# B Can someone explain the Aguirre-Gratton model & how it works?

Tags:
1. Dec 17, 2017

### relatively-uncertain

Hi, I'm slightly confused with the Aguirre-gratton model, and was wondering if someone could explain using simpler terms? I have read from : http://www.preposterousuniverse.com/blog/2007/04/27/how-did-the-universe-start/
" Aguirre and Gratton have presented a proof by construction that such a universe is conceivable; essentially, they demonstrate how to take an inflating spacetime, cut it near the beginning, and glue it to an identical spacetime that is expanding the opposite direction of time. This can either be thought of as a universe in which the arrow of time reverses at some special midpoint, or (by identifying events on opposite sides of the cut) as a one-way spacetime with no beginning boundary. In a similar spirit, Gott and Li suggest that the universe could “create itself,” springing to life out of an endless loop of closed timelike curves. More colorfully, “an inflationary universe gives rise to baby universes, one of which turns out to be itself.”

1. what does it mean by "cutting the space time near the beginning and glueing it to an identical spacetime that is expanding the opposite direction of time? " Doesn't time only go in one direction too?
2. What does "space-time with no beginning boundary" mean?
Could someone please give an explanation of the aguirre-gratton and how it creates the no-boundary state? thanks so much! (and if you have time, a brief explanation of the Gott-Li model) Thanks!

2. Dec 18, 2017

### kimbyd

As far as the fundamental laws are concerned, time is fully symmetric. The apparent direction of time only arises as a result of entropy: it indicates that there was a low-entropy condition in our distant past. Standard thermodynamic theory states that time would appear to flow away from that low-entropy event in each direction.

This model is basically a sketch of this idea that shows the basic concept is mathematically plausible. It doesn't suggest it's likely, just not mathematically forbidden.

If you take standard cosmology theory and extrapolate it back in time, it is forced to have some sort of beginning. This is the "beginning boundary". The space-time they've envisioned gets rid of that by stitching a different space-time on at some point, so that they're no longer extrapolating backward forever.

Imagine our universe like a cone. The sides of the cone spread further and further away from one another forever into the future. This represents the expansion of space. But go back in time, and the sides come together to a single point. This is the past boundary. The idea here is that they cut off the tip of the cone, and stitch it to another cone that expands outward forever in the other direction.

In this analogy, there's no future boundary because the universe expands forever, and the sides of the cone don't count as a boundary because they're just a heuristic for the expansion: they're not real. But the tip of the cone is a real boundary. This is the past singularity of our universe. By cutting off the tip, they get rid of the only boundary.

3. Dec 18, 2017

### relatively-uncertain

Thank-you so much for your help! I've got a few clarifications in case I have misunderstood anything here so I hope you don't mind.

If you cut the tip and glue another cone in the opposite direction, doesn't that leave you with the middle being lowest entropy? How can a high entropy decrease to a low entropy?

Does that mean in the other direction, the expansion somehow closed/collapsed into a smaller point and expanded again from the big bang?
So is it in any way suggesting the the universe we lived in was from something like a big bounce? Thanks again!

4. Dec 19, 2017

### Staff: Mentor

5. Dec 19, 2017

### relatively-uncertain

In this analogy, there's no future boundary because the universe expands forever,[/QUOTE]

if the model is like an hour-glass shape, and the middle was the lowest entropy...doesn't the past also expand forever into the past? I don't really understand this notion...