What if the early expansion of the Universe was not so rapid?

[kirtg]

Hi. If the early universe did not undergo rapid expansion but rather a more controlled growth like the human body, how old would the universe be given its present size?
thanks

 

[Orodruin]

It is not very clear what you would mean by the universe growing like the human body. The size of the Universe is also likely infinite. The observable Universe is something different but there it does not really make any sense to talk about growth like the human body either.

 

[phinds]

Hi. If the early universe did not undergo rapid expansion but rather a more controlled growth like the human body, how old would the universe be given its present size?
thanks

how to avoid breaking physics with your what if question/

 

[PeterDonis]

If the early universe did not undergo rapid expansion

What makes you think this is a possibility? Are you aware of the evidence for our best current cosmological model which says it's not?

 

[Ibix]

Hi. If the early universe did not undergo rapid expansion but rather a more controlled growth like the human body, how old would the universe be given its present size?
thanks

As others have noted, I don't think this question really makes sense on a number of levels.

First off, "size of the universe" doesn't really make sense. There are three distinct kinds of universe allowed by cosmological models in general relativity, two of which are infinite and always have been. The third does have a finite size. But we don't know which kind the one we live in is (our measurements don't currently rule out any of them), so we don't know if "size of the universe" is a meaningful thing. You do hear people talking about the size of the observable universe, and the current size of that would depend on the age and expansion history. But it is not the same as the size of the universe.

Next, "age of the universe" is a tricky concept. The 14 billion year figure you hear quoted is the time (technically, the "cosmological time") since the universe cooled to the point we can see through it. In a naive general relativistic model the universe is a couple of hundred thousand years older than that (a rounding error on 14bn years). But in inflationary models the universe is much, much older, I believe possibly even infinitely old.

Finally, the reason for invoking a rapid inflation phase in the early universe is to explain why the universe is the same temperature wherever we look. That is really difficult to understand if all the different parts of the universe were not in contact in the past, and without an inflationary phase the far distant parts we can see in opposite directions are too far apart to ever have been in contact. So we would expect them to be at different temperatures and we wouldn't see the more-or-less-isotropic universe we see today. Since that isotropy is a key part of how it develops, the universe would potentially be a very different place, not just bigger or smaller.

As almost always happens with physical models, "what if I change this small thing" can have quite complex implications down the line which can invalidate (or at least complicate) apparently simple questions.

 

[Jaime Rudas]

Hi. If the early universe did not undergo rapid expansion but rather a more controlled growth like the human body, how old would the universe be given its present size?
thanks

The rate of growth of distances due to the expansion of the universe is very slow compared to the rate of growth of the human body. For example, a height of 90 cm would take 7.8 billion years to reach 180 cm.

 

[kirtg]

As others have noted, I don't think this question really makes sense on a number of levels.

First off, "size of the universe" doesn't really make sense. There are three distinct kinds of universe allowed by cosmological models in general relativity, two of which are infinite and always have been. The third does have a finite size. But we don't know which kind the one we live in is (our measurements don't currently rule out any of them), so we don't know if "size of the universe" is a meaningful thing. You do hear people talking about the size of the observable universe, and the current size of that would depend on the age and expansion history. But it is not the same as the size of the universe.

Next, "age of the universe" is a tricky concept. The 14 billion year figure you hear quoted is the time (technically, the "cosmological time") since the universe cooled to the point we can see through it. In a naive general relativistic model the universe is a couple of hundred thousand years older than that (a rounding error on 14bn years). But in inflationary models the universe is much, much older, I believe possibly even infinitely old.

Finally, the reason for invoking a rapid inflation phase in the early universe is to explain why the universe is the same temperature wherever we look. That is really difficult to understand if all the different parts of the universe were not in contact in the past, and without an inflationary phase the far distant parts we can see in opposite directions are too far apart to ever have been in contact. So we would expect them to be at different temperatures and we wouldn't see the more-or-less-isotropic universe we see today. Since that isotropy is a key part of how it develops, the universe would potentially be a very different place, not just bigger or smaller.

As almost always happens with physical models, "what if I change this small thing" can have quite complex implications down the line which can invalidate (or at least complicate) apparently simple questions.

Thanks Jaime. What if the inflationary period took much, much longer than the explosive expansion theorized?

 

[Jaime Rudas]

Thanks Jaime. What if the inflationary period took much, much longer than the explosive expansion theorized?

That question doesn't make sense to me, because I don't see what difference there could be between what you call "the inflationary period" and what you call "the explosive expansion theorized".

 

[Ibix]

Thanks Jaime. What if the inflationary period took much, much longer than the explosive expansion theorized?

As in my penultimate paragraph, if you don't have a rapid inflationary phase then you don't get the homogenous universe we see. Inflation is specifically proposed to address that issue, and if you take away the "rapid" bit then it doesn't give us a homogenous universe. That might be a very different place to our universe.

 

[Vanadium 50]

What makes you think this is a possibility? Are you aware of the evidence for our best current cosmological model which says it's not?

This was a very good question, OP. I think answering it will help your understanding.

 

[PeterDonis]

What if the inflationary period took much, much longer than the explosive expansion theorized?

You need to answer question I asked you in post #4. It applies to this just as much as to what you said in your OP.

 

[ohwilleke]

Thanks Jaime. What if the inflationary period took much, much longer than the explosive expansion theorized?

Much, much longer is a relative concept. The most common inflation theories (there are literally hundreds of variant inflation theories) hypothesize it happening in a tiny fraction of a second, very shortly after the Big Bang indeed.

If inflation actually took an hour, or a year, or a millennium, or even a million years, this would mess up the means by which an inflation theory is supposed to help explain properties of the universe that we observe, but it wouldn't distinguishably change the apparent age of the universe.

Existing estimates of the age of the universe based upon astronomy observations have uncertainties on the order of 0.1%, i.e. about ± 100 million years.

 

[PeterDonis]

What if the inflationary period took much, much longer than the explosive expansion theorized?

Then the predictions of the inflation model would not match observations, so this hypothesis is ruled out.