Lecture 24: The end of the UniverseAstronomy 111

The end of the UniverseThe end of the Universe

• What is the ultimate fate of our Universe?

• Depends on how gravity affects matter in the Universein the Universe– Shape of the Universe (density)

Content of the Universe (dark matter)– Content of the Universe (dark matter)– Nature of the Universe (dark energy)

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Critical densityCritical density

• All galaxies attract each other via gravity.– Gravitational attraction slows the expansion.p

• How it behaves depends on the density:– High Density: Expansion slows, stops, & g y p , p ,

reverses.– Low Density: Keeps expanding forever.

• Dividing Line = “Critical Density”

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Density parameter: Density parameter:

average density of Universe

i i l d icritical density

• >1: High Density “Closed” Universe• =1: Critical Density “Open” Universe• =1: Critical Density Open Universe• <1: Low Density “Open” Universe

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2-D examples of curved spaces2-D examples of curved spaces

Closed Openp

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Flat

What is the value of ?What is the value of ?

Observers: • Best Estimate is =0.10.3 (or higher)Best Estimate is 0.1 0.3 (or higher)

– Hard to measure slowing of expansion rate– Galaxies evolve in an (as yet) unknown way( y ) y– Problem of accounting for Dark Matter

Theorists:Theorists:• (Some) would like =1, so introduce 0

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Closed Universe: >1Closed Universe: >1

• Gravity of all matter is enough to eventually overcome the expansion of the U iUniverse:– Expansion slows to a maximum size & stops.

Universe re collapses:• Universe re-collapses:– Galaxies get closer together– Get a cosmological blueshiftGet a cosmological blueshift– Universe grows hotter & denser– Collapses in the Big Crunch

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A phoenix from the ashes?A phoenix from the ashes?

• After the Big Crunch, what then?– Nothing. Or,Nothing. Or,– Another Big Bang creates a new Universe.

• Second Law of Thermodynamics:• Second Law of Thermodynamics:– Entropy increases in a closed system.

Next Big Bang starts with greater entropy– Next Big Bang starts with greater entropy.– Will expand for longer than previous one.

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Fate of a closed UniverseFate of a closed Universees

Gal

axie

etw

een

ClosedO ill ti ?an

ce B

e

>1 Oscillating?

Time

Dis

ta

~100 Billion Years

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100 Billion Years

Open (and flat) universes: <1 or =1<1 or =1

• Universe keeps expanding forever.• Gravity slows the expansion a little:Gravity slows the expansion a little:

– Lower the density, the less the expansion slowsslows.

– A Critical Density Universe slows to a stop at infinite time.at infinite time.

– A Sub-critical Density Universe approaches a constant speed at infinite

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pp ptime.

Evolution of an open UniverseEvolution of an open Universe

• As the Universe expands:– Space between galaxy clusters widens.Space between galaxy clusters widens.– Universe steadily cools down.– Expansion continues foreverExpansion continues forever.

• Details depend on:Stellar evolution– Stellar evolution

– Quantum mechanics

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Star formationStar formation

• Present time (t 1010 yrs):– Most stars are metal rich, and make moreMost stars are metal rich, and make more

metals ejected in supernova explosions.– Next generation starts with a little less g

Hydrogen and more metals.– Some fraction of the star’s matter is locked

up in stellar remnants: white dwarfs, neutron stars & black holes.

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End of star formationEnd of star formation

• t=1014 years:– Successively more matter is locked up in y p

stellar remnants, depleting the free gas reserves.C l f bi h d d h i b k– Cycle of star birth and death is broken.

– Nuclear fuel is exhausted.R d d f b t l hit d f– Red dwarfs burn out as low-mass white dwarfs

– Remaining matter is locked up in black dwarfs, cold neutron stars and black holes

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cold neutron stars, and black holes.

Solar system “evaporation”Solar system evaporation

• t=1017 years:– Weak gravitational encounters between

stars are rare, but slowly disrupt orbiting systems:Planetary systems get disrupted by stellar– Planetary systems get disrupted by stellar encounters and their planets scattered.

– Wide binary systems are broken apartWide binary systems are broken apart.– Close binary stars coalesce into single

remnants.

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Dissolution of galaxiesDissolution of galaxies

• t=1019 years:– Stellar remnants within galaxies interact over g

many many orbits.– Some stars gain energy from the interaction

d 90% j d f h land ~90% get ejected from the galaxy.– Others lose energy and sink towards the

centercenter.– These coalesce into a Supermassive Black

Hole.

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Hole.

Dissolution of matter?Dissolution of matter?

