what do we know about dark matter and dark energy? cas meeting, 14 january 2006 dr. uwe trittmann...
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What do we know about Dark Matter and Dark Energy?
CAS Meeting, 14 January 2006
Dr. Uwe Trittmann
Otterbein College
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
The Structure of MatterAtom: Nucleus and Electrons
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
Nucleons
Neutron udd charge 0
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
Nucleons
Neutron udd charge 0
(composite particles)
The Forces of the Standard Model
Force (wave)
Gravity: couples to mass
Electromagnetic force: couples to charge
Weak force: responsible for radioactive decay
Strong force: couples to quarks
Carrier (particle)
graviton (?)
photon
W+, W-, Z0
8 gluons
massless carriers long ranged
massive carriers short ranged
The particles of the Standard Model
Force carriers have integer spin (bosons)
Matter particles have half-integer spin(fermions)
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
We know almost everything about almost nothing!
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2
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25%
5%
70%
Dark EnergyDark MatterSM MatterSM Matter
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
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”)
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?
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
The silent majority: Dark Energy
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3
70%
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!
(Actually, it took Prof. Einstein 10 years to come up with that!)
Rμν -1/2 gμν R = 8πG/c4 Tμν
OK, fine, but what does that mean?
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: Sun Planet’s orbit
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 “size” of the Universe – depends on time!
Expansion wins!
It’s a tie!
Mass wins!
Time
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 expands now, so looking
back in time it actually shrinks 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 finite
0=1 zero (flat) plane open, marginal infinite
0<1 negative saddle open, unbound infinite
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
• Physical origin of 0
is unclear• Einstein’s biggest
blunder – or not !• Appears to be small
but not quite zero!• Particle Physics’
biggest failure
• Usually denoted 0, it represents a uniform pressure which either helps or retards the expansion (depending on its sign)
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 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
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 universeSupernovae are further away than expected for any decelerating (“standard”) universe
Supernova Data
redshift
magnitude
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 QuasarHighly 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
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3
1
2
3
1
2
3
1
2
3
1
2
3
-11.5
+24.5+11.5
Now
-7.5¼ size ½
2 size 4
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
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
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 2007