Download ASTR100 Class 01 - University of Maryland Astronomy

ASTR100 (Spring 2008)
Introduction to Astronomy
The Fate of the Universe
Prof. D.C. Richardson
Sections 0101-0106
Will the universe continue expanding
forever?
Does the
universe have
enough kinetic
energy to
escape its own
gravitational
pull?
Fate of
universe
depends
on the
amount
of dark
matter.
Lots of
dark
matter
Critical
density of
matter
Not enough
dark matter
Amount of matter is
~25% of the critical
density, suggesting
fate is eternal
expansion.
Not enough
dark matter
But expansion
appears to be
speeding up!
Dark
Energy?
Not enough
dark matter
old
older
oldest
Estimated age depends on both dark matter and energy.
Thought Question
Suppose that the universe has more
dark matter than we think there is
today — how would that change the
age we estimate from the expansion
rate ?
A. Estimated age would be older.
B. Estimated age would be the same.
C. Estimated age would be younger.
Thought Question
Suppose that the universe has more
dark matter than we think there is
today — how would that change the
age we estimate from the expansion
rate ?
A. Estimated age would be older.
B. Estimated age would be the same.
C. Estimated age would be younger.
Is the expansion of the universe
accelerating?
Brightness of distant white-dwarf supernovae tells us
how much universe has expanded since they exploded.
Accelerating universe is best fit to supernova data.
What is the fate of the Universe?
Fire & Ice
Some say the world will end in fire,
Some say in ice.
From what I’ve tasted of desire
I hold with those who favour fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice.
—Robert Frost
Solar System
 Few Myr: Moon moves away from us.
 Day becomes longer.
 Few Gyr: Sun ascends giant branch.
 5 Gyr: inner solar system consumed.
 Sun becomes white dwarf.
 10’s of Gyr: Passing star dislodges
remaining planets.
Galaxy
 100 Myr: LMC & SMC merge with MW.
 3 Gyr: MW & Andromeda collide.
 Together may become elliptical galaxy.
 10’s of Gyr: MW runs out of gas…
 Star formation ceases.
 Most material locked up in WD, NS, BH.
 Galaxy becomes fainter & redder.
Universe
 Expands forever.
 1012 yr: all matter consists of dead stars
or cold lumps (planets, meteorites, used
spaceships, etc.).
 1027 yr: Extremely rare events become
important.
 Stellar collisions  galaxy evaporation and
growth of central black hole.
 1031 yr: gravitational radiation 
galactic black holes merge to form 1015
MSun black holes.
Universe
 1040 yr: protons decay—all atomic
matter disintegrates into radiation and
subatomic particles (electrons,
neutrinos, etc.).
 1067 yr: stellar-mass BH’s evaporate.
 10100 yr: supermassive BH’s evaporate.
 Universe ends as just photons and
subatomic particles separated by
enormous distances—this is the end of
time… (not with a bang but a whimper).
ASTR100 (Spring 2008)
Introduction to Astronomy
The Big Bang
Prof. D.C. Richardson
Sections 0101-0106
What were conditions like in the
early universe?
The universe
must have
been much
hotter and
denser early
in time.
The early
universe must
have been
extremely hot
and dense.
Early universe was full
of elementary particles
and radiation because
of its high temperature.
Early universe was full
of elementary particles
and radiation because
of its high temperature.
Photons converted into
particle-antiparticle
pairs and vice-versa:
E = mc2
What is the history of the universe
according to the Big Bang theory?
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.
Four known forces
in universe:
Strong Force
Electromagnetism
Weak Force
Gravity
Thought Question
Which of the four forces keeps you from
sinking to the center of the Earth?
A. Gravity.
B. Electromagnetism.
C. Strong Force.
D. Weak Force.
Thought Question
Which of the four forces keeps you from
sinking to the center of the Earth?
A. Gravity.
B. Electromagnetism.
C. Strong Force.
D. Weak Force.
Do forces unify at high temperatures?
Four known forces
in universe:
Strong Force
Electromagnetism
Weak Force
Gravity
Yes!
(Electroweak)
Maybe
(GUT)
Who knows?
(String Theory)
Planck Era
Time: < 10-43 sec
Temp: > 1032 K
No theory of quantum
gravity.
All forces may have
been unified.
GUT Era
Time: 10-43 – 10-38 sec
Temp: 1032 – 1029 K
Era began when
gravity became
distinct from other
forces.
Era may have ended
with sudden burst of
inflation (more later).
Electroweak Era
Time: 10-10 – 10-10 sec
Temp: 1029 – 1015 K
Era ended when all
four forces became
distinct.
Particle Era
Time: 10-10 – 0.001 sec
Temp: 1015 – 1012 K
Amounts of matter and
antimatter are nearly
equal.
(Roughly one extra
proton for every 109
proton–antiproton
pairs!)
Era of
Nucleosynthesis
Time: 0.001 sec–5 min
Temp: 1012 – 109 K
Began when matter
annihilates remaining
antimatter at
~ 0.001 sec.
Nuclei began to fuse.
Era of Nuclei
Time:5 min–380,000 yr
Temp: 109 – 3,000 K
Helium nuclei formed at
age ~3 minutes.
The universe became
too cool to blast helium
apart.
Era of Atoms
Time: 380,000 years –
1 billion years
Temp: 3,000 – 20 K
Atoms formed at age
~380,000 years.
Background radiation
is released.
Era of Galaxies
Time: ~1 billion years
– present
Temp: 20 – 3 K
The first stars and
galaxies formed by
~1 billion years after
the Big Bang.
Primary Evidence
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.
How do we observe the radiation left
over from the Big Bang?
The cosmic
microwave
background—
the radiation
left over from
the Big Bang—
was detected
by Penzias and
Wilson in 1965.
Background radiation from the Big Bang has been
freely streaming across the universe since atoms
formed at temperature ~3,000 K: visible/IR.
Creation of the Cosmic Microwave Background
Background has perfect
thermal radiation
spectrum at temperature
2.73 K
Expansion of universe has redshifted thermal
radiation from that time to ~1,000 times longer
wavelength: microwaves.
Full sky in all wavelengths
WMAP gives us detailed baby pictures of structure
in the universe.
How do the abundances of elements
support the Big Bang theory?
Protons and neutrons combined to make long-lasting
helium nuclei when universe was ~ 3 minutes old.
Big Bang theory prediction: 75% H, 25% He (by mass).
Matches observations of nearly primordial gases.
Abundances of
other light
elements agree
with Big Bang
model having
4.4% normal
matter—more
evidence for
WIMPS!
Thought Question
Which of these abundance patterns is
an unrealistic chemical composition for
a star?
A. 70%
B. 95%
C. 75%
D. 72%
H,
H,
H,
H,
28% He, 2% other.
5% He, less than 0.02% other.
25% He, less than 0.02% other.
27% He, 1% other.
Thought Question
Which of these abundance patterns is
an unrealistic chemical composition for
a star?
A. 70% H, 28%
B. 95% H, 5%
other.
C. 75% H, 25%
D. 72% H, 27%
He, 2% other.
He, less than 0.02%
He, less than 0.02% other.
He, 1% other.