Welcome to Starry Monday at Otterbein Astronomy Lecture
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 -10 m | | 10 -14 m | | 10 -15 m |
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 W + , W -, Z 0 Strong force: 8 gluons couples to quarks 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 theory that describes all of this (the Standard Model) Great News !
Pie in the Sky: Content of the Universe 5% 25% 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 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 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 3 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
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: 2 d. F 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 Now +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 M 42
South Spring constellations: – Leo – Hydra M 44 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 3 rd 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/c 4 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 shri nks 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
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
- Slides: 74