# What is wrong with eternal inflation model?

• Dmitry67
In summary, the article discusses the difficulties in understanding entropy and whether or not eternal inflation makes sense.f

#### Dmitry67

I was thinking that "eternal inflation" model deals nicely with the problem of the beginning of the universe. If I understand this model correctly, Multiverse is a boiling bulk of some sort of very dense vacuum, creating inflationary "bubbles" (baby Universes), finite from the outside but infinite from the inside.

on p.12

"As shown by Borde, Guth & Vilenkin [65], in inflation must have had a beginning, i.e., cannot be eternal to the past".

Googling gives me hints that it is based on some theorem, but I am afraid it will be beyond my level of math, do you have any simple explanation for idiots like me?

An eternal universe in the past would be static. In the eternal inflationary model the universe has a finite not eternal beginning as it developed from a finite point into a bubble universe. Keep in mind their are over 60 still valid inflationary models. This is just one of them that still fits observational data. My signature has numerous inflationary articles under cosmology101 link. For further reading. the Inflationary encyclopedia has a listing of valid models

Thanks. Just to confirm, when you say "universe has a finite not eternal beginning" are you talking about one of the baby universes (particular bubble) (and I totally agree) or about the whole Multiverse? (if so I am confused)

The baby universes.

Googling gives me hints that it is based on some theorem, but I am afraid it will be beyond my level of math, do you have any simple explanation for idiots like me?
Place a single photon inside the inflating universe. Consider inflation to have a constant energy density. Extrapolating into the past, how long until that photon has a higher energy density than the inflaton?

I see, there is no problem anyway - the inflation of any baby Universe had a beginning (branching from the bulk), I have no problems with it. The important thing is the boiling bulk, constantly generating baby universes - that thing exists forever (I am not sure that the word "forever" is valid here because it is not clear if "time" is consistently and correctly defined in that boiling bulk)

I see, there is no problem anyway - the inflation of any baby Universe had a beginning (branching from the bulk), I have no problems with it. The important thing is the boiling bulk, constantly generating baby universes - that thing exists forever (I am not sure that the word "forever" is valid here because it is not clear if "time" is consistently and correctly defined in that boiling bulk)
Right. This is why eternal inflation is said to be future-eternal, but with a finite past.

Some have used entropy arguments to show that this future-eternal inflation may not make sense. But the problem is it's really, really hard to make that determination because any such estimates of entropy run against the problem of how to define a measure across different Hubble volumes.

Yes, and the article discusses the difficulty of defining entropy - not only across different Hubble volumes, but also across different MWI universes. I was happy to see the claim I was talking about a long time ago - that total entropy of the whole multiverse is 0.

So, what is wrong with eternal confusion? It makes as much sense.

what is wrong with eternal confusion?

It makes the Big Rip of the brain :)