Quarks to Quasars
Very Big Stuff
Recap
- Energy has mass and mass has energy.
- Matter makes spacetime curve.
- The curves of spacetime affect how matter moves.
- Our models of the universe are always incomplete.
Cosmology
The study of the universe in its entirety. It's structure and its evolution.
Cosmogony - the study of the origins of the universe.
The shape of the universe
What does the universe look like?
The shape of the universe
What does the universe look like?
Since spacetime is 4-dimensional and curved, that would be a lot easier to answer if you were a 5-dimensional being.
The shape of the universe
What does the universe look like?
Since spacetime is 4-dimensional and curved, that would be a lot easier to answer if you were a 5-dimensional being.
We can't visualise the shape of the universe.
But we can describe its geometry.
What happens to beams of light?
The shape of the universe
What about our universe?
No one is entirely sure.
Expansion
The universe cannot be infinite and have been around for ever.
Expansion
The universe cannot be infinite and have been around for ever.
But if it's not infinite, it should collapse under gravity.
Expansion
The universe cannot be infinite and have been around for ever.
But if it's not infinite, it should collapse under gravity.
Einstein proposed a weak, repulsive form of gravity to counteract this. Very unstable, however.
Expansion
1929 - After a decade of observations, Edwin Hubble showed that the speed that galaxies are moving away from us is proportional to their distance.
The universe is expanding.
L
L
2L
2L
Expansion
How do we know how far away stars are?
Distance to closest stars can be measured from parallax.
Hipparcos satellite gave accurate data for 120,000 stars.
Expansion
How do we know how far away stars are?
Distance to closest stars can be measured from parallax.
Hipparcos satellite gave accurate data for 120,000 stars.
From this information, we know that certain types of stars always give out the same amount of light. They can be used as 'standard candles'.
Expansion
How do we know how fast the stars are running away?
Doppler shift of light.
Expansion
There's a problem (as ever...).
Galaxies very far apart from each other are moving faster than the speed of light relative to each other.
How can this be?
Expansion
There's a problem (as ever...).
Galaxies very far apart from each other are moving faster than the speed of light relative to each other.
In fact, the galaxies are stationary. The space between them is expanding.
Expansion
There's a problem (as ever...).
Galaxies very far apart from each other are moving faster than the speed of light relative to each other.
In fact, the galaxies are stationary. The space between them is expanding.
We, the Solar System and our galaxy are not expanding. Gravity and molecular forces are much more significant.
A quick quiz
1. How big is space?
2. If all distant galaxies are moving away from us, are we at the centre of the universe?
3. A black hole is an area of space surrounding a point of infinite density where gravity is so strong that even light cannot escape. How big is a black hole? What would happen as you approached?
4. If the universe is 13.5 billion years old, how far away are the furthest objects in the sky that we can see?
What happens next?
This depends on the density of the universe.
Lots of stuff - universe will collapse. Spherical.
Some stuff - universe will just avoid collapse. Flat.
Not that much stuff - universe will continue to expand big-time. Hyperbolic.
What happens next?
Infinite is rather big
Define infinite - You can head off in any direction and go on forever.
Two interpretations of this:
Infinite is rather big
Imagine if you had a big box with every single even number in it.
2, 4, 6, 8, 10, 12, 14...
If you counted them, you'd have an infinite number.
Infinite is rather big
Imagine if you had a big box with every single even number in it.
2, 4, 6, 8, 10, 12, 14...
If you counted them, you'd have an infinite number.
Then you throw in all the odd numbers as well and count them:
1, 2, 3, 4, 5, 6, 7, 8, 9, 10...
You have more numbers but also the same (infinite) number.
Infinite is rather big
Imagine if you had a big box with every single even number in it.
2, 4, 6, 8, 10, 12, 14...
If you counted them, you'd have an infinite number.
Then you throw in all the odd number as well and count them:
1, 2, 3, 4, 5, 6, 7, 8, 9, 10...
