Cosmology

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# Cosmology - PowerPoint PPT Presentation

Cosmology. What is Cosmology?. Stellar Systems Stellar Clusters Galaxies Galaxy Groups and Clusters Galaxy Superclusters The Universe. Study of the universe as a whole Its past, present, and future Is the Universe finite or infinite? Did it have a beginning? Will it ever end?

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Cosmology

What is Cosmology?

• Stellar Systems
• Stellar Clusters
• Galaxies
• Galaxy Groups and Clusters
• Galaxy Superclusters
• The Universe
• Study of the universe as a whole
• Its past, present, and future
• Is the Universe finite or infinite?
• Did it have a beginning?
• Will it ever end?
• Where did everything come from?

The Shape of the Universe

• In Einstein’s General Theory of Relativity, space and time can be curved
• The amount of matter and itsmotion affects the curvature ofspace-time
• If there’s a lot, then theuniverse is like a sphere andfinite: “Closed”
• If there’s less than that, it’sinfinite: “Open”
• If there’s just the right amount,it’s infinite: “Flat”

The Ultimate Number: 

• The density, denoted , is the amount of stuff in the Universe
• The critical densityc, is the amount of stuff you need to make the universe flat
• The ratio is called :(aka 0)
• If  > 1, universe is closed andfinite
• If  < 1, universe is open andinfinite
• If  = 1, universe is flat andinfinite

What is ? First Attempt

• Add up contribution from everything we know of:

StuffCont. to 

Stars 0.012

Gas, Dust 0.038

Dark Matter 0.250

Neutrinos 0.001

Total 0.301

• Total  < 1*
• Universe is Open*
• Universe is Infinite*

*It’s lies, all lies

Hubble Expansion Revisited

• Universe is expanding
• In the past, everything was together
• When was everything together?
• Will it expand forever?

Age of the Universe: Round 1

• To find age of universe, find how long ago any galaxy left us
• Let its distance be d
• Hubble’s Law gives us the velocity:v = H0d
• If you move at constant speed, then: d = vt0
• Solve for t0:
• Compare: oldestknown stars

What’s wrong with this computation?

A) You assumed constant speed

B) You ignored peculiar velocities

C) Both A and B

D) There’s nothing wrong, this is correct

Age of the Universe: Round 2

• We assumed the Universe always expanded at a constant rate
• But there is gravity
• Gravity slows things down
• So the universe was expanding faster in the past
• Which means it should be younger than we previously calculated
• The formula gets alittle more complicated:
• If = 0.30, t0 = 11.7 Gyr
• If  = 1, t0 = 9.6 Gyr
• Younger than the oldest stars: The age problem

Age of the Universe: Round 2

Now

• If you ignore gravity, you get one age
• When you include it, universe is younger

No gravity

distance

Matter,  = 1

t = 9.6 Gyr

t = 14.4 Gyr

Measuring Deceleration

• White dwarf supernovae are so bright you can see them more than half way across the universe
• You are seeing the universe at an earlier era
• Universe may have been expanding at a faster rate
• Should be able to see this
• How you do this:
• Measure distance to very distant WD supernovae
• Compare to Hubble’s Law
• Fit to curve for various 
• Drum roll please . . .

And the Results Are In

• No model (with only ordinary matter) worked
• It looked like universe has recently been accelerating
• Best fit: Universe decelerated early on, but now accelerating

m = 0.0,  = 0.0

m = 0.3,  = 0.0

m = 1.0,  = 0.0

m = 0.3,  = 0.7

Dark Energy

• Universe is currently accelerating
• This is impossible with ordinary matter or dark matter
• There must be another unknown contribution to the universe’s gravity
• This mysterious stuff is called “Dark Energy”
• We know almost nothing about Dark Energy
• But dominant view is that it is
• Cosmological Constant
• Also known as Vacuum Energy Density

Vacuum Energy Density

• Best guess for nature of dark energy is vacuum energy density
• Particle physics: A combination of relativity and quantum mechanics
• Predicts you can create particles and anti-particles out of nothingness
• For a brief time
• For this reason, “empty” space (the vacuum) has energy
• We don’t know how to calculate how much
• Vacuum energy density predicted to have repulsive gravity
• Empty space repels itself – causes expansion

What is ? Second Attempt

• Add up contribution from everything we know of:

StuffCont. to 

Stars 0.012

Gas, Dust 0.038

Dark Matter 0.250

Neutrinos 0.001

Matter Subtotal 0.301

Dark Energy 0.701

Total 1.002

• Total  = 1, or close*
• Universe is Flat, or close*
• Universe is Very Large or Infinite*

*No longer lies

Age of the Universe: Round 3

• At present, universal expansion is accelerating
• Vacuum energy density is the most important contribution
• Therefore, in the past, the expansion was slower
• Early on, universal expansion was decelerating
• Matter had a high concentration and was most important
• Therefore, early on, the expansion was faster
• By a numerical coincidence, the average speed is almost exactly the current speed
• Our first estimate of age was pretty close: t0 = 13.8 Gyr
• Oldest stars: 13 Gyr
• No age problem

