Beamer: missing \endcsname inserted \end{frame}

I am getting this error while compiling this code: I have already tried checking the equations and looking for missing }, but without success. This is the first time I am using the beamer environment. Here is a minimum example which reproduces the error.

\documentclass{beamer}

\usepackage[utf8]{inputenc}
\usepackage{default}

\begin{document}
\begin{frame}\frametitle{Onde Gravitazionali da sistemi binari}
\begin{columns}
\begin{column}
\begin{itemize}
\item $P_{gw}=-\frac{dE_{orb}}{dt}$
\item $\dot{\omega}_{gw} = \frac{12}{5}\, 2^{\frac{1}{3}} \left(\frac{G M_c}{c^3}\right)^{\frac{5}{3}}\omega_{gw}^{\frac{11}{3}}$
\end{itemize}
\end{column}
\begin{column}
\includegraphics[scale=0.4]{separation(t).png}
\end{column}
\end{columns}
\end{frame}

\end{document}
• The width argument for column is missing: \begin{column}{0.4\textwidth} ... Beside this: don't load local files (default.sty, your graphic) in a minimal example. – Ulrike Fischer Mar 12 '15 at 15:02
• @UlrikeFischer There is a default.sty in TeX Live, dated 1998, but I don't recommend loading it. – egreg Mar 12 '15 at 18:46
• @UlrikeFischer: Would you write up an answer? Adding column widths solves the problem. – Werner Sep 10 '16 at 16:08