Starry Monday at Otterbein

Astronomy Lecture Series-every first Monday of the month-

May 1, 2006

Dr. Uwe Trittmann

Welcome to

Today’s Topics

• Introduction to Cosmology

• The Night Sky in May

On the Web

• To learn more about astronomy and physics at Otterbein, please visit– http://www.otterbein.edu/dept/PHYS/weitkamp.a

sp (Observatory)

– http://www.otterbein.edu/dept/PHYS/ (Physics Dept.)

Cosmology• The part of astronomy (and astrophysics) that deals with

the greatest structures in the universe – and the evolution of the universe itself!

• The “start” of the universe, a primordial fireball the early universe was very hot and dense

intimate connection between cosmology and nuclear/particle physics

“To understand the very big we have to understand the very small”

Big Bang

Questions, Questions, Questions

• Scientists want to know, so they ask questions:

– What is in the universe?

– How do these things interact?

– How does the universe change in time? • Is there a beginning?

• Is there an end?

What’s in the Universe?

• Answers come from observations

Let’s observe:

The Earth

PlanetsMercury Venus Mars

JupiterSaturn

The Sun (a typical star)

Stars

Galaxies

Clusters of Galaxies

What’s in the Universe?

THE UNIVERSE

clustersand

superclusters

voids

galaxieslike the

Milky Way

quasars

Starsnebulaemolecular cloudsstar clusters

Solar System

black holespulsars

Sun

planets

moonscometsmeteorsasteroidsdust

terrestrialjovian

Big ……………………………………..small

What’s in the Universe?

A lot of stuff !!! Scientific term: Mass

Observation II: It is dark at night!

• Big deal!

• Indeed: it has cosmological consequences!

• Let’s find out why!

Night sky: No sun – just stars

Look closer and find more dimmer stars

If the Universe is infinite…

There’smoreandmore…

dimmeranddimmerstars

Until finally…

…the view fills up completely

…and it’s as bright as the day!

So, why is the night sky dark?(Olbers’ Paradox)

• Conclusion: either

– Universe is not infinite or

– Universe

changes in time

Observation III: 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

The Universe expands!

• Where was the origin of the expansion?

Everywhere!

• Every galaxy sees the others receding from it – there is no center

Conclusions from our Observations

• The Universe has a finite age, so light from very distant galaxies has not had time to reach us, therefore the night sky is dark.

• 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.

How does the expansion work?

• Like an explosion (hot, dense matter in the beginning), but space itself expands!

• Slowed down by gravitational attraction

• Attraction is the stronger, the more mass there is in the universe

• Scientifically described by Einstein’s General theory of Relativity (1915)

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 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

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

    

                                             

The “size” of the Universe – depends on time!

Expansion wins!

It’s a tie!

Mass wins!

Time

So, how much mass is in the Universe?

• Can count all stars, galaxies etc. this gives the mass of all “bright”

objects

• But: there is also DARK 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

First evidence for dark matter: The missing mass problem

• Showed up when measuring rotation curves of galaxies

Is Dark Matter 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

• Good news: it still behaves like mass, so Einstein’s cosmology still works!

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?

Back to: 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 2.725 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 universeSupernovae are further away than expected for any decelerating (“standard”) universe

Supernova Data

redshift

magnitude

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

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

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

• Cosmology is one of the most exciting subfields of physics these days

• The is an intimate connection between cosmology and particle physics

• We live in a “golden age” of cosmology: lots of data available and being measured

• Today’s era is that of “precision cosmology”

• There is lot’s we don’t know interesting for young scientists!

The Night Sky in May

• Nights get shorter => later observing!

• Spring constellations are up: Leo, Big Dipper, Virgo

• Saturn dominates the evening, Jupiter early morning.

Moon Phases

• Today: Waxing Crescent 17%

• 5/5 (First quarter Moon)

• 5/ 13 (Full Moon)

• 5 / 20 (Last Quarter Moon)

• 5 / 27 (New Moon)

Today at

Noon

• Sun at meridian, i.e. exactly south

10 PM

Typical observing hour, early May

• Jupiter

Mars

Moon

Saturn

Zenith

• Big Dipper points to the north pole

West

• Saturn near Praesepe, an open star cluster

South-West

• Spring constellations:– Leo– Hydra

South

• Canes Venatici:– M51

• Coma-Virgo Cluster

• Globular Star Clusters– M3, M5

South

Virgo and

Comawith the

Virgo-Coma galaxy cluster

Virgo-Coma

Cluster

• Lots of galaxies within a few degrees

M87, M88 and M91

East

– Hercules– Corona

Borealis– Bootes

Globular Star Clusters:• M 3• M 13• M 92

M13: Globular Cluster

Mark your Calendars!

• Next Starry Monday: October 2, 2006, 7 (!!!) pm (this is a Monday )

• Observing at Prairie Oaks Metro Park: – Friday, May 5, 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 Wednesday, 3:30 pm

• Open to the public, everyone welcome!

• Location: across the hall, Science 256

• Free coffee, cookies, etc.