The Early Universe and Life in the Universe
Review for Astronomy 123 Midterm Exam 3

posted May 29, 2001, jeb

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Outline


• The Early Universe
• Life in the Universe

The Early Universe

Major Events of the Early Universe 10-35 seconds Inflation begins 10-32 seconds Inflation ends 10-4 seconds to about 1 minute Matter emerges as dominant over Anti-matter few minutes Nucleosynthesis - fusion of neutrons and protons  into light elements few thousand years Radiation Era ends Matter Era begins 100,000 years Light de-couples from matter Today Matter era has ended? Vacuum-energy era has begun? Dark energy dominates

The Big Bang

The early universe was radiation dominated

• In the early universe, matter and anti-matter were being created equally out of the radiation
• pair production

Anti-Matter
• anti-matter consists of anti-particles
  • Each type of particles is "matched" by an anti-particle of opposite charge
  • for example, the anti-electron(positron) carries a positive charge

Matter and anti-matter can be created in pairs from energy (or electromagnetic radiation)
E = m c²
E = energy
m = mass
c² = speed of light squared (here just a constant of proportionality)

Anti-matter and matter can also annihilate into energy

The Early Universe
• Due to very high temperature matter and anti-matter were continuously created initially
• As temperature decreased, eventually even the lightest particles could not be created
• Eventually, through interactions, matter came to dominate anti-matter
  • this occurred about a minute after the Big Bang

Evolution of the Universe

Evolution of Matter

• Radiation Era (ended a few thousand years after the Big Bang)
• Planck Epoch
• First 10^-43 seconds after the Big Bang
• No current theory of physics (quantum gravity) exists
• GUT (Grand Unified Theory) Epoch
• After 10^-43 seconds, temperature fell to 10³² K
• Hadron Epoch
• Creation of protons and neutrons continued for about 10^-4 seconds
• Temperature drops below 10¹³ K, and protons and neutrons are no longer produced in pairs
• Lepton Epoch
• Ends when the universe is about 100 seconds old
• During this epoch, the leptons (electrons, neutrinos, and other light particles) are still produced in pairs, because they are light
• Ends when temperature drops below 1 billion K
• Nuclear Epoch
• Protons and neutrons fuse into light nuclei
  • nucleosynthesis
• Last until the Universe is about 15 minutes old
• Matter Era (began a few thousand years after the Big Bang)
• Atomic Epoch
• Galactic Epoch
• Stellar Epoch
• Vacuum-energy Era (Dark Energy Era)
  • Last few billion years
  • Universe's expansion is accelerating
  • Our recognition of this era is too new to be in the textbook

Formation of Atoms and Nuclei

Light nuclei were formed during the Early Universe
• Notably helium and deuterium
  • helium = 2 protons + 2 neutrons (+2 electrons)
  • deuterium = 1 proton + 1 neutron (+1 electron)

Why is there so much Helium?
 Helium (and other light elements) was produced just after the Big Bang due to extreme temperature in the Primordial Fireball


Present abundances (like deuterium) tell us what was going on very early in the evolution of the universe

Deuterium Cosmology

• The amount of deuterium reveals the density of the early universe
• Implys that the density of "normal" matter in the universe is at most a few percent of the critical value
  • deuterium is not formed in stars in much quantity


Heavier nuclei were formed later in burning stars and supernovae explosions.

Inflation

 
Puzzles of the Early Universe
1. Why does the universe appear so flat?
• not clearly open or closed
• If the early universe differed slightly from critical density (flat), it would differ greatly today
• Flatness Problem
2. How did the universe become so homogeneous and isotropic?

