Is Inflation Eternal Into the Past?

[saleh]

HI,...
This is my first post here on this forum ...
I wonder if anyone here can help me to clarify some concepts to me in the paper of alan guth 2007 named "Eternal inflation and its implications"
and this is my question here ...
he says in the abstract the following :"Although inflation is generically eternal into the future,
it is not eternal into the past: it can be proven under reasonable assumptions that the
inflating region must be incomplete in past directions, so some physics other than
inflation is needed to describe the past boundary of the inflating region."

and in p.14 :"If the universe can be eternal into the future, is it possible that it is also eternal into
the past? Here I will describe a recent theorem [43] which shows, under plausible
assumptions, that the answer to this question is no"

that seems to me -if I understand correctly-that he suggests a past incompleteness which is synonimous to "beginning" ,But then he says
"There is of course no conclusion that an eternally inflating model must have a
[unique beginning], and no conclusion that there is an upper bound on the length of
all backwards-going geodesics from a given point. There may be models with regions
of contraction embedded within the expanding region that[ could evade our theorem]."

so any thoughts on the subjects ?!, It could really be helpful
Thanks

 

[Simon Bridge]

Welcome to PF;
Yike: I'll just tidy that up for you...

I wonder if anyone here can help me to clarify some concepts to me in the paper of alan guth 2007 named "Eternal inflation and its implications"

You mean this one?
Eternal inflation and its implications.
Alan Guth (2007) J.Phys.A40:6811-6826,2007

and this is my question here ...
he says in the abstract the following

"Although inflation is generically eternal into the future, it is not eternal into the past: it can be proven under reasonable assumptions that the inflating region must be incomplete in past directions, so some physics other than inflation is needed to describe the past boundary of the inflating region."​

...and in p.14

"If the universe can be eternal into the future, is it possible that it is also eternal into
the past? Here I will describe a recent theorem [43] which shows, under plausible
assumptions, that the answer to this question is no"​

...that seems to me -if I understand correctly-that he suggests a past incompleteness which is synonimous to "beginning"

But then he says

"There is of course no conclusion that an eternally inflating model must have a
[unique beginning], and no conclusion that there is an upper bound on the length of
all backwards-going geodesics from a given point. There may be models with regions
of contraction embedded within the expanding region that[ could evade our theorem]."​

...so any thoughts on the subjects ?!, It could really be helpful

What is your question?
The quoted sections are saying that the inflation had to start sometime, but the argument used does not tell us anything beyond that.

The square brackets in the second quote (which is from p16) were added by you - the original no brackets but otherwise was identical. You have to be careful about this because square brackets inside a quote usually indicates text that is added by the person doing the quoting.

You don't need to change the color or the font-size and, in general, you should avoid doing that.

 

[saleh]

OH thanks simon ! ...
I'm new in this site , so excuse me for any thing (like typing in red color or ...) , and the brackets were indeed added by me , just for drawing your attention to these words !
I actually send a message to Alan guth to clarify this to me , ... If you would I will post it here ...
my question was about the " no conclusion that an eternally inflating model must have a unique beginning"
and the "that the inflating region must be incomplete in past directions"

so as I understand correctly ... the inflation had a beginning , but we don't know yet what was before

 

[Simon Bridge]

The inflation had a beginning - the argument used in the paper does not tell us about the nature of the beginning, or of the inflation between then and now, or anything really, just that it has to be there if the current models are correct. Try not to draw any conclusions from the paper beyond that. The paper does not say anything about what anyone else may or may not know or what other models and arguments may or may not show.

 

[saleh]

Thank you simon , for the answer ...
I'm not drawing any conclusion , I just want a clarification on the subject ...