Plip.

And that was it. The last hydrogen atoms in the universe had fused, deep in the heart of the red dwarf, warming it one final millidegree. From here on out, the heat in the star would slowly drain to the all-welcoming entropy of the cooling universe, until even the most blue-shifted of observers would see nothing but endless black across all of space.

True, there was no actual sound in the moment, but observing the event, Luna reached deep into her ancient memories, to a time when there were planets and atmospheres, and living things that experienced phenomena through crude, biological mechanisms. In those nearly-forgotten corners of her mind, she recalled tiny bright specks settling into dark waters and disappearing into the depths. She remembered they went "plip."

"Plip." She thought again, this time with intent, with direction. The thought, some seconds and many eons later, reached some vaguely tangled wisps of atoms that constituted what Luna still thought of as her sister. At some point in deep time, they'd had bodies, they'd been creatures like the near-infinite quadrillions of sapients that had come before and since. But here, in the final winter of all things, minds (such as they were) had to spread across regions larger than galaxies just to find enough energy to even be recognizable as anything more than entropy.

By the time the thought had reached Celestia, that last red dwarf had cooled enough that those biological ponies of old could've trotted on its now solid surface. Celestia perked up as the message wound its way slowly through her nebulae. It took even longer for her to understand it, sluggish as she was. She'd let Luna occupy the lower-entropy superclusters near that last star, while she resigned her own self to spreading even thinner amongst the decrepit matter where even the protons were falling apart.

With the thought came her own recollection. Over millions of years, particles light years apart slowly bumped into each other. Those moved, and bumped others, sometimes interacting, sometimes not. After a few billion years, some slight macroscopic effects could be noticed, much like the charge in a single neuron can grow only after millions of individual atoms do their work. And, much like those neurons, it takes that, a billion times over to construct even a single thought between ear and mind. When the ear is several gigaparsecs away, it takes even longer. But even so, as the eons wound on, Celestia slowly remembered.

[hr]

They'd camped on the shore by the still lake after another seemingly endless day of walking. Both of the sisters were hungry, but they had no food, and precious little else, as they'd had to flee when their home had been attacked, and they'd lost their parents. As the cold of nightfall settled in on the newly orphaned ponies, Celestia had built a modest fire there on the shore. Most of the details were fuzzy with time (and space) save for the very end of the night, the end of a conversation as the two had lain by the fire.

"Tia," Luna had said. "I don't want to be alone out here."

"You're not alone! You have me, silly."

Luna had sniffled. "But what if you go away too?"

"Oh Luna, I would never leave you!"

"You promise?" Luna said, shivering.

Celestia moved a bit, curling up behind her sister, and nudging her in closer to the dying embers. "I promise. We'll be together forever and ever."

"How long is that?"

Celestia chuckled, then grabbed a cloud of embers from the fire with her magic, and scattered them through the air over the lake in front of them. "Until all the stars in the sky burn out... and then some!"

Luna watched in wonder as the hundreds of little specks of red light slowly settled and drifted down toward the dark water.

Celestia leaned in toward her sister's ear, and as each little spark of light touched the water and went out, she whispered. "Plip... plip... plip."

When the last of the embers had died, and the final ripple faded, what was left was a reflection of tens of millions of little points of light, gleaming down from the heavens.

The two sat in silence for a moment, snuggling close against the cold, before Luna finally broke the spell with a happy realization.

"That's a lot of plips."