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1、Possible Fate of the Universe,We already know that the universe is expanding.Only the pull of the gravitational force can slow it down.So,similar to the fate of the stars,how the universe evolves depends on how much matter we have in the universeRecollapsing(or closed)universe If the mass density is
2、 greater than the critical density,the gravitation pull will eventually pull all the matter back.,critical(or flat)universeIf we have just the right amount of mass per unit volume(the critical density),the universal expansion will eventually stops.coasting(or open)universeIf the mass density is less
3、 than the critical density,then universe will expand foreveraccelerating universe Some new observations seem to suggest that the expansion is not slowing down,but instead is accelerating,Chapter 16,Dark Matter,Dark Energy,and the Fate of the Universe,If we know how much matter we have in the univers
4、e,then we can predict how the universe is going to evolve in the futureIn our effort to measure the total mass of the universe and trying to determine how the universe is going to evolve,we have come to some surprising conclusions,that is,most of the mass of the universe are in the form of dark matt
5、er,and that there is an unknown force pushing the expansion of the universe in an ever-increasing rate.This unknown force is what astronomers refer to as the dark energy of the universe.In this chapter,we will examine the evidences supporting these conclusions,and look intoWhat is dark matter and da
6、rk energy?The fate of the universe,Dark Matter and Dark Energy,DefinitionsDark MatterDark matter is the name given to mass that we infer to exist through its gravitational effects but that emits no detectable radiation.Dark EnergyDark energy is the name given to an unseen influence that may be causi
7、ng the expansion of the universe to accelerate with time.,How do we measure the mass of Milky Way,or other galaxies?,The Rotation Curve of the Milky Way Galaxy,Like our measurement of the mass of solar system objects,or the mass of stars in binary systems,we can estimate the mass of a object from th
8、e rotation period of another object around it,using Newtons version of Keplers third law of planet motionBy observing the orbital motion of stars at different distance from the galactic center,we can estimate the mass contained by the Milky Way galaxy as a function of distance from the galactic cent
9、er.Note that this method assumes that the stars are in stable bound orbits!,Observed rotation curve of the Milky Way Galaxy,What Does the Flat Rotation Curve of the Milky Way Tell Us?,Lets first look at the rotation curves of a rigid body and a system with concentrated mass,What Does the Flat Rotati
10、on Curve of the Milky Way Tell Us?,The flat rotation curve of the Milky Way Galaxy implies thatThe mass of the Milky Way is not concentrated at the center of the galaxy.The mass enclosed in a sphere increases as we increase the radius of the sphereThe mass in a sphere inside the Suns orbit contains
11、about 100 billion times the mass of the Sun.The mass in a sphere twice the size of the Suns orbit contains twice as much mass(200 billion Msun).Most of our galaxys mass lies well beyond our Sun,tens of thousands of light-years from the galactic centermost of this mass is located in the spherical hal
12、o that surrounds the disk of our galaxy,and that the total amount of this mass might be 10 times the total mass of all the stars in the disk However,very little light is detected from these region!,The Rotation Curve of the Spiral Galaxies,We can estimate the mass contained in other spiral galaxies
13、from Doppler shift of the spectrumThe rotation curves of spiral galaxies are very similar to that of the Milky Way Galaxy,If we measure the Doppler shift of the galaxy along the red line,we would see a spectra like this,Dark Matter in Spiral Galaxies,All observations of the mass in spiral galaxies s
14、uggest that The majority of the masses in spiral galaxies are contained in a spherical halo that surrounds the disk of our galaxy,and 90%of the mass of the spiral galaxies do not emit light.They are DARK MATTER!Only about 10%of the mass in the spiral galaxies are in luminous form concentrated in the
15、 disk of the galaxies,10%,90%,Dark Matter in Elliptical Galaxies,We cannot measure the rotation rate of stars in Elliptical galaxies like we do with the spirals,because stars in elliptical galaxies do not rotate around the center in any organized wayHowever,At any particular location,some stars will
16、 be moving toward us,some will be moving away from us,while some will be moving perpendicular to the line-of-sight.This random motion results in a broadened absorption spectrum how large this broadening is can be used to measure their orbital speed.,Rotation curves of ellipticals measured from the b
17、roadening of the absorption lines yielded the same results that ellipticals also contain large amount of dark matter,about the same like the spirals!,Measuring the Mass of Galaxy Clusters,The mass of cluster of galaxies can be measured by three different methodsOrbits of Galaxies in ClusterHot Gas i
18、n ClustersGravitational Lensing EffectThe first method is obviously the application of the Newtonian dynamics methods(Keplers Third Law of Planet Motion),X-ray Observation of Intercluster Medium,The blue patch represent the X-ray emission from extremely hot gas in the cluster.,Assuming that the ther
19、mal pressure of the hot intercluster medium is in equilibrium with the gravitation pull(like the gas in a star),we can estimate the amount of matter in the cluster by measuring the pressure(temperature)of the intercluster medium.The estimated mass of the Intercluster medium in the space between the
