Cosmology

CosmologyWhat 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 UniverseIn 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

c

What is ? First Attempt

*It’s lies, all lies

Stuff Cont. to Stars 0.012Gas, Dust 0.038Dark Matter 0.250Neutrinos 0.001Total 0.301

•Add up contribution from everything we know of:

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

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

Hubble Expansion Revisited

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•About 13 Gyr

0

dt

v

0

d

H d

0

1

H 14.42 Gyr

What’s wrong with this computation?A) You assumed constant speedB) You ignored peculiar velocitiesC) Both A and BD) 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

00

1t

H 1 1 1

1 ln1 1

Now

dist

ance

t = 14.4 Gyr

No gravity

Matter, = 1

Age of the Universe: Round 2

t = 9.6 Gyr

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

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

m = 0.0, = 0.0

m = 0.3, = 0.0

m = 1.0, = 0.0

m = 0.3, = 0.7

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

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

*No longer lies

Stuff Cont. to Stars 0.012Gas, Dust 0.038Dark Matter 0.250Neutrinos 0.001Matter Subtotal 0.301Dark Energy 0.701Total 1.002

•Add up contribution from everything we know of:

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

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

Nowdi

stan

ce

t = 13.8 Gyr

No gravity

Matter, = 1

Matter & Energym = 0.3e = 0.7

Age of the Universe: Round 3

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•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 = 1.381010 yr

Recall: Universe in era of eternalexpansion

•Giant Asteroids•Global Warming•Death of Sun•Isolated Universe•Super Galaxies•No New Stars•Stars Die•Galaxies Evaporate•Matter Decays•Black Holes Decay

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 = 5109 yr – Sun becomes Red Giant

•Earth melts, all life eradicated•Leave Earth for other stars



Act 2: Galaxies Merge or Separate•1011 yr – Superclusters or clusters separate from each other

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

VirgoSupercluster

OtherSupercluster

OtherSupercluster

OtherSupercluster

OtherSupercluster

SuperGalaxy

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

•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

•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

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


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Cosmology