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~R_earth

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=10^32-45 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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