1. Cosmology
Dr Katy Lancaster

2. Overview The Big Bang The Expanding Universe The Fate of the Universe
Formation of the Universe
The Cosmic Microwave Background
The COBE experiment
The Expanding Universe
Redshift
Hubble’s Law
The age of the Universe
The Fate of the Universe
The critical density
Big crunch or big chill?

3. Cosmology: ‘The science or theory of the universe as an ordered whole, and of the general laws which govern it. Also, a particular account or system of the universe and its laws.’

4. The Cosmological principle: ‘The Universe, on average, looks the same from any point and in all directions.’
Can observe the local Universe and draw conclusions about the Universe as a whole

5. The Big Bang

6. BOOM!
IN THE BEGINNING…….
EVERYTHING!

7. The Big Bang Not really an ‘explosion’
Universe expanded rapidly as a whole
Universe is still expanding today as a result of the Big Bang
Matter was created in the form of tiny particles (protons, neutrons, electrons)
Too hot for normal ‘stuff’ to form (eg atoms, molecules)

8. Charged particles - photons scatter (like ‘fog’)
COSMIC ‘SOUP’
PROTON
NEUTRON
ELECTRON
Charged particles - photons scatter (like ‘fog’)

9. Much cooled, atoms form, photons escape
300,000 years later
Much cooled, atoms form, photons escape

10. Formation of the CMB The Universe is initially hot, dense and ionised
Photons continually scatter from charged particles until….
….temperature decreases and atoms form (neutral particles)
Photons ‘escape’ and stream freely through the Universe.
Observe the same photons today, much cooled, as the Cosmic Microwave Background

11. Discovery of the CMB
Penzias and Wilson record excess noise when observing the galaxy
Soon identified as the CMB
Isotropic to 1 part in 100,000 - recognised as evidence for the big bang

12. What is the CMB like?
Can observe the CMB photons today, 13.7Gyr after the Big Bang
Radiation has been highly redshifted by the Hubble Expansion (wavelength now longer)
Much cooled: 2.73 K (compare this with 3000K at recombination)
Conclusive evidence for the Big Bang theory - proves Universe was once in thermal equilibrium
So what does it look like?

13. The Cosmic Background Explorer (COBE)
First satellite dedicated to CMB research
Launched by NASA in 1989
Measured the spectrum of the CMB
Also measured temperature fluctuations
Scientists won the Nobel Prize in 2007! (15 years after releasing their results)

14. A perfect blackbody
Remnant heat of the creation of the Universe

15. Observe ‘blank’ sky with a radio telescope (eg COBE)
Rather than darkness, see Uniform, high-energy glow
High sensitivity measurements reveal......

17. Tiny temperature differences
When the CMB photons ‘escaped’, structures were starting to form
These structures have now become galaxies
The structure formation processes have affected the CMB and we see the imprint as ‘hot’ and ‘cold’ spots
Very difficult to measure!

18. What does the CMB tell us?
Measure the strength of the temperature differences on different scales, eg:

19. What does the CMB tell us?
Measure the strength of the temperature differences on different scales, eg:

21. Parameters
The function on the previous slide is complex and involves many parameters including:
The Hubble constant
The density of the Universe
The curvature of the Universe
The age of the Universe
And more…..
We can constrain some of the big questions in cosmology by observing the CMB
We will look at some of these in more detail later

22. Back to the expanding Universe…..

23. Edwin Hubble
Edwin Hubble discovered objects beyond the Milky Way - Andromeda
He realised that these objects are all moving away from us
By the cosmological principle, we see that all objects are moving away from each other

24. Balloon Analogy

25. Redshift
The light from a galaxy which is moving away from us will be Doppler Shifted
As the object is moving away from us, the light will be shifted towards the red end of the spectrum
We refer to this as the cosmological redshift for objects moving along with the general expansion of the Universe

26. Redshift - Blueshift

27. Redshift Equations
Redshift is defined as the change of wavelength as a fraction of the rest wavelength
Interpreting the redshift as a Doppler shift:

28. Measuring Redshift

Increasing distance
Increasing wavelength

29. Hubble’s Observations
Hubble made observations of 24 galaxies, measuring their distances and their recessional velocities
He realised that the more distant galaxies were moving away more quickly
In fact, he found that the two are directly proportional
The constant of proportionality is know as the Hubble Constant

30. Constant of proportionality - The Hubble constant
Recessional velocites (Doppler effect)
Distances derived using ‘standard candles’

31. The Hubble Constant
Hubble showed that the recessional velocity of an object (or indeed its redshift) is proportional to its distance from us.
We use to represent the Hubble constant, so can now write:
Measuring the Hubble constant has been a longstanding problem in cosmology!

32. In reality…..
We are actually quite unsure of the exact geometry of the Universe
Thus the simple relationships stated hold true only for objects at low redshift
For a flat Universe:
But we will stick with the low redshift scenario!

33. Age of the Universe From mechanics: From today: Thus we can write:
Where t is the time for which the galaxy has been moving away from the earth, ie the time since the Universe began expanding
(Remember that before the big bang, everything existed in a ‘singularity’!)

34. Fate of the Universe
How the Universe will end is determined by its density
Quite literally how much stuff it contains!
A very dense Universe will fall back in on itself in a ‘big crunch’
A very sparse Universe will continue expanding forever
Of course, it might be somewhere inbetween these extremes

35. Big Problem!!!!
We know that only a few percent of the Universe’s mass exists in material that we can see
Over 90% of the contents of the Universe is completely invisible!
We call this mysterious material ‘dark matter’
We STILL don’t know what it is!
This makes it difficult to determine the density of the Universe

36. Dark Matter Unknown compostion Does not emit or reflect EM radiation
Presence inferred from gravitational effects, e.g. lensing
May be exotic new particles
Or lots of undetected, dark astronomical bodies such as planets or dwarf stars
Its existence is crucial to our current cosmological models!

37. As for the fate of the Universe……

38. Critical density: Universe expands forever
Less dense: Expansion rate increases
More dense: Universe will collapse
Accelerating: Dark energy???

39. Critical Density
We can specify these scenarios via the critical density parameter
The critical density is the density at which the Universe is just closed
If the density of the Universe, is less than , the Universe will expand forever
If is greater than the Universe will stop expanding and collapse back on itself

40. Critical Density We can summarise these scenarios via the parameter
If , , expands forever
If , , critical Universe
If , , one day recollapses
We currently believe that the Universe is critical! Seems like quite a coincidence….

41. How can we measure ?
This is difficult, given that we know we can’t see most of the matter in the Universe!
However, measurements of the CMB can help us
The shape of the CMB power spectrum depends on (amongst other things).

43. What is being done? AMiBA Taiwanese project, based in Hawaii
Testing observations with 7 dishes
Ultimately: 19 dishes?
Will measure the CMB power spectrum

44. The next big thing…..?

45. Summary The Universe started with a Big Bang, and still expands today
We can observe ‘leftover’ radiation - CMB
The velocity of receding galaxies is proportional to their distance away from us
Light from galaxies is ‘redshifted’
Constant of proportionality is the Hubble Constant
Can use to determine the age of the Universe
The fate of the Universe depends on its density
We currently believe that the Universe is ‘critical’
This means the it will just continue expanding forever