Astro 105: The Milky Way

Lecture XVI:
A Short Course on Cosmology
Cool Cosmo Links
1,2 ,3



The most incomprehensible thing about the Universe is that it is comprehensible
albert einstein

In the beginning ...

Creation stories are as old as our creation.

Every culture has one (or two...)

They are a fundemental way we humans make ultimate sense of the ultimate questions.

What is truely amazing - and speaks to our theme of "Coming of Age..." is that astrophysics has one too.

Most important, the story which science proposes is just now becoming amenable to the scientific process.

We can actually test our ideas about the creation and evolution of the Universe!

We can go back almost all the way to

t = 0

WARNING: the Ulimate question - what started it all? what was going on at t = -1. is still a question which science can not answer

Still a "theological question". Will it remain so? (Quantum Fluctions?).

Today we review Cosmology - the theory of the Universe as a whole.

The main point of the day is: we have one and it seems to be on strong footing.

The Big Bang

The dominant idea of Cosmology is that the Universe had a beginning.

That in itself is radical.

Even more radical is the notion that in the beginning the Universe was infinately compressed into a single point of mass/energy/space/time.

Under these conditions the Universe has to expand - Explode in fact.

This is the Big Bang - (a term of derision originally).

Our ability to (tentitively) describe this ball of creation begins at

t = 10-43 sec

That is an unimaginably short period of time.

Before this our understanding of physics can't deal with the conditions.

Why? Lots of reasons but consider just the uncertainty principle

Space/Time Matter/Energy can only get so confined.

There is something called the Planck Length where space-time itself becomes a quantum foam.

We can't ask what came before the big bang because there was no "before".

In the ultra-dense ultra-hot early universe the four physical forces we know today

  • Gravity
  • Electromagnetism
  • Strong Nuclear Force
  • Weak Nuclear Force

were unified into a single Uni-force.

Aesthetic - One is better than more.

As the universe expanded and cooled the one Uni-force split into the 4 we see today.

Note that split could have happened differently. Perhaps in other universes it did.

As the Universe continued to expand and cool a number of important epochs occured such as Inflation and Recombination etc.

At first the universe is hot and smooth.

Matter and radiation are always interacting

When universe is cool enough for electrons and protons to combine into H atoms the universe becomes trasparent to the radiation.

This is the Recombination epoch.

As it cools tiny "perturbation" in the density begin to get enhanced by gravity.

Eventually these perturbations become the seed of structure formation

Galaxies,Clusters, Superclusters, etc

Why do we Believe the Big Bang?


There are three pieces of evidence which strongly suggest that the Big Bang occured.

I.E. The Universe started out much smaller and much hotter.

  • Expansion of the Universe (galaxy redshifts)
  • Cosmic Microwave Background Radiation
  • Abundence of primordial elements H, He, Li etc.

Each of these would be difficult to explain with some kind of big bang model.

Togeher they form a killer 1,2,3 punch for BB cosmology.

Expansion of Universe

We have already talked about the Hubble law which comes from galaxy redshifts.

The cosmological interpretation of this is that all of Space is expanding.

If everything is rushing away from everything else then...

Balloon Analog Of Expanding Universe - The coins taped to the surface of a spherical balloon recede from one another as the balloon inflates (left to right). Similarly, galaxies recede from one another as the universe expands. As the coins recede, the distance between any two of them increases, and the rate of increase of this distance is proportional to the distance between them. Thus, our balloon expands according to Hubble's law.

But if its space that is expanding why do we see redshifts.

The light itself is streached as space expands.

The more distant an object is the more space has expanded while it was traveling

Balloon Analog Of Cosmological Redshift - As the universe expands, photons of radiation are stretched in wavelength, giving rise to the cosmological redshift.

Note that Hubbles law also gives you the age of the Universe.


Recession Velocity = Ho distance
time = distance/velocity
= distance/(Ho distance)
= 1 /Ho

Also just look at the units

Ho = 75 km/s/Mpc = distance/time/distance = 1/time
t = 13,000,000,000 years.

But why is 1/Ho the age of the universe?