20、galaxies in a cluster is about 50 times that mass of the visible mass of the cluster.,Gravitational Lensing Effect,This galaxy is directly behind the cluster.Gravitational lensing produces the multiple copies of the same galaxy we see here.,In general relativity,gravity causes the distortion of spac
21、etime.Light travels along these distorted path.Thus,a large gravitational object sometime behave like a lens.It can form image or images of distant objects behind it for us to see if the alignment happens to be right.,If we know the distance to the galaxy being imaged,then we can calculate the mass
22、of the cluster.,Results of Mass Measurements,All observations consistently tell us that There are more mass then we can see!The mass-to-light ratio(M/L)of galaxies and galaxy clusters are Solar Neighborhood:6 Galaxies:10Galaxy clusters:50The higher the mass-to-light ratio,the more dark matter there
23、isThese results imply that the universe is full of dark matter!The total mass density(including dark matter)of the universe is only about 20%of the critical mass densityCritical Mass DensityThe precise average density for the entire universe that marks the dividing line between a recollapsing univer
24、se and one that will expand forever.,What is Dark Matter?,Possible Dark Matter Candidates:Ordinary Dark Matter:MACHOs(Massive Compact Halo Objects)left over ordinary materials from the formation of the Milky Way still in the halo?Brown dwarfs White dwarfs,Jupiter-sized planets,Stellar-size black hol
25、es,Observations using gravitational lensing effects showed that there are not enough MACHOs to account for the mass of the dark matterExtraordinary Dark Mater:WIMPs(Weakly interacting massive particles)hypothetical heavy subatomic particles Neutrinos?Massive only in atomic particles sensemore massiv
26、e than the neutrinosThis is still in a speculative stagewe dont know what they are or if they even exist,Gravitational lensing effect of MACHO,Is Dark Matter Real?,Do we really need dark matter to explain the observed rotation curves in galaxies?Remember Ether?This is our historical dark matter that
27、 was proven to be not in existence.In 19th century,physicists hypothesized on the existence of a omnipresence medium called Ether to carry the propagation of light!Recall that measurement of galactic mass are based on the assumptions thatThe strength of gravity is given by Newtons Law of Gravity.The
28、 stars and gases used to measure the rotation of the galaxies are in bound orbits.What ifNewtons theory of gravity is not correct at very large distance?Newtons law has only been tested to a distance at the order of the size of the solar systemWe dont know how gravity behave in quantum mechanical sc
29、ale eitherKeep an Open Mind!,Possible Fate of the Universe,We already know that the universe is expanding.Only the pull of the gravitational force can slow it down.So,similar to the fate of the stars,how the universe evolves depends on how much matter we have in the universeRecollapsing(or closed)un
30、iverse If the mass density is greater than the critical density,the gravitation pull will eventually pull all the matter back.,critical(or flat)universeIf we have just the right amount of mass per unit volume(the critical density),the universal expansion will eventually stops.coasting(or open)univer
31、seIf the mass density is less than the critical density,then universe will expand foreveraccelerating universe Some new observations seem to suggest that the expansion is not slowing down,but instead is accelerating,Evidence of the Acceleration of the Galaxies,Latest data suggest that the expansion
32、rate of the universe is not static,nor is it slowing down,but is actually acceleratingThese observations,if verified,point to an accelerating universe in which the distances between the galaxies will increase at an ever increasing rate.Eventually,we will not be able to see anything around us,but vas
33、t dark,cold,and empty space.,If the galaxies are accelerating away from each other,then some strange,unseen force must be acting on them(acceleration=force,Isaac Newton)this is what the astronomers refer to as the dark energy of the universe.,Supernovae from Distant Galaxies,These snapshots,taken by
34、 NASAs Hubble Space Telescope,reveal five supernovae,or exploding stars,and their host galaxies.The arrows in the top row of images point to the supernovae.The bottom row shows the host galaxies before or after the stars exploded.The supernovae exploded between 3.5 and 10 billion years ago.http:/ima
35、gine.gsfc.nasa.gov/docs/features/news/news_d.html,Dark Energy and the Cosmological Constant,“Greatest Blunder”of Einsteins Career Einstein believed that the universe should be standing still.In order to make his General Theory of Relativity predict a static,flat universe,Einstein“invented”the cosmol
36、ogical constant to act as a repulsive force to counteract the pull of gravity.After Hubbles discovery of the universal expansion of the universe,which contradicts Einsteins idea of static universe,Einstein referred to the addition of the cosmological constant to GR as the“greatest blunder”in his car
37、eer.With the new evidences showing that the expansion of the universe is accelerating,astronomers have proposed that Einsteins cosmological constant may be responsible for the acceleration of the expansion fo the universe.So,maybe Einstein was correct in adding the cosmological constant to his theoryLets wait and see.But while we are waiting,lets also examine if there are other possibilities,
