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

The Big Bang

IN THE BEGINNING…….

EVERYTHING!

BOOM!

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)

PROTONNEUTRON

ELECTRON

COSMIC ‘SOUP’

Charged particles - photons scatter (like ‘fog’)

300,000 years later

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

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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?

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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)

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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

Back to the expanding Universe…..

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

Balloon Analogy

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 - Blueshift

Redshift Equations

• Redshift is defined as the change of wavelength as a fraction of the rest wavelength

• Interpreting the redshift as a Doppler shift:

Measuring Redshift

Increasing wavelength

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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

Distances derived using ‘standard candles’

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Constant of proportionality - The Hubble constant

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!

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As for the fate of the Universe……

Critical density: Universe expands foreverLess dense: Expansion rate increasesMore dense: Universe will collapseAccelerating: 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

The next big thing…..?

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