Modern AstronomyModern AstronomyStars & GalaxiesStars & Galaxies

Lecture 9Lecture 9

CosmologyCosmologyGeraint F. LewisGeraint F. Lewis

University of Sydney 2007University of Sydney 2007

OutlineOutline

The Universe as we know itThe Universe as we know it Cosmological modelsCosmological models Observations of the UniverseObservations of the Universe The evolution of the UniverseThe evolution of the Universe The distribution of matterThe distribution of matter The future of the CosmosThe future of the Cosmos

The Universe as we know itThe Universe as we know it

www-ed.fnal.gov/projects/exhibits

Fundamental ForcesFundamental Forces

Strong: 1

EM: 10-2

Weak: 10-5

Gravity: 10-39

Forces: UnificationForces: Unification

As the energy of the interactions increases, these force look As the energy of the interactions increases, these force look more like each other.more like each other.

Quantum Electrodynamics: Quantum Electrodynamics: unifies EM & Weakunifies EM & Weak Quantum Chromodynamics: Quantum Chromodynamics: EM-Weak & StrongEM-Weak & Strong Currently trying to add gravityCurrently trying to add gravity

Superstrings?????

Forces: UnificationForces: Unification

hyperphysics.phy-astr.gsu.edu/hbase/astro/unify.html

Cosmological FoundationsCosmological Foundations

General RelativityGeneral Relativity

In Einstein’s view, gravity is not In Einstein’s view, gravity is not really a force!really a force!

Importantly, gravity is created not Importantly, gravity is created not only by mass, but also by only by mass, but also by energy.energy.

Remember, mass is energy E=mcRemember, mass is energy E=mc22

CosmologyCosmology

Imagine a universe filled Imagine a universe filled with stationary stars.with stationary stars.

If finely balanced, it If finely balanced, it remains stationary.remains stationary.

A single star out of place A single star out of place results in collapse.results in collapse.

Cosmological ConstantCosmological Constant

This worried Einstein as he thought This worried Einstein as he thought the Universe should be static.the Universe should be static.

He added He added , the cosmological , the cosmological constant, which acts as anti-constant, which acts as anti-gravity.gravity.

This acts against collapse, but has This acts against collapse, but has negative pressure and appears negative pressure and appears unphysical!unphysical!

Einstein’s Einstein’s Biggest Blunder!Biggest Blunder!

CosmologyCosmology

Friedmann & Lemaitre looked at Friedmann & Lemaitre looked at cosmology within the framework cosmology within the framework of General Relativity.of General Relativity.

The concluded that, in general, The concluded that, in general, universes must be expanding or universes must be expanding or contracting.contracting.

Just how the universe changes with Just how the universe changes with time depends upon the energies time depends upon the energies in the Universe.in the Universe.

When this original work was done, When this original work was done, the only energy considered was the only energy considered was matter!matter!

EvolutionEvolution Too little matter, and the Too little matter, and the

Universe never slows downUniverse never slows down Too much matter, and the Too much matter, and the

Universe collapsesUniverse collapses Just right, the expansion Just right, the expansion

slows down forever, but slows down forever, but never quite reaches zeronever quite reaches zero

But what does this scale factor But what does this scale factor mean? It is the change of mean? It is the change of separation between a pair of separation between a pair of objects. objects.

EvolutionEvolution The raisin cake picture The raisin cake picture

encompasses this pictureencompasses this picture As the cake expands, the As the cake expands, the

distance between raisins distance between raisins increasesincreases

But for this picture to be But for this picture to be accurate, there can be no accurate, there can be no edges to the cake!edges to the cake!

Where is the centre of the Where is the centre of the Universe in this picture?Universe in this picture?

GeometryGeometry

The ratio of the The ratio of the circumference to the circumference to the diameter of a circle is diameter of a circle is =3.141592….=3.141592….

But this is only true in But this is only true in the flat geometry of the flat geometry of EuclidEuclid

What about non-flat What about non-flat geometry??geometry??

Open, flat, closed?Open, flat, closed?

Measure Measure on the surface on the surface of the Earthof the Earth

From pole to equator = From pole to equator = 10000km10000km

Around the equator = Around the equator = 40000km40000km

= 40000/(2= 40000/(2££10000) = 210000) = 2

Open, flat, closed?Open, flat, closed?

