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Astronomy 141
Life in the Universe
Prof. Scott Gaudi

Lecture 2: Death in the Universe

Key Ideas

The Universe is Expanding
The Universe is Accelerating
Milky Way will collide with Andromeda
No other galaxies will be visible
Five Ages of the Universe:
--Primordial Era - big bang -> 106 years
--Stelliferous Era - 106 years -> 1014 years
--Degenerate Era - 1014 years -> 1045 years
--Black Hole Era - 1045 years -> 10100 years
--Dark Era - >10100 years
After the dark era, life likely becomes impossible.

Death on Earth

After ~12.4 billion years

Sun ends up as a White Dwarf with R~Rearth
Mass loss over, the remaining planets settle into their final orbits:
--Venus at 1.34 AU
--Earth at 1.85 AU
--Mars at 2.8 AU
Earth has lost its
Sun as white dwarf begins to slowly cool and fade away into a long night...

Cosmic Collisions

In a few billion years, the Milky Way will collide with Andromeda.
Will form one big Galaxy.

The Empty Empty Sky

As the Universe expands:
--Expansion continues forever at a faster rate
--Space between galaxy clusters widens
--Universe cools down at a faster rate
The "Cosmological Horizon"
--Distance beyond which we can't observe due to finite speed of light
--Nearest galaxies not bound to local group will disappear after ~1011 years
--The sky will be devoid of Galaxies

A Dying Universe

Five Ages of the Universe
Primordial Era - big bang -> 106 years
Stelliferous Era - 106 years -> 1014 years
Degenerate Era - 1014 years -> 1045 years
Black Hole Era - 1045 years -> 10100 years
Dark Era - >10100 years

Stelliferous Era: 106 years -> 1014 years

Encompasses the present day
--Star formation continues
--Only stars with M>0.8M? have had time to use up their hydrogen
Future evolution
--Star formation will eventually cease
--Low-mass stars will dominate

End of Star Formation

After t=1014 years:
Successively more matter is locked up in stellar remnants, depleting the free gas reserves.
Cycle of star birth & death is broken:
Nuclear fuel is exhausted
Red dwarfs burn out as low-mass white dwarfs
Remaining matter is locked up in black dwarfs, cold neutron stars, and black holes
The last stars fade into a long night...

The Littlest Stars

Smallest mass star capable of fusing hydrogen is ~0.08 Solar Masses
These stars have a lifetimes of 1013 years

End of Life?

Without nuclear energy generation in stars, sustaining life is more difficult
Still some hydrogen, so fusion still possible
Could 'harvest' brown dwarfs

Degenerate Era: 1014 years -> 1045 years

Solar System "Evaporation"

After t=1015 years:
Gravitational encounters between stars are rare, but disrupt orbiting systems:
--Planetary systems get disrupted by stellar encounters and their planets scattered.
--Wide binary systems are broken apart.
--Close binary stars coalesce into single remnants.

Dissolution of Galaxies

After t=1019 years:
Stellar remnants within galaxies interact over many many orbits.
Some stars gain energy from the interaction and ~90% get ejected from the galaxy.
After t=1024 years
Others lose energy and sink towards the center.
After t=1030 years
The last 10% of remnants coalesce into Supermassive Black Holes.

Dissolution of Matter?

After t=1032-45 years: Some particle models predict that protons are unstable. Protons decay into electrons, positrons, to neutrinos. All matter not in Black Holes comes apart.

End of Life?

Without nuclear energy generation, life as we know it becomes effectively impossible
Only sources of energy are extremely weak background radiation and gravity
Would have to learn how to harvest gravitational energy

Black Hole Era: 1045 years -> 10100 years

Only black holes left
Two kinds
--Supermassive black hole in the center of the Galaxy
--Stellar-mass black holes

Evaporation of Black Holes

After t=1067 years:
--Remaining stellar-mass black holes evaporate by emitting particles via Hawking Radiation.
After t=10100 years:
--Supermassive Black Holes evaporate one-by-one.
End of the epoch of organized matter

Dark Era: >10100 years

After black holes have all evaporated:
--Universe continues to cool off towards a Radiation Temperature of absolute zero.
--Only matter is a thin, formless gas of electrons, positrons, neutrinos.
--Only radiation is a few increasingly redshifted photons.

The end is cold, dark, and disordered...

End of Life?

Everything is at the same temperature and same entropy
According to second law of thermodynamics, there are then no sources of energy

Life becomes effectively impossible.

The End

See A Note about Graphics to learn why the graphics shown in the lectures are generally not reproduced with these notes.

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