What would be the most time/energy efficient way of converting entire galaxy into unusable heat ?
Expose it to the big bang of another universe.
If you subscribe to multiverse theory, you can search through the other universes until you find one that is in the process of having its Big Bang moment. If you can open a gateway in between these two universes, I would think that the energy involved in the rapid expansion of a brand-new universe would handily wipe out a comparatively tiny galaxy.
(I leave the creation of said gateway -- of a size comparable to the target galaxy and able to be held open in the face of the forces passing through it -- as an exercise to the reader.)
If you are thinking of someone intentionally destroying a galaxy as a plot device, this is far beyond having any possible real scientific basis, even if you imagine almost any possible future technology.
So I think it becomes a question of what explanation might sound fairly plausible in a story?
Your story could have a premise that the universe is not what it seems in some fundamental way. Therefore the basic laws of nature don't apply. "The whole universe is a computer simulation" is an example of one such scenario, albeit a very tired one.
An organism that eats matter and poops energy could eat the galaxy. You could have really small, viral organisms that proliferate rapidly, or one mega-organism that eats the whole thing. In either case, faster than light travel would be necessary for this to happen in a reasonable amount of time, since galaxies are at least tens of thousands of light years across. All sorts of other science would go out the window, too. But it might be a cool story nonetheless.
- Something based on multiverse theory might be interesting. Multiverse theory is highly speculative, with a lot of variety, and few people really understand it well, so it won't be so easy to challenge your premise. Plus, it opens up interesting possibilities. What if you just figure out a way to move people, a planet, etc., to a different universe where that galaxy doesn't exist? Or you could come up with a way to combine two universes, and they both have galaxies in the same spot, causing mayhem (galaxies are sparsely distributed overall, so most of the rest of them survive).
The most efficient way to get rid of baryonic matter is to have it meet its counterpart - antimatter. Source.
So the answer would be: Collide it with an anti-galaxy. It's immediate, and completely efficient (100% energy conversion rate.)
Just some numbers to consider:
- 1 kilogram of antimatter joined by an equivalent amount of normal matter produces roughly 1.7974×1017 joules, the equivalent of 42.96 megatons of destructive force.
- The total mass of the Milky Way is estimated to be 1011 solar masses.
- That converts to 1.988435×1041 kg (kilograms).
- The total energy of a collision with an anti-Milky Way would be 8.54231676×1042 megatons (8,542,316,760,000,000,000,000,000,000,000,000,000,000,000).
That would make for quite pretty fireworks if you're in Andromeda.
Now, how would you grab and move around an anti-galaxy? As kindly mentioned by @TheNate in the comments, how to align all particles and prevent some from sneaking out? To that, I reply - very good topics for another question, indeed. ;)
Assuming no magic, there is no efficient method. These ones came to my mind:
- Collapsing a star (or the galactic black hole), by adding mass;
- Removing mass from a star, placing carefully positioned masses nearby to pull it by gravity);
- Doing pinball with stars, making them collide and destroy themselves by interference.
All these methods require much more energy than the stars themselves provide, and are by nature slow (a few million years, give or take).
Most efficient (active) method not invoking an alteration of the current assumed physical laws: (which includes acts of God)
Cut off any potential users. Without users who have access to that galaxy, there is no usable heat. You just need to kill or banish all life that could use it.
(That could actually use the Berzerkers, straight, come to think of it.)
To become the ultimate evil genius, you need a few things.
You need to create your own race of Beserkers that have a loftier goal than simply killing all life, they must kill the stars, and everything else too.
1) On arrival at a new star system, first clone the Berserker and send it to the next star system.
2) Create a large quantity of antimatter. A large solar collector that supplies energy to a super-collider can generate anti-matter. You will need a lot of it. Once your have collected enough anti-matter, create an implosion that makes the black hole you need to drop into the star. Drop it into the star -- it will take a while to actually collapse the star.
3) Gather up all the planets, asteroids, Kuiper-belt object, and Oort cloud. Drop all of the bits into the star that is collapsing. Ideally you allow scavenge all of the interstellar gas in the vicinity and drop this into the star too. This step is recommended, but very difficult to actually accomplish.
4) Proceed to the next system, or if your Beserker is at its final system throw itself into the collapsing star.
5) Be patient. You have not actually caused the heat death of a galaxy, but you have given it a good head start. Not much is heat is usable, but you have not actually caused a true heat death.
True heat death is not possible as long as the cosmic microwave background (CMB) radiation is warmer than your black holes. In fact your black holes are colder than the CMB and will thus actually grow until the CMB cools off further. Then your black holes can start undergoing Hawking evaporation for untold billions of years and then go off with a spectacular bang. Wait for heat to even out again.
The black hole at the center is around 4.1 million solar masses which corresponds to a lifespan of 1.4e87 years. You need to be very patient if you want true heat death.
In a Greg Egan novel, the scientists create a new state of vacuum that grows and destroys everything (eventually). In real physics, tunneling to a lower vacuum state is a real possibility, and this would destroy the galaxy (initially).
So, do something like that, but build-in a way to have it peter out with some half-life so it doesn't destroy the whole universe. One idea: surface area has a maximum due to holographic principle and then the rest of the universe can re-assert dominance and makes the bubble change state again.
Another Greg Egan novel, Diaspora, had an event energetic enough to kill everything in the galaxy.
James Blish ended things by colliding with another dimension, annialating every atom with its counterpart.
One famous series has photino birds, dark-matter life forms, age all the stars in a few million years rather than billions.
Another famous series has terraforming bugs go out of control and turn all star systems into dyson swarms of algae, whether we were "using" it already or not.