AST 301Introduction to AstronomyJohn LacyRLM [email protected] LiRLM [email protected] JeonRLM [email protected] site: www.as.utexas.eduGo to Department of Astronomy courses,AST 301 (Lacy), course websiteTopics for this weekWhat does Hubble’s law tell us about the Universe?What is the Universe made of?What are dark matter and dark energy?Is the rate of expansion of the Universe changing?Will the Universe pull back together some day?Hubble’s LawEdwin Hubble found that a galaxy’s speed away from us isproportional to its distance from us.v = H dWhat does this mean?If galaxies are objects like the Milky Way, why would theyall be moving away from the Milky Way?And are they accelerating, so they move faster as theymove away? What would cause this?We conclude that the galaxies are accelerating, but that isto explain deviations from Hubble’s law. If Hubble’s lawis accurate, the galaxies are not accelerating.Big Bang explanation for Hubble’s lawThe galaxies don’t have to accelerate as they move awayfrom us to explain the Hubble law.If different galaxies started out moving away from us fasterthan others did, and they all started at the same time, theones moving away faster would have gotten farther bynow.The explosion that started everything is called the BigBang.This still sounds like it means that the big bang occurredhere, and we are at a special place in the Universe.That is wrong.No matter where the explosion occurred, an observer onany galaxy would see other galaxies moving away fromhim.Sample table for homeworkABCD QuizHubble’s law state that:A. The Milky Way is expanding.B. The Universe is expanding.C. The redshifts of distant galaxies are proportional to theirdistances from us.D. The redshifts of distant galaxies are proportional to theirdistances from the center of the Universe.Another ABCD QuizFor the redshifts of distant galaxies to be proportional totheir distances from us, it must be thatA. Galaxies increase their speeds as they move away fromus.B. Galaxies keep their speeds constant as they moveaway from us.C. We are at the center of the expansion of the Universe.D. We are not at the center of the expansion of theUniverse.The age of the UniverseWe can calculate when the big bang occurred by asking howlong it would have taken distant galaxies to get to where theyare.If different galaxies started out moving away from us faster thanothers did, and they all started at the same time, the onesmoving away faster would have gotten farther by now.If the galaxies’ speeds have not changed we can calculate howlong ago they started moving.The time to travel a distance d at speed v is given bytime = distance / speedIf we use Hubble’s law that speed = H x distance, we gettime = distance / (H x distance) = 1 / HUsing our best number for H, we get the time since the bigbang = 13-14 billion years.Turning Hubble’s law aroundOnce we know that more distant galaxies have greaterredshifts, we can use a galaxy’s redshift to estimate itsdistance.speed = Hubble’s constant x distance, sodistance = speed / Hubble’s constantFrom their redshifts, we know that most of the galaxies inthe Hubble (Space Telescope) “Deep Field” picture arevery distant.Since light took a long time to get to us, we are seeingthem as they were long ago.Expanding spaceEven if Hubble’s law would look the same from all places, isthere actually some galaxy that we’re all moving away from?Does the Universe have a center or edges?What is the Universe expanding into?This isn’t a problem if the Universe is infinite.It would have no edges then.But the Universe might be finite. We don’t know.Even then it would not have a center or edges.Like the surface of a sphere has no center or edges.The best way to look at the expansion of the Universe is to saythat space is expanding.Stretching photonsSaying that space is expanding does not mean that galaxiesare expanding or the solar system is expanding.Gravity keeps the stars orbiting in a galaxy and the planetsorbiting in the solar system. The sizes of their orbits don’tchange.But the expansion of space does affect photons as theytravel to us across space from distant galaxies.As they travel through expanding space, the wavelength ofthe light in a photon is stretched.If space stretches by a factor of 2 while a photon is traveling,the wavelength of the light also stretches by a factor of 2.This is another way of looking at the redshifts of distantgalaxies.What is the Universe made of?Stars and planets are made of protons, neutrons, andelectrons. These particles are called baryonic matter[although strictly speaking only protons and neutrons arebaryons (heavy particles); electrons are leptons (lightparticles)].But we have reasons to believe that there are other unseenkinds of matter in the Universe.One is dark matter, which seems to dominate the mass ofthe Milky Way and other galaxies.The first evidence for dark matter was the motions ofgalaxies in clusters, observed by Fritz Zwicky.Although galaxies don’t move on circular orbits, we can usa form of Kepler’s 3rd law to calculate the mass that theyorbit around, and it appears to be larger than the mass ofthe stars we can see in the galaxies.Dark Matter in galaxiesMore convincing evidence ofdark matter was found byVera Rubin, who measuredmasses of galaxies from theorbital speed of stars inspiral galaxies.She found that as much as90% of the masses ofgalaxies is in somethingother than the stars we see.Rotation curves for solar system and Milky WayRotation curves for other galaxiesMeaning of flat rotation curvesThe fact that rotation curves are flat means that stars orbitaround galaxies at about the same speed at alldistances from the center.Kepler’s 3rd law says that this means that there is as muchmass in a 1000 pc wide ring at the outer edge of agalaxy as within 1000 pc of the center, even thoughthere are many fewer stars near the outer edge.Especially in the outskirts of galaxies, most of the mass ingalaxies must be in something other than stars.Fritz Zwicky’s measurements indicate that even more darkmatter is found in clusters of galaxies, in between thegalaxies.Dark Matter in galaxies and galaxy clustersThe dark matter could be in many small black holes, orconceivably even in rocks in space, but we now think it isin some unknown form of matter which doesn’t respondto either the electromagnetic
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