Download A time travel of 14 billion years

A time travel of 14 billion years
Stellar formation
Elements formation
The big
bang
Planets formation
The origin of life
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The big bang
•It occurred right here, nearly 14 billion years ago.
• All matter and energy of the Universe were concentrated in a very
small space region.
•At the beginning temperature was extremely high. Nuclei and atom
constituents formed a primordial soup.
•Since that moment the Universe expanded and cooled down.
Ordered structures were formed: nuclei, atoms, galaxies,
planets…and human beings.
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The big bang
pillars
There are three strong evidences of the big bang theory:
• Universe expansion
• Primordial nucleosynthesis
He
• Cosmic microwave background radiation
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Universe
expansion
• We see that galaxies receede from us, and that
each of them is at a distance D proportional
to its velocity V (Hubble’s law):
D= V t
• If this law was valid also in the past,
distances tend to zero when t=0,
that means the universe reduced to a point.
• The present value, t= 14 Billion years,
tells us how much time passed
after the big bang occurred.
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*
Universe
explosion
• As in every explosion, objects with greater velocity travel
longer distances.
• But where did the explosion happen?
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Why right here?
• Every point in the Universe
is considered to be equivalent
(Cosmological principle).
• When the Big Bang occurred
the whole space was
concentrated in a single point.
• Hence the Big Bang
happened right here.
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Three minutes of
cooking
• In the first three minutes, when
temperature was nearly
1 Billion degrees, protons and
neutrons bound together giving
origin to the nuclei of the lightest
elements: deuterium and Helium
(He).
• Abundance measurements of
these elements, created in the
primordial nucleosynthesis, are
one of the confirmations of the big
bang theory.
p
n
d=p+n
3He=2p+n
4He=2p+2n
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Why does primordial
primordial nucleosynthesis
stop ?
•
•
In the first minutes after the Big
Bang neutrons bind to protons
through nuclear capture reactions,
as example:
p+ n →d+ 
d+p →3He+ 
3He +n→4He+ 
The net result is the
transformation:
2p+2n →4He + energy
In the intermediate stages stable*
nuclei, are formed, which can wait
for the arrival of another particle
to be captured.
*same if unstable, however with half lives larger
than a few minutes…
1
Z
•The series of reactions ends with 4He since for
A=5 there are no stable systems*
*[ Elements heavier than 4He will be formed in stars
formed by means of 3alpha reactions
4He+ 4He +4He →12C + energia
This occurs for densities large enough to allow for a
3 body reaction] .
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“Cosmic concordance”
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Element abundances in the
solar system
•
•
•
•
•
The figure shows number
abundances, with respect to
hydrogen
Hydrogen is the most abundant
element.
Most of it is in the form 11H, with
Deuterium at the level of 10-5 .
Next comes Helium, with number
of atoms of about 1/12 with
respect to Hydrogen
Relative abundances decrease while
Z increases
1
Z
• Heaviest elements, as Uranium, have
abundances as low as 10-12 with respect to
Hydrogen.
• In the solar system, mass abundances are
X=aH=73%, Y=aHe=25% , whereas elements
with Z>2, generically indicated as “metals”
total to about 2% (Z=SZ>2aZ =2%).
There must be other kitchens in the universe…
10
p
Just an appetizer..
n
d=p+n
• Be careful: during the big bang
only the lightest nuclei were
formed.
3He=2p+n
4He=2p+2n
• Electrons could not firmly bind to
nuclei because the temperature was
too high.
• The matter that we see is made
of atoms, molecules and contains
elements that are much heavier
than helium. There must be other
kitchens….
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The first atoms
H=p+e
He=2p+2n+2e
• 300.000 years after the big bang
the temperature was nearly 1000
degrees and electrons can bind
themselves to the nuclei giving
origin to Hydrogen and Helium
atoms.
• The Universe became transparent
to light and heat.
• The background radiation
permeated the whole universe
giving us a trace of the big bang.
George Gamow
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The cosmic background radiation
• The entire universe cooles like
an expanding gas.
• The background radiation, that
comes from interaction with
matter at a temperature of nearly
1000 degrees, now has now a
temperature of -270 degrees.
• This radiation was seen for the
first time in 1964….
Penzias e Wilson
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Cosmic microwave spectrum
• The spectrum of the
cosmic radiation is
perfectly that of a black
body (Planck law)
• It shows that at early
times the universe was
much hotter than now
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The primordial universe
structure
• If we observe the cosmic background
radiation we can observe the baby universe.
• If we look towards different
directions in the sky, we see that the
radiation has very small non uniformities.
COBE 1992
• These are the first signs of
the formation of structures
in the universe.
PLANCK 2013
1992
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COBE (1992)
Twenty years of progress
Cobe 1992
Boomerang 2002
The resolution in the images of the cosmic
microwave radiation is strongly increasing.
