The Beginning of Time

The Big Bang

Our goals for learning:• What were conditions like in the early

universe?• What is the history of the universe

according to the Big Bang theory?

To what extent can we apply the Scientific Method?

Test theory against observation and experiment.

We observe the universe to be expanding. So, it must have started out much smaller.

Run the Hubble Expansion backwards.

If you go all the way back to when the universe was an infinitesimal object – that moment was the beginning of space and time, the Big Bang. [Remember “space” is expanding between galaxies.]

The universe must have been much smaller, hotter and denser earlier in time. How much hotter?

• Note: 1/H0 is the “age” of the universe for a constant rate of expansion

THE BIG BANG

First, the Big Bang Theory

The Early Universe must have been extremely hot and dense, with emphasis on the word “extreme”.

What happens under such conditions?

Temperature versus Time

Now

1 second old

We know how the laws of physics work up to about 1015 K or 10-10s

Photons converted into particle-antiparticle pairs and vice-versa

E = mc2

in reverse

PARTICLE CREATION

The early universe was full of particles, anti-particles and radiation because of its very high temperature. Look at a SPACE-TIME diagram.

The Planck Era

Before the Planck time (~10-43 sec)we don’t know what happened because we have no theory of quantum gravity.

Because space is so compressed there are huge energy fluctuations from point to point. Energy and mass are equivalent, which implies huge random gravitational fields that warp space and time, the opposite from Einstein’s smooth spacetime.

At extremely small sizes (~10-33 cm), Einstein’s Theory of Gravity and the Quantum Theory are not in agreement.

Strong Force: binds nuclei – like a restraining spring that prevents the (3) quarks that make up protons and neutrons from escaping Electromagnetism: binds atoms

Weak Force: responsible for radioactive decay

Gravity: holds planets together

The Forces of Nature

Four known forces in universe:

Strong Force Electromagnetism

Weak Force

Gravity

Why FOUR forces? Do these forces perhaps unify at high temperatures?

Observations at giant particle accelerators have proved that these force unite at high energy.

Four known forces in universe:

Strong Force Electromagnetism

Weak Force

Gravity

Do the forces unify at even high temperatures?

We are currently looking for evidence that the first three forces merge around 1029 K (10-38s).GUT = Grand Unified Theory

Four known forces in universe:

Strong Force Electromagnetism

Weak Force

Gravity

What about the temperature of the Big Bang?

Superstring theory is based on the idea that everything is made from tiny ribbons of energy. Strings are much smaller than we can detect.

GUT EraLasts from Planck time (~10-43 sec) to end of GUT force (~10-38 sec). Perhaps the simplest elementary particles form.

But, a critical transition may cause enormous expansion.

When the forces separate it is like a change of phase from liquid to a solid. Energy was released that drove an ultra rapid expansion of space.

Major Events since the Big Bang

Electroweak Era

Lasts from end of GUT force (~10-38 sec) to end of electroweak force (~10-10 sec)

Intense radiation fills all of space. Particles create and annihilate. The universe is expanding and cooling. Eventually it cools to 100 million times the core of the Sun. Suddenly weak & electromagnetic forces separate. From this time/ temperature onwards we have direct experimental evidence.

Particle Era

Photons convert into all sorts of matter.Amounts of matter and antimatter nearly equal; ends ~1 milliseconds.

(Roughly 1 extra proton for every 109 proton-antiproton pairs!)

We don’t know why there was such an excess, but it is the reason we are all here now.

Era of Nucleosynthesis

Begins when matter annihilates remaining antimatter at ~ 0.001 sec

Nuclei begin to fuse; temperature is now a few billion degrees.

Era of Nuclei

Helium nuclei form at age ~ 3 minutes

Universe has become too cool to blast helium apart

Expansion and cooling continue 75% H and 25% HeBut it is still a hot, opaque plasma

Era of Atoms

Neutral atoms form at age of ~380,000 years.

At 3,000 K the mix becomes transparent to radiation, instead of being an opaque plasma.

