Download Dark Matter - Otterbein University

Welcome to
Starry Monday at Otterbein
Astronomy Lecture Series
-every first Monday of the monthApril 7, 2008
Dr. Uwe Trittmann
Today’s Topics
• Dark Matter and Dark Energy – The
Dark Side of the Universe
• The Night Sky in April
Starting Point
• Before we can say anything about the “dark
side”, we have to answer the following
questions:
• What is “bright” matter?
• What do we know about “bright” 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
Atom: Nucleus and
Electrons
The Structure of Matter
Nucleus: Protons and
Neutrons (Nucleons)
Nucleon: 3 Quarks
| 10-10m |
| 10-14m |
|10-15m|
Elementary Particles
All ordinary nuclear matter is made out of quarks:
Up-Quark
Down-Quark
(charge +2/3)
(charge -1/3)
In particular:
Proton
uud  charge +1
Neutron
udd  charge 0
Nucleons
(composite particles)
The Forces of the Standard Model
Force (wave)
Carrier (particle)
Gravity: couples to mass
graviton (?)
Electromagnetic force:
photon
massless
carriers 
long ranged
couples to charge
Weak force:
responsible for radioactive decay
Strong force:
couples to quarks
W+, W-, Z0
8 gluons
massive
carriers 
short ranged
The particles of the Standard Model
Matter
particles
have halfinteger spin
(fermions)
Force
carriers have
integer spin
(bosons)
Conclusion
• We know a lot about the structure of matter!
• We know a lot about the forces between
matter particles
• We know al lot about the theory that
describes all of this (the Standard Model)
 Great News !
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!
What is the dark stuff?
Dark Matter is the stuff we know nothing
about (but we have some ideas)
Dark Energy is the stuff we have absolutely
no idea about
Conclusion
• If we don’t know anything about it, it is
boring, and there is nothing to talk about.
•  End of lecture!
Alternate Conclusion
• If we don’t know anything about it, it is
interesting because there is a lot to be
discovered, learned, explored,…
•  beginning of lecture!
So what do we know? Is it 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
• Let’s have a look!
First evidence for dark matter:
The missing mass problem
• Showed up when measuring rotation curves
of galaxies
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 precisely:
• What does Dark matter consist of?
–
–
–
–
–
Brown dwarfs?
Black dwarfs?
Black holes?
Neutrinos?
Other exotic subatomic particles?
Classification of Dark Matter
• Classify the possibilities
–
–
–
–
Hot Dark Matter
Warm Dark Matter
Cold Dark Matter
Baryonic Dark Matter
You could have
come up with this,
huh?!
Hot Dark Matter
• Fast, relativistic matter
• Example: neutrino
– Pro:
• interact very weakly, hard to detect  dark!
– Con:
• Existing boundaries limit contribution to missing mass
• Hot Dark matter cannot explain how galaxies formed
• Microwave background (WMAP) indicates that
mastter clumped early on
• Hot dark matter does not clump (it’s simply too fast)
Baryonic Dark Matter
• “Normal” matter
– Brown Dwarfs
– Dense regions of heavy elements
– MACHOs: massive compact halo objects
• Big Bang nucleosynthesis limits contribution
Cold Dark Matter
•
•
•
•
Slow, non-relativistic particles
Most attractive possibility
Large masses (BH, etc) ruled out by grav. lensing data
Major candidates:
–
–
–
–
–
Axions
Sterile neutrinos
SIMPs (strongly interacting massive particles)
WIMPs (weakly …), e.g. neutralinos
All of the above are “exotic”, i.e. outside the SM
Alternatives
• Maybe missing mass, etc. can be explained
by something else?
–
–
–
–
Incomplete understanding of gravitation
Modified Newtonian Dynamics (MOND)
Nonsymmetric gravity
General relativity
What General Relativity tells us
• The more mass there is in the universe, the more the
expansion of the cosmos slows down
• So the game is:
Mass
vs. Expansion
And we can even calculate who wins!
The “size” of the Universe –
depends on time!
Expansion
wins!
It’s a tie!
Mass wins!
Time
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!
The silent majority: Dark Energy
1
2
3
70%
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
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 3Kelvin 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
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
Best fit: 75% Dark Energy, 25% Matter
redshift
Redshift: 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
Example: Spectrum of a Quasar
Highly redshifted spectrum
 the quasar is very far away –and keeps going!
