I'm not going to tell you what a topos is; I'll just say some stuff about them. They were invented by Grothendieck in the 1960's as part of his quest to prove some conjectures in number theory. The solutions of a bunch of polynomial equations give, indirectly, a topos, and he thought of this topos as a generalization of a space, hence the name 'topos'. Later he and other realized that a topos is like a mathematical universe, since the world of set theory is one topos, but there are many more. So, you can think of topos theory as a grand generalization of set theory.
But then people realized that a topos is also like a theory. For many familiar mathematical gadgets - groups, rings, etcetera - there is a topos called the classifying topos of that gadget, which acts like an axiomatic theory of that gadget. In particular, any particular instance of that gadget in the world of sets - for example, any particular group - gives a map from the topos of sets to the classifying topos of that gadget, and conversely.
Olivia Caramello is deeply focused on classifying topoi and how they can be used as 'bridges' to relate seemingly different fields of math. She has an easy-to-read introduction to this idea here:
My friend Colin McLarty, who also attended this conference, has written a nice history of topos theory that gives a good flavor of what it's about, without revealing the - somewhat terrifying, at first - definition of a topos:
It's taken me a long time to learn what little I know about topos theory - in part because I'm interested in lots of other things, and in part because I found it a bit intimidating for the first few decades. A long time ago, I wrote this introduction:
whose main virtue is that it points you to 2 free books on the subject: the easy one by Goldblatt, and the vastly more profound)book by Barr and Wells. But now I've reached the point where topos theory makes a lot more sense! So, I should probably expand this page a bit someday.
If you know topos theory and/or a fair amount of logic, you can get a good intro to Caramello's work here:
• Olivia Caramello, The unification of mathematics via topos theory, http://arxiv.org/abs/1006.3930.
However, this is not for the faint-hearted! Here's a sample, just so you can decide if this is for you:
"Now, the fundamental idea is the following: if we are able to express a
property of a given geometric theory as a property of its classifying topos then we can attempt to express this property in terms of any of the other theories having the same classifying topos, so to obtain a relation between the original property and a new property of a different theory which is Morita-equivalent to it. The classifying topos thus acts as a sort of ‘bridge’ connecting different mathematical theories that are Morita-equivalent to each other, which can be used to transfer information and results from one theory to another. The purpose of the present paper is to show that this idea of toposes as unifying spaces is technically very feasible; the great amount and variety of results in the Ph.D. thesis  give clear evidence for the fruitfulness of this point of view and, as we shall argue in the course of the paper, a huge number of new insights into any field of Mathematics can be obtained as a result of the application of these techniques."
(A 'geometric theory' is not one about geometry; it's a technical term for an axiom system obeying certain properties,which Caramello explains. As I hinted, many different kinds of mathematical gadgets can be described using geometric theories. She also explains the concept of "Morita equivalence".)
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