Quasars In 1963 Martin Schmidt was trying to understand some unidentified lines in the optical spectra from a star that had a strong radio signal He realized that the lines were Balmer lines that were normally in the UV that had been shifted to the visible by the stars veleocity of recession which was about 15% of the speed of light This star turned out to be very distant and not a star at all Quasi-stellar object - quasar Thousands of quasars have been found and they all show very large redshifts The largest shows λ/λ = 5 94% of the speed of light! Distances to Quasars To figure out how far away quasars are astronomers looked for quasars associated with galaxies that to which they could measure the distance This is difficult because quasars outshine entire galaxies by a lot Then astronomers used previous techniques to measure the distance to the galaxy and hence the quasar 1 3 The Luminosity Puzzle The very large redshifts of quasars means that they are very far away and because we can see them they must be very luminous The quasars also seemed to vary in luminosity over a period of months This luminosity variation suggested that the quasars were large objects 2 Seyfert Galaxies Active galaxies produce abnormal amounts of energy, mostly in their centers Active galactic nuclei, AGN One example is Seyfert galaxies Seyferts are spiral galaxies Seyfert galaxies produce emission lines rather than absorption lines indicating hot gas clouds Seyfert galaxies have luminosity variations on the scale of months like quasars and have pointlike bright centers that are brighter than the sum of individual stars The central region of Seyfert galaxy NGC 1068 4
Active Elliptical Galaxies Elliptic galaxies are also observed to have active nuclei Elliptic galaxy M87 has such an active center Jets of ionized gas are visible coming from the center The Mass of the Quasar in M87 The period of material orbiting the center of M87 was calculated by measuring the redshift of material circling the center The rotation speed was measured to be 550 km/s Using Kepler s law we get a mass of about 2.5 billion M sun There is evidence for black in the center of many galaxies Once formed, these black holes continue to absorb material and grow 5 7 The Power Behind Quasars Astronomers are now convinced that quasars have massive black holes at their centers We can say a black hole exists if we can demonstrate that there is a very massive object such that no star or cluster could account for that mass We can determine the mass of an object using Kepler s law as before We simply measure the period of an object orbiting the quasar and calculate the mass 6 Radio Jets Material falling into a black hole forms an accretion disk Models show that these accretion disks can lead to jets along the axis of the disk These jets glow with radio, light, and x-rays 8
Evolution of Quasars When we see a distant object we see it as it was long ago If we see more quasars far away, there must have been more quasars long ago The theory is that quasars are black holes with enough fuel around them to make bright accretion disks This theory leads to the conclusion that quasars should have formed early in the history of the universe This theory leads to the conclusion that quasars should have formed early in the history of the universe 9 The Distribution of Galaxies in Space Looking at distant galaxies is like looking back in time When we look at astronomical objects we find they are seldom alone The question arises: do galaxies cluster also? Hubble used the 100 inch Palomar telescope to sample the sky in 1283 places He found that number of galaxies visible is about the same He found that the number of galaxies increased with faintness More evidence for a constant density of galaxies 11 Gravitational Lenses and Quasars The quasars brilliance and immense distance makes it ideal for the study of deep space Gravitational lensing was first discovered using quasars in 1979 when identical images of a quasar were observed On the right one can see four identical optical images of a quasar (top) and an Einstein ring of a quasar made with radio waves using the VLA The Cosmological Principle The universe is the same everywhere The universe appears to be isotropic and homogeneous Without the cosmological principle, we could not make progress in understanding the universe around us Hubble had simply counted the number of galaxies Recently astronomers have measure the distances of thousands of galaxies and have built up a picture of the distribution of galaxies 10 12
The Milky Way is a member of a small group of galaxies called the Local Group containing more than 40 members The Local Group 13 Superclusters and Voids Galaxy clusters form superclusters Among the superclusters are giant voids The Milky Way is located in the Local Supercluster One conclusion we can draw is the space is mostly empty The clusters occupy only about 5% of the space 15 Neighboring Groups and Clusters Galaxies form clusters Rich galaxy cluster have thousands of galaxies The nearest rich galaxy cluster is called the Virgo Cluster An much larger galaxy cluster is the Coma cluster with a diameter of 10 million LY Large galaxy clusters such as Coma have few spirals in the center but have many ellipticals Slices of the Universe Enormous volumes of space lie beyond the Local Supercluster This space has not been completely mapped One striking structure that has been found is the Great Wall There are obvious sheets and filaments separated by huge voids 14 16
