since a universal singularity existed (the big bang).

 

Assume that instead of the universe having always expanded but it goes through phases of acelleration and contraction akin to a sine function, that would place the current universe somewhere after a trough approaching an inflexion.

 

In this question it does not matter what would cause such an oscillation, but I am curious about what observations contradict such an idea?

[Martin]

it is also commonly believed that it has been acellerating since a universal singularity existed (the big bang).

...

 

I don't know of any scientist who believes this. It is not the common opinion of cosmologists, or any kind of astronomer.

 

In the usual model that is used professionally, the expansion decelerates for at least the first 6 or 7 billion years. Then it gradually starts accelerating.

 

So it is not believed that expansion has been speeding up the whole time. Much of the time it has been slowing.

 

Also there is no reason to imagine that a singularity actually existed.

All the model can say is that when you project back in time you find conditions of very high density.

 

We can't say that the density was ever *infinite* however. Some recent models show a bounce----a prior phase of the universe contracts down to very high (but not infinite) density and then rebounds.

 

This issue is not yet resolved. All one can say is that there is no scientific reason to suppose that time "began" at the big bang. The universe can very well have been there before, evolving according to its laws.

 

You talk about some kind of cycle: contracting---then rebounding---then expanding for a while and then starting to contract again. There are some conjectured mechanisms that would cause that, but it's too complicated for me to find interesting right now. What interests me is whether or not a model with one single rebound might be right. That would have to be tested by detailed study of the cosmic microwave background and any other relevant data, to see if it agrees with observation.

[BigMoosie]

Thanks Martin, your response gave me the insight I was after.

 

I didn't realise the big bang had little evidence, I always thought it was generally agreed upon.

 

Also, it should have been clear to me that under a big bang situation it would have had to decellerate before accelerating, since the big bang would have had asymptopically infinite acceleration at the start.

 

Regards,

-Moosie

[Martin]

...

 

I didn't realise the big bang had little evidence, I always thought it was generally agreed upon.

...

 

Failures to communicate due to terms like B.B. are not uncommon. You and I are experiencing a failure of that sort.

 

To me it seems obvious that the Big Bang is generally agreed on.

 

I can't understand how you can imagine that it is not, or that what I said implied that it is not.

 

The question is not about whether there was a big bang. The question that many people are investigating these days is what conditions preceded it and what led up to it.

 

Talk about "singularity" is, and always was bullshît. Nature does not have singularities (they are recognized to be breakdowns in manmade theory). There is no scientific reason to believe "time began" with the B.B. and it is difficult to imagine what that could mean.

 

A good clear up-to-date exposition of Big Bang concepts and terminology is at the Einstein Online website. The link is in my sig. Please check out their essay titled "A Tale of Two Big Bangs"

[Piltdown man]

Are you, Martin, saying that you are unaware what the event was that led to the circumstance that resulted in the "big bang?"

[Martin]

Are you, Martin, saying that you are unaware what the event was that led to the circumstance that resulted in the "big bang?"

 

Well there are several modern models of what led up to the big bang. None of them involve a "singularity", of course.

 

At this point it's hard to choose. Not enough data. But there is a kind of stampede of researchers going into studying the Loop Cosmology "bounce" model. And abandoning some of the others. That doesn't mean the bounce is true, just that it is currently the most attractive to researchers. Progress is being made, new papers come out frequently. We'll see.

 

I should make the general observation that in science you don't get absolutely certain answers.

 

What you get are models, which if they pass observational tests, gradually accumulate credibility---based largely on their track record of making correct predictions. And if they fail a test they are ruled out.

 

Preference also depends somewhat on their simplicity and consistency. If two models make equally good predictions people tend to prefer to use the simpler one (with the fewer assumptions and adjustable parameters.)