Teaching the Big Bang
Pat Hall, York University
Department of Physics and Astronomy
www.yorku.ca/phall/OUTREACH/bbstao.p
pt
Evidence for the Big Bang and Inflation
The Observable Universe
Summary
• Hubble’s Law: all galaxies are moving away
from each other: space is expanding
• Our 3-D universe is expanding into a 4th
dimension [time] like the 2-D surface of a
balloon expands into a 3rd dimension
Summary
• Hubble’s Law: all galaxies are moving away
from each other: space is expanding
• Our 3-D universe is expanding into a 4th
dimension [time] like the 2-D surface of a
balloon expands into a 3rd dimension
• The Big Bang was a point in time, not in
space. It happened 13.7 billion years ago.
Summary
• Hubble’s Law: all galaxies are moving away
from each other: space is expanding
• Our 3-D universe is expanding into a 4th
dimension [time] like the 2-D surface of a
balloon expands into a 3rd dimension
• The Big Bang was a point in time, not in space.
It happened 13.7 billion years ago.
• Our Observable Universe: a sphere centered
on us, 13.7 billion light-years in radius
• Physical Universe: effectively infinite in size
Steps to the Big Bang
• From our viewpoint in the Milky Way Galaxy, we
see almost all galaxies moving away from us
(Edwin Hubble and others, 1920s-1930s)
By measuring
distances to
galaxies,
Hubble found
that redshift and
distance are
related in a
special way
Steps to the Big Bang
• From the Milky Way, we see almost all other
galaxies moving away from us (Edwin Hubble)
• Either we’re at the center of the Universe…
Either we’re
at the center
of the
universe, or...
Steps to the Big Bang
• Here in the Milky Way, we see almost all
galaxies moving away from us (Edwin Hubble)
• Either we’re at the center of the Universe…
• …or galaxies are ALL moving away from
each other, and the Milky Way is just
another one of those galaxies.
Galaxies are all
moving away
from each other.
Rewind the
movie; galaxies
must have been
closer together
in the past.
Galaxies are all
moving away
from each other.
Rewind the
movie; galaxies
must have been
closer together in
the past.
Universe was
denser in the
past. The Big
Bang was when
the density was
“infinite”.
Steps to the Big Bang
• Here in the Milky Way, we see almost all
galaxies moving away from us (Edwin Hubble)
• Space is expanding, carrying all galaxies
away from each other, and the Milky Way is
just another one of those galaxies.
• The universe has no center.
Steps to the Big Bang
• Here in the Milky Way, we see almost all
galaxies moving away from us (Edwin Hubble)
• Space is expanding, carrying all galaxies
away from each other, and the Milky Way is
just another one of those galaxies.
• The universe has no center.
• Think of rewinding the movie of the
universe: The Big Bang was a point in time,
not in space.
Surface of a balloon expands but has no center or edge
Steps to the Big Bang
• Here in the Milky Way, we see almost all
galaxies moving away from us (Edwin Hubble)
• Galaxies are all moving away from each other,
and the Milky Way is just another one of those
galaxies. The 3-D universe has no center, just
like the 2-D surface of a balloon has no center.
• Our 3-D universe is expanding into a 4th
dimension [time] like the 2-D surface of a
balloon expands into a 3rd dimension.
• The Big Bang was a point in time, not in
Aside: Hubble’s Law and Vectors
• Galaxies moving away from each other (save
for some of each galaxy’s closest neighbors).
Aside: Hubble’s Law and Vectors
• Galaxies moving away from each other (save
for some of each galaxy’s closest neighbors).
• Vector formula: V=Hd where H is Hubble’s
constant and V & d are the velocity & distance
vectors of a distant galaxy relative to our own.
Aside: Hubble’s Law and Vectors
• Galaxies moving away from each other (save
for some of each galaxy’s closest neighbors).
• Vector formula: V=Hd where H is Hubble’s
constant and V & d are the velocity & distance
vectors of a distant galaxy relative to our own.
