Precision Cosmology and Headline Writing
Quanta Magazine recently published a great article with a silly headline: "Laws of Logic Lead to New Restrictions on the Big Bang". Such a title seems to suggest that an otherwise unrelated idea - some new logical argument - has somehow imposed new order on Cosmology. As if philosophers have somehow wrestled back the narrative in Cosmology. Far from it.
For the experts, the "laws of logic" means Unitarity. That all the sum of all probabilities for any outcome of given event adds up to one. And I don't know a single, reasonable physicist who doesn't assume Unitarity (although some may occasionally stumble) as a given. The aforementioned trouble with the Big Bang was that nobody knew what Unitarity meant in an exponentially expanding universe.
Cosmic Inflation is a model for the Big Bang that has enjoyed much success in the past two decades of precision cosmology. Inflation is similar to - but distinct from - Dark Energy, which involves a far less understood model for the current state of the universe. Both of these ideas resemble the simple structure of de Sitter Spacetime.
The geometry of de Sitter space is something akin to the sphere. It his highly symmetric and easy to describe. Despite this, de Sitter has one major complication: time. Our understanding of time in de Sitter space - or causality, more precisely - is far from complete.
In the standard, flat, Minkowski spacetime, causality is intimately connected with the speed of light. This fact is baked in to the structure of physical law - in particular quantum field theory - from which all sorts of related ideas find a natural setting. Notably, antimatter.
At the microscopic level, collisions of particles look essentially (although not completely) the same going backwards and forwards in time. If you filmed them, you wouldn't be able to know the difference. Our macroscopic sense of time is then understood in terms of entropy: the incidental rise in disorder. Glasses break, water spills, and someone has to expend energy to clean that mess up again.
Microscopically, time works very differently in de Sitter space. de Sitter is an odd because causality is violently distorted by the exponential expansion of spacetime. Two objects that might be in causal contact in one moment, could be ripped apart faster than the speed of light by this expansion. Watching a movie of particle collisions in de Sitter is much different if played forwards or backwards.
To summarize, Charlie Wood at Quanta Magazine reports on recent progress in understanding causality in de Sitter spacetime. Notably, Paper et. al.'s development of a cosmological versionof the famous optical theorem.
A better title for Charlie's piece might be "Theoretical physicists find a better understanding of what they're doing in Cosmology."
