Prof. Mel Ulmer

DEPT OF PHYSICS

&

ASTRONOMY

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

No discussion section meetings this week

Next week begin observing, no signups necessary

Big Bang Myth

• 15 billion years ago universe was hot and dense and expanding

• It cooled and got less dense until it got to about 3000 K [273 K = 0 Celsius]• Then the gas became transparent = emits light

• We see this light today as the Cosmic Microwave Background (CMB) = really cold = 3 K

Brick Wall, 15 billion lt yrs

Basic Concepts• Space and time are interrelated

• Far away = look back in time. =>

• Brick wall about 15 billion light years away from us =>

• far away means looking at younger universe

• Brick wall = 500,000 to 1 million years old.

• The further away, more rapid expansion

• = The larger the shift to the red

• “redshift” = 1+z =>

Big Bang Myth Details

• Seeing to the brick wall rushing away from us so it has a“redshift” of 1+z =1000 and T=T_o(1+z)

• T=temp of brick wall, T_o=Observed today

• Look far away, we see universe when it was denser!

•Too dense to see through

• The brick wall concept… we see back until the universe is too dense for light to penetrate!

• Far away means long ago and high velocity of recession goes with z and redshift

• Long ago means the universe was hot and dense then

•But hot then but looks cold now

• Key concepts:

•Expansion, distance, and “time travel” are all related

•Far way = long ago = objects moving away from us faster the further away they are.

•Long ago means (in our Myth) hot and dense

•Today means old and “sparse”

•At Brick Wall density = about 10-21 g/cm3)

• Today, density = about 10-30 g/cm3

Brick Wall Facts:

•If Brick Wall density much less than air (about a billion, billion times less, why can’t we see through it?)

• Because it is about 1,000 light years thick and physics (deferred to later in the course)

•Brick Wall (a.k.a. the CMB) was found to be too smooth to be explained by ordinary matter. Why? (again more deferred to later)

• Ordinary matter affects light with electrons (think electricity, radio, TV etc).

• If we assume initial lumps made of of enough ordinary matter to make clusters, CBM would have been lumpier

• How much?

•About 10-100 times too much!

• “Simple” calculation to “see this”:

• Go backwards in time.

• Clusters don’t contract (aren’t expanding now, this is reverse)

•Universe does get more dense take ratio of densities cluster [lump] to average = about 5/10,000 = 500/1,000,000 versus observed smoothness to 50/1,000,000

Model Universe Based on GR• Model is that we live on a 4-dimensional surface

• The more matter, the more tightly curved the surface:

• Works “locally,” and for the entire universe =>

• Concept of curvature of Universe

• It is related to the “mass density of the universe

• Curvature (k)

• k = +1, closed (balloon)

• k = 0 flat (sheet)

• k= -1 open (saddle)

k=-1

k=0

k=+1

GR Concepts, Cont.

> 1; k =+1

0=1 ; k = 0

0<1 ; k =-1

•Additional concept: “density parameter = 0; the

subscript 0 means “today”

• stands for a ratio of the density of the universe to some “critical value” (later)

and k are intimately related:

•Basic Holy grails of our Myth:

k and 0

• Other one, does universe expand forever or not?

• knowledge of value of 0 ALONE used to guarantee we could predict the future

• “Ain’t necessarily so”!!!

complex case is ( greater than 0 )

value k value surface future

Greater than 1 +1 closed expand forever#

1 0 flat expand forever

Less than 1 -1 open expand forever

#unless m > 3, just barely positive = 0.01

• Because of dark energy!

• Energy and mass are equivalent through E = Mc2

•Dark Energy contributes to

but

• Dark Energy can act as a pressure to push the universe apart more strongly than the Dark Energy mass pulls things in

Basic Concepts Cont.•Additional holy grail: amount of Dark Energy =>•We need to know

• (Dark Energy mass ratio),

• m (total matter [= “stuff”] ratio),

• Total gives us k

• Generally, positive means will expand forever.

Bottom Line:

= 0.7, m = 0.3, k= ?

0

Why do we believe this?

Observations plus “faith” in GR