• t=1032 years:– Some GUTs theories predict that protonsSome GUTs theories predict that protons

are unstable.– Protons decay into electrons, positrons, to y , p ,

neutrinos.– All matter not in Black Holes comes apart.– Current experimental limits on the proton

decay time may be much larger than 1032

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years.

Evaporation of black holesEvaporation of black holes

• t=10100 years:– Black Holes slowly evaporate by emitting y p y g

particles and photons via Hawking Radiation.– Supermassive Black Holes evaporate

l l b i l fi l kcompletely one-by-one in a last final weak flash of gamma rays in ~10100 years.

• The end of the epoch of organized matter

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The Big ChillThe Big Chill

• After black holes all evaporate:– Increasingly redshifted photons.Increasingly redshifted photons.– Universe continues to cool off towards a

radiation temperature of absolute zero.p– Only matter is a thin, formless gas of

electrons, positrons, neutrinos.

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

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

Fate of an open UniverseFate of an open Universees

Very Open

1

Gal

axie Critical/Open

=1<<1

etw

een 1

ance

Be

Time

Dis

ta

~100 Billion Years

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100 Billion Years

Possible fates of the UniversePossible fates of the Universees

Very Open

1

Gal

axie Critical/Open

=1<<1

etw

een 1

ClosedO ill ti ?an

ce B

e

>1 Oscillating?

Time

Dis

ta

~100 Billion Years

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100 Billion Years

Effects of Dark Energy and Dark Matterand Dark Matter

• Dark Matter and Dark Energy are not well understood, but are major constituents of th U ithe Universe– Many experiments underway to characterize

bothboth• LHC• HETDEX

S• DES• LSST• Others: JDEM, BigBOSS

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g

Effects of Dark Energy and Dark Matterand Dark Matter

• Both can alter the fate of the Universe

• Dark Energy counteracts gravity th h i ithough, increasing the expansion

Probably makes the– Probably makes the Universe more likely to be open

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The role of Dark MatterThe role of Dark Matter

• How much matter exists in the Universe also affects the densitythe density– Both light and dark

• We now think that 95% of• We now think that 95% of matter in the Universe is dark!– We still don’t know what it

is—ongoing research in ti l h i d

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particle physics and astronomy

The role of Dark EnergyThe role of Dark Energy

• Recent experiments have shown that 74% of the Universe is composed of “Dark Energy”

Wh t i it?– What is it? – How do we measure it?– What are its effects on the– What are its effects on the

Universe?– Another subject of modern

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research

The new Big Bang modelThe new Big Bang model

• The Universe is (nearly) flat– No spacetime curvatureNo spacetime curvature

• The Universe is acceleratingThe expansion rate in the of the Universe– The expansion rate in the of the Universe is increasing

• The Universe is made largely of Dark• The Universe is made largely of Dark Matter and Dark Energy

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Solving the mystery of Dark EnergyDark Energy

• Recent observations of distant supernovae show that the Universe is pexpanding at an increasing rate– Unexpected!Unexpected!

• Changes our understanding of how the• Changes our understanding of how the Universe is evolving

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Solving the mystery of Dark EnergyDark Energy

• What is the connection between dark energy, gravity and the fate of the universe?universe?

• What is dark energy, and how does it affect us?– Are there mysterious particles permeating

space?– Can they be useful to us in some way?– Can they be useful to us in some way?

• Ongoing experiments are now trying hard

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Ongoing experiments are now trying hard to answer these questions…

HETDEXHETDEX

• Hobby-EberlyTelescope Dark Energy Experiment atEnergy Experiment at McDonald Observatory

• Maps large area of the sky spectroscopicallyp p y– Look for distant

galaxies and measure size of Universe in the

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past

DESDES

• Dark Energy Survey• World’s largest digital

camera to go on thecamera to go on the CTIO 4 m telescope in Chile next year

• Large area imaging• Large area imaging survey of the sky to search for supernova, study weak lensingstudy weak lensing, measure size of Universe, study galaxy clusters

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galaxy clusters

LSSTLSST

• Large Synoptic Survey Telescope

• 8 4m telescope to8.4m telescope to image entire sky every three nights!

• Look for supernovae• Look for supernovae, study weak lensing, measure size of Universe studyUniverse, study galaxy clusters– Even earth-killer

asteroids!

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asteroids!

SummarySummary

• The Fate of the Universe depends on the density of matter.y

• Closed Universe:– Enough matter to stop the expansionEnough matter to stop the expansion– Collapses in a “Big Crunch”

• Open Universe:• Open Universe:– Expands forever

Ends in a cold disordered state

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– Ends in a cold, disordered state.

SummarySummary

• The Universe is composed of mainly Dark Matter and Dark Energygy

• We live in a flat Universe that will expand foreverexpand forever

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