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Quasars• Quasars are small, extremely

luminous, extremely distant galacticnuclei

– Bright radio sources– Name comes from Quasi-Stellar Radio

Source, as they appeared to be stars!– Can have clouds of gas near them, or

jets racing from their cores– Spectra are heavily redshifted, meaning

they are very far away– Energy output is equivalent to one

supernova going off every hour!• The HST was able to image a quasar,

showing it to be the active core of adistant galaxy

Energy Source for ActiveGalactic Nuclei

• Active galactic nuclei emit atremendous amount ofradiation over a broad rangeof wavelengths

• A black hole can be bothvery small, and have anaccretion disk that can emitenough radiation

• Likely that at the centers ofthese galactic nuclei, thereare supermassive black holes

• Intense magnetic fields inthe accretion disk pumpsuperheated gas out into jetsthat leave the nucleus

• There are still manyquestions to be answered…

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Energy Source for ActiveGalactic Nuclei

• Active galactic nuclei emit atremendous amount ofradiation over a broad rangeof wavelengths

• A black hole can be bothvery small, and have anaccretion disk that can emitenough radiation

• Likely that at the centers ofthese galactic nuclei, thereare supermassive black holes

• Intense magnetic fields inthe accretion disk pumpsuperheated gas out into jetsthat leave the nucleus

• There are still manyquestions to be answered…

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Seyferts and quasars are both types of active galaxies,harboring powerful luminous nuclei. Quasar nucleiappear to be more luminous, and therefore their blackholes a) are accreting matter at a higher rate. b) are more massive. c) are less obscured along our sightline. d) any of the above.

Unification of AGNSee into the beast Partially obscured

obsecured

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Many galaxies have flat rotation curves!

Dark matter is not unique to the Milky Way!

Figure 78.03

• 99 percent of the stars in a galaxy arewithin 20 kpc of the center

• Gas extends far out into the disk, butis not very massive!

• Galaxies are now thought to beembedded in a dark matter halo thatsurrounds the entire galaxy

• Unfortunately, dark matter cannot bedetected directly.

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Dark Matter in Clusters of Galaxies

• Missing mass is also aproblem in clusters ofgalaxies!– Not enough visible mass

to hold the clusterstogether by gravitation,and to keep hot gas intheir vicinity

– Cluster mass must be 100times greater than thevisible mass!

– Once again, dark matterseems to be the solution

Gravitational Lenses

• Dark matter warps space just like ordinarymatter does

• The path of light rays bends in the presenceof mass

• A galaxy or other massive object can bendand distort the light from objects locatedbehind it, producing multiple images

• This is called gravitational lensing

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Figure 78.06

Which of the following observations about the nature ofthe universe can be made without using any specialequipment? a) The universe is expanding. b) Most of the matter in the universe does not emitlight. c) Luminous matter in the universe occurs in clumpsrather than being evenly distributed. d) There is background radiation from the Big Bang.

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The expanding Universe

• V = H × d• The expansion of the universe and the

increasing distance between galaxies issimilar to the increase in distance betweenraisins in a rising loaf of raisin bread.

• Problem with these analogies – loaves andrubber bands have edges!– We have seen no ‘edge’ to the Universe;

there are an equal number of galaxies inevery direction!

– Also, galaxies can move relative tospace, as sometimes gravity canaccelerate one galaxy toward anotherfaster than space expands!

The expanding Universe

• V = H × d• Looks like the Milky Way is at the

center of the Universe, and allgalaxies are moving away from us

• Hubble’s Law can be applied to anyobserver in any galaxy

• No matter where you are, anexpanding Universe will give thisappearance! This is the cosmologicalprinciple

• The expansion of the Universe is notlike the explosion of a bomb sendingfragments in all directions

• Space itself is expanding!• No Center, No Edge!

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Suppose the Universe were not expanding, but was insome kind of steady state. How should galaxy recessionvelocities correlate with distance? They should a) be directly proportional to distance. b) reverse the trend we see today and correlateinversely with distance. c) show a scatter plot with most recession velocitiespositive. d) show a scatter plot with equal numbers of positiveand negative recession velocities.

The Meaning of Redshift• As light waves travel through

space, they are stretched byexpansion

• This increases the wave’swavelength, making it appearmore red!

• An objects redshift, z, is

Here, Δλ is the change inwavelength, and λ is the originalwavelength of the photon

• This is equivalent to:!

!"=z

galaxiesbetween distance Average

galaxiesbetween distance averagein Change=z

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If the universe is expanding, won't the solar system eventually expandapart? a) The solar system may actually be shrinking now, which makesthe Universe LOOK like it's expanding. b) No, its gravity holds it together. c) No, because there is no planetary redshift. d) Eventually, but only after a very long time.