When Did Galaxies Form? We can study old, distant galaxies and get information about times near the beginning of the universe Most galaxies we can see are least a few billion years old We can learn about a galaxy by measuring its color Blue means young, hot stars Yellow or red means old stars Another way to learn about a galaxy is to study its shape Spiral galaxies are young Elliptical galaxies are old 17 Colliding Galaxies Galaxies can collide which stimulates star formation Indivdual stars are not affected much because of the large distance between stars 19 The Ages and Compositions of Galaxies Nearly all galaxies are old The Milky Way contains stars that about the age of the universe Distant galaxies show evidence for heavier elements that were not present at the beginning of the universe A least one generation of stars has passed Star formation has stopped in elliptical galaxies while it continues in spiral galaxies Elliptical are poor in interstellar gas but galaxy clusters have a large amount Galaxies in clusters collide! The Life History of Galaxies Elliptical galaxies formed early and turned all the gas and dust to stars in the first 3 billion years Spirals converted gas and dust at a much slower rate and are still producing stars today The peak of star formation occurred when the universe was between 3 and billion years old When the universe was 3-6 billion years old, the galaxies were small Galaxies have merged to form larger galaxies since that time 18 20
Cosmology The study of the universe as a whole is called cosmology How did the universe come into being? What will its ultimate fate be? What have we observed about the universe? All galaxies show a redshift proportional to distance, implying that the universe is expanding The distribution of galaxies on the largest scale is isotropic and homogeneous The contents of the universe evolve with time: hydrogen and helium are changed into heavier elements inside stars Gravity warps the fabric of space-time Estimates of the Age of the Universe The estimates of the age of the universe depend on our knowledge of the distance of galaxies An error of a factor of 2 in the distance would mean a factor of 2 in the age of the universe Over the past 20 years debate has raged among astronomers about the value of H It has varied from 15 to 35 km/s per million LY Recent data from the Hubble Space Telescope have yielded 20 ± 2km/s per million LY 70 ± 7 km/s per million parsecs 21 23 The Age of the Universe The universe cannot be static The universe must either be contracting or expanding If we had a movie of the expanding universe and ran it backwards, we would see the galaxies moving together until they were all in one place The big bang We can estimate how long the galaxies would take to be back in the same place v = Hd, Hubble s Law From physics we know d = vt t = d/v = d/(hd) = 1/H Hubble time H = 20 ± 2 km/s per million LY t = 15 ± 1.5 billion years 22 Deceleration/Acceleration The Hubble time is the correct age of the universe only if this expansion has been constant throughout the age of the universe Constant H Gravity creates attraction and should slow the expansion of the universe Deceleration The universe would actually be younger than the Hubble time New measurements of type Ia supernovae can interpreted to mean that the universe is accelerating The universe is expanding more slowly now than in the past The universe would be older than the Hubble time Based on the observation that distant type Ia supernovae are 20% dimmer than they should be if expansion were constant Other explanations? 24
The Age of the Universe Astronomers generally agree that the modifications to the Hubble time for acceleration and deceleration make the age of the universe 15 ± 5 billion years Another way to estimate the age of the universe is to find the oldest objects whose age we can measure Computer models show that the age of globular clusters is about 13 billion years and assuming that it took a billion years for stars to form, the oldest stars are younger than the age of the universe This agreement has only occurred in recent years Previously the Hubble time was shorter Previously the age of globular clusters was longer 25 A Balloon Universe If you go in one direction, you get back to where you started There is no center, all points on the balloon are the same If the balloon grows, all points move away from each other The points move away from other other because the balloon is growing, not because any point is doing something special This example represents a closed universe An open universe is also possibility but harder to visualize 27 The Geometry of Spacetime The gravity from the matter of the entire universe warps spacetime We must consider a fourth dimension of space Thinking in 4 dimensions is difficult so we will think in 3 dimensions (2 + 1) In the world of 2 dimensions the third dimension if curvature A 2-dimensional observer would observe odd things in his 2-dimensional curved world Let s use a balloon as an example The Expanding Universe If the universe is dense enough, it will stop expanding and collapse If the universe not dense enough, it continue to expand forever At a critical density, the universe will just stop expanding at infinite time 1. Closed universe 2. Open universe 3. Universe with critical density 4. Universe with less than critical density and positive cosmological constant 26 28
