The firewall is a new term in an extremely provocative paper

http://arxiv.org/abs/arXiv:1207.3123

Black Holes: Complementarity or Firewalls?

by Ahmed Almheiri, Donald Marolf, Joseph Polchinski, James Sully that claims that an observer who falls into a black hole gets burned at the horizon, after all. So the event horizon – the surface of a black hole – is a burning place you don't ever want to hit, therefore called "firewall".

Most other physicists – including Susskind, Banks, Fischler, and others – disagree with the conclusion. After all, we've been explaining for many decades why the "burnout" at the would-be "firewall" is exactly what doesn't happen. See the followups:

http://inspirehep.net/search?ln=en&p=find+title+firewall&f=&action_search=Search

These disagreeing physicists usually point out that the black hole complementarity – the idea that the degrees of freedom (fields) inside a black hole aren't quite independent from those outside but they're highly scrambled versions of them – has everything it needs to preserve the conclusion of classical general relativity, namely that nothing special happens at the horizon (to a large enough observer), without violating any insights about the quantum theory (such as the existence of the Hawking radiation and its ability to preserve the information).

Whether the bizarre argument by Almheiri et al. would apply to other horizons, such as the cosmic horizons in de Sitter space, is an interesting question. That would be pretty bad because the firewall could be everywhere around us. ;-)

I have written a more detailed review of the followups as of today here:

http://motls.blogspot.com/2012/09/are-black-holes-surrounded-by-firewalls.html?m=1

A few days later, I decided that Raphael Bousso's resolution was right:

http://motls.blogspot.com/2012/09/raphael-bousso-is-right-about-firewalls.html?m=1

## Best Answer

There is a surface more or less at 13.7 billion light years from us which is where we see back to the big-bang (looking back in time, if you define "now" in a global way, although there is no reason that we should't define "now" by what we are seeing "now", i.e. along a past light cone). This surface is analogous to a black hole horizon, except it surrounds us instead of being localized in a region.

This thing is called the "cosmological horizon", and the general idea of the holographic principle suggests that everything inside the cosmological horizon is described by oscillations of this horizon. This is hard to make precise because the horizon has a finite area and growing, and so has a finite maximum entropy associated with it (which is growing), and this is paradoxical seeming, because it suggests that the Hilbert space for our universe is growing.

The number of states in a quantum mechanical system can't increase, so this leads many people to renounce the idea of string theory in our kind of universe, choosing instead to describe the dynamics in terms of the asymptotic future, where presumably the universe will vacuum decay to a supersymmetric state. This is one approach, another is to try and formulate a real theory with a finite Hilbert space. I think a possible third approach is to consider finite area horizons as somehow density-matrix like, so that they, unlike black holes, have fundamental decoherence. Nobody knows the answer, and this is the major unsolved problem of string theory today.