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 Expansion Milky Way

The Expanding Universe This expansion pattern (speed proportional to distance) actually implies that galaxies are all moving away from each other Expansion Milky 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 2d2d

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 e x p a n d s now, so looking back in time it actually s h r i n k s 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 spacetimeEinstein field equations

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

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 Relativity Bending 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 H 0 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) planeopen, marginal infinite  0 <1 negative saddleopen, 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)