University of Wisconsin – Eau Claire

Continuing Education

Dr. Nathan MillerDepartment of Physics & Astronomy

To the Stars and Beyond

WELCOME BACK!

Appearance and motions of night sky

objects Visit to the planetarium to see sky motions

in 3D (we will walk over together) Telescopes: design and basic use The Lives of the Stars The Universe and the Big Bang Life in the universe and planets where it

may be found

Main topics of Course

Our Home Galaxy: The Milky Way

The Milky Way

The Milky Way in Infrared Light (COBE)

William Herschel (1738-1822)

• Made first systematic study of nebulae and star distributions

• Discovered planet Uranus (1781)

One of Herschel’s Telescopes

Herschel’s Drawing of the Milky Way

Shapley (1885-1972) mapped globular clusters MW is big, Sun not at center

Radio Data – Hydrogen emits radiation with wavelength of 21 cm

The Great Debate:“The Scale of the Universe”

Debate between Harlow Shapley and Heber Curtis,

April 26, 1920

Is the Milky Way the Entire universe? Is the Milky Way big (S) or small

(C)? Are the “spiral nebula” galaxies

like the Milky Way (C), or are they in the Milky Way (S)?

Shapley: The Milky Way is the entire universe

Curtis: The Milky Way is but one of many galaxies

The Milky Way Galaxy Basic shape:

– disk 80,000 ly across, 1011 stars– spherical halo surrounding disk– central bulge

Our sun is– 28,000 ly from the center– orbital speed of 220 km/s– completes one revolution every 200

My

The Expansion of the Universe& Hubble’s Law

Nearby Galaxy Image

Galaxy spectrum is aggregate of the spectra of all the stars/nebulae of which it is composed

Spectrum of a whole nearby galaxy

Compare wavelengths of observed lines to where those lines are in the laboratory

Zoomed In Spectrum of Nearby Galaxy

In most galaxies observed, lines shifted to longer wavelengths “redshifted”

Simplest interpretation – Doppler effect

Redshift means galaxies are all moving away from us

Other galaxies don’t seem to like us very well.

Far Away Galaxy (we assume it is far away because it appears much smaller)

Spectrum of Far Away Galaxy

Zoomed In Spectrum of Far Galaxy – Larger Redshift

The farther away a galaxy is, the quicker it is fleeing from us

This is Hubble’s Law.

Does this mean the Milky Way is special (i.e. especially disliked)?

What would astronomers in other galaxies measure?

The graph is a straight line with a y-intercept of zero.

Mathematical form is y = mx

Galaxy velocity = slope times galaxy distance

If the middle, labeled line shown below corresponds to 72 km/s/mpc, which line would correspond to a Hubble constant of 144 km/s/mpc?

The slope is referred to as Hubble’s Constant (=H0). The current best value is 72 kilometers per second per megaparsec (km/s/Mpc)

V (km/s) = H0 x Distance (Mpc)

Note H0 (km/s/Mpc) really has units of inverse time, because km and Mpc really measure the same thing.

The Cosmic Microwave Background Radiation

(CMBR): some of the strongest

evidence for the Big Bang

Look-back time:

As you look farther from Earth, you are looking back in time due to light travel time

You might think that you would see all the way back to the Big Bang, but the universe was so hot that it was fully ionized and not transparent

Big Bang Timeline (starting 12.7 billion years ago):

10-43 sec- shortest time measurable 10-6 sec – 1013 K – Protons and

Neutrons can form from quarks 15 minutes of deuterium and helium

formation 379,000 years – electrons combine

with ions to form atoms, universe becomes transparent

http://map.gsfc.nasa.gov/ContentMedia/MAPevo1588_72.jpg

http://www.youtube.com/watch?v=17jymDn0W6U&feature=popw00us07

Penzias and Wilson with “The Big Ear” (discovered CMBR 1964)

COsmic Background

Explorer (COBE)

1989

Wilkinson Microwave Anisotropy Probe (WMAP) 2001

Universe consisted of gas, all at mostly the same temperature

Universe was not transparent

Universe was emitting a blackbody spectrum

If Cosmic Background Radiation appears like a blackbody emitting in the microwave region, it corresponds to:

A. A. A high temperature (e.g. stars)B. B. A medium temperature (e.g.

people)C. C. A low temperature (colder than

Minnesota on a day in January)

CMB Blackbody – 2.7 Kelvin

But the universe must have been very hot (thousands of degrees) when atoms recombined and the universe became transparent.

What happened?

A Doppler-shifted blackbody curve is just a blackbody curve of a

higher/lower temperature

What was originally a roughly 3000 Kelvin blackbody curve was redshifted so much it now appears like a 2.7 Kelvin curve

CMB is a remnant of when the universe was in a hot, dense state soon after the Big Bang

We can’t see all the way back to the Big Bang because the universe wasn’t transparent to light

Interpreting the Cosmic Expansion

Why “space expanding” and not simple Doppler shift?

1. Cons. of energy – what would cause other galaxies to accelerate?

2. The acceleration would make other galaxies non-inertial reference frames.

3. If Doppler effect, distant objects would be exceeding the speed of light.

Substructure of the CMBR

“All-Earth/All-Sky Temperature Maps”

WMAP results:Hubble Constant: 71 km/s/MPCAge of Universe: 13.7 Billion Yr4% of the universe is made up of

atoms

Take home message These fluctuations are tiny (CMB is very

smooth) but the fluctuations are the seeds of all structure in the

universe.

The Future of the Universe

Fundamental Competition:Objects are moving away from

each other

Gravity tends to pull them back together

Think of tossing ball upwards from earth’s surface

usually, ball will slow down, stop and turn around

If you can throw faster than escape speed, the ball will keep going

There is a borderline case where the ball has just enough energy to escape, no more, no less

What determines what happens to the ball?

How fast the ball is thrown upwards (compare to the Hubble Constant)

The mass of the Earth (compare to the density of the Universe)

In a mass free-universe, Hubble’s Constant would not change over time (F = ma with no force -- >

constant velocity)

In a universe with a lot of mass, Hubble’s Constant must have been much greater in the past (i.e. the

universe is slowing down).

Note: the most important data points are also the hardest to

measure

Open –universe expands foreverFlat – universe asymptotes to a

stopClosed – universe glides to a stop,

starts collapsing

This just in: Actually, the universe is accelerating!

Distant supernova measurements indicate that the universe is expanding faster now than ever before.

This is attributed to dark energy which we know even less about

than dark matter.

But WMAP says dark energy makes up 73% of the universe

Universe made of:

73% dark energy 23% cold dark

matter 4% normal matter

(atoms)

So to sum up:

We know 4% of the universe pretty well

We know some things about the distribution of the dark matter

We know almost nothing about the dark energy which is 73% of the Universe!