Presently, Most accurate detector is
PLANCK
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Map 2003
Other background radiations
• The cosmic microwave radiation takes a
picture of the universe at an age of 300.000
years.
• There are other radiations in the cosmos,
big bang’s remainders that we are not (yet)
able to detect:
-The background neutrinos that provide a
picture of the universe a second after its birth.
- The gravitational waves that provide a
picture of the universe at 10-43 seconds after
the Big Bang.
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Galaxies origin
• Nearly after 1Billion years, matter
started to gather in big stuctures under
the effect of gravitational interaction.
• Galaxies clusters are considered to
have initially formed.
• Each of them would have been
separated into galaxies.
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The Galaxy
• In the Virgo cluster
there’s the Galaxy with
capital G, that’s the one
in which we live.
• Light needs 100.000
years to go from an end
of the Galaxy to another.
• The Galaxy contains nearly 100 billions stars: one of them, in
the outskirts, is our Sun.
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Stars birth
• The biggest galaxies
inhabitants are giants clouds of
gas, each of them containing
the material that will form
milions of stars.
• Due to gravity these clouds
break into fragments around
some gravitational
accumulation centres, giving
rise to the stars.
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Stars energy
source ?
•Kelvin 1800: Gravitational
energy can sustain sun’s
luminosity for nearly 30.000.000
years.
• It’s too short to justify the
evolution of biological and
geological processes.
• Understanding the stars energy
source was the scientific problem
of the XIX century.
Which kind of energy
source can sustain
the sun for billion
years?
(see later lectures…)
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Birth of nuclear astrophysics
Einstein (1905):
E=mc2
Aston (1918):
m(He)<4 m(H)
Eddington (1920): If a star initially
consists of hydrogen, that gradually is
transformed into heavier elements,
then we understand the energy source
of stars…and…*
*...If this is true, then we are closer to the dream of controlling this latent power,
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to the benefit of the human race or for its suicide (1920)
The imprints of
nuclear reactions
in the sun
• Gallex experiment in the underground
laboratories of Gran Sasso detected
neutrinos coming from the nuclear
fusions inside the sun.
•Gallex has demonstrated that the energy
of the sun is produced by nuclear
reactions taking place inside it.
(see later…)
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The stars:
The nuclear kitchen
• Gravity leads a star to shrink
towards its centre.
4He=2p+2n
12C=4He
+ 4He + 4He
16O=12C
+ 4He
…
• The star balances gravity with the
pressure originated from matter heated
by nuclear reactions.
• These reactions tansform Hydrogen
into Helium and, if the star is heavy
enough, into Carbon and then Oxygen.
In this way all chemical elements up to
Iron are produced.
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The first
dish is ready
Iron
• Stars release energy by
fusion of atomic nuclei.
• This process ends with
the creation of Iron,
that’s the most strongly
bound nucleus.
• The main meal is ready: nuclei up to Iron are produced by nuclear
fusion. But where the heavier nuclei are formed?
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Stars life
• Each star has its own
history and its future.
• The life-time of a star and
its destiny depends on its
mass.
Temperature
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The end of stars
• Heavier stars have a violent end.
• When the nuclear fuel finishes gravitation
shrinks the star which begins to implode.
before
• The outer parts bounce on the stellar core
giving rise to an enormous explosion.
• This process gives life to a collapse
supernova, the most luminous objects in the
galaxies.
•What occurs inside a Supernova? See
later…
dopo
after
dopo
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Supernovae
• The nuclear kitchen
completes with the
formation of supernovae in
which elements heavier
than Iron are produced.
• The produced material is injected into the circumstant gas.
• The shockwave explosion triggers the formation of new stars.
• These stars contain the elements formed in the primordial Big28
Bang and in stars previously exploded.
The Allende meteorite
• We believe that the solar
system birth was preceded by the
explosion of a near supernova,
creating a shockwave which
compressed the circumstant gas.
• The Allende meteorite contains inclusions coming from the
radioactive decay of nuclei produced in a supernova with half life
of milion years.
• This means that the explosion happened nearly 10 milion years
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before the formation of the solar system.
The solar
system
ingredients
• At this point we have all
the ingredients necessary
to form the sun and the
planets.
Abundances of elements in the
Solar system
Atomic number
• 74 % of Hydrogen
• 24 % of Helium
• 2% of heavier elements, mainly Carbon,
Nitrogen, Oxygen and Iron, the so called
«metals» by astrophysicsts
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Birth of the
solar system
• The cloud shrinks to form a star in
its centre.
• The rotation of the cloud produces a
disk.
• In the disk rocky planetesimals
form near the star.
• Ice made planetisimals in the outer
parts.
