the expanding universe
Post on 15-Mar-2016
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The Expanding Universe• 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)
The Expanding Universe
This expansion pattern (speed proportional to distance) actually implies that galaxies are all moving away from each other
ExpansionMilky Way
The Expanding Universe
This expansion pattern (speed proportional to distance) actually implies that galaxies are all moving away from each other
ExpansionMilky Way
Twice as far away, so moves twice as fast
The Expanding Universe
This expansion pattern (speed proportional to distance) actually implies that galaxies are all moving away from each other
A while later:
d
Start:
d 2d
The Expanding Universe
• 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!
A while later:
Start:
The Expanding Universe
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)
The Expanding Universe
Cosmological Red-Shift
• Not really a Doppler effect• Space itself is being stretched between
galaxies
Conclusions from our Observations• 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 expands now, 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.
Big Bang
• 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”
How does the expansion work?• Like an explosion (hot, dense matter in the
beginning), but space itself expands!
• 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)
More General
• 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
Do pumpkins fall faster than apples?
No!
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
The Idea behind General Relativity
– 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”
Planetary Orbits
Sun Planet’s orbit
Effects of General RelativityBending of starlight by the Sun's gravitational field (and other gravitational lensing effects)
The Dynamics of the Universe
• 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
The “size” of the Universe – depends on time!
Expansion wins!
It’s a tie!
Mass wins!
Time
The Fate of the Universe
• 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
What General Relativity tells us
• 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!
Assumption: Cosmological Principle
• 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 no edge and no center – the ultimate Copernican principle!
The Fate of the Universe – determined by a single number!
• 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 > 1 means the universe will recollapse (Big Crunch)
Mass wins! 0 < 1 means gravity not strong enough to halt the expansion
Expansion wins!
• And the number is: 0 < 1 (probably…)
The Shape of the Universe
• In the basic scenario there is a simple relation between the density and the shape of space-time:
Density Curvature 2-D example Universe Time & Space
0>1 positive sphere closed, bound finite
0=1 zero (flat) plane open, marginal infinite
0<1 negative saddle open, unbound infinite
So, how much mass is in the Universe?
• Can count all stars, galaxies etc. this gives the mass of all “bright”
objects
• But: there is also DARK MATTER
“Bright” 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
First evidence for dark matter: The missing mass problem
• Showed up when measuring rotation curves of galaxies
Is Dark Matter real?
• 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!
Properties of Dark Matter• 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
What is Dark Matter?
• i.e. what does Dark matter consist of?– Brown dwarfs?– Black dwarfs?– Black holes?– Neutrinos?– Other exotic subatomic particles?
Back to: Expansion of the Universe
• 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!
Enter: The Cosmological Constant
• Physical origin of 0 is unclear
• Einstein’s biggest blunder – or not !
• Appears to be small but not quite zero!
• Particle Physics’ biggest failure
• Usually denoted 0, it represents a uniform pressure which either helps or retards the expansion (depending on its sign)
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