Lecture 40 Cosmology IV

• In spite of all this, there are arguments that Omega must be 1

• There is observational evidence that space is Euclidean, not curved

• “Inflation” (see p634) requires Omega to be 1

• If Omega is approximately 1, it is probably exactly 1 (??!!!***)

Simply stated: if Omega =1, it stays 1 through the history of the universe. If Omega < 1 or Omega > 1, the value depends on the Cosmic Scale Factor a.

Since a has changed enormously (infinitely) throughout the history of the universe, it shouldn’t be anywhere near 1 now if it wasn’t very, very close to 1 early on.

So if it is close to 1 now, it is probably exactly 1

Important new piece of information in and about 1997

• If you have a “standard candle”, an object with constant, known M, you can measure how a(t) has changed throughout the history of the universe

• Basic idea: expanding universe says light from a source has to “fan out” into a larger volume than in a static universe. Fall-off of intensity is steeper than the inverse square law

• The degree of extra “fan out” depends on how a(t) has changed with time

• For Omega =1, the fan out is the smallest• For Omega =0.33, it is larger• For Omega = 0, it is larger still, and the maximum for a Friedmann universe

Use cosmological model to calculate m of z

Now need objects to fill up the plot. Real standard candles

• Type Ia supernovae….white dwarfs “shoved over the edge”

• http://antwrp.gsfc.nasa.gov/apod/ap000312.html

What do the data on Type Ia supernovae show?

• http://www-supernova.lbl.gov/public/misc/forrosen/sciencesnpop.pdf

• Distant Type Ia supernovae are even fainter than in an empty universe

• The scale factor has increased even more than for Omega = 0

Points to the following form for a(t)

An accelerating universe How is it possible?