• Matter accumulates near these
planetisimals while the solar wind
sweeps the circumstant space.
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Extrasolar planets
• Planets around other suns were discovered
studying the perturbations of stellar orbits.
•As of today some 3000 exo-planets are
discovered
• In the 80’s observation started about
planets a big as Jupiter or Saturn.
• More sensitive instruments allow now
to observe planets of Earth size.
• The next step is the search of life
traces.
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The travel steps
Temperature
Time
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Problems and additional readings
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Edwin Hubble
•
•
•
•
•
•
Edwin Hubble was a man who changed our view of the Universe. In 1929 he showed that
galaxies are moving away from us with a speed proportional to their distance. The
explanation is simple, but revolutionary: the Universe is expanding.
Hubble was born in Missouri in 1889. His family moved to Chicago in 1898, where at High
School he was a promising, though not exceptional, pupil. He was more remarkable for his
athletic ability, breaking the Illinois State high jump record. At university too he was an
accomplished sportsman playing for the University of Chicago basketball team. He won a
Rhodes scholarship to Oxford where he studied law. It was only some time after he returned
to the US that he decided his future lay in astronomy.
In the early 1920s Hubble played a key role in establishing just what galaxies are. It was
known that some spiral nebulae (fuzzy clouds of light on the night sky) contained individual
stars, but there was no consensus as to whether these were relatively small collections of stars
within our own galaxy, the 'Milky Way' that stretches right across the sky, or whether these
could be separate galaxies, or 'island universes', as big as our own galaxy but much further
away. In 1924 Hubble measured the distance to the Andromeda nebula, a faint patch of light
with about the same apparent diameter as the moon, and showed it was about a hundred
thousand times as far away as the nearest stars. It had to be a separate galaxy, comparable in
size our own Milky Way but much further away.
Hubble was able to measure the distances to only a handful of other galaxies, but he realised
that as a rough guide he could take their apparent brightness as an indication of their distance.
The speed with which a galaxy was moving toward or away from us was relatively easy to
measure due to the Doppler shift of their light. Just as a sound of a racing car becomes lower
as it speeds away from us, so the light from a galaxy becomes redder. Though our ears can
hear the change of pitch of the racing car engine our eyes cannot detect the tiny red-shift of
the light, but with a sensitive spectrograph Hubble could determine the redshift of light from
distant galaxies.
The observational data available to Hubble by 1929 was sketchy, but whether guided by
inspired instinct or outrageous good fortune, he correctly divined a straight line fit between
the data points showing the redshift was proportional to the distance. Since then much
improved data has shown the conclusion to be a sound one. Galaxies are receding from us,
and one another, as the Universe expands. Within General Relativity, the theory of gravity
proposed by Albert Einstein in 1915, the inescapable conclusion was that all the galaxies, and
the whole Universe, had originated in a Big Bang, thousands of millions of years in the past.
And so the modern science of cosmology was born.
Hubble made his great discoveries on the best telescope in the world at that time - the 100inch telescope on Mount Wilson in southern California. Today his name carried by the best
telescope we have, not on Earth, but a satellite observatory orbiting our planet. The Hubble
Space Telescope is continuing the work begun by Hubble himself to map our Universe, and
producing the most remarkable images of distant galaxies ever seen, many of which are
available via the World Wide Web.
Hubble 1929___ Stima odierna --
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Problem: the Hubble law and
Hubble constant
The Hubble law is generally written as v=HD, where
v is the recession velocity and D is the distance of
the Galaxy.
The most precise value of the Hubble constant is H=
(70 +- 1) km/sec/Mpc
- Show that [ H] = [t]-1
- Compute 1/H in years
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Notes about nuclear fusion and
fission
Binding energy per nucleon  grows
with A for A<60, where it reaches a
maximum (≈9 MeV), and then slowly
decreases.
• This means that energy can be liberated
by fusion of two nuclei A1 e A2 as long
as A= A1 + A2 <60.
• Indeed the mass of A nucleus is
•
Z
M(A)c2 =(Z1+Z2)mpc2 + (N1+N2)mnc2 -Eb(A)
The binding energy of this nucleus is
Eb(A)=A (A)= A1 (A) + A2 (A)> A1 (A1) + A2 (A2)
so that
M(A)c2< (Z1+Z2)mpc2 + (N1+N2)mnc2 - A1 (A1) A2 (A2)= M(A1)c2+M(A2)c2
Since M(A)< M(A1)+M(A2) the reaction A1+A2→A liberates energy .
•
•
Conversely, the fission of a nucleus with large A, is energetically favored, i.e. the
fission fragments are more bound and the fission process liberates energy
Note: this is why nuclear fusion reactors use lightest elements, whereas 37
nuclear fission reactors burn the heaviest elements