So, depending upon the density in the UniverseSo, depending upon the density in the Universe

Open >Open >: Infinitely large & expand forever : Infinitely large & expand forever Flat = Flat = : Infinitely large & expand forever: Infinitely large & expand forever Closed <Closed <: Finite & eventually collapse: Finite & eventually collapse

Critical density = 1 Critical density = 1 ££ 10 10-26-26 kg/m kg/m33

= 6 hydrogen atoms / m= 6 hydrogen atoms / m33

Observing: The redshiftObserving: The redshift

Hubble’s expansionHubble’s expansion

Hubble found a relation Hubble found a relation between distance and between distance and velocityvelocity

V = HV = Hoo d d

where d is the distance to where d is the distance to the galaxy and v is its the galaxy and v is its velocity.velocity.

Hubble’s constant has Hubble’s constant has been measured to bebeen measured to be

HHoo = 72 km/s/Mpc = 72 km/s/Mpc

Hubble’s expansionHubble’s expansion

Imagine the Earth expands overnight. We might feel squashed in our beds, but when we awake in the morning, all the distances would have increased! What if we lived in the surface?

What is the redshift?What is the redshift?

The redshift is not a Doppler shift.

As the Universe expands, the wavelength of the radiation is also stretched.

Mathematically

where z is the redshift and R is the distance between galaxies.

Which universe is ours?Which universe is ours?The cosmological models make specific predictions on how faint something should look at a particular redshift.

This depends upon the content of the Universe.

Observations of distant supernova show that we do not live in a universe containing only matter.

In fact we live in a universe dominated by Something Else!!!!

Cosmological time dilationCosmological time dilation

Cosmological supernovae are like Cosmological supernovae are like clocks, they brighten and fade in clocks, they brighten and fade in a fixed time.a fixed time.

The cosmological equations The cosmological equations predict that we should see the predict that we should see the distant universe run slowly, a distant universe run slowly, a time dilation effect.time dilation effect.

This has now been observed, This has now been observed, providing further evidence that providing further evidence that the cosmological models are a the cosmological models are a good description of the cosmos.good description of the cosmos.

Putting it all togetherPutting it all together Dark MatterDark Matter

• Interacts mainly via gravityInteracts mainly via gravity• Not stars, gas, rocks etcNot stars, gas, rocks etc• Not large black holesNot large black holes• Possibly an elementary particlePossibly an elementary particle

Dark EnergyDark Energy• Not dark matter!Not dark matter!• Looks like Einstein’s Looks like Einstein’s • Must be exotic: strings, defectsMust be exotic: strings, defects• Is beginning to dominateIs beginning to dominate• Accelerates the expansionAccelerates the expansionThe Universe is flat

The Universe is 13.7 billion yrs old

Accelerated expansionAccelerated expansion

Running backwardsRunning backwards

Running the Universe backwards, we see Running the Universe backwards, we see there was a point where the scale factor was there was a point where the scale factor was zero.zero.

This marks the This marks the Big BangBig Bang, or start of the , or start of the Universe.Universe.

Currently, science cannot answer what caused Currently, science cannot answer what caused this event.this event.

But science can describe the universe in detail But science can describe the universe in detail from 10from 10-43-43 seconds after the event to the seconds after the event to the present day!present day!

At the beginningAt the beginning The energy from inflation The energy from inflation

became particles and radiationbecame particles and radiation 1010-6 -6 secs, protons and neutrons secs, protons and neutrons

formedformed 1 sec, protons & neutrons join to 1 sec, protons & neutrons join to

make deuterium and heliummake deuterium and helium 3 mins, cooking ceased with the 3 mins, cooking ceased with the

universe 75% hydrogen and 25% universe 75% hydrogen and 25% heliumhelium

300,000 yrs, the Universe is cool 300,000 yrs, the Universe is cool enough for electrons to join enough for electrons to join atomsatoms

There should be a background of There should be a background of radiation left over from this radiation left over from this event!!event!!

The smaller the scale factor, the hotter the universe was. The very early universe was very hot!

Background RadiationBackground Radiation

In 1941, Herzberg saw that In 1941, Herzberg saw that molecules in interstellar space molecules in interstellar space were too energetic.were too energetic.

He concluded that they must be He concluded that they must be bathed in a radiation of bathed in a radiation of temperature 2.3K.temperature 2.3K.

““a rotational temperature of 2.3k a rotational temperature of 2.3k follows, which has of course only a follows, which has of course only a very restricted meaning”very restricted meaning”

Penzias & Wilson confirmed this in Penzias & Wilson confirmed this in 1965 (and won the Nobel in 1978)1965 (and won the Nobel in 1978)

Cosmic microwave backgroundCosmic microwave background

The universe is bathed in cool radiation The universe is bathed in cool radiation left over from its energetic start.left over from its energetic start.

