1Quasars• Quasars are small, extremelyluminous, extremely distant galacticnuclei– Bright radio sources– Name comes from Quasi-Stellar RadioSource, as they appeared to be stars!– Can have clouds of gas near them, orjets racing from their cores– Spectra are heavily redshifted, meaningthey are very far away– Energy output is equivalent to onesupernova going off every hour!• The HST was able to image a quasar,showing it to be the active core of adistant galaxyEnergy 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…2Energy 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…3Seyferts 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 beastPartially obscuredobsecured4Many 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.5Dark Matter in Clusters of Galaxies• Missing mass is also aproblem in clusters ofgalaxies!– Not enough visible massto 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 solutionGravitational 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 lensing6Figure 78.06Which 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.7The expanding Universe• V = H × d• The expansion of the universe and theincreasing 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 thecenter 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!8Suppose 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 throughspace, they are stretched byexpansion• This increases the wave’swavelength, making it appearmore red!• An objects redshift, z, isHere, Δλ is the change inwavelength, and λ is the originalwavelength of the photon• This is equivalent to:!!"=zgalaxiesbetween distance Averagegalaxiesbetween distance averagein Change=z9If 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.10Olber’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 Paradox11A 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