Quasar
Lab
Large Scale Structure of the Cosmos
• Large scale
structure of the
universe can be
explained only
by models
which include
Dark Matter
and Dark
Energy
Experiments: 2dF GRS, SDSS
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
The Pie changes - As time goes by
-11.5
¼
size
½
-7.5
1
Now
2
3
1
2
3
1
2
3
+11.5
2
size
4
+24.5
Why does the Pie change?
• Dark energy density stays constant
• Matter density falls of like volume
– Volume grows, mass stays constant
Big Question: why do we live in an era
where the content is rather democratic?
Because we are here to observe!
(Dangerous answer)
What is Dark Energy?
• We have a few ideas what it could be
• Unfortunately none of these makes fits our
“job description”
• Wanted: “Dark Energy Candidate”
Dark Energy Candidates
• Global Vacuum Energy
• Local Vacuum Energy
• Dynamical Dark Energy
• Modified Gravity
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
• Need more ideas
– No problem! That’s what theorists produce
every day
• Need more data
– Some space missions (Planck, etc) are on the
way
– LHC probing SUSY will start operation in 2008
The Night Sky in April
• Nights are getting shorter!
• Spring constellations: Leo, Virgo, Big Dipper,
Bootes, Canes Venatici, Coma  lots of galaxies!
• Mars & Saturn are visible most of the night
Moon Phases
• Today (Waxing Crescent)
• 2 / 12 (First Quarter Moon)
• 4 / 20 (Full Moon)
• 4 / 28 (Last Quarter Moon)
• 5 / 5 (New Moon)
Today
at
Noon
Sun at
meridian,
i.e.
exactly
south
10 PM
Typical
observing
hour,
early
February
Saturn
Mars
Star
Maps
Celestial
North Pole –
everything
turns around
this point
Zenith – the
point right
above you &
the middle of
the map
40º
90º
Due
North
Big Dipper
points to the
north pole
West
Perseus,
Auriga &
Taurus
with Plejades
and the
Double
Cluster
West
• Orion
• Canis
Major &
Minor
• Beautiful
open star
clusters
• Orion
Nebula
M42
South
Spring
constellations:
– Leo
– Hydra
M44 Beehive
(open star
cluster)
Saturn
East
• Virgo &
Coma
High up
in the
East
• Big
Dipper
• Bootes
Mark your Calendars!
• Next Starry Monday: May 5, 2008, 7 pm
(this is a Monday
)
• Observing at Prairie Oaks Metro Park:
– Friday, May 9, 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 Monday, 3:00 pm
Open to the public, everyone welcome!
Location: across the hall, Science 244
Free coffee, cookies, etc.
The Mass of the Galaxy
• Can be determined using Kepler’s 3rd Law
– Solar System: the orbital velocities of planets determined by
mass of Sun
– Galaxy: orbital velocities of stars are determined by total
mass of the galaxy contained within that star’s orbit
• Two key results:
– large mass contained in a very small volume at center of our
Galaxy
– Much of the mass of the Galaxy is not observed
• consists neither of stars, nor of gas or dust
• extends far beyond visible part of our galaxy (“dark
halo”)
Aside: Standard Cosmology
• Based on Einstein’s theory of Gravity, aka
General Relativity
• Assumes isotropic, homogeneous universe
• This “smeared out mass” property is
approximately valid if we average over
large distances in the universe
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)
The Universe expands!
• Where was the origin of the expansion?
Everywhere!
• Every galaxy sees the others receding from
it – there is no center
Big Bang
• The universe expa
nds now, so looking
back in time it actually
shrin
ks until…?
Big Bang model: The universe is
born out of a hot dense medium
13.7 billion years
ago.
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 (probably…)
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
Maybe pigeons?
• Proposed error: pigeon
crap in antenna
• Real reason: a signal
from the Big Bang
Pigeon trap
 Horn antenna
Global Vacuum Energy
• Cosmological constant
– Constant in space and time
– Same across the universe
• Pro:
– Could be explainable from first principles
• Con:
– No known explanation yet
Local Vacuum Energy
• Constant in the observable universe, but
different in very distant parts of cosmos
• Pro
– Maybe explains why cosmological const. is so
small “here”
• Con
– Requires different domains
Dynamical Dark Energy
• Quintessence
– Slowly varying energy source
• Pro
– Testable
– Can gradually go to zero energy
• Con
– Has not been detected
Modified Gravity
• Modification of general relativity on large
scales
• Pro
– Does not need “dark energy”
• Con
– Hard to modify and still explain existing data