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

• 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

• 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 Relativity- Gravitational 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?

• 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