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The Expanding Universe

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- Except for a few nearby galaxies (like Andromeda), all the galaxies are seen to be moving away from us
- Generally, the recession speed of a galaxy is proportional to its distance from us; that is, a galaxy that’s twice as far away is moving twice as fast (aside from local motions within galaxy clusters)

This expansion pattern (speed proportional to distance) actually implies that galaxies are all moving away from each other

Milky Way

Expansion

This expansion pattern (speed proportional to distance) actually implies that galaxies are all moving away from each other

Milky Way

Expansion

Twice as far away,

so moves twice as fast

d

This expansion pattern (speed proportional to distance) actually implies that galaxies are all moving away from each other

Start:

A while later:

2d

d

- Each galaxy sees the others moving away with the same pattern (further → faster)
- As though the galaxies ride on a rubber band that is being stretched!

Start:

A while later:

In three dimensions, imagine the galaxies are raisins in an expanding loaf of bread

- Appears the universe “exploded” from a state in which matter was extremely dense and hot – the Big Bang
- Where did the expansion begin? Everywhere!
- Every galaxy sees the others receding from it – there is no special point (center)

Not really a Doppler effect

Space itself is being stretched between galaxies

The Universe has a finite age, so light from very distant galaxies has not had time to reach us, therefore the night sky is dark.

The universe expandsnow, so looking back in time it actually shrinks until…?

Big Bang model: The universe is born out of a hot dense medium

13.7 billion years ago.

- The “start” of the universe, a primordial fireball
the early universe was very hot and dense

intimate connection between

cosmology and nuclear/particle

physics

- “To understand the very big
we have to understand the

very small”

Like an explosion (hot, dense matter in the beginning), but space itselfexpands!

Slowed down by gravitational attraction

Attraction is the stronger, the more mass there is in the universe

Scientifically described by Einstein’s

General theory of Relativity (1915)

- General Relativity is more general in the sense that we drop the restriction that an observer not be accelerated
- The claim is that you cannot decide whether you are in a gravitational field, or just an accelerated observer
- The Einstein field equations describe the geometric properties of spacetime

Galileo: In the absence of air, all objects experience the same acceleration (change in motion) near Earth’s surface

http://www.youtube.com/watch?v=5C5_dOEyAfk

We view space and time as a whole, we call it four-dimensional space-time.

It has an unusual geometry, as we have seen

Space-time is warped by the presence of masses like the sun, so “Mass tells space how to bend”

Objects (like planets) travel in “straight” lines through this curved space (we see this as orbits), so

“Space tells matter how to move”

Sun

Planet’s orbit

Bending of starlight by the Sun's gravitational field (and other gravitational lensing effects)

- There is a competition between Hubble expansion and gravity, which attempts to pull things together
- The ultimate fate of the universe depends on how fast it is expanding and how much mass is in it
- More mass means more slowing of the expansion

- If there is too little mass the expansion “wins” and the universe will expand for ever – “unbound” universe
- If there is enough mass gravity “wins” and the universe will eventually re-collapse (Big Crunch) – a “bound” universe

Expansion

wins!

It’s a tie!

Mass wins!

Time

- The Hubble constant H0 tells us how fast the universe is expanding
- We also need to know the density of matter in the universe
- “Critical” density is the density required to just barely stop the expansion
- We’ll use 0 = actual density/critical density
- 0 = 1 is then the critical density
- 0 > 1 means the universe will re-collapse (Big Crunch)
- 0 < 1 means gravity is not strong enough to halt the expansion

- And the number is: 0 = 1 ± 0.02

The more mass there is in the universe, the more “braking” of expansion there is

So the game is:

Mass vs. Expansion

And we can even calculate who wins!

The Cosmological Principle: on very large scales (1000 Mpc and up) the universe is homogeneous and isotropic

Reasonably well-supported by observation

Means the universe has noedge and nocenter – the ultimate Copernican principle!

Critical density is the density required to just barely stop the expansion

We’ll use 0 = actual density/critical density:

0 = 1 means it’s a tie

0 > 1means the universe will recollapse (Big Crunch) Mass wins!

0 < 1means gravity not strong enough to halt the expansion Expansion wins!

And the number is: 0 < 1 (probably…)

In the basic scenario there is a simple relation between the density and the shape of space-time:

DensityCurvature2-D exampleUniverseTime & Space

0>1 positive sphere closed, bound finite

0=1 zero (flat) planeopen, marginal infinite

0<1 negative saddleopen, unbound infinite

- Can count all stars, galaxies etc.
- this gives the mass of all “bright” objects
- But: there is also DARK MATTER

- All normal or “bright” matter can be “seen” in some way
- Stars emit light, or other forms of electromagnetic radiation
- All macroscopic matter emits EM radiation characteristic for its temperature
- Microscopic matter (particles) interact via the Standard Model forces and can be detected this way

- Showed up when measuring rotation curves of galaxies

- It is real in the sense that it has specific properties
- The universe as a whole and its parts behave differently when different amounts of the “dark stuff” is in it
- Good news: it still behaves like mass, so Einstein’s cosmology still works!

- Dark Matter is dark at all wavelengths, not just visible light
- We can’t see it (can’t detect it)
- Only effect is has: it acts gravitationally like an additional mass
- Found in galaxies, galaxies clusters, large scale structure of the universe
- Necessary to explain structure formation in the universe at large scales

- i.e. what does Dark matter consist of?
- Brown dwarfs?
- Black dwarfs?
- Black holes?
- Neutrinos?
- Other exotic subatomic particles?

- Either it grows forever
- Or it comes to a standstill
- Or it falls back and collapses (“Big crunch”)
- In any case: Expansion slows down!

Surprise of the year 1998

(Birthday of Dark Energy):

All wrong! It accelerates!

- Usually denoted 0, it represents a uniform pressure which either helps or retards the expansion (depending on its sign)

- Physical origin of 0is unclear
- Einstein’s biggest blunder – or not !
- Appears to be small but not quite zero!
- Particle Physics’ biggest failure