Astro 103 - Lecture 26

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1. Historical Backdrop

Early 20th century

Einstein and almost all scientists believed . . .

. . . Universe is static

. . . unchanging, not expanding

But then ...

Einstein's field equations

(General Relativistic version of Newton's Equations of Motion)

Universe is expanding!

. . . oops . . . add `cosmological constant:'

Hubble (1930's):

Discovers galaxies are receding from us

Measures the expansion

Einstein claims cosmological constant was the biggest blunder of his life. But recent evidence (1990's) indicates he might have been right after all!

2. Two Competing Theories of the Universe

Big Bang vs. Steady State

Both assume a homogeneous, isotropic Universe

no edge

no center

every part looks the same (on average, i.e. over a large enough volume)

Big Bang -

initial `singularity' (beginning)



cooling, transformation of energy into matter, formation of structure, all through normal physical processes.

Steady State -

expansion (in current theory; original theory did not!)

no evolution

no beginning

no end

matter (Hydrogen) spontaneously created ``in between'' Galaxies

NB: Both theories call for space to be spontaneously created ``in between'' Galaxies.

3. Which Theory is Right?


Which theory has better predictive power?

and is the simplest ....

and invokes the fewest ``tooth fairies.''


The microwave background

The remnant of a primordial explosion or what?

Elemental Abundances

Major triumph of the Big Bang is nucleosynthesis.

A ``primordial'' composition of elements, formed in the early, hot phase of the Universe, before further processing via fusion in stellar cores and supernova explosions.

Formation and evolution of structure (galaxies, clusters)

A natural part of the Big Bang.

Questions and Problems:

- Are redshifts real? i.e., are they cosmological ``recession'' velocities? If not, one could call into question the evidence for evolution.

(no longer an issue)

- What about the ``Horizon Problem'' and the ``Flatness Problem'' ? (still an issue)

Needed: One more ingredient to Big Bang model


4. The Cosmic Microwave Background Radiation (CMBR)


early 1940's (in Big Bang scenario)


Penzias and Wilson (1964)

Bell Labs (ATT), New Jersey


COBE satellite measurements

T = 2.7 degress (K)

isotropic to 1 part in 105 (0.00001)

Two ways to think about what this radiation is:

(i) Primordial fireball was hot

(think: black body radiation)

- Universe cooled during expansion.

- What we see today is this ``cooled'' radiation left over from the Bang.

(ii) Or, we are looking back to just before the epoch when the Universe cooled enough to become 'transparent' to radiation.

We are seeing the `wall' which represents Universe when it was still opaque, but redshifted.

Credit: WMAP Science Team

Historical Relevance

Today almost all astronomers believe the Big Bang is the correct theory . . .

. . . to the point of being `gospel.'

That's ok, since the theory works pretty well.

But there are still some very substantial problems and questions not answered by the theory.

What isn't particularly healthy is the lack of a competing theory to question and challenge the status quo.


The debate between the Sun-centered and Earth-centered view of the Universe (or, at least the solar system).

The Sun-centered theory was better than the Earth-centered one, but still left a few observations left unexplained.

(e.g. Mercury's orbit General Relativity)

The point is: Keep questioning until the theory is simple, elegant, and continues to correctly predict new observations.

Q26.1 Which theory predicts that Hydrogen atoms are spontaneously generated in between galaxies as the galaxies expand away from each other? Do we think this is correct or incorrect?

(a) The Big Bang Theory, erroneously.

(b) The Steady State Theory, erroneously.

(c) The Inflation Theory, correctly.

(d) The Earth-centered Theory, erroneously.

(e) The Theory of General Relativity, correctly.

Q26.2 Why is it important to keep questioning the current paradigm of the Big Bang theory?

(a) What else would we have to do? (It wouldn't be much fun if there weren't some controversy.)

(b) The theory of a static Universe could again gain wide-spread acceptance.

(c) There are still some observations and questions left unexplained and unanswered by the Big Bang theory.

(d) If we can't question the theory, it can't be right.

(e) The Big Bang must be questioned because the Steady State theory appears to better fit our observations.

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Last updated: Aug 23, 2011 Matthew A. Bershady