The Big Bang● Primordial explosion creating our universe

– all matter and space, “beginning” of time.

● Hot and dense conditions => like a “fireball”

● Specific prediction for initial amounts of hydrogen and helium

● Source of Cosmic Microwave Background Radiation

– 2006 Nobel Prize in Physics

Primordial Nucleosynthesis

• Creation of base level of Helium (~25%) in the

early universe via Hydrogen fusion

• What conditions allowed this to occur, and why

did it eventually stop?

Primordial Nucleosynthesis

• Creation of base level of helium (~25%) in the

early universe. Supports big bang model!

• Stopped because temperature and density became

too low for fusion to continue

The Formation Of Atoms

• Early universe hotter and opaque to radiation● Universe expanded and cooled

– Electrons & nuclei combined => neutral atoms

• Epoch of Decoupling– Universe becomes transparent to EM radiation.

The Epoch of Decoupling

The Horizon Problem• Cosmic Microwave Background Radiation (CMBR)

- Leftover from Big Bang• Blackbody spectrum• Cooled by expansion of universe• More energy than all stars and galaxies that ever existed!

• CMBR is uniform across entire sky• Why is this a problem?

The Horizon Problem• Cosmic Microwave Background Radiation (CMBR)

- Leftover from Big Bang• Blackbody spectrum• Cooled by expansion of universe• More energy than all stars and galaxies that ever existed!

• CMBR is uniform across sky• No reason why CMBR should look the same in regions A and B. Each lies outside the other’s communication horizon

The Flatness Problem• The cosmic density appears to be fairly close to the critical value

• In terms of space-time curvature, the universe is remarkably close to being flat

• Why is this a problem?

The Flatness Problem• The cosmic density appears to be fairly close to the critical value

• In terms of space-time curvature, the universe is remarkably close to being flat

• No obvious reason why our universe should have exactly the critical density!

The Theory of Inflation• A short period of unchecked

expansion

• The universe swelled in size by a factor of about 1050 in only 10-32 seconds!

• Solves both horizon and flatness problems! How?

Inflation and the Flatness Problem• Solved by taking a curved surface

and expanding it enormously in size.

• Example: As balloon expands, curvature becomes less pronounced on small scales• Earth looks flat to us.

• The universe appears close to being flat because of inflation.

Inflation and the Horizon Problem• Initially, regions A and B could communicate

● Inflation => expand at speeds much greater than that of light => no time for densityand temperature fluctuations toform

● Possible Mechanisms● Cosmological constant?● Inflaton – repulsive particle

that decayed almost immediately?

Variable Speed of Light (VSL) Theories● Much larger 'ultimate speed limit' in early universe

● Solves horizon problem

● Speed of light must have decreased to current value

Noether's theorem:

Time-evolving laws of physics => mass-energy not conserved● VSL predicts mass-energy is destroyed in closed and created in open universe

=> flat universe becomes an attractor!

● Same argument also explains why universe is homogeneous● Feature of universe not explained by inflation

Extraterrestrial Life (as We Know It)

• “Life as we know it” is generally taken to mean

carbon-based life that originated in a liquid-water

environment.

• Requirements?

Extraterrestrial Life (as We Know It)

• “Life as we know it” is generally taken to mean

carbon-based life that originated in a liquid-water

environment.

• Requirements?• Liquid water, protection from radiation, temperate climate

• What are some important characteristics of living organisms?

Characteristics of a Living Organism

Reacting to environmental stimuli

Taking in nourishment and growth/healing

Reproducing and passing on characteristics

Genetic changes (mutations/evolution)

Extraterrestrial Life

Assumptions of mediocrity

Life on Earth depends on just a few basic molecules.

Elements are common to all stars.

Laws of physics the same everywhere => life fairly common in the cosmos.

The Miller-Urey experiment attempts to recreate the chemical conditions of the primitive Earth in the lab, and synthesize building blocks of life.

Review Question

● The point in time when the temperature of the ● universe first dropped low enough to allow atoms ● to form is called the epoch of ______.

– A) inflation

– B) expansion

– C) contraction

– D) decoupling

Review Question

The properties of the cosmic background

radiation imply that in the past the universe

was ___ and ___.

– A) transparent, cooler

– B) transparent, hotter

– C) opaque, cooler

– D) opaque, hotter

Review Question

Which of the following is NOT a characteristic

of “life as we know it?”

– A) the capacity to heal and grow

– B) reproduction without mutation

– C) carbon-based chemistry

– D) responsiveness to external stimuli

ET Life in the Solar System?

Besides the Earth, which bodies in our solar system are most likely to harbor life?

The Moon and Mercury lack liquid water,

protective atmospheres, and magnetic fields.

• Venus has far too much

atmosphere.

Planet most likely to harbor life

• Mars, but still● No Liquid water● Atmosphere is thin● Lack of magnetism and ozone layer

• Jovian moons Titan and • Europa also good

candidates

The Drake Equation

number of technological, intelligent civilizations in the Milky Way

rate at which new stars are formed (avg. over galactic lifetime)

= x

fraction of stars having planetary systems

x

average number of habitable planets within those planetary systems

xfraction of those habitable planets on which life arises

xfraction of those life-bearing planets on which intelligence evolves

fraction of those planets with intelligent life that develop technological society

average lifetime of a technological civilization

xx

Each term is less certain than the preceding one! Only in last twenty years have we addressed the second term. Only in the last five years have weaddressed the third term.

Contact?

How might we go about trying to determine

whether or not technologically advanced

civilizations exist in our galaxy?

SETI• Search for ExtraTerrestrial Intelligence

- Listens for radio signals from alien worlds

• Clear window on the cosmos? - Electromagnetic spectrum, as viewed from Earth,

is a noisy place

• Microwave Window - in radio part of EM spectrum - Photons travel relatively unhindered through the interstellar

medium, at the speed of light

- Natural noise is at a minimum

The Water Hole

Barney Oliver dubbed the spectral region between H and OHthe Cosmic Water-Hole.

"Where shall we meet our neighbors?" he asked."At the water-hole, where species have always gathered."

The Water Hole

The Fermi Paradox

•More stars than sand grains (~ 1022 vs. 1018) • Most conservative estimates (1st three terms in Drake)

• In Milky Way• 100 billion stars• 5 billion Sun-like stars• 1 billion Earth-like planets

• Assuming 1% (or 1% survival rate in humanity’s

lifetime) => 100,000 intelligent civilizations

•So, where is everybody?

•Link to Fermi Paradox Article