The Universe
The Expanding Universe
• Edwin Hubble, 1920’s and 1930’s: Galaxies
receding from us
• The farther away they are, the faster they
recede
• Velocity increase seems to be linear
• Implies that at some point in the past,
everything was in a small space
• 24 km/sec per million l.y.  1019 km/24
km/sec  4.5 x 1017 sec = 13 b.y.
The Alternative
• British Astronomer Fred Hoyle derisively
coined the term “Big Bang”
• He proposed that matter was continuously
created in space
• Rate very small: One H atom per cubic meter
per billion years
• Universe always has same density
• Problem: Where do light nuclei come from?
• Problem: Where Are All the Baby Galaxies?
Cosmic Background Radiation
• 1965: Universe is filled with faint microwave
radiation, temperature 3K
• Highly diluted radiation left over from origin of
universe (Cosmic Microwave Background:
CMB)
• Smaller Universe would be hotter
– One million l.y. radius = 1300 x 1300 x energy/area
of sky = 170 million times present CBR
– Stefan-Boltzmann Law: radiation = T4
– T = 3K x 4√ 170 million = 340 K
Cosmic Microwave Background
Cosmic Microwave Background
Time Line
• T proportional to 1/√time
• > 1 b.y. (T=11k) Stable stars and galaxies
• 300,000 years (T=600 K): atoms form
– Space becomes transparent
– The CMB dates from this time
• 70,000 years (T=1300 K): Matter dominates
over radiation
• 3-20 minutes (T=50 million K):
Nucleosynthesis of He and Li
Time Line
• 1 second – 3 minutes: Leptons (electrons) sort
out
• 10-6 – 1 second: Hadrons (protons, neutrons)
form
• 10-12 – 10-6 second: Quarks dominate
• Before that: Electromagnetic, weak nuclear
and strong nuclear forces become distinct
• Inflation
Travel to the Stars?
• Kinetic Energy = 1/2 Mv2
• What does it take to get a 1000-ton spaceship
to 10% of the speed of light? (43 years to
Alpha Centauri)
• M=106 kg, v = 3 x 107 m/sec
• KE = 1/2 x 106 x 9 x 1014 = 4.5 x 1020 joules
• Equals U.S. Energy Use for 4.5 years
• Once you get there, you have to stop.
• You are planning on coming back, right?
Relativity
• Speed of Light is Independent of Source
• Michelson and Morley, 1887 - Speed of
Light Independent of Observer
• “One of the Most Unexpected Results in
the History of Science” - Isaac Asimov
• Conclusion: Speed of Light is the Same for
All Observers
• Implication: Space and Time Must Change
to Keep Speed of Light Constant
Pioneers of Relativity
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•
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Joseph Larmour
Hendrick Lorentz
Henri Poincare
They worked out the mathematical
implications of light always having a constant
speed
• Einstein took it to the next level
Frames of Reference
Frames of Reference
• Frames of Reference that are moving at
constant speed and direction are called
inertial
• Laws of physics are the same in all inertial
reference frames (Galilean invariance)
• There is no “right” frame of reference
What is Light?
Catching the Wave
What Would Einstein See?
Electrical and Magnetic Fields
Riding A Light Wave
• You’d See Alternating Strips of + and – Electric and
Magnetic Fields
• We Don’t See Anything Like That In Our World
• The Alternating Strips of + and – Electric and
Magnetic Fields Are Just There
• All Electric and Magnetic Fields in Our World Come
From a Source
• The Electric and Magnetic Fields of Light Generate
Each Other – Cannot Be Static
• Conclusion: There is no reference frame where light
is at rest
Why the Speed of Light is a Speed
Limit
• Energy of a Moving Object in Relativity:
E = mc2
 1 - v2/c2
-------------------------------------------------------------------------------
• One consequence: as v approaches c,
Energy goes to infinity
• Also, as we approach c, Energy goes up very
fast
Beating c?
Beating c?
Beating c?
Faster Than
Light
(But Not c)
Beating c?
• Lots of things can travel faster than light
– They are not material objects
– They do not carry information between two points
faster than light
• In a material where light is slower than c, it is
possible to travel faster than light
• Superluminal Jets
– Appear to travel faster than light
– Perspective effect of coming at us at an angle
What is Relativity?
• All the relationships observed in science hold
for all observers (cause-effect, conservation of
energy, etc.)
– This is why it’s called “relativity.”
• There is no frame of reference where light is at
rest.
Doppler Shift = Time Dilation
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Source = green (1.8 x 1015 sec per wave)
We see it as red (2.4 x 1015 sec per wave)
It’s the same wave
Things take 50 per cent longer for us than for
the source
• Our clock is faster, theirs is slower
Time Dilation and Muons
• Muons are short-lived particles created in the
upper atmosphere by cosmic ray collisions
• Their lifetime is 2 microseconds
• At c they could travel 600 m and should decay
before reaching the surface
• Actually observed in abundance
• At 0.998c, time dilation = 16x
Special and General Relativity
• You might think the “special” theory is more
“special” and the “general” theory is generic
• “Special” means “special case”: No changes in
velocity, direction, etc.
