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UT AST 301 - AST 301 Lecture notes

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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 weekIs the rate of expansion of the Universe changing?What could make the expansion slow down? What couldmake it speed up?How did the temperature of the gas in the Universe changewith time since the big bang?What caused the temperature to change?What particles and atoms were made at different times inthe early Universe?What is the cosmic microwave background? How was itemitted and why is it seen at the wavelengths it is?What do the fluctuations in the microwave background tellus about the early Universe?Schedule for this weekI will be gone tomorrow and Wednesday. (Prof. Jaffe willbe here for class on Wednesday.)The TAs will have their usual office hours and reviewsession this week.I will have office hours Thursday 10-12.We will try hard to have the tests graded by Monday.The scores will be posted with the formulas to calculateyour final grade if you don’t take the final.Tests will be available in my office.We will have multiple office hours Monday and Tuesday.The final is on Wednesday of next week.The interpretation of Hubble’s lawThe best way to look at the motion of the other galaxiesaway from us is not to say that the galaxies are flyingthrough space.Instead, we think that space is stretching.It’s like the stretching surface of a balloon.The writing on the balloon isn’t moving on the rubber.But the distance between letters increases as the rubberstretches.It’s easier to understand the stretching of a balloon since asthe 2-D surface of the balloon stretches it moves into adifferent place in 3-D space.It’s hard to understand how 3-D space can be stretching.Maybe it is moving into a different place in a 4th dimensionthat we aren’t aware of.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.Flat rotation curvesAbout 30 years ago, Vera Rubin found 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 force or the strong force.Neutrinos fit this description, but we don’t think they haveenough mass to be the dark matter.Maybe there is some kind of a particle like the neutrino, butwith much more mass.These hypothesized particles are often called WIMPs, orweakly interacting massive particles.We can also ask how much matter is in the Universe byasking how it affects the expansion of the Universe.We would expect gravity to slow the expansion.Hubble diagram for not too distant galaxiesExpansion of Universe vs. timeThe measured accelerationInstead of greater speeds in the past, as would beexpected if gravity is causing the expansion of theUniverse to decelerate, smaller speed were observed.Apparently the expansion is accelerating.There seems to be some sort of negative gravity affectingthe expansion of the Universe.According to general relativity this could happen if creationand destruction of particles in a vacuum gives thevacuum a negative pressure (like a stretched piece ofrubber).The cause of this negative pressure is called dark energy(although vacuum energy may be a better name).There are theories about its cause, but we really don’tknow which (if any) are right.The inventoryAs a fraction of the critical density, which would haveenough gravity to stop the expansion:visible stars: 1%unseen normal matter: 2%dark matter: 24%vacuum energy 73%(Note: vacuum energy accelerates, rather than slows theexpansion, but we include it in the inventory since thesum of these four stays constant as the Universeexpands.)What is the Universe made of?Hubble’s Law and the Big Bang TheoryDistant galaxies are moving away from us with speedsproportional to their distances from us.Our favored interpretation is that the Universe is expanding,so the space between the galaxies is expanding.But other ideas have been suggested.Do we have other evidence that the Universe actually beganwith an explosion?Our best evidence involves microwave radiation discovered45 years ago.Radiation from the Early UniverseIn ~1950 George Gamow predicted that there should beradiation left over from the early Universe.About 400,000 years after the big bang, the Universe wasfilled with ionized gas at a temperature of 3000 K, andwas 1000 times smaller than


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