Will the Universe eventually exhaust its energy? This question, posed by Dennis O’Brien, invites us to contemplate our cosmic destiny, particularly the potential fate of a “big freeze,” where even black holes may evaporate. As dark energy is thought to expand uniformly with space, could it stabilize the Universe's temperature, or will it continue to plummet towards absolute zero?

Billions of years ago, the cosmos was a hotbed of stellar activity, with stars forming at an unprecedented rate. Energy release is fundamental for various processes, and that energy can be harnessed for interesting phenomena. For example, sunlight energizes chlorophyll, enabling photosynthesis in plants, while hydrothermal vents in the ocean provide energy in the absence of sunlight.

However, as time progresses, star formation rates have significantly declined, currently at just a fraction of what they were 11 billion years ago. The Universe continues to cool and expand, with the leftover radiation from the Big Bang shifting to longer wavelengths and lower temperatures, currently at about 2.725 K.

Stars, while still shining, are finite in their life spans. Massive stars will meet their end in spectacular supernovae, while less massive stars, like our Sun, will evolve into white dwarfs over billions of years.

As time passes, even neutron stars and white dwarfs will radiate away their energy, eventually fading from existence. After hundreds of trillions of years, the cosmic landscape will shift dramatically, with remnants left behind as gravitational interactions eject most objects from galaxies into interstellar space.

The glow from the Big Bang will diminish, leaving behind a Universe where atoms rest in their lowest energy states, with only three primary sources of energy remaining:

1. Gravitational radiation: As celestial bodies orbit one another, they emit energy through gravitational waves, which can lead to the eventual spiraling of planets into stellar remnants over vast timescales.
2. Black hole radiation: Black holes will absorb matter but will also decay through Hawking radiation, ultimately leading to their disappearance over time.

3. Dark energy: This enigmatic force appears to be a constant energy component of the Universe, necessary to explain its accelerated expansion. If dark energy behaves as a cosmological constant, it suggests that the Universe will not run out of energy, though this energy is not usable for work.

For energy to be harnessed, a transition from a higher-energy state to a lower-energy state is essential. In a perfectly uniform Universe, no such transitions exist, making energy extraction impossible.

The presence of dark energy implies a positive, finite zero-point energy state, yet it does not contribute to temperature. As the Universe expands, quanta decay and redshift, leading to a temperature that trends toward absolute zero.

Despite the daunting prospect of a big freeze, there is hope that dark energy may not represent the ultimate low-energy state. If it can transition to a lower-energy state, there could be potential for energy release.

Observatories like Euclid and Vera Rubin are set to measure the nature of dark energy, which may reveal whether it is a constant or evolving. While a big freeze seems likely, the future remains uncertain until we obtain critical measurements.

Send your Ask Ethan questions to startswithabang at gmail dot com!

Starts With A Bang is now featured on Forbes and republished on Medium after a 7-day delay. Ethan has authored two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.