ISP205 Lecture #27, April 17, 2001
- Announcements:
- The winners of the spring break astronomy story contest
are ...
each receives 4 joker bonus points
- Last Chance for Homework Bonus Point Project #4
(find black hole property on the web)
- There will be an assessment quiz on 4-19.
1 point on the quiz will count as: 1 quiz bonus point
1/2 homework bonus point
(max available 20 points)
- Bonus Point Overview
- Quiz:
max 20 for Quiz 1, max 20 for quiz on 4-19
- Homework: max 3x6
points from bonus point projects so far
plus max 10 for quiz on 4-19
- Joker (Exam): 2 for story + 4 for
winners
are applied to exam
- Review:
- Active Galaxies
- Examples: Quasars, Seyfert Galaxies, Radio Galaxies
- Common feature: lots of non thermal emission
(for example lots of radio waves)
- Model: Supermassive black hole at center of the
Galaxy
- Large scale structure of the universe
- Stars are grouped in Galaxies (~200 billion per
Galaxy)
(our Galaxy is the Milky Way)
- Galaxies are grouped in Clusters (~10-10,000 per
Cluster)
(our Cluster is the Local Group)
- Clusters are grouped in Superclusters
(our Supercluster is the Local Supercluster)
- The universe is expanding because space is expanding
therefore objects seem to move away from us
- Light from distant objects (for example) is redshifted
because photons get streched.
(If interpreted as a Doppler effect, one still gets the right
velocity)
- The further away the object the larger redshift
(velocity)
It obeys Hubbles law: (picture)
Hubble Law: velocity = H * distance
H: Hubble constant. Current value 20 km/s/Mly
- Definition of Redshift:
redshift = change in wavelength / actual wavelength
maximum redshifts observed for quasars: 5
(means an UV hydrogen line at 397nm is shifted by 5*397nm
= 2000nm - new wavelength 2400 nm is infra red !)
redshift = 5 corresponds to ~90% speed of light
- In addition to the motion due to the expansion of the
universe,
stars, galaxies, clusters, and superclusters move individually
driven by gravities pull
- We all move (with our local supercluster) towards the
Great Attractor
- The Age of the Universe
- The Hubble time: Assume constant expansion
- At beginning of the universe two places were at
same point in space
- after a time t they are at a distance d, so
velocity = d/t.
but also Hubble: velocity = H * d
Therefore H * d = d / t or t = 1/H
- t = 1/H is called Hubble time.
With H = 20km/s/Mly it is 15 billion years
- What if expansion changes over time ?
- Decelerating universe: Universe was faster at
beginning
so it took less time to become as big as it is:
Age of the universe is younger than Hubble Time
- Accelerating universe:
Age of the universe is older than Hubble Time
- There is now evidence that the universe is
accelerating:
very distant type Ia supernovae (Standard candles) seem
fainter than they should according to redshift.
- Old Stars in globular clusters
- Using HR diagram one finds that oldest globular
clusters
are about 13 billion years old. (example)
- A simple model of a "closed" Universe
- The Universe has a finite size but no border or center
(every
location is equal)
- The "balloon model"
- The 4th dimension is time
- The Big Bang
- First 10-43 s (Planck time) - 1032
K
We don't have a theory for the physics at these conditions.
From then on quarks, gluons, electrons, neutrinos, ...
- After 1 ms, Temperature: 12
trillion K, Size: 1.4 light days
quarks and gluons condense into protons and neutrons
- After 1 s, 10 billion K, 4 ly
ratio of neutrons and protons becomes fixed as ~ 1:7
- After 100 s - 5 minutes, 1-0.4 billion K, ~55 ly
neutrons and protons form helium, deuterium, and lithium
most protons are left over (hydrogen) (picture)
need 2 neutrons and 2 protons to make one Helium;
7 protons per neutron: 12 protons are left over !
final result therefore:
93% Hydrogen (by number)
7% 4Helium (by number)
0.003 % Deuterium
0.001 % 3Helium
2x10-8 % Lithium
- After 500,000 yrs, 3000 K, 1.5 Million Ly
Neutral Atoms formed and Photons can move freely around
- After 1 billion years, 20 K
Galaxies form
- Today: 2.726 K
- Evidence for the Big Bang:
- Expansion
(Redshift and time dilation of distant supernova
light curves)
- Cosmic Microwave Background (CMB)
- Photons created 500,000 years after the big
bang from
the hot universe roam freely since then and form the
cosmic microwave background.