Facing the Future If the mass density of the universe is high enough, the expansion of the universe will reverse and the universe will collapse The Big Crunch If the mass density of the universe is low enough, the universe will expand forever and slowly die out At critical density, the universe can just barely expand forever Flat universe Zero curvature The Big Bang The big bang theory states that the universe began as a gigantic explosion This idea has entered popular culture 29 31 Who s s Winning? The observed matter density is too low to close the universe Dark matter may play a role The ages of stars suggest that we live in an open universe Type Ia supernovae suggest that the universe if accelerating The lookback time is how long ago the light from an object was emitted Depends on our model of the universe History of the Idea of the Big Bang Georges Lemaitre proposed a big bang-like theory in the early 1920s involving fission In the 1940s, George Gamov proposed the a big bang model incorporating fusion Since that time, many astronomers and physicists have added their work to what is now known as the standard model of the big bang Three main ideas underlie the big bang model The universe cools as it expands In very early times, the universe was mostly radiation The more hotter the universe, the more energetic photons are available to make matter and anti-matter 30 32
The Evolution of the Early Universe With the three previous ideas in mind, we can trace the evolution of the universe back to when it was 0.01 s old and had a temperature of 100 billion K We can go back farther but not all the way to zero time At 10-43 s most of our physical laws become impractical At times before 0.01 s, the universe was filled with quarks and gluons Learning from Deuterium All the deuterium in the universe was formed in the first 3 minutes If the universe was very hot and dense when the deuterium formed, it would have been broken up If the universe expanded and then out thinned out rapidly, deuterium would survive The density extracted from the surviving deuterium is 5 x 10-31 g/cm 3 Suggests a low enough mass that the universe is open Dark matter may still play a role 33 35 After 0.01 s Our picture after 0.01 s is that the universe was filled with radiation and with types of matter that exist today Protons and neutrons Photons and neutrinos The temperature was no longer hot enough to create neutrons and protons in collisions of photons At about 3 minutes, nuclei begin to form 75% hydrogen, 25% helium, some lithium The Universe Becomes Transparent For several hundred thousand years the universe resembled the interior of a star After that time, atoms began to form The universe became transparent Radiation and matter decoupled 1 billion years after the big bang, stars and galaxies began to form The radiation from the big bang faded but it left an indelible fingerprint, the cosmic radiation background 34 36
The Cosmic Radiation Background In the 1940s Adler and Herman realized that just before matter and radiation decoupled, the universe must have been radiating like a blackbody at a temperature of 3000 K That was 15 billion years ago, and the universe has expanded, leaving an afterglow of the big bang with a temperature of 3 K In the 1960s, Penzias and Wilson were using a microwave antenna to study the sky They could not make their receiver work without background noise that seemed to be coming from everywhere in the sky They thought is was their detector but soon realized that it was real and was coming from space Penzias and Wilson got in touch with some cosmologists from Princeton and who realized that this radiation was the cosmic background radiation (CBR) 37 Problems with the Big Bang Model The standard big bang model explains many things but there are remaining issues It does not explain why there is more matter than antimatter in the universe It does not explain the observed uniformity of the universe Parts of the universe could never have been in contact yet they show the same background temperature It does not explain why the density of the universe is close to the critical density 39 Properties of the CBR The first accurate measurements of the CBR were made by the COBE satellite They observed that the CBR matched perfectly with a blackbody with a temperature of 2.73 K Astronomers concluded that the universe we see today evolved from a hot, uniform state We live in an evolving universe The universe looks uniform in all directions but not completely uniform Grand Unified Theories There are 4 forces Gravity, weak, electromagnetic, nuclear At high temperatures, these forces become one force Theories exist that unify weak, electromagnetic and nuclear Grand unified theories (GUTs) No theory yet exists incorporating gravity 38 40
The Inflationary Hypothesis GUTs predict that at 10-35 s, a rapid, early expansion took place Prior to this inflation, the universe was small enough to communicate with itself After inflation, parts of the universe were beyond each other s horizon The inflationary model also predicts that the universe is exactly at critical density 41 Lucky Accidents The temperature of the radiation emitted when the universe became transparent varies by about 16 millionth of a K Smaller variations would have led to no galaxies Larger variations would have led to black holes The fine balance between expansion and contraction The existence of only matter and not anti-matter The production rate of nuclei in the big bang produced only hydrogen and helium, and did not go all the way to iron Neutrinos have to have just the right interaction properties with matter to allow supernovae Anthropic principle 42