Horizon problem
• Background radiation nearly the same in all directions
• Therefore - entire universe must have been at uniform temperature near beginning
• But - different regions are not in contact and never have been
3. Why no magnetic monopoles seen?
• What is a magnetic monopole?
• A particle with just one magnetic pole (either north or south)
• none have been found in Nature, so far
Answers from Grand Unified Theory (GUTS).
• Inflationary Universe

 
Grand Unified Theory (GUTS) predicts period of inflation

Inflation Solves Mysteries

1. Why is universe so flat? (flatness problem)
• Expansion flattens it
• Like balloon suddenly expanding becomes very flat locally
2. Why so homogeneous and isotropic? (horizon problem)
• Universe was in contact before inflation
• but not now
3. Why no monopoles?
• Since they are very heavy, they had to be created while universe was very hot
• before their freeze-out time (the heaviest particles freeze out first)
• this would have been before inflation
• Inflation spread them out - so we won't see many now

Fundametal Basis forInflation

Basic Building Blocks of the Universe (particles)

Matter can be probed to the fundamental building blocks

We are aware of three types of matter:

• quarks
• protons, neutrons, and all nuclei of atoms are built from quarks
• six types of quarks have been discovered
  the top quark is the heaviest
• leptons
• electrons and neutrinos are examples of leptons
• dark matter
• what is it?


Matter interacts through four forces (see note below)

• gravity
• weak nuclear force
• electromagnetism
• strong nuclear force

Note - these four forces can be thought of as just three, since the weak nuclear and electromagnetic force have been unified into the electroweak force

Forces of Nature

.	Relative Strength	Role	.
Strong Nuclear	1	Binds Nucleus	(short range)
Electromagnetic	10-2	Binds Atoms	(long range)
Weak Nuclear	10-13	Causes Radioactivity	(short range)
Gravitational	10-40	Binds earth to Sun Binds galaxies	(long range)


GUTS - Grand Unified Theories

States that strong, electromagnetic, and weak forces are manifestations of the same force
In other words, unifies the forces
• Mathematical Theory of GUTS ------> predicts Inflation
• When universe cooled to point that strong force separated from electroweak force
• From 10^-35 sec. to 10^-32 sec, universe expanded in size by 10⁵⁰
Inflation Solves:
• Horizon problem
• Flatness problem
• missing magnetic monopoles

Remnant of Inflation might be cosmic strings

Structure

Dark Matter
• Recall, most of the universe is dark matter (90 % or more)
• So fluctuations in dark matter should be more important than baryonic.


What is dark matter?
 

Hot Dark Matter
• lightweight particles
• neutrinos? (if they have mass)
Cold Dark Matter
• heavy particles
• photinos?


Best models:

• Cold Dark Matter
and
• Hot Dark Matter
 


How does Dark Matter relate to microwave background?

• Density fluctuations appear first in the dark matter
  then create some ripples in visible matter- but small ripples
•  1992 - ripples finally observed by COBE
• at about 1 part in 100,000.

Cosmological Constant ( or )

What is the cosmological constant?
• Einstein's general theory of relativity (1915) showed the universe should be expanding
• Einstein assumed it was not expanding
• To "correct" this feature of the theory he invented a constant to stop the expansion
• In 1929 Hubble discovered the universe was expanding, and Einstein called the cosmological constant the greatest blunder of his career
• Since then the cosmological constant has been thought to be zero


The expansion of the universe depends on the value of the cosmological constant

But now measurements have suggested the cosmological constant is not zero

• the vacuum energy of the universe is accelerating the expansion of the universe


Where does the cosmological constant come from?

• vacuum energy density
• or the Dark Energy

Life in the Universe

Our understanding of life in the universe requires we understand how life evolved in the one place we know for sure it did - on the Earth

Life on Earth emerged as the end of an evolutionary sequence:

Life appeared on Earth about 3.5 billion years ago

Carbon based

• based on complex molecules (amino acids and nucleotide bases)
• early Earth provided energy sources which combined basic elements into complex molecules
  • basic elements
    • ammonia (NH₃)
    • methane (CH₄)
    • carbon dioxide (CO₂ )
    • water (H₂O)
  • energy sources
    • natural radioactivity
    • lightning
    • volcanism
    • solar ultraviolet radiation
    • meteoritic impacts

Liquid water essential

Life Elsewhere in the Solar System?