Radiation streams to every part of the cosmos, establishing a kind of background glow – we detect this today as the Cosmic Microwave Background.

Era of Galaxies

Galaxies form at age ~1 billion years.

In principle, we could “see” this happen using the next generation of big telescopes in space and on the ground.

And here we are, ~14 billion years later, thinking about all of this!

What have we learned?• What were conditions like in the early universe?

– The early universe was so hot and so dense that radiation was constantly producing particle-antiparticle pairs and vice versa

• What is the history of the universe according to the Big Bang theory?– As the universe cooled, particle production stopped,

leaving matter instead of antimatter– Fusion turned remaining neutrons into helium (first 3

minutes)– Radiation traveled freely after formation of atoms (no

more electron scattering); age = 380,000 years

Evidence for the Big Bang

Our goals for learning:• How do we observe the radiation left over

from the Big Bang?• How do the abundances of elements support

the Big Bang theory?

Primary Evidence(in addition to Hubble expansion)

1) We have detected the leftover radiation from the Big Bang – the Cosmic Microwave Background.

2) The Big Bang theory correctly predicts the abundance of helium and other light elements (Deuterium, Lithium).

The cosmic microwave background – the radiation left over from the Big Bang – was detected by Penzias & Wilson in 1965.

(Both engineers, working at Bell Labs in Murray Hill, New Jersey.)

Background radiation from Big Bang has been freely streaming across universe since atoms formed at a

temperature ~3,000 K. Thermal Spectrum: Visible/IR photons with wavelength of 1 millionth of 1 meter.

Scattering by electrons makes the plasma opaque

No free electrons means the gas is transparent

Expansion of the universe has redshifted thermal radiation from that time to ~1000 times longer wavelengths: the photons now have mm wavelengths. These are microwaves.

Background has perfect thermal radiation spectrum at temperature 2.73 K

The young universe had a THERMAL SPECTRUMPeak wavelength = 1 micrometer

But …

Patterns of structure observed by WMAP tell us “genetic code” of universe. WMAP is the Wilkinson Microwave Anisotropy Probe satellite which has been mapping the cosmic microwave background.

Protons and neutrons combined to make long-lasting helium nuclei when universe was ~ 3 minutes old – because the (rapidly dropping) temperature was just right.

REMEMBER THIS REACTION IN THE SUN’S CORE?

Big Bang theory prediction: 75% H, 25% He (by mass)No free neutrons around, all bound in atomsMatches observations of nearly primordial gases

Abundances of other light elements agree with Big Bang model having 4.4% normal matter – more evidence for WIMPS!

More detailed evidence – the deuterium (heavy hydrogen) abundance was established with the Keck Observatory by UCSD researchers.

What have we learned?• How do we observe the radiation left over from

the Big Bang?– Radiation left over from the Big Bang (when the

plasma became neutral atoms at 3,000 K) is now in the form of microwaves (due to expansion)—the Cosmic Microwave Background—which we can observe with a radio telescope.

• How do the abundances of elements support the Big Bang theory?– Observations of helium and other light elements

agree with the predictions for fusion in the Big Bang theory; we started with 25% He, stars add some more

The Big Bang and Inflation

Our goals for learning:• What aspects of the universe were

originally unexplained with the Big Bang theory?

• How does inflation explain these features of the universe?

• How can we test the idea of inflation?

Mysteries Needing Explanation1) Where does structure come from, that eventually

leads to galaxy formation? The structure begins with tiny, tiny fluctuations in

energy at the quantum level

2) Why is the overall distribution of matter so uniform? How can distant parts of the universe be almost

identical in temperature?

3) Why is the density of the universe so close to the critical density? Or why does space look FLAT?

INFLATION is a process that can make all the structure by stretching tiny quantum ripples to enormous size.

Start with quantum ripples in spacetime

Expansion occurs in about 10-36 s

Spacetime expands much faster than the speed of light, which is OK because it is NOT matter.These ripples in density then become the seeds for all structures.

