Prof. Mel Ulmer

DEPT OF PHYSICS

&

ASTRONOMY

http://www.astro.edu/astro/faculty/ulmer/a01_fall2002.html

Discussion sections begin meetings this week

Begin observing

• Dark Energy

• Two kinds of weakly interacting matter we haven’t detected here on earth (yet).

=>We are living in a very exciting time and are on the verge of making new discoveries in physics or we’ve got the Myth all wrong.

We’ve evoked “New Physics”

•What is Dark Energy?

•Answer: from a key theoretician working in this area, and I quote: “We haven’t a clue.”

•Simple answer is Einstein’s biggest blunder, Lambda ()= the energy of the vacuum (whatever the heck that means)

Why have we done this?• Because we don’t know any other way to proceed

• New forms? dark energy, non-baryonic matter (perhaps we need 2 kinds it seems; self interacting and non-interacting [except gravity])

• Within the context of theory, all 3 observational methods agree with each other

SN Ia CMB

clusters

• Review and back track:

• Big bang concept and smoothness of the CMB lead to required new forms of matter

• Detailed observations of CMB + supernovae + clusters of galaxies + interpretation leads to need for Dark Energy

Next 2 weeks or so, first CMB, then Supernovae and then Clusters: “who, what, where, how”(more or less)

• Smoothness issue:

• Why are we here?

• Or why is the universe lumpy today when it was smooth as seen in the CMB?

Concept: gravity acts the same in all directions => need to get “seeds” to start lumps.

If lumps from after CMB, then explosions are needed and would be visible [not seen]

Step one, seeds form before CMB, therefore seeds are visible at some level in the CMB

•If seeds made from normal matter, then electrons:

•then affects CMB so much that CMB would be “rough” to 1 part per thousand

•but observed is “rough” only to between 1 part per million and 50 parts per million

Concept calculation:

Start with galaxy today = density of about 50 million times the critical density today

Go back to time of CMB, (1+z = 1000)

Density goes up by 1 billion (109) but galaxy stays same => 50 x 106/109 =

5 x 10-2 = roughness!

The real calculation is beyond the scope of the course.

Now what about the CMB?

• How do we know the temperature?

• Because when we measure the intensity versus color, we find a very specific shape we named “BLACK BODY”

•A black body has a very definite shape and peak intensity color, so that there is a one-to-one correspondence between temperature and peak intensity.

• How do we know this isn’t more local?

• We can use motion and red/blue-shift

• We know these relative motions:

• Earth-Sun (30 km/sec)

• Sun local stars (20 km/sec)

• Local stars and the Galaxy ( 250 km/sec)

• The Galaxy to Others (400 km/sec)

The detected motion plus expectation gives us confidence the CMB is from the Big Bang

Look for Earth motion relative to CMB

Detected motion in expected direction and size (magnitude) assuming the CMB is well beyond local galaxies =>

CMB, red = cold; blue = hot, we are moving towards blue

(remember concept of redshift (or blueshift)

• Result of turning up the magnification

• These are COBE (Cosmic microwave Background Explore) results = 10 degree scale