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Lecture 29

Cosmology

December 12, 2018

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Structure of the Universe

• Does clustering of galaxies go on forever?

– Looked at very narrow regions of space to far

distances.

– On large scales the universe appears

homogeneous (smooth, same in all directions)

Figure 26.1, Chaisson and McMillan,

6th ed. Astronomy Today,

© 2008 Pearson Prentice Hall

Cosmological Principle

• The universe looks homogeneous on the

largest scales

• The universe is isotropic

• Both are assumptions about the universe

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Implications

• The universe can have no edge

• The universe can have no center

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Obler’s Paradox

• Assume

– cosmological principle

– universe is infinite

– universe is unchanging

in time.

• Results

– The entire sky should

be bright (BUT this is

not observed!!)

Figure 26.3, Chaisson and McMillan,

6th ed. Astronomy Today,

© 2008 Pearson Prentice Hall

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Motions of Stars

• Transverse (Proper) Motion

– Motion of star across the sky.

– Measured by observing

change in position of star over

a long period of time.

– Only detectable for nearby

stars.

• Radial Motion (Radial Velocity)

– Motion along the line of sight

– Measured using Doppler shift of spectral lines.

– Can measure for very distant stars

Figure 17.4, Chaisson and McMillan,

6th ed. Astronomy Today, © 2008 Pearson Prentice Hall

The Expanding Universe

• The universe is not unchanging over time, but

is expanding

– Primary evidence comes from Hubble’s Law

• Hubble’s Law

– The further away an

object is, the faster it is

moving away.

– predicts universe had a

beginning ~13 billion

years ago

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• How does the expanding universe solve the

paradox?

– We can see only a finite distance (~13 billion

light years).

– For more distant objects the light has not had

time to reach us.

– Edge of universe (from our perspective) is the

cosmic light horizon

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Why does the observable universe have an “edge”?

A. We can only see as far as light has traveled over the

age of the universe.

B. The density of neutrinos at the “edge” becomes so

large that photons cannot penetrate this barrier.

C. Dust prevents us from seeing beyond a certain

distance.

D. There are so many galaxies in the universe that they

block our view.

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The Big Bang

• Universe starts out almost infinitely small

• Goes through period of inflation, increases

size by factor of 1050

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Inflation Explains Why the

Universe Appears Flat

• The universe does not appear to be curved.

• Perhaps it is so large that we cannot observe

the curvature at this time.

Figure 27-3, Freedman, Geller, Kaufmann

9th ed. Universe,

© 2011 W. H. Freeman and Company

From what evidence do astronomers deduce

that the Universe is expanding?

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A. They can see the disks of galaxies getting

smaller over time.

B. They see a redshift in the spectral lines of

distant galaxies.

C. They detect cosmic background X-ray

radiation.

D. They can see distant galaxies dissolve,

pulled apart by the expansion of space.

E. All of the above.

What is meant by inflation in the early

Universe?

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A. The force of gravity suddenly grew stronger in the

distant past.

B. Protons expanded to the size of stars, which was

how our Sun formed.

C. The Universe increased dramatically in size in an

extremely brief period of time.

D. The number of galaxies that we see at large

distances is much greater than the number we can

see near to us.

E. The diameter of distant galaxies is much greater

than the diameter of galaxies near to us.

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Evidence for the Big Bang

• Universe would be very hot (> 1 trillion K) right

after the Big Bang (photon dominated).

• Early universe would emit black body radiation.

• As universe expanded, wavelengths were stretched

• Radiation would appear much cooler = Cosmic

Microwave Background Radiation.

Figure 26.6, Chaisson and McMillan,

6th ed. Astronomy Today,

© 2008 Pearson Prentice Hall

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Cosmic Microwave Background

• CMB has

now cooled

to about 3 K

Figure 17.7, Arny and Schneider,

5th ed. Explorations,

© 2008 The McGraw-Hill Companies

Wilkinson Microwave Anisotropy

Probe (WMAP)

• Launched June

2001

• Measured

fluctuations in

Cosmic

Microwave

Background

Image by NASA/WMAP Science Team

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Temperature and Density

Fluctuations of Early Universe

Image by NASA/WMAP Science Team

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What is meant by the cosmic microwave

background?

