1. Quasars and Their Evolution (in a nut-shell)

(summary of Lecture 39)

Quasars are extremely luminous active galactic nuclei

- their luminosities imply 0.1-1000 M/year fall into a massive black hole

- they are rare objects

Two possible evolution scenarios:

(i) quasars are long-lived (large fraction of age of Universe),

Ultra-massive central black holes in a few galaxies today

(ii) quasars are short-lived (1 Gyr or less)

Moderately massive central black holes in most galaxies today

Difficult to study quasar (or galaxy) evolution because astronomers tend to collect apparent-brightness limited samples

nearby objects are low-luminosity

distant objects are high-luminosity

This effect masks or mimics the effects of evolution

2. Historical Sketch of the Universe

From start to present . . . an overview

Hot Big Bang ( t = 0 sec )

- the primordial fireball, expansion and cooling

inflation ( t = 10^-34 sec )

- a period of exponentially fast expansion

baryogenesis ( t = 10^-15 sec )

- formation of fundamental particles via pair-production and annihilation

nucleosynthesis ( t = 100 sec )

- formation of atomic nuclei from fundamental particles

decoupling ( t = 1 Myr )

- formation of neutral atoms and the subsequent free-streaming of photons (``light'')

galaxy and quasar formation ( t = 1 Gyr )

- formation of structure on the largest scales

today ( t = 10 Gyr )

- formation of lecture notes on the world-wide-web

Times are in round numbers !

3. The current cosmological paradigm

The Big Bang produced a Universe which was initially . . .

hot

homogeneous

isotropic

. . . and expanding. ``Concept'' of expansion:

not an explosion ``into'' space, but an explosion of space

The associated concept of cooling:

From the primordial fireball, to . . .
. . . the cosmic microwave background radiation (CMBR)

The expansion stretches photons,
makes hot black-bodies appear cool

Notice how we stop short of explaining the actual ``creation'' event - the ``Bang'' itself.

Overwhelming observational evidence:

the expansion is real !

(In lecture 41 we will examine the evidence in more detail.)

4. The Age and Fate of Universe

Age (first pass at calculation):

Assume the Universe has always expanded at the same rate (speed).

- Recall Hubble's law:

velocity = H₀ * distance

time (age) = distance / velocity = 1 / H₀

- Plug in the numbers:

Assuming H₀ = 75 km/s / Mpc . . .

age = 1 Mpc / 75 km/s

= ( 3.1 x 10¹⁹ km / 75 km ) s

= 4.1 x 10¹⁷ s

. . . now in years . . .

= 4.1 x 10¹⁷ s / x 10⁷ s/yr

= 1.3 x 10¹⁰ yr

= 13 Gyr

Fate

Will the Universe expand forever ?

Or does the rate of expansion change ?

Speed up or slow down?

i.e., acceleration or deacceleration ?

What happens if the Universe stops expanding?

density, curvature, and geometry (Lecture 42!)

(a) The expansion stretches the wavelengths of the photons left over from the Big Bang.

(b) The back-body radiation gets refrigerated by the contraction of space.

(c) The Universe radiates away its excess heat.

(d) The expansion causes a sudden blue-shift in the temperature.

(e) The photons become less interactive.

(a) The estimated age of the Universe will remain the same.

(b) The estimated age depends only on the curvature.

(c) The Universe will be estimated to be younger.

(d) The Universe will be estimated to be older.

(e) The Universe will be estimated to be younger, but only if there is no expansion.