Cosmic Microwave Background

The CMB was discovered by accident by two physicists at Holmdale New Jersey.

It had actually been predicted years before by George Gamow!

Everyone said "No Way" and forgot about it.

No matter where you point your microwave antenna you pick it up.

It is almost a perfect blackbody with T = 2.7 K.

Why is it here?

Before the recombination era matter and radiation where perfectly coupled.

The Universe was opaque.

Then recombination occured and matter and energy went their seperate ways.

At the instant of recombination the photons where left with nothing to interact with.

They became fossils of the birth of the Universe.

As space expanded so did the photons.

They cooled down to 2.7 K.

What does the CMB look like.

Pretty smooth expect for the doppler shift due to the Earths motion

But CMB must remember the seed "perturbations" which produced all the structures.

At some small level there should be "bumps" in the CMB due to the little aggregates of mass in the early universe.

All our theories of evolution demand it!

In 1990 it was finally found with the COBE satellite

These "bumps" correspond to deviations in temperature of

1 part in 100,000.

This was a major triumph for big bang cosmology because it links the CMB, which is a fossil of the big bang with structures we see today.


Recently the WMAP satellite improved our understanding of the CMB



Primordial Abundence of Elements

Only the lightest elements could be formed in the big bang (H, He, Li).

How much of these elements is produced is very tightly constrained by theory.

The predicttion of the ratios of He/H, He/Li etc has been matched very well with observations.


Fate of the Universe

Open or Closed? It Depends on the amount of Mass.

Is there enough matter to slow the expansion of the Universe.

Is there enough matter to halt the expansion of the Universe and get it to recollapse?

What about new data showing that Universe is accelerating!

Three Scenarios For Future Of Universe - Distance between two galaxies as a function of time in each of the three possible universes discussed in the text: unbound, bound, and marginally bound. The point where the three curves touch represents the present time.

Scenario For High-Density Universe - A high-density universe (a) has a beginning, an end, and a finite lifetime. The lower frames (b) illustrate its evolution, from explosion to maximum size to recollapse.

Scenario For Oscillating Universe - An oscillating universe has neither a beginning nor an end. Each expansion-contraction phase ends in a "bounce," which becomes the "Big Bang" of the next expansion. There is currently no information on whether or not this can actually occur.

Shape of Universe and Fate

Omega_0 is critical matter density of Universe.

We now believe however that the Universe is accelerating which means there exists a cosmological constant, an energy which exists everywhere pushing space apart


The Dark Energy will compise most of the energy content of the Universe!!!


Formation of structure

gravity makes structure
movie 1
movie 2

The Horizon Problem
and Cosmic Inflation



The uniformity of cosmic background radiation--varying by no more than one part in 10,000, where ever you look--is a problem to Standard Big Bang cosmology.

Suppose the universe began 14 billion years ago. We look to the west, we detect cosmic background radiation. We turn our radio antennas to the east, we detect cosmic background radiation--at exactly the same temperature. The radiation from the east and the radiation from the west are separated by 28 billion light years.

The radiation from the east could not possibly be causally connected to that from the west, because information cannot travel faster than the speed of light. Nor could the regions they traveled from ever have been in communication.

It's as though 200 students one of those huge introductory chemistry classes were taking a test in a huge auditorium. And yet each student scores exactly 93% on the test. There had to be some cheating going on. But how?

The horizon problem gets worse if we travel back to the time when radiation was released from matter. The universe was 100,000 years old--meaning that the horizon was 100,000 light years across. But the east and west photons reaching our radio antennae today were then separated by 10 million light years. That's 100 times the horizon!

Solution?  Inflation Theory

The instant before inflation began, 10-35 seconds after the Big Bang, the "stuff" that expanded to become our universe was only about 10-24 centimeter in diameter. All matter and energy were in close and uniform contact.

Within the briefest instant, the universe expanded exponentially by a factor of about 1050, stretching once
intimately connected matter and energy to the farthest reaches of the universe. The information contained in the pre-inflationary universe didn't have to travel the speed of light--indeed it couldn't have--it traveled at the speed of inflation.