This made sense to Newton because the gravity of a finite number of stars would eventually cause the universe to fall together into a compact blob - clearly this isn’t/hasn’t happen[ing]/[ed]
Need segue - perhaps now that there is evidence that the universe is expanding, need an explanation/change to ideas for the origin of the universe. Einstein dimissed lemaitre’s ideas, thinking that he did not understand the laws of physics properly. After Hubble’s discovery that provided evidence of expansion, Lemaitre gave a lecture presenting his idea, and Einstein admitted his error (cosmological constant) - beautiful and satisfying interpretation
This paper was published as Alpher (student), Bethe (Gamow’s friend), and Gamow as a joke - referred to alpha beta gamma paper. Due to his sense of humour, much of gamow’s early work was forgotten.
Tried everything to get rid of this noise, including cleaning all of the pigeon droppings off of their antenna.
Explain what BB radiation is
Today, we still don’t really know that much about dark matter except that it doesn’t interact with light but still has mass. Example: our galaxy supercluster is moving towards a giant clump of dark matter, known as the great attractor.
Due to expansion, dark energy is dominant now.
The Big Bang Theory:
Origin & Evolution of the Universe
UB PASI 2010
Newton’s Static Universe
• Universe is static and composed of an infinite
number of stars that are scattered randomly
throughout an infinite space.
• Universe is infinitely old and will exist forever
without any major changes.
• Time and Space are steady and independent
of one another and any objects in existence
If universe is as how Newton describes,
then why is the sky dark at night?
• If space goes on forever with stars
scattered randomly throughout, then in
any line of sight in any direction will
eventually run into a star.
• Using this logic, the sky should be the
average brightness of all of these stars;
the sky should be as bright as the sun,
even at night.
But isn’t the sky dark at night…?
Yes, of course - that is what we observe
now and have always observed.
Something is wrong with Newton’s idea of
a static, infinite universe.
• Einstein overturned part of Newton’s
theory with his theories of special and
general relativity - time and space were
indeed related, as were the objects
existing within them.
Time and Space and their rates are
intertwined and depend on the motion
of the observer (1905).
Gravity bends the fabric of space time -
the matter that occupies the universe
influences the overall shape of space
and the rate of time (1916).
Implications of Einstein’s Ideas
• Based on the general relativity
equations, the structure of universe is
either always expanding, always
contracting, or always static.
• To agree with the ideas of the time
(Newton’s), Einstein added a
“cosmological constant” which yielded a
• Represents the pressure that allows the
universe’s expansion to directly balance
gravitational collapse due to the objects
existing within the universe, thus yielding a
• Without this idea of a “cosmological
constant”, Einstein could’ve been the first to
predict that the universe is not static.
• Edwin Hubble’s
observations of remote
galaxies, and the
redshift of their spectral
• Hubble noticed that the
further away the galaxy,
the greater the redshift
of its spectral lines.
• This linear relationship
is called Hubble’s Law.
• The wavelengths of
the light emitted by
distant objects is
elongated as it
travels to earth.
• Longer the light
travels, the more it
v = H0d
v = recessional velocity of the galaxy
H0 = Hubble constant
D = distance of galaxy to earth
Galaxies are getting farther apart as time
progresses, therefore the universe is
• Expansion rate measured using Type
• The age of the universe can be derived
from Hubble’s constant:
• T0 = d → T0 = 1
For example, if H0 = 73 km/s*Mpc, then
T0 = 13.4 Billion years old
Age of Universe
• Currently, after taking into account
differences in expansion rate over time
and our movement through space:
T0 ~ 13.7 ± 0.2 byo
• Age of stars: ~13.4 byo ± 6%
Therefore, oldest stars are younger than
the age of universe.
How the Universe Expands
• The space between
galaxies expands, not the
objects held together by
their own gravity are always
contained within a patch of
• Example: raisins in a loaf
– As the dough rises, the
overall loaf of bread
expands; the space
between raisins increases
but the raisins themselves
do not expand.
Center of Universe?
• There is NO CENTER to the universe
– Expansion looks the same regardless of
where you are in the universe.
– Every point appears to be the center of the
expansion, therefore no point is the center.
– The universe is infinite.
Evidence for Expansion
• The light from remote galaxies and
other objects is redshifted.
• This redshift is called cosmological
redshift because it is caused by the
expansion of the universe, not by the
actual movement of the object (doppler
• The degree of
tells you how far into
past you are seeing the
object due to the finite
speed of light; this value
is called Lookback time.
