u/Ghost-of-Carnot

If we follow the most commonly accepted theory for the "end of the universe", heat death, the universe will last for approximately 10^100 years after the big bang.

Given this, the stelliferous era is estimated to last up to 10^14 years, or 100 trillion years. From my understanding, that means that the habitable period of the universe should also last that long, or at least somewhere in the ballpark of that.

The thing is, we are currently at 13.8 billion years after the big bang. In other words, 13,800,000,000/100,000,000,000,000, or 0.0138% of the period that we believe that the universe will be habitable.

I understand that isn't an impossibly small percentage, but that seems unusually *soon*. How likely is it that we just so happened to evolve and pop into life at what is essentially the beginning of this period?

Could this indicate that perhaps the universe, or at least the period of habitability in the universe will end much sooner than we believe?

And yes, I know that we can't just make an assumption based on a likelihood- like if something has even the tiniest chance of happening, it probably will happen *somewhere*.

However, humanity has always assumed that we're special in some way, like being the center of the universe, and have been proven wrong time and time again. It seems a lot more likely to me that we'd be closer to an average in the universe than a crazy outlier like being the first 0.01% of life.

Our sun may have a few billion years left in its existence. So does planet Earth. And the universe may go on for trillions of years, or ever.

But Earth's habitability for carbon-based life may be curtailed at just over half a billion years.

The sun's luminosity grows about 10% every billion years. This on its own will trigger a run-away greenhouse effect in about a billion years leading to evaporation of our oceans (and the end of most life with it).

But well before this, a more subtle process will be occurring. The increased temperatures from the sun's rising lumonisity will accelerate silicate rock weathering - which leads to increased absorption of CO2 from the atmosphere into the soil and rock.

CO2 will effectively drain from the atmosphere to a point where photosynthesis is no longer possible. With the loss of most plants, oxygen will no longer be refreshed into the atmosphere. And life - at least as we know it - will not be sustained.

Note: There is active scientific debate on how long this process will take - and whether ancillary effects of loss of photosynthesis (such as a lower atmospheric pressure) may lengthen the runway for certain types of plants to continue to exist. But the long-term trend of loss of CO2 in the atmosphere to silicate weathering has been happening over geologic time and will continue to happen.

Mars might be the most off-Earth "hospitable" place for humans in the solar system. And yet it's really really inhospitable.

Mars looks alluringly like parts of the desert southwest of North America, with sunny days and sometimes blue skies. But photos belie the extreme inhospitability of the planet, and the fact that the pressure on Mars is for most intents and purposes smilar to that of outer space.

Lack of atmospheric pressure (it's well less than 1% of Earth's on Mars's surface) is perhaps the most immediate and consequential harsh condition for human life. Exposing our bodies to this pressure (which is essentially like exposing them to the full vacuum of space) would have some immediately terrible consequences, including:

Simultaneous vomiting, defecation, and urination as our pressurized insides push bodily fluids outward.

Boiling away of bodily surface liquids like saliva and eye fluid (water can't exist as a liquid at this pressure, which is a problem for water-based beings).

Extreme swelling of our bodies as dissolved gasses come out of solution and water-based liquids evaporate as internal pressure drops (again no liquid water at this pressure).

Deoxygenation of our blood as oxygen comes out solution, potentially permanently damaging our lungs and in any case leading to unconsciosness within seconds and death within minutes.

Theoretically it might be possible. Practically it seems ridiculous. 4 quadrillion is about 22,000 Mount Everests in mass.

For more reading: Making Mars Habitable