You have more numbers but also the same (infinite) number.
Infinity is a strange place.
Infinite is rather big
Does the universe go on forever?
We can't tell.
Our observable universe is limited by:
- Stretching of space.
- Speed of light.
- Age of the universe.
Infinite is rather big
Does the universe go on forever?
We can't tell.
Our observable universe is limited by:
- Stretching of space.
- Speed of light.
- Age of the universe.
We can see stuff that's (presumably now) 46 billion light-years away.
But we're seeing what it looked like when it was 36 million light-years away.
Infinite is rather big
What we do know:
The universe looks the same whichever way we look.
If it didn't go on for a very long way indeed, this wouldn't be the case.
Infinite is rather big
What we do know:
The universe looks the same whichever way we look.
If it didn't go on for a very long way indeed, this wouldn't be the case.
If it's not infinite, it's still stupidly, stupidly enormous...
Spherical, flat or saddle-shaped?
Wilkinson Microwave Anisotropy Probe (WMAP)
Space-based telescope which detects microwave radiation left over from the Big Bang.
Spherical, flat or saddle-shaped?
Wilkinson Microwave Anisotropy Probe (WMAP)
Space-based telescope which detects microwave radiation left over from the Big Bang.
Microwaves are almost uniform but have been travelling for a long time.
Size of visible fluctuations is affected by the geometry of the universe.
Spherical, flat or saddle-shaped?
Wilkinson Microwave Anisotropy Probe (WMAP)
Space-based telescope which detects microwave radiation left over from the Big Bang.
Microwaves are almost uniform but have been travelling for a long time.
Size of visible fluctuations is affected by the geometry of the universe.
The universe is flat. (To within a 2% margin of error.)
Contact
Contact
How accurate was the initial pull away from Earth?
What are the chances of there being intelligent life out there?
What problems would be involved in making contact?
Dark Energy
By looking at distant galaxies, it's possible to determine how the expansion of the universe has varied over time.
Dark Energy
By looking at distant galaxies, it's possible to determine how the expansion of the universe has varied over time.
Further away means further back in time.
Rate of expansion should have been higher in the past.
Therefore, greater red-shift.
Dark Energy
By looking at distant galaxies, it's possible to determine how the expansion of the universe has varied over time.
Further away means further back in time.
Rate of expansion should have been higher in the past
Therefore, greater red-shift.
But expansion should have slowed, preventing them separating from us so fast. Light didn't have so far to travel.
Standard candles in distant galaxies should seem brighter than red-shift suggests.
Dark Energy
Bizarrely, the supernovae appear dimmer than they would if the rate of expansion were constant.
The rate of expansion is increasing.
Dark Energy
Bizarrely, the supernovae appear dimmer than they would if the rate of expansion were constant.
The rate of expansion is increasing.
This requires a repulsive force and lots of energy.
Dark Energy
Bizarrely, the supernovae appear dimmer than they would if the rate of expansion were constant.
The rate of expansion is increasing.
This requires a repulsive force and lots of energy.
Empty space exerts outward pressure.
Dark Energy
Bizarrely, the supernovae appear dimmer than they would if the rate of expansion were constant.
The rate of expansion is increasing.
This requires a repulsive force and lots of energy.
Empty space exerts outward pressure.
No one knows how or why...
...but plenty of people have a theory.
Dark Matter
Geometry of the universe suggests it has critical density.
- Just enough to keep on expanding (without dark energy).
Unfortunately, we can't see enough stuff.
Dark Matter
Geometry of the universe suggests it has critical density.
- Just enough to keep on expanding (without dark energy).
Unfortunately, we can't see enough stuff.
We know it's there because of the movement of galaxies.
We just don't know what it is.
Dark Matter
Two possibilities:
Dark Matter
Two possibilities:
MACHOs - Ordinary matter which is simply too faint to see.
Dark Matter
Two possibilities:
MACHOs - Ordinary matter which is simply too faint to see.
