From the Big Bang to Planets a brief history of the Universe

Background material – Chapter 1 in textThings I expect you to know, but will not be tested on

Intro to some useful physics – Chapters 4 & 5Things you will be tested on

The Hot Big Bang Theory

The Universe started from a very hot, dense state around 14 billion years ago and is expanding and cooling down with time.

How do we know?

1. Hubble’s law – distant galaxies are all moving away from us, the farther away they are, the faster they are moving

2. The Cosmic microwave background radiation – things that are hot emit light and the universe today is filled with this light emitted when it was much hotter

Hubble’s Law Raison bread analogy

As dough expands all raisons move away from all other raisons

Cosmological Redshift

Light, which is an electromagnetic wave, gets stretched as it travels though the expanding space to reach us from far away galaxies.

Because red light has longer wavelengths than blue light, we say the light from these galaxies is redshifted, meaning shifted to longer wavelengths.

The cosmological redshift

Misconception

The cosmological redshift is not a Doppler shift.

Doppler shifts are caused by the motion through space, cosmological redshifts are caused by the expansion of space.

But there is no way to tell the difference from observations

1st rule to memorize

Cosmological redshifts depend on the distance

Doppler shifts do not depend on the distance

See Chapter 5

Discussion

If all of the galaxies are today getting farther away from each other, what does this tell you about the galaxies in the past?

Hubble time

If we know how fast the universe is expanding, we can trace it back to find a time when all the galaxies we see were at the same place.

This happened about 14 billion years ago.

Discussion

What does this tell you about the size of the universe in past?

How big is the Universe?

Universe – size unknown, may be infinite

Observable universe – 14 billion light years in all directions

Speed of light and Distances

Light travels through space at of fixed speed.

3.00 X 108 m/sec

We can use this as a unit for measuring distances. A light year is the distance light travels in a year.

The Cosmic Horizon

If the universe is only 14 billion years old, then the only light that we can see is the light that has had time to reach us.

The farthest we can see is therefore 14 billion light years away, anything farther the light has not had enough time to reach us.

Cosmic Horizon

1st Law of Astronomy 201

If you squish something it gets hotter

Discussion

Can you think of any examples from everyday life of something becoming hot as it is compressed?

Diesel vs. gasoline engines

Gasoline – air fuel mixture ignited by sparkplug

Diesel – air fuel mixture ignited by compression

Discussion

Diesel engines are about twice as efficient as gasoline engines, so why are the vast majority of cars gasoline cars?

Think about how the engines need to be different.

What is temperature?

Temperature is average random speed of atoms or molecules that make up an object.

Higher average random motions = higher temperatures

Air molecules at 0 C have an average velocity of about 485 m/s or 1090 miles/hour.

Temperature

Discussion

If temperature is a measure of how fast the atoms are moving, how can we have a cold wind? Shouldn’t all winds by definition be warmer than still air?

Why do compressed objects heat up?

Consider a super ball bouncing on a table. What happens to the bouncing ball if you drop a book on it?

Electrostatic charges

Like charges repel (+,+) or (-,-)

Opposite charges attract (+,-)

Discussion

If like charges repel each other, how came the nucleus of an atom can contain more than one positively charged protons? Why don’t all atoms heavy than hydrogen fly apart?

Protons and Neutrons

You can consider a neutron to be a proton + an electron.

A neutron decays into a proton and an electron.

A proton smashed together with an electron will turn into a neutron.

Elements and isotopes

The element is defined by the number of protons in the nucleus.

The isotope is defined by the number of protons + neutrons in the nucleus.

1st Law of astronomy 201

If you squish something, it heats up

The opposite is also true, if something expands it cools.

Example – a refrigerator

Space is very cold, now

As the universe expands, it cools. Currently its temperature is 2.73 K or –455 degrees F.

But, as we run time back, the universe was much denser and much hotter.

Hydrogen

The early universe consisted of entirely of a plasma of hydrogen nuclei (protons), neutrons, electrons and photons.

More complex nuclei would be busted up by high speed collisions

Nucleosynthesis

As the universe cooled

Proton – neutron collisions produced deuterium

deuterium – deuterium collisions produced 3He

deuterium – 3He collisions produced 4He.

The first 5 minutes

At the end of nucleosynthesis the universe consisted of 75% hydrogen and 25% helium with a trace of lithium and beryllium.

Also there were left over traces of deuterium and helium-3

Neutral atoms

After about 380,000 years of expansion the universe cooled to about 3000 K and electrons could remain bound to protons forming the first neutral atoms.

Discussion

What happens when a cloud of hydrogen and helium contracts because of gravity?

Cosmic Microwave Background

The early universe contained no life

Without carbon, life could not exist in the early Universe.

Planets like the Earth could not form without iron, silicon and oxygen.

Andromeda Galaxy

What are stars?

Stars are enormous balls of mostly hydrogen and helium gas.

Internal pressure halts gravitational collapse

The core of the star gets so hot that hydrogen can fuse into helium. The heat released by this reaction stops the star from contracting.

The struggle to avoid collapse

When the hydrogen in the core runs out, the star starts to collapse again and becomes even hotter.

If the star is massive enough it will become hot enough to fuse helium into carbon.

Ring nebula

How do the elements got out of the stars?

Once enough iron builds up in the core of a massive star, it suddenly collapses. The outer layers of the star “bounce” off the core and are blasted into space in a supernova explosion.

Supernovae

Supernovae explosions are the most energetic events in the Universe. A single star can outshine 100 billion stars.

The explosion of the outer layers is so violent that all the elements heavier than iron can be produced, including many unstable radioactive isotopes.

Supernova 1987a

The Crab nebula

Supernovae can trigger star formation

The expanding shock wave of a supernova explosion, can cause a nearby cloud of gas to collapse and form new stars.

Thus, its heavy nuclei get incorporated into the next generation of stars.

Summary

Squishing things makes them hot

Temperature is a measure of the average random speed of atoms

The universe started out with mostly hydrogen and helium with all other elements were created during supernovae explosions of stars more massive than the Sun