• Change your location to a distant Coma cluster
galaxy which has distance vector c relative to
Milky Way. You can show that for aliens in that
galaxy, Hubble’s law is Vc=Hdc where dc=d-c is
the distance from their galaxy to a galaxy with
distance vector d relative to the Milky Way.
Evidence for the Big Bang and
Inflation
• Here in the Milky Way, we see almost all
galaxies moving away from us (Edwin Hubble)
• Galaxies are all moving away from each other,
and the Milky Way is just another one of those
galaxies. (Exceptions: galaxies very near to
each other can orbit each other or crash into
each other due to gravity.)
• Galaxies all moving away from each other, so…
…the universe
must have been
much denser (and
hotter) early in its
history.
…the universe
must have been
much denser (and
hotter) early in its
history.
At very early
times, only the
elements helium
and hydrogen
existed. (Other
elements were
made later, inside
stars.)
What is everything made of?
• Water (H2O) is made up of molecules, as are many
other everyday substances.
• A water molecule is made up of atoms: 2 hydrogen
atoms (H) and 1 oxygen atom (O).
What is everything made of?
• Water (H2O) is made up of molecules, as are many
other everyday substances.
• A water molecule is made up of atoms: 2 hydrogen
atoms (H) and 1 oxygen atom (O).
• Every atom consists of two parts:
– One or more electrons surrounding ...
– the nucleus: ≈ 100,000 times smaller than the
electron cloud, but 10,000 to 100,000 times heavier.
What is everything made of?
• Water (H2O) is made up of molecules, as are many
other everyday substances.
• A water molecule is made up of atoms: 2 hydrogen
atoms (H) and 1 oxygen atom (O).
• Every atom consists of two parts:
– One or more electrons surrounding ...
– the nucleus: ≈ 100,000 times smaller than the electron
cloud, but 10,000 to 100,000 times more massive.
If you increase the density and temperature enough,
you can break apart molecules into atoms, and even
atoms into electrons and nuclei.
Hydrogen & helium are transparent as atoms, but are
opaque when split into nuclei and electrons.
Aside: hydrogen nuclei (protons) and neutrons combined into
stable helium nuclei when universe was ~3 minutes old.
Leftover neutrons became protons. (Before the protons &
neutrons, there were quarks. Before that, we’re not sure!)
Not long after the Big Bang, the universe was very hot.
Hydrogen and helium were in the form of nuclei plus
electrons, so the universe was opaque.
When the universe cooled to a temperature of about
3000 degrees, hydrogen and helium formed into
atoms, and the universe became transparent.
The infrared light from when the universe was 3000
degrees hot has traveled through the universe ever since
it became transparent: cosmic microwave background.
Expansion of universe stretches wavelengths of light, causing
a cosmological redshift similar to the Doppler effect.
Cosmic background has a
nearly uniform temperature
2.73 degrees above
absolute zero (2.73 Kelvin
= -270 degrees Celsius)
The expansion of the universe has redshifted infrared
light from when the universe became transparent to
~1000 times longer wavelength now: microwaves
The cosmic
microwave
background –
the radiation left
over from the
Big Bang – was
detected by
Penzias &
Wilson in 1965,
for which they
later won the
Nobel Prize.
Cosmic background has a
nearly uniform temperature
2.73 degrees above
absolute zero (2.73 Kelvin
= -270 degrees Celsius)
Cosmic microwave background has a nearly uniform
temperature, but not quite uniform...
Wilkinson Microwave Anisotropy Probe (WMAP)
• The WMAP satellite has shown that when our
universe became transparent (when it was
about 380,000 years old), it was uniform in
temperature to within a maximum difference of
plus or minus 1 part in 5,000.