Metereorites often contain organic molecules

Three large bodies in the solar system besides the Earth have possiblities:
 

• Titan, Saturn's largest moon
• atmosphere of methane, ammonia and nitrogen
• Cassini will drop a probe into the atmosphere of Titan
• Europa, Jupiter's moon
• liquid water below the icy surface
• Mars
• explored by Viking -> no life
• meterorites from Mars reach Earth

So far, life has only been observed on Earth

Origin Of Life - Can It Happen Spontaneously?

Life - as we know it - is based on complex molecules built as carbon chains
 
Can these molecules be created in the lab?
• 1953 - Harold Urey and Stanley Miller
• Mixed H₂O (water), CH₄ (methane), NH₃ (ammonia), CO₂ (carbon dioxide), and applied sparks
• They found amino acids were formed - the basic building blocks of life
• More recent experiments have produced amino acid, complex sugars, and nucleotide bases
• DNA?
• Still no life
• This step is a mystery
• But research continues

Possible Alternate Origin Of Life On Earth

Not from the primordial soup on Earth, but from organic material produced in interstellar space that came to Earth as interplanetary dust and with meteors
 
 

Drake Equation

1961 - A small group of astrophysicists (including Carl Sagan) gathered at the National Radio Astronomy Observatory, Green Bank, WV
 
• Frank Drake presented his equation
 
• to estimate the number of technological civilizations that presently exist in the Milky Way
 

N = R_*f_pn_ef_ef_if_cL



• R_* = rate of star formation in the Milky Way
• f_p = fraction of stars with planets
• n_e = # of habitable planets/star
  planet in habitable zone; cannot include binary systems
• f_e = fraction with life evolved
• f_i = fraction where life develops intelligence
• f_c = fraction which choose to communicate and develop advanced technology
• L = lifetime of such civilizations


(Note: This original form is a little different from the textbook)
• The Drake Equation is an excellent method for us to fomulate our thinking,
• but it does not lead to a well defined answer, because there are many uncertainties in the factors

UFOs (Unidentified Flying Objects)

Occam's Razor 
• Fundamental principle in science - "the simplest explanation that satisfies all the evidence is the best". 

Occam's Razor leads to the conclusion that reported UFO's are not good evidence for extraterrestrial life

The Scientific Search

1. We are looking for intelligent, advanced life since we assume primitive life is too difficult to detect

 
2. We assume that intelligent life wants to communicate
• Communication will take time
  • Round trip for radio signal over 50 light years takes 100 years
  • Round trip for radio signal over 100 light years takes 200 years
  • Round trip for radio signal over 10,000 light years takes 20,000 years

 
3. Two avenues for discovery

 
1. We discover them
SETI - search for extraterrestrial intelligence
2. They discover us
CETI - communication with extraterrestrial intelligence

We Discover Them (SETI)

SETI - search for extraterrestrial intelligence

What should we look for?

• Popular guess among scientists

- 21 cm spin-flip line of hydrogen
    (1420 MHz)
hydrogen is the simplest atom
and 21 cm is a "quiet band" known as the water hole

Searches at 21 cm (1420 MHz) have failed so far

They Discover Us (CETI)

CETI - communication with extraterrestrial intelligence

We have been beaming radio signals to outerspace now for about 50 years

1974 - Arecibo - Puerto Rico

message from people on earth beamed at globular cluster M13 (many stars) - distance 24,000 light years
1679 bits of information (0000001010101000000000000101......
only factors of 1679 are 23 and 73 (1679 = 23 x 73)

One motivation for this is to learn from an advanced technological civilization


Placed messages aboard space vehicles

• Pioneer
• Voyager

Carbon Chauvinism

Usual assumption of SETI is that life is based on carbon chemistry, like on earth

Why Not Silicon?

1. We don't have an existence proof - like for carbon
2. Life on earth is based on molecules containing millions of atoms (DNA)
Silicon
• short molecules at room temperature
• and we think life would not be able to function at colder temperatures

So silicon does not look like as likely a basis for life as carbon

Search for Extrasolar Planets

One element of the Drake equation is the existence of planets around stars

We can detect the wobble of a star caused by the pull of orbiting planets

This search is biased toward large planets orbiting close to a star

Many stars with at least one planet have been identified

Some stars with detectable multiple planets have been identified