• However, these values
are not always certain
because of the
expansion of universe
was not always
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
• Olber’s Paradox is solved:
due to the finite speed of
light, the observable
universe does not include
the entire universe.
• Radius of the observable
universe depends on the age
of the universe and the
speed of light: ~47 billion
• Result: Sky is dark at night
with points of light (stars,
galaxies, etc.) scattered
Origins of the Big Bang Theory
• Georges Lemaître (1927) expanded on idea of
expanding universe, realizing that the universe
was smaller yesterday than today, and so on until
a “day that would not have had a yesterday”: the
moment of creation.
– The moment of creation would be the sudden
expansion that started the expansion of the universe
as we know it today.
• This idea wasn’t widely accepted at first: Fred
Hoyle dismissed “this hot Big Bang”, noting that
there wasn’t any record or remnants. He argued
for a “steady state” universe.
Origins of the Big Bang Theory
• George Gamow (1948) suggested that if the
universe was created with a “hot Big Bang”, then:
– Various elements, such as H and He, would be produced for
a few minutes immediately after the Big Bang due to the
extremely high temperatures and density of the universe at
– The high density would cause rapid expansion.
– As the universe expanded, H and He would cool and
condense into stars and galaxies.
– Today, due to continued cooling, radiation left over from the
epoch of recombination, when neutral atoms formed
(~380,000 years after Big Bang) should be about 3K.
– Production of H and He during this time instead of just in H-
burning in stars would explain why the H:He ratio of the
universe is higher than what could’ve been produced by
Evidence for the Big Bang
• Gamow’s theory was revisted in the
1960’s by Bob Dicke and Jim Peebles
of Princeton University.
– Believed that this cooled radiation would
be redshifted to the microwave region of
the electromagnetic spectrum.
– Made a receiver to detect this radiation, but
Evidence for the Big Bang Theory
• The radiation, so far undetected
by the Princeton team, was
posing a problem for NJ Bell
Telephone Labs, where Arno
Penzias and Robert Wilson
were developing a new
for phone calls.
– Puzzled by steady hiss that
they received no matter where
in the sky they pointed their
– This faint background noise
they were trying to get rid of
was exactly what the Princeton
team was trying to detect:
evidence of the Big Bang.
• Detection of this radiation, called
Cosmic Microwave Background
radiation, won Penzias and Wilson the
Nobel Prize for Physics in 1978.
• CMB radiation can be detected by your
tv as well - 1% of static seen on a
channel that your tv doesn’t receive is
from the birth of the universe.
• Intensity of CMB Radiation reveals origins of universe.
– However, difficult to detect intensity from Earth- the
atmosphere is opaque to wavelengths 10 µm to 1 cm (CMB
~ 1 mm).
• COBE (Cosmic Background Explorer) 1989: detector
outside the atmosphere:
– Measured the blackbody spectrum of CMB radiation to be at
T = 2.725 K - consistent with theory.
– CMB radiation almost entirely isotropic; CMB is slightly
warmer in direction of Leo and slightly cooler in direction of
• WMAP (Wilkinson Microwave Anisotropy Probe) (2002)
improved picture of CMB Radiation.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
• Radiation appears to be
mostly smooth, but there are
slight variations in
temperature that show that
matter had started to clump
in the early universe -
clumps of matter formed the
galaxies and stars see
• Sound waves in early
universe are recorded in this
radiation; by studying the
characteristics of these
sound waves, we can find
out about the conditions of
the early universe.
• Despite all of the success with the Big Bang
Theory so far, the horizon problem was still
yet to be solved.
– The temperature of the CMB radiation was the ~same no
matter where you look in the sky, indicating that some how
information linking all parts of the sky was traveling faster
than the speed of light.
– Also, information from one side of the sky at 100,000 years
old (horizon is 100,000 light years in diameter) differed from
the other side of the sky by 10 million light years - 100 times
the diameter of the horizon.
How is this possible?
• Alan Guth (1970s) had a solution:
– The universe must have expanded
exponentially very early for a short period
– This would account for the clumping of
Evidence for Inflation Theory
• Guth predicted that the average density
of the universe should be equal to the
critical density (6 protons/m3
– This was confirmed by powerful
• Evidence from WMAP shows that the
clumping of matter is consistent with the
amount of accelerated expansion
Extent of Inflation
Today, evidence and theory show that:
• At T = 10-35
sec, universe d = 10-24
• Between T = 10-35
sec and T = 10-32
the universe expanded exponentially by
a factor of 1050.
• For the briefest moment, the universe
expanded faster than the speed of light.
