# Observable (in principle) signal of a bubble collision in eternal inflation

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Assuming a scenario of eternal inflation with a lot of "bubble universes" expanding, Lenny Susskind explains here that a potential signal of a collision of our universe with another bubble could be a spot of warmer or colder temperature in the cosmic microwave background and the light coming from this direction would be linearly polarized and pointing in circles around the spot of higher or lower temperature (I hope you know what I mean, maybe I should draw a picture).

What are the physical reasons that such a bubble collision would lead to this particular characteristic, in principle observable signal? In particular concerning the polarization issue I have no clue why it would look like this.

recategorized Mar 24, 2014
I know that the idea of eternal inflation as a whole is quite speculative and far outlying and that there exists physical reasoning that speaks against it. By this questions I just want to hear about and understand the physics reasoning that explains the characteristic signal Lenny Susskind mentions in the lecture.

This post imported from StackExchange Physics at 2014-03-12 15:23 (UCT), posted by SE-user Dilaton
Hi Dilaton - I slightly edited your title to what I thought you meant. Hope this is correct. Will be interesting to hear the answer to the question!

This post imported from StackExchange Physics at 2014-03-12 15:23 (UCT), posted by SE-user twistor59
Thanks @twistor59 the title looks better now :-). Lenny Susskind said in addition that there is actually such a candidate cold spot observed, but I guess it can have many different causes ...

This post imported from StackExchange Physics at 2014-03-12 15:23 (UCT), posted by SE-user Dilaton

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See http://arxiv.org/abs/0810.5128 for the gory (and they are very gory!) details.

Basically, it's unlikely that the inflaton field has the same value in the two bubbles. When the bubbles collide, the bubble with the lower energy inflaton field will see the collision as a hot spot, and the bubble with the higher energy inflaton field will see the collision as a cold spot.

The paper also calculates the geometry of the effects seen in the CMB. I couldn't follow much of this, though the general result is that the collision will have SO(2,1) symmetry and this creates a cone. We see see a cross section of this that is a circle.

This post imported from StackExchange Physics at 2014-03-12 15:23 (UCT), posted by SE-user John Rennie
answered Mar 4, 2013 by (470 points)
Thanks John, for the explanations and the link +1. I will look into the paper.

This post imported from StackExchange Physics at 2014-03-12 15:23 (UCT), posted by SE-user Dilaton
Good luck, I understood about 10% of it :-)

This post imported from StackExchange Physics at 2014-03-12 15:23 (UCT), posted by SE-user John Rennie
Yeah, I will see what I can take out of it, since it is a HEP-th paper after all ... :-D

This post imported from StackExchange Physics at 2014-03-12 15:23 (UCT), posted by SE-user Dilaton

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