They had a temperature of 3000K and cooled to
2.726 K today because of the expansion of the universe.
- CMB first discovered by Arno Penzias and Robert
Wilson
accidentally as background noise in the antenna ~1965
1978: Nobel Prize
- COBE: Satellite did very accurate measurements
and showed:
- CMB is black body curve
- CMB is exactly 2.726 K
- CMB is very smooth
- There is a red/blue shift of CMB due to
motion of earth in
the universe.
This is the "absolute" velocity of the earth and
is ~500 km/s
towards the great attractor
- Better experiments recently from balloons:
BOOMERANG, MAXIMA
- Primordial element abundances (H,He,d)
- We can measure the abundances of He, d, and Li
as they were produced
in the big bang
- Deuterium has been measured in highly
redshifted gas clouds
towards distant quasars
- Other elements can be observed together with
heavier elements that have
been made later in stars. The fewer of the heavier elements are
around
the closer the observation should be to the original big bang
abundance
(example)
- All observations show that the element
abundances agree exactly with
what the Big Bang theory predicts
- Elementary particle physics experiments show that
there are 3 only types of neutrinos - this is exactly
what one needs for a big bang that works.
- The future of the Universe - closed, flat, or open ?
- Nature of Universe depends on total density (picture
space time)
- larger than critical: closed
- equal critical: flat
- less than critical: open
- Possibilities for the future of the universe (picture):
- Gravity slows expansion down, halts, reverses
motion: Big Crunch
(used to be associated with closed universe - now we know
that gravity can repel at large distances (accelerating
universe))
- Gravity slows expansion down - universe just comes
to halt
after infinite time
(used to be associated with flat universe)
- Gravity can't slow down expansion enough - universe
keeps expanding
(used to be associated with open universe)
- Critical density is 10-23 g/cm3 or:
6 Atoms per cubic centimeter !
- Composition of the Universe / dark matter
- Visible stars: 0.5% of critical density !
- Dark, normal matter (made of protons and neutrons)
4.5%
- Some unknown dark matter 35%
(from gravitational lensing, rotation of stars and galaxies)
- This sums up to ~40% of the critical density, most
of it dark
- 60% is missing - "dark matter problem"
for theorists
that predict that universe has critical density
(inflation)
- BUT - Now we know that the Universe is flat from
BOOMERANG and MAXIMA CMB experiments (picture)
- Accelerating universe solves then the problems that
were associated
with a flat universe before:
- Dark matter problem to get a flat universe
If space is repulsive at large distances it must have energy
and therefore mass - "dark energy" makes remaining 60%
of matter to critical density
- Age problem for flat universe
Without dark energy a flat universe would slow down
and would therefore be much younger than Hubble Age.
It would be younger than globular cluster !
Dark energy prevents slow down but keeps universe flat.
For most cosmologists these are strong arguments for
the accelerating universe in addition to evidence from
the apparent brightness of type Ia supernovae
- Why is the universe the way it is ?
- There are a lot of "accidents" that allow
humans to exist in this universe:
- Universe does expand just right (fast enough to
prevent everything
from forming a giant black hole, but slow enough to allow stars
to form)
- Hydrogen and Helium cannot fuse into iron during
the big bang
because there is no stable nucleus that can be made of 2 helium
nuclei. Therefore stars can shine.
- Helium can fuse into carbon at the conditions in
stars, but not
quickly enough into oxygen, so that some carbon is left
- ... many more ...
- The Anthropic Principle: The Universe is the way it is,
BECAUSE
only then humans exists that can discover its properties
- There are theories proposing that countless universes
exist with
different physical laws and properties.