Big Bang Theory: Timeline of
• Hubble’s Law shows that the universe
has been expanding for billions of years
- the universe is denser the further back
in time you look.
• At some point, you reach an infinitely
dense point at which
Tage of universe= 0 → Big Bang
T = 0 seconds to 10-43
• BIG BANG occurs.
• Something causes infinitely dense point to
expand (into Nothing).
• Density of universe is so high that time and
space are curled up and the laws of physics
that we know today do not apply.
• All four forces in nature were unified.
• This is time is called the Planck Time.
Separation of Forces
After the Planck time, the temperature
had decreased 1032
K and gravity was
the first force to separate.
The remaining three forces were still
united - these are the conditions that
particle physicists today try to replicate.
T = 10-35
• Inflation caused the size to the universe
to increase exponentially by a factor of
• This time is called the inflationary
After Inflation Stops
• Matter is created:
– Photons collide and produce pairs of elementary
particles such as electrons and positrons, and
quarks and antiquarks.
– Pair production continues until one of particle
could no longer be produced - pair annihilation
happens - result: symmetry breaking.
– Reason for slight excess of matter over antimatter
is because of an unknown reaction known as
baryogenesis, in which conservation of baryon
number is violated.
– Pair Production occurred until T = 6E9K, but pair
annihilation happens independent of temperature.
Particle Production in Early Universe
• As the size of the universe increases and the
temperature decreases, the particles produced
are of decreasing energy.
• The fundamental forces and parameters of
elementary particles at the time that symmetry
was broken are the same as they are today.
• The time between the birth of the universe and t
is rather unknown, but we can speculate
what is happening based on other observations;
beyond this time is less speculative as these are
conditions that particle physicist try to replicate.
T = 10-6
• Temperature has cooled enough for baryons
(Protons, Neutrons) to form.
• Like the leptons, baryons form in pair production.
• Once the temperature has decreased past the
point at which baryons can no longer be
produced, pair annihilation occurs again, leaving
a slight excess of baryons over antibaryons.
• Also, at this temperature, all particles are no
longer moving relativistically, so the universe
becomes dominated by the higher energy
photons (radiation-dominated universe).
T = few minutes
• Temperature ~ 1 GK, density ~ that of air.
• Neutrons combine with protons making
deuterium and helium nuclei, and some
protons remain independent (hydrogen
• Called Big Bang nucleosynthesis.
• Temperature is still too high to form atoms as
they would be ionized immediately.
• The universe would appear opaque during all
this time because photons and matter would
be interacting due to high temperatures.
T = 379,000 years
• Universe is now cool enough that matter
energy is greater than radiative energy, thus
allowing atoms to form.
• Radiation is decoupled from matter and
photons are free-streamed throughout space
- origin of CMB radiation.
• This time is known as the epoch of
• Universe is now matter-dominated.
T ~ 400 million years
• Since epoch of recombination, slightly denser
regions attracted matter nearby and the first
stars begin to form.
• Regions continue to acquire matter and other
objects like galaxies and gas clouds form.
• Universe begins to look like how we know it
today (still expanding and still cooling).
Matter in the Universe Today
• Evidence gathered from WMAP shows that all
of the matter in the universe is composed of
three types of matter:
– Cold dark matter
– Hot dark matter
– Baryonic matter
– Cold dark matter accounts for ~82% of all matter
and hot dark matter and baryonic matter combined
account for the remaining ~18%.
Nature of Expansion Today
• Evidence of Type 1a supernovae and CMB
radiation show that the expansion is accelerating,
driven by dark energy.
• Dark energy comprises ~72% of all energy and
permeates all space.
• It is likely that this dark energy has always been
throughout the universe, but when the universe
was younger and much smaller, gravity was
stronger than dark energy.
• This acceleration could be described by
Einstein’s cosmological constant.
• Today, dark energy is still very misunderstood.
Expansion & Fate of Universe
Fate of the Universe
• Today, particle accelerators such as the LHC
are trying to replicate conditions just after the
Big Bang so that we understand how the
• Currently, all cosmic evolution after
inflationary epoch can be modeled and
described pretty accurately, but the time
before this (10-15
sec) is basically unknown;
understanding this time remains one of the
greatest mysteries in physics.
• What is dark matter?
• What is dark energy?
• Can dark energy and matter be detected and
studied in labs?
• What happened from the birth of the
universe, at the instance of the Big Bang, until
the end of the inflationary epoch?
• What caused the Big Bang?
• What is the ultimate fate of the universe?