• “General” means it covers everything:
acceleration, gravity, etc.
The Equivalence Principle
• Gravity and Uniform Acceleration are
equivalent
• Nobody knows exactly why, but it is true to
very high precision
• Gravitational and uniformly accelerating
frames of reference are also inertial
– After all, every inertial framework we’ve ever
experienced has included gravity
The Equivalence Principle
Einstein and Gravity
• Common Sense: What we observe from
gravity, we will also see in an accelerating
spaceship
• No Surprises Here
• Einstein: What we see in an accelerating
spaceship, we will also see from gravity.
• We See Unexpected Things
– Redshift
– Time Dilation
Redshift and
Acceleration
Gravitational
Redshift
E = mc2
E = mc2
• Stationary observer sees light emitted in both
directions
• Moving observer sees more energy emitted in
one direction than the other
• Kinetic energy of emitter as seen by moving
observer must change
• Velocity doesn’t change
• Therefore mass of emitter must change
Fuel Economy of a Starship
• At 0.1 c, energy is 0.7% greater than
Newtonian formula
• At 0.5 c, 24% greater
• At 0.9 c, 3 x greater
• At 0.99 c, 12 x greater
• At 0.999 c, 43 x greater
• Each extra 9 more than triples the energy
• Getting our 1000 ton ship to 0.9 c takes 1.1 x
1023 joules = U.S. energy use for 1100 years
Another Consequence of RelativityGravitational Lenses
“Curved Space”
• Distance relationships in relativity aren’t
always the same as in ordinary life
• The overall geometry of the Universe might
not be the same as small-scale geometry
• The rules for measuring distance in a given
system are called a metric
• We use different metrics all the time, even
outside of relativity.
The
Urban
Metric
Warped
Space
Gravity and “Curved Space”
What Does The Universe Look Like?
What Does The Universe Look Like?
As Far Out as We Can See - Ten Days
With the Hubble Telescope
The Faint Outer Universe
• For nearby stars and galaxies, brightness
varies as 1/r2
• Distant galaxies are fainter than we’d expect
from distance
– Redshift: Photons packing less energy
– Time Dilation: Galaxies seem to be emitting
photons more slowly than they really are
– Apparent distance from brightness is hundreds of
billions of light years
A New Name for the “Big Bang?”
(Sky and Telescope, 1995)
• The Big Boot
• God’s Log-On
• Fred Withair Day (“Nobody ever named
anything else after me, so why not?”)
• What Happens If I Push This Button?
• You’re Never Going To Get It All Back In
There Again
90% of the Universe is “Missing”
• Outer Stars in Galaxies revolve faster than
expected
• What holds clusters of galaxies together?
• Conclusion: There must be a lot of invisible
mass in the Universe
• Not really “missing”, just non-luminous
Astronomers Are Not As Upset As One
Might Expect
• MACHO’s (Massive Compact Halo Objects):
faint stars, brown dwarfs, planets
• Cool non-luminous gas
• Massive Neutrinos?
• WIMP’s (Weakly Interacting Massive
Particles)
• Magnetic monopoles
• Exotic objects: strings, mini-black holes
Fine Tuning the Universe
• Density
– Much greater and the Universe would already
have collapsed in on itself
– Much less and stars could not have formed
• Nuclear Forces
– Fusion impossible
– Fusion too easy
• Anthropic Principle
– Cosmology has to be able to explain why we exist
Fine Tuning the Universe
• It just came out that way
• It has to be that way for reasons we haven’t
yet discovered
• Maybe there are an infinity of universes but
only those with certain parameters develop
intelligent life (Multiverse)
• Engineered or designed
Fine Tuning the Universe
• Designer?
• Doesn’t Explain Anything
• If the Designer can create a Universe, why is c
= 300,000 km/sec instead of 400,000?
• If Designer had to have c = 300,000 km/sec,
why? What (who?) dictated that, and why?
• Who says Designer is anything pictured by any
religion?
Misinterpreting Relativity
• There are no absolutes
– Speed of light is absolute
– It may not be possible to be absolutely right but it
is very easy to be absolutely wrong
Where Will It All End?
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Trillions of years: Star formation ends
10-100 trillion years: Stars stop radiating
1015 – 1020 years: Planetary orbits decay
1032 – 1041 years: Protons decay?
Heat Death?
Big Crunch?
Big Bounce?
What Does Cosmology Imply for Philosophy?
• Nothing
• At Least, Not Yet
• It Is Hopelessly Premature To Try To Create A
Philosophy Based on Cosmology
– We can say what the fate of the Universe will be
based on specific assumptions
– We are a long way from knowing the assumptions
are complete or correct
• If you support your philosophy with science,
be prepared for science to prove it wrong