Download April 2006 - Otterbein University

Welcome to
Starry Monday at Otterbein
Astronomy Lecture Series
-every first Monday of the monthApril 3, 2006
Dr. Uwe Trittmann
Today’s Topics
• Introduction to Cosmology
• The Night Sky in April
On the Web
• To learn more about astronomy and physics at
Otterbein, please visit
– http://www.otterbein.edu/dept/PHYS/weitkamp.a
sp (Observatory)
– http://www.otterbein.edu/dept/PHYS/ (Physics
Dept.)
Cosmology
• The part of astronomy (and astrophysics) that deals with
the greatest structures in the universe – and the evolution
of the universe itself!
Big Bang
• The “start” of the universe, a primordial fireball
 the early universe was very hot and dense
 intimate connection between cosmology and
nuclear/particle physics
 “To understand the very big we have to
understand the very small”
Questions, Questions, Questions
• Scientists want to know, so they ask questions:
– What is in the universe?
– How do these things interact?
– How does the universe change in time?
• Is there a beginning?
• Is there an end?
• Answers come from observations
 Let’s observe:
What’s in the Universe?
The Earth
Planets
Mercury
Venus
Mars
Jupiter
Saturn
The Sun (a typical star)
Stars
Galaxies
Clusters of Galaxies
What’s in the Universe?
Stars
nebulae
molecular clouds
star clusters
THE UNIVERSE
clusters
and
superclusters
galaxies
like the
Milky Way
quasars
voids
Sun
planets
terrestrial
jovian
Solar System
black holes
pulsars
moons
comets
meteors
asteroids
dust
Big ……………………………………..small
What’s in the Universe?
A lot of stuff !!!
Scientific term: Mass
Observation II: It is dark at night!
• Big deal!
• Indeed: it has cosmological
consequences!
• Let’s find out why!
Night sky: No sun – just stars
Look closer and find more dimmer stars
If the Universe is infinite…
There’s
more
and
more…
dimmer
and
dimmer
stars
Until finally…
…the view fills up completely
…and it’s as bright as the day!
So, why is the night sky dark?
(Olbers’ Paradox)
• Conclusion: either
– Universe is not
infinite or
– Universe
changes in time
Observation III: Everything is
moving away from us!
• Measure spectrum of
galaxies and compare to
laboratory measurement
• lines are shifted towards red
• This is the Doppler effect:
Red-shifted objects are
moving away from us
The Universe expands!
• Where was the origin of the expansion?
Everywhere!
• Every galaxy sees the others receding from
it – there is no center
Conclusions from our Observations
• The Universe has a finite age, so light from very
distant galaxies has not had time to reach us,
therefore the night sky is dark.
nds now, so looking back in
• The universe expa
time it actually
shri
nks until…?
Big Bang model: The universe is born out
of a hot dense medium
13.7 billion years ago.
How does the expansion work?
• Like an explosion (hot, dense matter in the
beginning), but space itself expands!
• Slowed down by gravitational attraction
• Attraction is the stronger, the more mass
there is in the universe
• Scientifically described by Einstein’s
General theory of Relativity (1915)
General Relativity ?! That’s easy!
Rμν -1/2 gμν R = 8πG/c4 Tμν
OK, fine, but what does
that mean?
(Actually, it took Prof. Einstein 10 years to come up with that!)
The Idea behind General Relativity
–
In modern physics, we view space and time as a
whole, we call it four-dimensional space-time.
–
Space-time is warped by the presence of masses like
the sun, so “Mass tells space how to bend”
–
Objects (like planets) travel in “straight” lines
through this curved space (we see this as orbits), so
“Space tells matter how to move”
Still too complicated?
• Here is a picture:
Planet’s orbit
Sun
Effects of General Relativity
• Bending of starlight by the Sun's gravitational
field (and other gravitational lensing effects)
What General Relativity tells us
• The more mass there is in the universe, the
more “braking” of expansion there is
• So the game is:
Mass
vs.
Expansion
And we can even calculate who wins!
The Fate of the Universe –
determined by a single number!
• Critical density is the density required to just barely
stop the expansion
• We’ll use 0 = actual density/critical density:
– 0 = 1 means it’s a tie
– 0 > 1 means the universe will recollapse (Big Crunch)
 Mass wins!
– 0 < 1 means gravity not strong enough to halt the
expansion  Expansion wins!
• And the number is:
0 = 1
The Shape of the Universe
• In the basic scenario there is a simple relation between the
density and the shape of space-time:
Density Curvature 2-D example
Universe
Time & Space
0>1
positive
sphere
closed, bound
0=1
zero (flat)
plane
open, marginal
infinite
0<1
negative
saddle
open, unbound
infinite
finite
The “size” of the Universe –
depends on time!
Expansion
wins!
It’s a tie!
Mass wins!
Time
So, how much mass is in the
Universe?
• Can count all stars, galaxies etc.
•  this gives the mass of all “bright” objects
• But: there is also DARK MATTER
“Bright” Matter
• All normal or “bright” matter can be “seen”
in some way
– Stars emit light, or other forms of
electromagnetic radiation
– All macroscopic matter emits EM radiation
characteristic for its temperature
– Microscopic matter (particles) interact via the
Standard Model forces and can be detected this
way
First evidence for dark matter:
The missing mass problem
• Showed up when measuring rotation curves
of galaxies
Is Dark Matter real?
• It is real in the sense that it has specific
properties
• The universe as a whole and its parts
behave differently when different amounts
of the “dark stuff” is in it
• Good news: it still behaves like mass, so
Einstein’s cosmology still works!