WIMPs - Sparticles! Everywhere but difficult to detect.
Dark Matter
Two possibilities:
MACHOs - Ordinary matter which is simply too faint to see.
WIMPs - Sparticles! Everywhere but difficult to detect. Only interact via weak force and gravity.
Not enough MACHOs detected. Therefore WIMPs currently winning.
Here be dragons
The universe -
Ordinary Matter: 4.6%
Dark Matter: 23%
Dark Energy: 72%
Gravitational Waves
Gravitational waves provide a way of investigating some of the dark matter.
They are ripples in the fabric of space-time created by objects accelerating.
Gravitational Waves
Gravitational waves provide a way of investigating some of the dark matter.
They are ripples in the fabric of space-time created by objects accelerating.
Useful because:
Gravitational Waves
Gravitational waves transmit energy.
Two stars orbiting around each other will create gravitational waves and lose energy. They slow down and spiral inwards.
This effect was observed between two pulsars in 1974 using the Arecibo Radio Observatory in Puerto Rico. The observed slow down since then matches extremely closely with the predictions of General Relativity.
Gravitational Waves
Direct Observation:
Passing waves cause the distance between objects to change rhythmically.
Tiny actual distances.
Gravitational Waves
Direct Observation:
Passing waves cause the distance between objects to change rhythmically.
Tiny actual distances.
The Laser Interferometer Gravitational-Wave Observatory (LIGO) detected the collision of two black holes in 2015.
The black holes are 1.3 billion light years away. The 4 kilometre long experiment changed in length by 1/1000 of the width of a proton.
Gravitational Waves
Modelling
Relativity and quantum mechanics.
How closely is any of this related to reality?
Modelling
Let's go back to Archimedes.
Remember him?
Modelling
He also came up with this principle:
Put something in a liquid and it will feel lighter. The liquid pushes it up. The size of the push is equal to the weight of liquid moved out of the way.
4N
6N
Spring balance
Weight of displaced liquid = 2N
Modelling
Leads to an explanation of why stuff floats
Up and down forces on a submerged object.
Which wins?
Pressure of liquid
Pressure of liquid
Gravity
Modelling
The pressure is different at different depths. It depends on the amount of liquid above.
If the displaced weight of liquid is more than the weight of the object, then the object will go up.
Pressure of liquid
Pressure of liquid
Gravity
Modelling
Great! We've gone from an observation, to a theory and now we can predict that anything with a mean density lower than water will float.
Modelling
Great! We've gone from an observation, to a theory and now we can predict that anything with a mean density lower than water will float.
All well and good but what assumptions are we making?
What have we simplified, ignored or taken for granted?
Modelling
Great! We've gone from an observation, to a theory and now we can predict that anything with a mean density lower than water will float.
All well and good but what assumptions are we making?
What have we simplified, ignored or taken for granted?
Flat surfaces, no currents or other forces, we're not using a sponge, that kind of thing.
Modelling
We're probably safe, though. Stuff floats. Reality and the model match up pretty well.
It doesn't take much to separate the two a little, though.
Take the behaviour of gases as an example.
Modelling
You can draw pretty graphs like this:
Temperature
Pressure
The Pressure Law
Modelling
The behaviour can even be derived mathematically using the following assumptions:
Modelling
Most gases aren't this ideal.
Real gases do behave this way at low pressures and at temperatures well above the point they start to liquify.
The 'laws' are useful and the maths is relatively easy.
Modelling
It's easy to increase the complexity of the model. Doing the calculations can be the hard part.
Still, if we're making assumptions, how do we know we're not picking and choosing like Aristotle?
Modelling
For that we need the Scientific Method. (See Week 9!)
Recap
- The universe has critical density. Light beams stay parallel.
- This requires Dark Matter. (Probably WIMPS.)
- It's expanding.
- It's rate of expansion is increasing. This requires Dark Energy.
Next week
The evolution of the universe.
Stars.
The scientific method - sorting the crazy but true from the simply crazy.