Wilkinson Microwave Anisotropy Probe (WMAP)
• The WMAP satellite has shown that when our
universe became transparent (when it was
about 380,000 years old), it was uniform in
temperature to within a maximum difference of
plus or minus 1 part in 5,000
• If everyone on Earth were the same height
to within plus or minus 1 part in 5000, the
tallest person on Earth would be only 1 mm
taller than the shortest person on Earth.
Remember that every point on this map of the microwave sky
represents light that traveled 13.7 billion light years to reach us…
How can temperature be nearly identical on opposite
sides of the sky, which didn’t know about each other?
Inflation: regions now on opposite side of the sky were
close together before inflation pushed them far apart
Inflation (Alan Guth, Stanford, 1980s)
• When the universe was almost unimaginably
young, the forces between particles got out of
balance ... we call this symmetry breaking.
• Analogy: you can balance a pencil on end if you
provide a force to balance gravity. Take that
force away, and the force of gravity is converted
into energy of motion in the pencil.
Inflation
• When the universe was almost unimaginably
young, the forces between particles got out of
balance ... we call this symmetry breaking.
• Analogy: you can balance a pencil on end if you
provide a force to balance gravity. Take that
force away, and the force of gravity is converted
into kinetic energy in the pencil.
• At symmetry breaking, the unbalanced force
was converted into kinetic energy that made the
universe expand by a mind-boggling amount
• Analogy: microwave a potato in its skin!
Inflation can
make all the
structure in the
microwave
background by
stretching subatomic “ripples”
to enormous
size. These
ripples in
density then
become the
seeds for all
structures in
today’s universe
Inflation flattened
the overall geometry
of the universe like
the inflation of a
balloon flattens the
balloon’s surface –
however the
universe started out,
it ended up almost
exactly flat.
(Could there be
island universes in a
long, thin balloon?)
Observable Universe versus
Physical Universe
Observable Universe has a finite size.
Physical Universe is either infinite or
has a finite size so large it’s effectively infinite.
Structure Formation
Structure means:
Galaxies and Satellite Galaxies
Groups, Clusters, Superclusters of Galaxies
WMAP gives us detailed baby pictures of structure in
the universe
Observed patterns of structure in universe – from WMAP
etc. - agree (so far) with what inflation should produce
Time in billions of years
0.5
2.2
5.9
8.6
13.7
13
35
70
93
140
Size of expanding box in millions of lt-yrs
Gravity pulls mass into denser regions – universe grows
lumpier with time. But there’s not enough normal matter to
explain the lumpiness. So we think there’s dark matter...
matter that exerts gravity but hardly any other force.
Dark Matter and Dark Energy:
Will the universe continue
expanding forever?
Does the
universe have
enough
energy of
motion
(kinetic
energy) to
escape its
own
gravitational
pull?
Lots of
dark
matter
Not
enough
dark
matter
Fate of universe depends on amount of dark matter
Lots of dark
matter
Critical density
of matter
Not
enough
dark
matter
Fate of universe depends on amount of dark matter
Amount of dark matter
is ~25% of the critical
density suggesting fate
is eternal expansion
Not
enough
dark
matter
But expansion
appears to be
speeding up!
Dark
Energy?
Not enough
dark matter
old
older
oldest
Estimated age depends on dark matter and dark energy
The Big Bang and our Universe
• The Big Bang was a point in time, not in
space
• The physical universe has no center or edge
The Big Bang and our Universe
• The Big Bang was a point in time, not in space
• The physical universe has no center or edge
• The observable universe is centered on us.
We observe light from everything that was
within 13.7 billion light years of our current
location when the universe became
transparent. Next year, we’ll see 1 light-year
farther away.
The Big Bang and our Universe
• The Big Bang was a point in time, not in space
• The physical universe has no center or edge
• The observable universe is centered on us: we
observe light from everything that was within
13.7 billion light years of our current location
when the universe became transparent. Next
year, we’ll see 1 light-year farther away.
• Energy conservation: the universe’s positive
mass-energy & kinetic energy plus its negative
potential energy (three kinds: gravitational,
electroweak, strong-force) can sum to zero.
