Ian M George PHYS 105 (2005 Spring) 2005may02 Announcements Final homework assignment: “Fate of the Universe” tutorial Strict deadline, due Weds May 11

Ian M George PHYS 105 (2005 Spring) 2005may02

AnnouncementsAnnouncementsFinal homework assignment:Final homework assignment:

““Fate of the Universe” tutorial Fate of the Universe” tutorial Strict deadline, due Weds May 11 5pm Strict deadline, due Weds May 11 5pm

As noted on www.jca.umbc.edu/~turner/course105.html

Final Exam 30%

Homework 30%

Quizzes plus Telescope Night 20%

Mid-term Exam 20%

A > 85% B 70-84% C 55-69% D 45-54% F < 45%


Final Exam Final Exam

• Monday May 23 1-3pm LH1Monday May 23 1-3pm LH1

-there will be plenty of spare time-there will be plenty of spare time

Covers chapters 15-23Covers chapters 15-23

As noted on www.jca.umbc.edu/~turner/course105.html


An Aside:An Aside:

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

April 30 2005 press release, ESO scientists using the Very Large Telescope facility in Chile

The planet, 2M1207b, is ~x5 The planet, 2M1207b, is ~x5 the size of Jupiter orbiting at the size of Jupiter orbiting at a distance nearly twice as far a distance nearly twice as far as Neptune is from our Sun.as Neptune is from our Sun.

Star and planet are more than Star and planet are more than 200 light-years away near 200 light-years away near the southern constellation of the southern constellation of Hydra.Hydra.

Star which is brown dwarfStar which is brown dwarf

Planet 'seen' around distant sun

An artist's impression of the planet around the brown dwarf


DMDMRecap last timeRecap last time

DM is a generic term for any matter that DM is a generic term for any matter that is:is:

not radiating or reflecting light, but is not radiating or reflecting light, but is exerting grav influence exerting grav influence

some of it (tiny bit) is due to MACHOs, some of it (tiny bit) is due to MACHOs, tiny bit due to neutrinos, tiny bit due to neutrinos, but vast majority thought to be WIMPs... but vast majority thought to be WIMPs...


Large Scale Structure Large Scale Structure ofof UniverseUniverse

On scales of 10On scales of 1088 ly: ly: galaxiesgalaxies distributed in gigantic distributed in gigantic chainschains & & sheetssheets surrounding great surrounding great voidsvoids..

• Chains+sheetsChains+sheets from initial regions of from initial regions of density enhancementdensity enhancement• VoidsVoids from initial regions of from initial regions of density depletiondensity depletion

On scales of several x 10On scales of several x 1099 ly: ly: galaxiesgalaxies appear appear evenly distributedevenly distributed..

slice of the Universe out to 7 x 108 ly slice of the Universe out to 4 x 109 ly


Simulation: Different TimesSimulation: Different Times

simulation courtesy of Prof. Nickolay Gnedin (Univ. Colorado)

redshiftz =Δ

=λλ

Movie. Click to play.

brown color represents neutral Hydrogen


The Critical DensityThe Critical DensitySo gravitational attraction between galaxies can overcome expansion of So gravitational attraction between galaxies can overcome expansion of

Universe in localized Universe in localized regionsregions

How strong must gravity be How strong must gravity be

to stop theto stop the entireentire Universe from expanding?!? Universe from expanding?!?- - it depends on the total it depends on the total mass densitymass density of the of the

UniverseUniverse

mass density required for this gravitational pull to equal the mass density required for this gravitational pull to equal the kinetic energy of the Universe as the “kinetic energy of the Universe as the “critical density”critical density”

• mass < critical densitymass < critical density, Universe , Universe will will expand foreverexpand forever

• mass > critical densitymass > critical density, Universe will , Universe will stop expandingstop expanding and then and then

contractcontract


The Critical DensityThe Critical DensityTo calculate the current To calculate the current kinetic energykinetic energy of the Universe of the Universe

need to knows its need to knows its speedspeed

this can be obtained from value of this can be obtained from value of HHoo

(see Hubble’s Law v= (see Hubble’s Law v= HHoo d) d)

• this being known, this being known, critical density is 10critical density is 10–29–29 g / cm g / cm33 (v.small density!)(v.small density!)

• All All luminous matterluminous matter we observe we observe accounts for accounts for < 1% of critical density< 1% of critical density

(!!!!!)(!!!!!)