Properties of Dark Matter
• Dark Matter is dark at all wavelengths, not
just visible light
• We can’t see it (can’t detect it)
• Only effect is has: it acts gravitationally like
an additional mass
• Found in galaxies, galaxies clusters, large
scale structure of the universe
• Necessary to explain structure formation in
the universe at large scales
What is Dark Matter?
• More precise: What does Dark matter consist of?
–
–
–
–
–
Brown dwarfs?
Black dwarfs?
Black holes?
Neutrinos?
Other exotic subatomic particles?
Back to: Expansion of the Universe
•
•
•
•
Either it grows forever
Or it comes to a standstill
Or it falls back and collapses (“Big crunch”)
In any case: Expansion slows down!
Surprise of the year 1998
(Birthday of Dark Energy):
All wrong! It accelerates!
Enter: The Cosmological Constant
• Usually denoted 0, it represents a uniform
pressure which either helps or retards the
expansion (depending on its sign)
• Physical origin of 0
is unclear
• Einstein’s biggest
blunder – or not !
• Appears to be small
but not quite zero!
• Particle Physics’
biggest failure
Effects of the “Cosmological Constant”
• Introduced by Einstein, not necessary
• Repulsive  accelerates expansion of universe
Hard to
distinguish
today
Triple evidence for Dark Energy
• Supernova data
• Large scale structure
of the cosmos
• Microwave
background
Microwave Background:
Signal from the Big Bang
• Heat from the Big Bang should still be around,
although red-shifted by the subsequent expansion
• Predicted to be a blackbody spectrum with a
characteristic temperature of 2.725 Kelvin by
George Gamow (1948)
 Cosmic Microwave Background
Radiation (CMB)
Discovery of Cosmic Microwave
Background Radiation (CMB)
• Penzias and Wilson
(1964)
• Tried to “debug” their
horn antenna
• Couldn’t get rid of
“background noise”
 Signal from Big Bang
• Very, very isotropic (1
part in 100,000)
CMB: Here’s how it looks like!
Peak as expected from 3 Kelvin warm object
Shape as
expected
from black
body
Maybe pigeons?
• Proposed error: pigeon
crap in antenna
• Real reason: a signal
from the Big Bang
Pigeon trap
 Horn antenna
Latest Results: WMAP
(Wilkinson Microwave Anisotropy Probe)
• Measure fluctuations in microwave background
• Expect typical size of fluctuation of one degree if
universe is flat
• Result:
Universe is flat !
Experiment and Theory
Expect
“accoustic
peak” at l=200
 There it is!
Supernova Data
• Type Ia Supernovae are
standard candles
• Can calculate distance
from brightness
• Can measure redshift
• General relativity gives us distance as a
function of redshift for a given universe
Supernovae are further away than
expected for any decelerating (“standard”)
universe
Supernova
Data
magnitude
redshift
Pie in the Sky: Content of the
Universe
5%
25%
1
2
3
70%
Dark Energy
Dark Matter
SM Matter
We know almost everything about almost nothing!
Properties of Dark Energy
• Should be able to explain acceleration of
cosmic expansion  acts like a negative
pressure
• Must not mess up structure formation or
nucleosynthesis
• Should not dilute as the universe expands
 will be different % of content of universe
as time goes by
Threefold
Evidence
Three independent
measurements
agree:
•Universe is flat
•30% Matter
•70% dark energy
Measuring Dark Energy
Dark energy acts
like negative
pressure, and is
characterized by
its equation of
state, w = p/ρ
 We can
measure w!
Conclusion
• Cosmology is one of the most exciting
subfields of physics these days
• The is an intimate connection between
cosmology and particle physics
• We live in a “golden age” of cosmology: lots
of data available and being measured
• Today’s era is that of “precision cosmology”
• There is lot’s we don’t know  interesting for
young scientists!
The Night Sky in April
• Nights still long, but EDT => later observing!
• Spring constellations are up: Cancer, Leo, Big
Dipper
• Saturn dominates the evening, Jupiter early
morning.
Moon Phases
• Today: Waxing Crescent 34%
• 4/5 (First quarter Moon)
• 3/ 13 (Full Moon)
• 3 / 20 (Last Quarter Moon)
• 3 / 27 (New Moon)
Today
at
Noon
Sun at
meridian,
i.e.
exactly
south
10 PM
Typical
observing
hour,
early
March
Jupiter
Saturn
Mars
Moon
Zenith
Big Dipper
points to the
north pole
West
Perseus and
Auriga
with Plejades
and the
Double
Cluster
West
The Winter
Constellations
–
–
–
–
–
Orion
Taurus
Canis Major
Gemini
Canis Minor
SouthWest
• Saturn
near
Praesepe,
an open
star
cluster
South
• Spring
constellations:
– Leo
– Hydra
East
• Canes
Venatici:
– M51
• ComaVirgo
Cluster
• Globular
Star
Clusters
– M3, M5
East
Virgo and
Coma
with the VirgoComa
galaxy
cluster
VirgoComa
Cluster
• Lots of
galaxies
within a
few
degrees
M87, M88
and M91
East
– Hercules
– Corona
Borealis
– Bootes
Globular Star
Clusters:
•M3
• M 13
• M 92
M13: Globular Cluster
Mark your Calendars!
• Next Starry Monday: May 1, 2005, 8 (!!!) pm
(this is a Monday
• Observing at Prairie Oaks Metro Park:
– Friday, May 5, 9:00 pm
• Web pages:
– http://www.otterbein.edu/dept/PHYS/weitkamp.asp (Obs.)
– http://www.otterbein.edu/dept/PHYS/ (Physics Dept.)
)
Mark your Calendars II
•
•
•
•
Physics Coffee is every Wednesday, 3:30 pm
Open to the public, everyone welcome!
Location: across the hall, Science 256
Free coffee, cookies, etc.