The ripples in density then become the seeds for all structures

How can microwave temperature be nearly identical on opposite sides of the sky (smooth to 1 part in 100,000)?

Spacetime Diagram

Regions now on opposite sides of the sky were once VERY close before INFLATION pushed them far apart

The answer …

The overall geometry of the universe is closely related to total density of matter & energy. (Einstein’s General Theory – matter tells the universe how to curve and the curvature tells matter how to move.)

Spacetime can have curvature.

These pictures are only 3-dimensional representations of spacetime.

Density =Critical

Density >Critical

Density <Critical

Inflation of the universe flattens the overall geometry, like the inflation of a balloon, causing overall density of matter plus energy to be very close to critical density, that is the geometry of spacetime is flat.

The real universe is bigger than we can see – cosmic horizon – distance light can travel since time of Big Bang

Patterns of structure observed by WMAP show us the “seeds” of universe.

Observed patterns of structure in universe agree (so far) with the “seeds” that inflation would produce.

“Seeds” Inferred from CMB

• Overall geometry is flat – Total mass+energy has critical density

• Ordinary matter ~ 4.4% of total• Total matter is ~ 27% of total

– Dark matter is ~ 23% of total– Dark energy is ~ 73% of total

• Age of 13.7 billion years

In excellent agreement with observations of present-day universe and models involving inflation and WIMPs!

What have we learned?

• What aspects of the universe were originally unexplained with the Big Bang theory?– The origin of structure, the smoothness of the

universe on large scales, the nearly critical density of the universe

• How does inflation explain these features?– Structure comes from inflated quantum ripples– Observable universe became smooth before

inflation, when it was very tiny– Inflation flattened the curvature of space,

bringing expansion rate into balance with the overall density of mass-energy

What have we learned?• How can we test the idea of inflation?

– We can compare the structures we see in detailed observations of the microwave background with predictions for the “seeds” that should have been planted by inflation

– So far, our observations of the universe agree very well with models in which inflation planted the “seeds” for future gravitational collapse and the formation of galaxies and stars

• How is the “flatness” problem of the universe explained?• By the Hubble expansion• By faster-than-light motion of the first particles in the early universe• By space experiencing enormous inflation in size over very short time• By accident. It just happens to be so.

Observing the Big Bang for Yourself

Our goals for learning:• Why is the darkness of the night sky

evidence for the Big Bang?

Olbers’s Paradox

If universe were

1) infinite

2) unchanging

3) everywhere the same

Then, stars would cover the night sky

Olbers’s Paradox

Analogy is with a forest of trees that is so dense with more or less identical trees that every line of sight is blocked.

To solve a paradox, question the assumptions!

Night sky is dark because the universe changes with time!

As we look out in space, we can look back to a time when there were no stars!

The universe started out with no stars and galaxies 13.7 billion years ago. It is not infinite, eternal and unchanging!

This is strong evidence for a Big Bang.

What have we learned?

• Why is the darkness of the night sky evidence for the Big Bang?– If the universe were eternal, unchanging, and

everywhere the same, the entire night sky would be covered with stars

– The night sky is dark because we can see back to a time when there were no stars

Science Fact to Science Fiction• We know that the Sun is a thermonuclear furnace• We know that the Sun will not go supernova, instead it will

become a Red Giant star and then a dense White Dwarf the size of Earth

• But heavier stars will explode and become ultra-compact spinning objects made entirely of neutrons, or if even heavier, they will collapse to become black holes

• There is evidence for black holes from a few solar masses to billions of solar masses

• The universe began 13.7 billion years ago in a Big Bang event that caused the enormous expansion of space we see today

• None of this is science fiction … any more

Final Review – Topics Covered Since Midterm (2/3rds of final)

• Sun & Stars• Star Birth• Exoplanets• Death of High and Low Mass Stars• White Dwarfs, Neutron Stars and Black Holes• Our Galaxy• Other Galaxies and Active Galaxies• Dark Matter, Dark Energy and the Big Bang