• for for dark matterdark matter to stop Universal expansion, to stop Universal expansion,

average M/L of Universeaverage M/L of Universe would have to be would have to be 1,000 1,000 MM/ / LL

… … a few times greater than clustersa few times greater than clusters

This line of argument This line of argument suggests Universe will expand foreversuggests Universe will expand forever


Mass Density & ExpansionMass Density & Expansion


Is Gravity alone ?Is Gravity alone ?Recent observations of white dwarf supernovae in very Recent observations of white dwarf supernovae in very

distant galaxies have yielded unexpected results.distant galaxies have yielded unexpected results.(remember, white dwarf supernovae make very good standard (remember, white dwarf supernovae make very good standard

candles)candles)

these these supernovaesupernovae are apparently are apparently fainter fainter than than predictedpredicted for their for their redshifts!redshifts!

At a given cosmological At a given cosmological redshiftredshift• galaxies should be galaxies should be closercloser to us to us (i.e. shorter lookback (i.e. shorter lookback time)time)

……for for greatergreater Universal Universal mass densitiesmass densities

• these supernova are farther back in time than even the these supernova are farther back in time than even the models for an ever-expanding (coasting) Universe models for an ever-expanding (coasting) Universe

predictpredict

This implies that the This implies that the UniversalUniversal expansion is expansion is accelerating!accelerating!


““Dark Energy” ?!?Dark Energy” ?!?

must be an as yet must be an as yet unknown force which repelsunknown force which repels galaxies !!!! galaxies !!!!

((aka aka “dark energy”“dark energy”))


Mass Density Mass Density andand Dark Dark EnergyEnergy


4 Models for the Future4 Models for the Future

1) Recollapsing Universe:1) Recollapsing Universe: expansion will halt & reverse expansion will halt & reverse

2) Critical Universe:2) Critical Universe: will not collapse, but expands more will not collapse, but expands more

slowly with timeslowly with time

3) Coasting Universe:3) Coasting Universe: will expand forever with little slowdown will expand forever with little slowdown

4) Accelerating Universe4) Accelerating Universe:: expansion will accelerate with time expansion will accelerate with time

currently favoredcurrently favored


4 Models for the Future4 Models for the Future


What have we learned?What have we learned?• Does dark matter really exist?Does dark matter really exist?

YesYes, inferred from its , inferred from its gravitational effectsgravitational effects on visible matter on visible matter evidence for its existence is evidence for its existence is overwhelmingoverwhelming

—— ifif we we correctlycorrectly understand understand theory of gravitytheory of gravity (can never be 100% sure of course)(can never be 100% sure of course)

• How does the distribution of dark matter compare to the distribution How does the distribution of dark matter compare to the distribution of luminous matter in spiral galaxies?of luminous matter in spiral galaxies?luminous matterluminous matter is concentrated in the is concentrated in the diskdiskdark matterdark matter is distributed throughout the is distributed throughout the spherical halo & spherical halo &

beyondbeyond

• How do we determine the distribution of mass in distant galaxies?How do we determine the distribution of mass in distant galaxies? spiral galaxy: rotation curvespiral galaxy: rotation curve - - flatflat at large distances from center: at large distances from center: ellipticals: orbital speedsellipticals: orbital speeds of its stars at different distances from of its stars at different distances from

center,center,(measured from broadening of spectral (measured from broadening of spectral

lines) lines)


• How does a galaxy’s M/L ratio tell us how much dark matter it contains? How does a galaxy’s M/L ratio tell us how much dark matter it contains? • What have we learned about galaxies from their mass-to-light ratios?What have we learned about galaxies from their mass-to-light ratios?

M/LM/L ratio gives how many ratio gives how many solar massessolar masses of matter galaxy contains of matter galaxy contains for each for each solar luminositysolar luminosity of light output. of light output.

can estimate can estimate M/LM/L ratio ratio ifif galaxy were made galaxy were made onlyonly of stars, of stars, any excess mass must be dark matterany excess mass must be dark matter

Find: indeed Find: indeed M/LM/L of galaxies is of galaxies is much highermuch higher than than stars-onlystars-only estimate estimate

• Describe 3 Describe 3 (independent)(independent) ways to measure total mass of a cluster of ways to measure total mass of a cluster of galaxies.galaxies.(1) (1) orbital speedsorbital speeds & & positionspositions of the galaxies of the galaxies (2) (2) temperaturetemperature & & distributiondistribution of its hot, of its hot, intracluster medium intracluster medium (3) distortion of more distant galaxies by (3) distortion of more distant galaxies by gravitational lensing gravitational lensing

What have we learned?What have we learned?


