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!

1

2

3

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

                                                                                                 

                                                                 

1

2

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

1

2

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