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Galaxy NGC4414 from HST Astronomy 162:
Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30

Lecture 39:
"This is the way the world ends..."
The Fate of the Universe

Readings: Ch 28, Section 28-7

Key Ideas

Matter-Dominated Universes:
High-density: expansion stops and collapses ("Big Crunch")
Low-density or Flat: expands forever ("Big Chill")

Cosmological Constant
Evidence from Supernova distances
Suggests we live in a spatially flat, accelerating, infinite Universe

The Fate of an Accelerating Universe
Expands forever at an ever-increasing rate
Ends in a cold, dark, distordered state

Matter-Dominated Universes

Theses are the simplest class of Universe models to consider.

Their futures depend on the density of the matter within them:

High-Density:

Low-Density or Flat:

The idea is simple: "Density is Destiny" in such a Universe. All we have to do is measure the matter density, and we know the fate of the Universe (in outline, if not in detail).


What is the matter density?

There are three sources of matter and energy density:

Baryonic Matter: (gas and stars)

Radiation: (photons)

Dark Matter:

Adding these all up gives: Wm = 0.2 - 0.4


Expansion Forever?

If this really were a matter-dominated Universe, then This means W0 < 1:

Future: The Universe would expand forever and a steadily decreasing rate.


What about L?

If, however, there is a cosmological contant, the density parameter W0 becomes:
Density Parameter with matter and Lambda
where:
Wm = Density of Matter & Energy
WL = Density of the Vacuum Energy

Now the fate is tied to knowing both the density and the cosmological constant (the vacuum energy of the Universe).

Density is no longer Destiny...


What does WL do?

If WL = 0, matter slows the expansion rate:

If WL is large enough the expansion will accelerate:

To measure this, you need to make a Hubble Diagram (plot of recession velocity versus distance) for very distant galaxies in deep space, where we look back to when the Universe was younger.

Effect of Lambda in a Hubble Diagram

Distant Type Ia Supernovae

Type Ia Supernovae are excellent standard candles:

A number of projects are underway to search for and characterize distant Type Ia SNs.

Hubble diagrams created from the SNIa results show compelling evidence of accelerated expansion of the Universe:


The Accelerating Universe

The SNIa results combined with constraints from complementary observations of details of the cosmic microwave background and galaxy clusters give the following numbers:
Wm = 0.3 ± 0.1
WL = 0.7 ± 0.1
Taken together, they give W0 = 1, with a range of about 10-15%.

We live in a spatially flat, accelerating, infinite Universe.


The Once & Future Universe

As the Universe expands:

The details of the future Universe depend upon:


Epoch of Star Formation

The Present Time (t = 14 Gyr): Some fraction of the mass cycled into stars, however, gets locked away in stellar remnants:

End of Star Formation

After t=1012 years:

Successively more matter gets locked up in stellar remnants, depleting the free gas reserves.

The Cycle of star birth, death, and birth is broken:

The last stars fade into a long night...


Solar System "Evaporation"

After t=1017 years:

Gravitational encounters between stars are rare, but when they do happen, they can disrupt orbiting systems:

While rare, the thing to remember is that an eternally expanding Universe has plenty of time to wait for rare occurances ("rare" doesn't mean "never").


Dissolution of Galaxies

After t=1027 years:

These coalesce into a Supermassive Black Holes at the center of what was once a galaxy.

The bigger the galaxy, the bigger the black hole.


Dissolution of Matter?

After t=1032 years: Current experimental limits on the proton decay time may be much larger than 1032 years, but the Universe has plenty of time...

Evaporation of Black Holes

After t=1067 years:

After t=10100 years:

Marks the end of the epoch of organized matter.


The Big Chill

After the black holes have all evaporated:

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


Possible Fates of the Universe

On that note, I'll give the last word to the obligatory poets...
"Some say the world will end in fire.
Some say in ice.
From what I've tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice."

Robert Frost, Fire and Ice (Harper's Magazine, Dec 1920)

"This is the way the world ends
This is the way the world ends
This is the way the world ends
Not with a bang but a whimper."

T.S. Eliot, The Hollow Men (1924)


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Updated: 2006 February 25
Copyright Richard W. Pogge, All Rights Reserved.