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## Cosmology

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### Cosmology

Dr Katy Lancaster

Overview

- The Big Bang
- 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?

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.’

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

- 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)

Much cooled, atoms form, photons escape

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

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

- 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?

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)

A perfect blackbody

Remnant heat of the creation of the Universe

Observe ‘blank’ sky with a radio telescope (eg COBE)

- Rather than darkness, see Uniform, high-energy glow
- High sensitivity measurements reveal......

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!

What does the CMB tell us?

- Measure the strength of the temperature differences on different scales, eg:

What does the CMB tell us?

- Measure the strength of the temperature differences on different scales, eg:

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

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

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

Redshift Equations

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

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

Constant of proportionality - The Hubble constant

Recessional velocites (Doppler effect)

Distances derived using ‘standard candles’

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!

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!

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’!)

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

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

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!

Critical density: Universe expands forever

Less dense: Expansion rate increases

More dense: Universe will collapse

Accelerating: Dark energy???

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

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….

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).

What is being done?

- AMiBA
- Taiwanese project, based in Hawaii
- Testing observations with 7 dishes
- Ultimately: 19 dishes?
- Will measure the CMB power spectrum

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

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