Some of the snow on tv is this CMBSome of the snow on tv is this CMB

Two important predictions Two important predictions CMB was hotter in the pastCMB was hotter in the past CMB should not be completely smoothCMB should not be completely smooth

Cosmic microwave backgroundCosmic microwave background

Relativistic cosmology predicts that the Relativistic cosmology predicts that the temperature of the CMB should scale with the temperature of the CMB should scale with the size of the universe. size of the universe.

Astronomers have looked at distant molecules in Astronomers have looked at distant molecules in the early universe and have mentioned the early universe and have mentioned temperature of the CMB, finding it to be hotter temperature of the CMB, finding it to be hotter in the past, in agreement with the in the past, in agreement with the cosmological model.cosmological model.

www.eso.org/outreach/press-rel/pr-2000/pr-27-00.htmlwww.eso.org/outreach/press-rel/pr-2000/pr-27-00.html

The CMBThe CMB

While the CMB has a While the CMB has a mean temperature of mean temperature of 2.7K, there should be 2.7K, there should be small (10small (10-5-5) variations ) variations superimposed on it.superimposed on it.

These variations These variations reflect the distribution reflect the distribution of matter in the very of matter in the very early Universe.early Universe.

AnisotropyAnisotropy

Inflation predicts a Inflation predicts a very specific pattern very specific pattern on the CMB (red line)on the CMB (red line)

Data from COBE and Data from COBE and now WMAP have now WMAP have revealed this pattern revealed this pattern matches the theory!matches the theory!

In the beginning…In the beginning… … … the Universe was very smooththe Universe was very smooth

Inflation results in very small Inflation results in very small ripples in the matter at the level ripples in the matter at the level of 1 part in 10000 (as seen in the of 1 part in 10000 (as seen in the CMB)CMB)

Gravity causes material to flow Gravity causes material to flow into denser regions, forming the into denser regions, forming the large scale structure of galaxy large scale structure of galaxy groups, clusters and groups, clusters and superclusterssuperclusters

Again need a Universe in a Again need a Universe in a computercomputer

The growth of large scale structureThe growth of large scale structure

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Large Scale StructureLarge Scale Structure

Surveys of the Universe reveal galaxies to lie on the foamy structure predicted from inflation!

Sloan galaxy survey (www.sdss.org)

Present: 14Present: 14££101099 yrs yrs

Stars are metal richStars are metal rich More metals made in supernovaMore metals made in supernova New stars have less and less New stars have less and less

hydrogen as they become hydrogen as they become polluted with metalspolluted with metals

Mass is getting locked up in Mass is getting locked up in white dwarfs, neutron stars and white dwarfs, neutron stars and black holes!black holes!

Future: 16Future: 16££101099 yrs yrs

The Milky Way and Andromeda The Milky Way and Andromeda collide and form a single large collide and form a single large ellipticalelliptical

Future: 17Future: 17££101099 yrs yrs

The Sun diesThe Sun dies

Future: 7Future: 7££10101111 yrs yrs

With the accelerated expansion, With the accelerated expansion, galaxies beyond the Local Group galaxies beyond the Local Group fade from view.fade from view.

The sky outside the Local Group The sky outside the Local Group will be black!will be black!

Future: 10Future: 101313 yrs yrs

Nuclear fuel is exhaustedNuclear fuel is exhausted Small red stars finally burn outSmall red stars finally burn out All star light begins to fadeAll star light begins to fade

All potential raw material for star All potential raw material for star formation is locked away in formation is locked away in stellar remnantsstellar remnants

Future: 10Future: 101616 yrs yrs

Stellar interactions finally eject Stellar interactions finally eject all planets all planets

Future: 10Future: 102828 yrs yrs

Galaxies finally dissolve, with Galaxies finally dissolve, with 90% of stars ejected into 90% of stars ejected into intergalactic spaceintergalactic space

The remaining stars spiral into The remaining stars spiral into the central black hole the central black hole

Future: 10Future: 103232 yrs yrs

Protons decay and matter Protons decay and matter dissolves into radiation and dissolves into radiation and electrons. electrons.

Future: 10Future: 106767 !! 10 10100100 yrs yrs

Stellar and supermassive black Stellar and supermassive black holes evaporate via Hawking holes evaporate via Hawking radiationradiation

Nothing left by a sea of ever Nothing left by a sea of ever cooling radiationcooling radiation

The universe is now a cold, dark, The universe is now a cold, dark, lifeless place, a little bit like lifeless place, a little bit like Canberra. Canberra.

See you next week!See you next week!