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A. It is radiation from distant quasars.

B. It is radiation from hot gas in intergalactic

space.

C. It is radiation from the first stars formed

when the Universe was young.

D. It is radiation created during the early days

of the Universe.

E. It is the explanation of Olbers’ paradox.

History of Universe19

Creating Matter

• Universe is initially dominated by photons

• Matter and anti-matter created via pair production.

– Most pairs annihilate each other.

– Inflation is so fast that some pairs are separated

– But more matter than anti matter survives – Why?

• By 300,000 yrs from Big Bang, universe becomes matter dominated.

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Galaxies Form

• Galaxies form from density

fluctuations in the early universe.

– Early universe would have been

very uniform.

– Minor fluctuations in density form

stars and galaxies.

– First stars form ~ 100 to 400

million years after Big Bang

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Formation of Large-Scale Structure22

Figure 18.14 Bennett et al, The Essential Cosmic Perspective, 7th ed. (Pearson, 2015)

Small irregularities in the cosmic microwave background are important because

A. they show us that the Big Bang explosion was very

nonuniform.

B. they were the seeds of the quasars we can still see

today.

C. they are thought to have led to the creation of galaxy

superclusters.

D. they are thought to contain most of the elusive

“missing matter.”

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Results from WMAP and Planck

• Universe is “flat” (parallel lines never meet)

• Universe is composed of:

– Baryonic matter (matter made of atoms)

– Cold dark matter (massive particles as yet

undiscovered)

– “Dark Energy”????

• Universe is 13.7 billion

years old

Figure 18.17 Bennett et al, The Essential Cosmic

Perspective, 7th ed. (Pearson, 2015)

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Image credit: http://chandra.harvard.edu/xray_astro/dark_energy/

See also NASA, Wikipedia, National Geographic

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Is the Universe Accelerating?

Figure 17.10, Arny and Schneider,

5th ed. Explorations,

© 2008 The McGraw-Hill Companies

• Recent observations of supernova indicate

that the universe may be accelerating

instead of slowing down.

• Universe may expand forever.

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How Will the Universe Evolve?

• Depends on density (gravitational pull) and

velocity of galaxies.

• Critical density = density of universe

needed to stop expansion

Future of Universe27

Figure 18.15 Bennett et al, The Essential Cosmic Perspective, 7th ed. (Pearson, 2015)

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• Open Universe (unbound, or low density)

– Density Critical Density

– Not enough gravity to stop expansion.

– Universe expands forever.

Figure 26.14,

Chaisson and McMillan,

6th ed. Astronomy Today,

© 2008 Pearson Prentice Hall

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• Closed Universe (bound, or high density)

– Density > Critical Density

– Gravity is stronger than expansion.

– Universe will initially expand, stop, and eventually

fall back into itself.

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Predicting the Fate of the

Universe

• If we know the density of the universe, we

can predict the future of the universe

– Measure mass in large volume of space and

divide mass by volume

• Current Estimate: The Universe is open; it

will expand forever. It may also be

accelerating.

Evidence for acceleration31

Figure 18.16Bennett et al, The Essential Cosmic

Perspective, 7th ed. (Pearson, 2015)

What will happen if the universe is closed ?

A. The universe will expand past its maximum size, then

fragment into mini-universes.

B. The universe will eventually fall back in on itself,

heading toward a “Big Crunch.”

C. The universe will expand forever.

D. The universe will reach a maximum size and remain

there, like a balloon being blown up.

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Your mission• Be informed, speak, watch, and listen in an

informed manner

• Stay curious!

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Galaxy Arp 188, The Tadpole Galaxy

APOD 11 December 2018

The Mars InSight Lander takes a “selfie”

photo on 12/06/18 after landing 11/26/18. More information at NASA Jet Propulsion

Laboratory and from APOD 10 December 2018

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