Age of the Universe: Round 3

Now

Matter & Energym = 0.3e = 0.7

No gravity

distance

Matter,  = 1

t = 13.8 Gyr

The Standard Cosmological Model

• The standard cosmological model: CDM
• Big bang started it all
• The universe is formed from:
• Dark energy ()
• Cold (slowly moving) dark matter (CDM)
• Ordinary matter
• The total density parameter is  = 1 (or close to 1)
• All structure grew from tiny density perturbations
• With a simply described distribution

Composition of the Universe

H0 = 21 km/s/Mly

t0 = 13.8 Gyr

 = 1

The Future of Expansion

• “Vacuum” is the biggest part of the mass density now
• As the universe expands, you have more vacuum, but no more matter – vacuum will be forever the dominant part
• Vacuum causes universe to accelerate its expansion
• Universe should expand exponentially – forever
• Are we sure?
• No – recall, we don’t know what the dark energy is
• If it behaves differently, there are other possibilities
• Modest eternal growth
• Big Crunch
• Big Rip

Cosmic Eschatology – End Times

• Giant Asteroids
• Global Warming
• Death of Sun
• Isolated Universe
• Super Galaxies
• No New Stars
• Stars Die
• Galaxies Evaporate
• Matter Decays
• Black Holes Decay
• What events will destroy life on Earth?
• Can we survive the destruction of theEarth?
• How long can we live around other stars?
• How long will energy continue to beproduced somewhere in the universe?
• Current age – 13.8 Gyr = 1.381010 yr
• Recall: Universe in era of eternalexpansion

Act 1: Destruction of Life on Earth

• 100 Myr = 108 yr – Giant Asteroid impact
• Probably can deflect it with minimal improvement in technology
• 1 Gyr = 109yr – Runaway Global Warming
• Sun is gradually getting brighter
• If it gets too bright, Earth gets like Venus
• Move Earth (?) or leave for space
• 5 Gyr = 5109 yr – Sun becomes Red Giant
• Earth melts, all life eradicated
• Leave Earth for other stars
• Giant Asteroids
• Global Warming
• Death of Sun
• Isolated Universe
• Super Galaxies
• No New Stars
• Stars Die
• Galaxies Evaporate
• Matter Decays
• Black Holes Decay

Act 2: Galaxies Merge or Separate

• Giant Asteroids
• Global Warming
• Death of Sun
• Isolated Universe
• Super Galaxies
• No New Stars
• Stars Die
• Galaxies Evaporate
• Matter Decays
• Black Holes Decay
• 1011 yr – Superclusters or clusters separate from each other

Other

Supercluster

Other

Supercluster

Virgo

Supercluster

SuperGalaxy

Other

Supercluster

Other

Supercluster

• 1012 yr – All galaxies in a cluster or supercluster merge into super-galaxies

Act 3: The Stars Die

• Giant Asteroids
• Global Warming
• Death of Sun
• Isolated Universe
• Super Galaxies
• No New Stars
• Stars Die
• Galaxies Evaporate
• Matter Decays
• Black Holes Decay
• 1014 yr – No gas left for making stars
• Will have to switch to long lived stars
• 1014 yr later – Low mass stars die
• Low mass stars much dimmer
• They burn fuel more efficiently

What remains – White Dwarfs, Neutron Stars, Black Holes, and Brown Dwarfs

Act 4: The Galaxies Die

• Giant Asteroids
• Global Warming
• Death of Sun
• Isolated Universe
• Super Galaxies
• No New Stars
• Stars Die
• Galaxies Evaporate
• Matter Decays
• Black Holes Decay
• 1019 yr – Near collisions of dead stars cause some to get thrown out of galaxy, others thrown into center
• Black hole at center grows, gets super big
• Galaxies disappear

Act 5: Matter Dies

• Giant Asteroids
• Global Warming
• Death of Sun
• Isolated Universe
• Super Galaxies
• No New Stars
• Stars Die
• Galaxies Evaporate
• Matter Decays
• Black Holes Decay
• 1034 - 1046 yr – Protons and neutrons decay
• Speculative, but most theorists agree

What remains – Small and large black holes, isolated electrons and anti-electrons

Act 6: Black Holes Die

• Giant Asteroids
• Global Warming
• Death of Sun
• Isolated Universe
• Super Galaxies
• No New Stars
• Stars Die
• Galaxies Evaporate
• Matter Decays
• Black Holes Decay
• According to Quantum mechanics, black holes don’t last forever, they spontaneously emit radiation – very slowly
• 1060 years – Stellar black holes evaporate
• 1099 years – Galactic black holes evaporate

What remains – Isolated electrons and anti-electrons in a super-cold universe