# What is the cardinality of the set of all topologies on R\mathbb{R}?

This was asked on Quora. I thought about it a little bit but didn’t make much progress beyond some obvious upper and lower bounds. The answer probably depends on AC and perhaps also GCH or other axioms. A quick search also failed to provide answers.

Let $X$ be any set of some infinite size $\kappa$. A topology on $X$ is a set of subsets of $X$. $X$ has $2^\kappa$ subsets and there are $2^{2^\kappa}$ collections of subsets of $X$. This is an upper bound for the number of topologies on $X$.

Now, choose a point $x_0\in X$ and let $Y=X\setminus\{x_0\}$. Since $X$ is infinite, $Y$ is of size $\kappa$, too. Let $\beta Y$ be the Stone-Čech compactification of the space $Y$ with the discrete topology.
$\beta Y$ can be thought of as the space of all ultrafilters on $Y$, with the ultrafilter generated by a singleton $\{y\}$ identified with $y$. $Y$ is dense in $\beta Y$. The space $\beta Y$ is of size $2^{2^\kappa}$. For each $y\in\beta Y\setminus Y$ let $\tau_y$ be the topology on $X$ that makes the map mapping $x\in Y$ to $x$ and $x_0$ to $y$ into a homeomorphism.

This gives you $2^{2^\kappa}$ different topologies on the set $X$.
In the case of $X=\mathbb R$ we get $2^{2^{2^{\aleph_0}}}$ topologies. (Wow!)

Now, you ask about different topologies, and these are different topologies, even pretty good ones, in terms of separation axioms. What about homeomorphism classes of topologies? I am almost certain that you can construct $2^{2^\kappa}$ ultrafilters on $Y$
that give you $2^{2^\kappa}$ pairwise non-homeomorphic topologies.
But this needs some more thought.

Ok, I thought about this some more. Let $y,z\in\beta Y\setminus Y$ and let $f:(X,\tau_y)\to(X,\tau_{z})$ be a homeomorphism.
For both topologies, $x_0$ is the only non-isolated point of $X$.
Hence $f$ restricts to a bijection from $Y$ to $Y$.
There are $2^\kappa$ bijections from $Y$ to $Y$.
It follows that for each $y\in\beta Y\setminus Y$ there are at most $2^\kappa$ points $z\in\beta Y\setminus Y$ such that $(X,\tau_y)$ and $(X,\tau_z)$ are homeomorphic.
In other words, in the class of topologies of the form $\tau_y$,
the homeomorphism classes are of size at most $2^\kappa$.

But there are $2^{2^\kappa}$ topologies of this form. It follows that there are $2^{2^\kappa}$ pairwise non-homeomorphic topologies on the set $X$.