The Age of the Universe

• Thanks to the Hubble Law, we canestimate the age of the universe

• At some point in the distant past,matter in the universe must havebeen densely packed.

• From this point, the universewould have expanded at some highspeed to become today’s universe

• Assuming a constant expansionover time, we find that the age ofthe universe is around 14 billionyears.

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Olber’s Paradox

• Over very large distances, galaxiesin the universe are more or lessuniformly distributed(homogeneous)

• If there are galaxies in everydirection, however, why do we nothave a fully-lit sky? We shouldsee a star in any direction we look!– This is called Olber’s Paradox

• If there is an edge to the universe,we should be able to see our way“out of the woods”

Olber’s Paradox

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A Solution? • In a sense, there is an edgeto the universe, an edge intime

• Light travels at a finite(though fast) speed

• The size of the visibleuniverse is defined as thedistance light can travel inthe age of the universe

• Galaxies exist at greaterdistances, but light fromthem has not reached usyet.

• The edge is called thecosmic horizon

• If we wait long enough, thenight sky might becomebright!

Suppose you've accepted that the universe is expanding, andwill always expand. You must then accept that a) there is no center to the universe b) the observable universe is infinite c) the distance between stars in the galaxies grows. d) either the density of the universe always decreases or newmatter must be continuously created.

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The Last Scattering Epoch

• Minutes after the Big Bang,the Universe was opaque– High temperatures kept all matter

ionized– Photons could only travel a short

distance before being absorbed

• After 400,000 years, theUniverse cooled enough forelectrons and ions torecombine, allowing light topass

• Now the Universe wastransparent!

Light from the Early Universe

• So what should light from 400,000years after the Big Bang look like?– It should have a spectrum that

corresponds to the temperature of theUniverse at that time, 3000 K.

– Expansion of space will stretch thislight, however

• The Universe has expanded by afactor of 1000 since this time, so thewavelength will have stretched by thesame amount

– Spectrum will correspond to atemperature of 3K.

• This light from the early Universehas been found, and is called theCosmic Microwave Background

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Clumpiness in the CMB

Clumpiness in the CMB

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Clumpiness in the CMB

The Curvature of the Universe • Remember that mass andenergy can curve the spacearound it.

• As the Universe expands,the distances between thegalaxies increases, likegalaxies painted on thesurface of an inflatingballoon

• If the universe was like anexpanding balloon (butwith the galaxiesdistributed in threedimensions), travel in anydirection would eventuallybring you back to yourstarting place (a closeduniverse)

• No Center, No Edge!

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Other Possible Curvatures of Space

• In addition to a closed, or positive curvature ofspace, there are two other options– Space could be flat, or have zero curvature– Space could be curved away from itself, or have

negative curvature– Geometry behaves differently with each curvature!

Expansion Forever? Or Collapse?

• The fate of the universe isultimately controlled by itstotal amount of energy– Energy of expansion (positive)– Gravitational energy that can

slow the expansion (negative)– Binding energy

• If the total energy is positiveor zero, the expansioncontinues forever

• If the total energy is negative,the expansion will halt, and theuniverse will contract andeventually collapse.

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Density of the Universe

• If we can measure thedensity of the universe,we can predict howmuch gravitationalenergy the universe has,and therefore whether itwill collapse or keepexpanding

• The critical density ofthe universe, ρC, is thedensity at which thetotal energy of theuniverse is zero

• ΩM = ρ/ρC, where ρ is the measured densityof the universe

• If ΩM > 1, the universe will recollapse• If ΩM < 1, the universe will expand forever• If ΩM = 1, the universe is exactly at the

critical density

G

H

C

!"

8

32

=

If the Big Bang theory is correct, and there is not enough mass toclose the universe, then a) more Big Bangs will occur. b) there is no "dark matter". c) the universe will eventually be entirely cold. d) the expansion will slow to a halt.

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Supernova Type Ia Findings

• We also need to know how theuniverse is expanding – this can helpus determine the value of ΩM

• We can measure the recessionvelocity of distant galaxies usingType Ia supernovae as standardcandles

• It appears that the expansion rate at atime when the universe was half itscurrent size (z=1) was slower than itis today!

• This shows that the expansion rate isincreasing with time! Very puzzling!

Dark Energy!

• Dark energy may provide thesolution to the mystery

• Dark energy remains constanteverywhere, regardless of theuniverse’s expansion

• Provides an outward push toaccelerate expansion:antigravity!

• In order for dark energy tobalance the equation, itmust make up around 70%of all of the energy in theuniverse

• Much work remains to bedone on this frontier…