A notice from the Returners: The total mass of our universe has decreased to below the critical threshold. The universe will turn from being closed to open, and die a slow death in perpetual expansion. All lives and all memories will also die. Please return the mass you have taken away and send only memories to the new universe.
The Returners, a group of intelligent lifeforms who seek to prevent the Universe turning from a closed system to an open one (which would prevent the Big Crunch and cause a Big Freeze to happen instead) send out a messege across the supermembrane to the many mini-universes so that they make the sacrifice to return back to the great universe so that a new universe can be born. The book itself let's us know just how much energy a messege across multiple universes like this would require...
They also cared about the answer to one question in particular: Could the mini-universe communicate with the great universe? Sophon told them that there was no way for the mini-universe to transmit any information to the greater universe, but it was possible for the mini-universe to receive broadcasts from the great universe. She explained that all the universes were bubbles above a supermembrane—this was a fundamental conceptual image from Trisolaran physics and cosmology, and she could explain it no further. The great universe had enough energy to propagate information across the supermembrane. However, this was difficult and required a great expenditure of energy—the great universe would have to convert a Milky Way’s worth of matter into pure energy. As a matter of fact, the monitoring systems in Universe 647 often received messages from other great universes on the supermembrane. Some were natural phenomena; some were messages from intelligent beings that could not be decoded—but they had never received any message from the particular great universe they had come from.
The energy required to send a messege across the supermembrane like this would require the mass-energy of an entire Milky Way. The mass of the Milky way is
around 1.5 trillion solar masses (that's 1500000000000), and one solar mass is around
2×10^30kg (or 2.0e30kg).
M = 2.0e30 X 1500000000000
= 3.0e42kg
E = mc^2
= 3.0e42 X 299792458^2
= 2.69626554e59/10^44
= 2.696265540000000 PETAFOE (2696265540000000 FOE)
Final Results
The Returners messege across the supermembrane = 2.696 PETAFOE
If that's not enough, there are also other intelligent lifeforms in other great universes who can achieve this level of energy output, as well as natural forces.