Olbers’ Paradox:
The night sky tells us that the
observable universe isn’t
infinitely old and unchanging
Olbers’ Paradox
If universe were
1) infinitely old,
and infinite in all
directions in space
2) unchanging
3) everywhere
the same
Then stars would
cover the night sky
Olbers’ Paradox
If universe were
1) infinite in space
and time
2) unchanging
3) everywhere
the same
Then, stars would
cover the night sky
Night sky is
dark because
the universe
changes with
time, and
because the
observable
universe is
neither
infinitely big
nor infinitely
old
Night sky is
dark because
the universe
changes with
time, and
because the
observable
universe is
neither
infinitely big
nor infinitely
old
Cosmological
Horizon
Universe’s age
of 13.7 billion
years limits how
far we can see,
and 13.7 billion
years ago there
weren’t any stars
around yet.
Thinking about a simpler universe
• Our universe has 3 (large) dimensions of
space: up/down, left/right, forward/backward.
Thinking about a simpler universe
• Our universe has 3 (large) dimensions of
space: up/down, left/right, forward/backward.
• Our universe has 1 dimension of time
(past/future, but we move only forward in time).
Thinking about a simpler universe
• Our universe has 3 (large) dimensions of
space: up/down, left/right, forward/backward.
• Our universe has 1 dimension of time
(past/future, but we move only forward in time).
• It’s easier to discuss a simpler universe -- one
with 2 dimensions of space and 1 of time – and
then extend that picture to 3 dimensions.
• Flatland by Edwin Abbott
Thinking about a simpler universe
• A universe with 1 dimension of time but only
2 dimensions of space might have left/right and
front/back, but no up/down (like a board game).
Or it might have left/right and up/down, but no
front/back (like some video games [Asteroids]).
• This two-dimensional universe could be:
• Flat & infinite (infinite piece of paper)
• Positively curved & finite (surface of a balloon)
• Negatively curved & infinite (infinite Pringle)
Thinking about a ‘simpler’ universe
• A universe with 1 dimension of time but only
2 dimensions of space might have left/right and
front/back, but no up/down (like a board game).
Or it might have left/right and up/down, but no
front/back (like some video games [Asteroids]).
• This two-dimensional universe could be:
• Flat & infinite (infinite piece of paper)
• Flat & semi-finite (infinitely long cylinder)
• Flat & finite (surface of a donut [a torus])
• Positively curved & finite (surface of a balloon)
• Negatively curved & infinite (infinite Pringle)
• Negatively curved & finite (can’t be drawn!)
Inflation of universe
flattens overall
geometry like the
inflation of a balloon
– however the
universe started out,
it will end up almost
exactly flat.
(Could there be
island universes in a
long, thin balloon?)
Thinking about our universe
• Our three-dimensional universe appears very
close to flat (<=1 part in 50 at the moment), as
well as infinite (infinite in all spatial directions)
• Inflation predicts our universe is very close to
flat (probably at the level of 1 part in 100,000)
• If the universe is that flat, for all intents and
purposes it is exactly flat, and we may never
know whether our universe started out exactly
flat or curved (positively or negatively)
• (2-dimensional analogies: zero curvature =
infinite piece of paper; positive curvature = like
a balloon; negative curvature = like a Pringle)
Aside: Spherical Geometry
• Flat geometry means angles of a triangle in that
space add up to 180 degrees (this is the normal
‘Euclidean’ geometry taught in school)
• Positively curved geometry means angles of a
triangle add up to more than 180 degrees
(From the north pole, draw two lines to the
equator: one through 0 degrees longitude and
another through 90 degrees longitude. Connect
them along the equator to make a triangle with
three 90-degree angles.)
• Negatively curved: angles of a triangle add up
to less than 180 degrees
Density =
Critical
Density >
Critical
Density <
Critical
Overall
geometry of the
universe is
closely related
to total density
of matter &
energy