A lot happened in the 1920's. First, Edwin Hubble, using the new 100 inch telescope on Mt. Wilson, discovered Cepheid variables in M31, thereby establishing, definitely, that this spiral nebula was, in fact, another galaxy external to our own. Our view of the universe thus expanded to one in which our own Milky Way galaxy was but one of billions, not the sole central object in an otherwise empty universe.

Then Hubble noticed that on the scale of galaxy clusters, all clusters were moving away from us and, the more distant clusters were receding proportionately faster. These observations were correctly interpreted as evidence of an expanding universe in which all galaxy clusters are receding from each other as the spaces between them increase in size; from inside any cluster one would see all other clusters moving away in all directions, thus giving the illusion that one is in the center of everything. Of course one would also see the same effect from inside any other galaxy cluster.

The relationship between recessional velocity and distance became known as Hubble's Law.

Cepheid variables are large stars which vary their brightness due to a periodic expansion and contraction. They can be used to calculate distance, because they exhibit a definite relationship between the period over which they vary their output and their luminosity or total power output (period — luminosity relation). The period - luminosity relation for Cepheid variables has already been determined using stars in the Small Megellanic Cloud and within our own Milky Way galaxy, and since the physical characteristics of Cepheids appear to be universal, the same assumptions and analyses can then be applied to Cepheids in other galaxy clusters to determine their distance.

Universe has constant density for all time

Perfect cosmological principle: universe same in all directions all the time

New matter created spontaneously to maintain constant density

Generally not accepted today due to discovery of 3K background radiation

Big Bang Theory

Began w/ singularity 13.7 billion years ago

Exploded - expansion

Heisenberg Uncertainty Principle - Planck time - 10⁴³ Sec

Density = 10⁹⁰ kg/ CM³ @ P.T.

Temp. = 10³¹ OK (10,000 billion, billion, billion OK)

Radiation & neutrinos, few electrons, protons, neutrons, etc.

Hadrons (heavier particles) from primordial quarks in 1 millionth sec.

Leptons (lighter particles) ist second

Slightly unequal matter & antimatter

Gravitons, primordial black holes

Hawking - black holes evaporate

BH's > 10 billion tons still surviving (diam. 1/100 billion m.m.)

<@ 1/2 million years after B.B. radiation decoupled

Elementary particles associate into atoms, molecules normal matter

Galaxies, stars, solar systems?

.'.Universe becomes transparent

Radiation freed at that time - 3K background radiation found in universe today - cooler due to continued expansion

Large scale fluctuations in matter density existing @ decoupling remained - 1,000 trillion - 10,000 trillion solar masses

Supercluster-sized blobs collapsed due to gravity to form clusters of galaxies & individual stars

Superclusters probably largest structures existant in universe

Residual background radiation from decoupling: 3K

Today visible universe = approximately 13.7 billion L.Y.

Most distant objects - quasars - QSOs

Quasars very old - looking back in time

Possibly galaxies in early stage of development

Possibly contain massive black holes

Background radiation - Perzias & Wilson:Nobel Prizes .Robert Dicke,Prinston

Critical density

Density of universe > c.d. fall back on itself - closed universe or possibly oscillating universe

Density < c.d. - continued expansion - entropy death (open universe)

Everything burns out, runs down = 1/10 c.d. found at present

But: Other possibilities

- neutrinos w/ mass

- black holes

- dark stars

- giant coronas surrounding galaxies

- exotic particles

Now, acceleration: Dark energy, quintessence?