dark things and the cosmos.ppt - unf.edu

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Copyright © 2010 Pearson Education, Inc.Copyright © 2010 Pearson Education, Inc.

Chapter 16

Dark Matter, Dark Energy, and

the Fate of the Universe


Normal matter: The kind of matter we are familiar

with, which is made of atoms and responds to the

electromagnetic force and gravity.

Dark matter: An undetected form of mass that

emits little or no light but whose existence we

infer from its gravitational influence

Dark energy: An unknown form of energy that

seems to be the source of a repulsive force

causing the expansion of the universe to

accelerate


Contents of Universe

• Normal matter: ~ 4.9%

• Dark matter: ~ 26.8%

• Dark energy: ~ 68.3%

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Rotation curve

A plot of orbital

speed versus

orbital radius

Solar system’s

rotation curve

declines because

Sun has almost

all the mass.

Rotation Curve of the Solar System


The rotation

curve of the

Milky Way stays

flat with

distance.

Mass must be

more spread out

than in the solar

system.

Rotation Curve of a Spiral Galaxy


The mass in the

Milky Way is

spread out over

a larger region

than the stars.

Most of the

Milky Way’s

mass seems to

be dark matter!

Encircled Mass as a Function of Distance for a Spiral Galaxy

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The visible

portion of a

galaxy lies

deep in the

heart of a

large halo of

dark matter.


We can

measure

orbital

velocities in

other spiral

galaxies

using the

Doppler

shift of the

21-cm line

of atomic H.


Spiral galaxies all tend to have orbital velocities that

remain constant at large radii, indicating large amounts

of dark matter.

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We can

measure the

velocities of

galaxies in a

cluster from

their Doppler

shifts.


The mass we

find from

galaxy

motions in a

cluster is

about

50 times

larger than

the mass in

stars!


Clusters contain

large amounts of

X ray–emitting hot

gas.

The temperature of

hot gas (particle

motions) tells us

cluster mass:

85% dark matter

13% hot gas

2% stars

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Gravitational lensing, the bending of light rays by gravity,

can also tell us a cluster’s mass.


All three methods of measuring cluster mass indicate

similar amounts of dark matter.


Does dark matter really exist?

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Our Options

1. Dark matter really exists, and we are observing the effects of its gravitational attraction.

2. Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter.


Our Options

1. Dark matter really exists, and we are observing the effects of its gravitational attraction.

2. Something is wrong with our understanding of gravity, causing us to mistakenly infer the existence of dark matter.

Because gravity is so well tested, most

astronomers prefer option #1.


What might dark matter be made of?

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How dark is it?


How dark is it?

… not as bright as a star.


Two Basic Options

• Ordinary Matter (MACHOs)

— Massive Compact Halo Objects:

dead or failed stars in halos of galaxies

• Exotic Particles (WIMPs)

— Weakly Interacting Massive Particles:

mysterious neutrino-like particles

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Compact starlike

objects occasionally

make other stars

appear brighter

through lensing…


Compact starlike

objects occasionally

make other stars

appear brighter

through lensing…

… but there are not

enough lensing

events to explain all

the dark matter.


Why WIMPs?

• There’s not enough ordinary matter.

• WIMPs could be left over from the Big Bang.

• Models involving WIMPs explain how galaxy

formation works.

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Will the universe continue expanding forever?


Fate of

universe

depends on

the amount

of dark

matter.

Critical

density of

matter

Not enough

dark matter

Lots of

dark

matter


Amount of matter is ~25% of

the critical density, suggesting

fate is eternal expansion.

Not enough

dark matter

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But expansion

appears to be

speeding up!

Dark energy?Not enough

dark matter


Estimated age depends on both dark matter and dark energy.

old older oldest


The brightness of distant white dwarf supernovae tells us

how much the universe has expanded since they exploded.

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An accelerating universe is the best fit to supernova data.


Chapter 17

The Beginning of Time


The universe

must have

been much

hotter and

denser early

in time.

Estimating the Age of the Universe

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The early

universe

must have

been

extremely

hot and

dense.


Photons converted into

particle–antiparticle

pairs and vice versa.

E = mc2

The early universe was

full of particles and

radiation because of its

high temperature.


Defining Eras of the Universe

• The earliest eras are defined by the kinds of forces present in the universe.

• Later eras are defined by the kinds of particles present in the universe.

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Four known forces

in universe:

Strong Force

Electromagnetism

Weak Force

Gravity



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Four known forces

in universe:

Strong Force

Electromagnetism

Weak Force

Gravity

Do forces unify at high temperatures?


Four known forces

in universe:

Strong Force

Electromagnetism

Weak Force

Gravity


Yes!

(Electroweak)


Four known forces

in universe:

Strong Force

Electromagnetism

Weak Force

Gravity


Maybe

(GUT)

Yes!

(Electroweak)

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Four known forces

in universe:

Strong Force

Electromagnetism

Weak Force

Gravity


Maybe

(GUT)

Yes!

(Electroweak)Who knows?

(String Theory)



Planck Era

Time: < 10-43 s

Temp: > 1032 K

No theory of quantum

gravity

All forces may have been

unified

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GUT Era

Time: 10-43–10-38 s

Temp: 1032–1029 K

GUT era began when

gravity became distinct

from other forces.

GUT era ended when strong

force became distinct from

electroweak force.

This was accompanied by

inflation.


Electroweak Era

Time: 10-38–10-10 s

Temp: 1029–1015 K

Strong force and

electroweak force now

separate


Particle Era

Time: 10-10–0.001 s

Temp: 1015–1012 K

Amounts of matter and

antimatter are nearly equal.

(Roughly one extra proton

for every 109 proton–

antiproton pairs!)

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Era of Nucleosynthesis

Time: 0.001 s–5 min

Temp: 1012–109 K

Began when matter

annihilates remaining

antimatter at

~ 0.001 s.

Nuclei began to fuse.


Era of Nuclei

Time: 5 min–380,000 yrs

Temp: 109–3000 K

Helium nuclei formed at age

~3 minutes.

The universe became too cool

to blast helium apart.

Radiation could not travel far

The universe was “opaque”


Era of Atoms

Time: 380,000 years– 1Gy

Temp: 3000–20 K

Atoms formed at age

~380,000 years.

The universe became

“transparent”.

Cosmic background

radiation is released.

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Era of Galaxies

Time: ~1Gy–present

Temp: 20–3 K

The first stars and galaxies

formed by ~1 billion years

after the Big Bang.


Primary Evidence for the Big Bang

1. We have detected the leftover radiation

from the Big Bang.

2. The Big Bang theory correctly predicts the

abundance of helium and other light

elements in the universe.


The cosmic

microwave

background—

the radiation left

over from the

Big Bang— was

detected by

Penzias and

Wilson in 1965.

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Background radiation from the Big Bang has been

freely streaming across the universe since atoms

formed at temperature ~3000 K: visible/IR.


Expansion of the universe has redshifted thermal

radiation from that time to ~1000 times longer

wavelength: microwaves.

Background has

perfect thermal

radiation spectrum

at temperature 2.73

K.


Protons and neutrons combined to make long-lasting helium nuclei when the universe was ~5 minutes old.

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Big Bang theory prediction: 75% H, 25% He (by mass)

Matches observations of nearly primordial gases

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