• What have we learned about dark matter in galaxy clusters?What have we learned about dark matter in galaxy clusters?• All three methods agree: All three methods agree:

large amounts of dark matter in clusterslarge amounts of dark matter in clusters

• What do we mean when we ask whether dark matter is ordinary or What do we mean when we ask whether dark matter is ordinary or extraordinary matter?extraordinary matter?Ordinary matter is made from protons, neutrons, and electrons; Ordinary matter is made from protons, neutrons, and electrons;

referred to as “baryonic” matter since protons & neutrons are referred to as “baryonic” matter since protons & neutrons are both baryons. both baryons.

But baryonic matter does not account for all the dark But baryonic matter does not account for all the dark mattermatter

Possible that most dark matter is made of Possible that most dark matter is made of “nonbaryonic”“nonbaryonic” particles particles

(yet to be discovered). (yet to be discovered).

i.e. form of matter very different from that encountered in daily life i.e. form of matter very different from that encountered in daily life (so (so

“extraordinary”)“extraordinary”)



• What are MACHOs, and can they account for dark matter? What are MACHOs, and can they account for dark matter? MAMAssive ssive CCompact ompact HHalo alo OObjectsbjects

‘‘ordinary objects’… ordinary objects’… (dim stars, brown dwarfs, planet-size bodies)(dim stars, brown dwarfs, planet-size bodies) … … populating galactic halo without being visible to our populating galactic halo without being visible to our

telescopes telescopes MACHOs existMACHOs exist (seen via gravitational lensing)(seen via gravitational lensing) but probably but probably not in large enough numbers not in large enough numbers

to account for all dark matterto account for all dark matter

• What are WIMPs, and can they account for dark matter? What are WIMPs, and can they account for dark matter? WWeakly eakly IInteracting nteracting MMassive assive PParticlesarticles undiscoveredundiscovered particles of extraordinary ( particles of extraordinary (nonbaryonicnonbaryonic) matter ) matter

do not interact with light do not interact with light leading candidate for dark matterleading candidate for dark matter

• Why can’t neutrinos account for dark matter in galaxies?Why can’t neutrinos account for dark matter in galaxies? are weakly interacting particles but are weakly interacting particles but too lighttoo light & & travel too fasttravel too fast to to be be gravitationally bound in galaxies.gravitationally bound in galaxies.



• How does structure appear to be growing in the universe?How does structure appear to be growing in the universe?grew from grew from regions ofregions of slightly enhanced densityslightly enhanced density in in early universeearly universe. .

GravityGravity in these in these higher density regionshigher density regions drew matter together to form drew matter together to form galaxiesgalaxies,,drew those galaxies together to form drew those galaxies together to form clustersclusters, , currently currently superclusterssuperclusters are just beginning to form. are just beginning to form.

• What does the universe look like on very large scales?What does the universe look like on very large scales? Galaxies distributed in Galaxies distributed in gigantic chainsgigantic chains & & sheetssheets surrounding surrounding voidsvoids..

• What is the critical density?What is the critical density?average matter density universeaverage matter density universe must have in order for strength of must have in order for strength of

gravity to be enough to someday gravity to be enough to someday halthalt the the expansion of universeexpansion of universe (assuming today’s expansion rate).(assuming today’s expansion rate).

Although Although maymay be dark matter unaccounted for, be dark matter unaccounted for, appears that appears that overall matter densityoverall matter density is only is only

about 30% of critical densityabout 30% of critical density..



• Describe the four general models for the future expansion of the Describe the four general models for the future expansion of the universe. Which model is currently favored? universe. Which model is currently favored? (1) (1) Recollapsing universeRecollapsing universe: : expansion will someday expansion will someday halt and reversehalt and reverse. . (2) (2) Critical universeCritical universe: : universe will never collapse but will universe will never collapse but will expand expand

more and more slowly with timemore and more slowly with time.. (3) (3) Coasting universeCoasting universe: : universe will universe will continue to expand forevercontinue to expand forever, ,

with little change in rate of with little change in rate of expansionexpansion. .

(4) (4) Accelerating universeAccelerating universe: : expansion of universe will expansion of universe will accelerate with accelerate with timetime. .

Recent observations favor the accelerating universeRecent observations favor the accelerating universe. .

• Do we know what might be causing the universe to accelerate? Do we know what might be causing the universe to accelerate? No!!!No!!! although people give names to the mysterious force that could be although people give names to the mysterious force that could be

causingcausing acceleration acceleration (“dark energy”, “quintessence”, or “cosmological (“dark energy”, “quintessence”, or “cosmological constant”)constant”)

— — no one (yet) really knows what it isno one (yet) really knows what it is !!!!!!!!


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Ian M George PHYS 105 (2005 Spring) 2005may02 Announcements Final homework assignment: “Fate of the Universe” tutorial Strict deadline, due Weds May 11