Welcome to Starry Monday at Otterbein Astronomy Lecture Series -every first Monday of the month- April 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 Nucleons Neutron udd charge 0 (composite particles)
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 The Forces of the Standard Model massless carriers long ranged massive carriers short ranged
The particles of the Standard Model Matter particles have half-integer 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% We know almost everything about almost nothing! Dark Energy Dark Matter SM Matter 25% 70%
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!
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 0is 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 -7.5 ¼ size ½ Now 2 size 4 +11.5 +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