The 19 th Century Universe Olbers’ Paradox Standard Candles Galaxies and Quasars The Big Bang

1. CHAPTER 16Cosmology • The 19th Century Universe • Olbers’ Paradox • Standard Candles • Galaxies and Quasars • The Big Bang • The Universe • The Missing Mass • The Cosmological Constant • Inflation • The Future Edwin Hubble (1889-1953) I too can see the stars on a desert night, and feel them. But do I see less or more? The vastness of the heavens stretched my imagination—stuck on this carousel my little eye can catch one-million-year-old light. - Richard Feynman

2. 19th Century Cosmology 19th-century scientists knew of the solar system, other stars, and a wide range of other dim, fuzzy objects that they couldn’t resolve, which they decided were interstellar clouds—nebulae—and some of which they called “spiral nebulae.” They also thought the universe was static and unchanging.

3. The 19th Century Universe Aside from the few nearby planets and moons and some nebulae, the earth and sun were surrounded by infinitely many stars, roughly uniformly spaced and extending out to infinity. The light intensity reaching earth from a star of power P a distance R away will be I=P/(4pR2), so the other stars were dim.

4. R Olbers’ Paradox Calculate the total amount of light reaching the earth from all the stars in the universe. Let the density of stars be r. Take it to be uniform throughout the universe. The number of stars from R to R+dR away will be: The light intensity reaching earthfrom a star of power Pa distance Raway will be I=P/(4pR2). So the total starlight power reaching earth will be: But this is clearly not the case!

5. The big problem in cosmology is determining how far away objects are. If an object is close, parallax measurements relative to the “fixed stars” (far away) can determine its distance.

6. The 20th Century Universe: Standard Candles Standard candles are celestial objects whose output power, P, is known. Because intensity diminishes as 1/r2: measuring the intensity reaching earth of a standard candle, it’s possible to measure its distance, r, away.

7. Standard Candle #1: Cepheid Variables In 1912, American astronomer Henrietta Leavitt studied Cepheid variables, stars that have exhausted their hydrogen and pulsate. One needs a distance measurement from some other method for at least one Cepheid. The “original” Cepheid variable, Delta Cephei, is close enough that we have a parallax measurement for it.

8. Standard Candle #2: Supernovae Supernovae are gigantic stellar explosions. • The Crab supernova occurred in 1054 and was recorded by the Chinese and Japanese. It was bright enough to see during the daytime. • Other supernovae in our galaxy occurred in 1572, 1604 and 1987. Supernova remnant in the Small Megallanic Cloud galaxy 190,000 light-years away Supernova 1604, also known as Kepler's Supernova, was a supernova that occurred in the Milky Way and was observed by Kepler in 1604.

9. Standard Candle #2: Supernovae Supernovae are classified according to their spectra: Type I: supernovae WITHOUT hydrogen absorption lines Type II: supernovae WITH hydrogen absorption lines. Type I leaves behind a gaseous supernova remnant (and no stellar corpse at the center), very rich in iron. Tycho’s Supernova remnant is of a Type Ia supernova (no helium lines, strong silicon lines). Type Ia supernovae are all very similar! Tycho’s supernova remnant

10. Super-novae The Crab Nebula is the remnant of a Type II supernova; it contains a neutron star at its center. Type II leaves a gaseous supernova remnant, containing elements heavier than iron and a compressed stellar core, a neutron star or black hole. Type II supernovae aren’t as similar as Type Ia. But they’re worth mentioning.

11. Hubble’s Law Hubble measured the intensities of many standard candles, yielding the objects’ distances. This showed that some objects were very far away—the so-called “spiral nebulae” were really distant islands of stars: galaxies. Edwin Hubble (1889-1953)

12. Okay, so what does our neighborhood of the universe look like?

13. Galaxies Galaxies are collections of stars bound by gravitational attraction. The Milky Way as seen from earth. Our galaxy is the Milky Way with 200 billion stars. The Milky Way as seen from the outside.

14. Stream from Canis Major Sun Canis Major Milky Way Nearby galaxies Sagittarius Dwarf Elliptical Galaxy, discovered in 1994, is 50,000 light-years away. It’s passed through the Milky Way several times so far. Canis Major Dwarf Galaxy, discovered in 2003, is about 42,000 light years from the centre of the Milky Way and only 25,000 light years away from the solar system. It’s doomed by the massive Milky Way. They’re hard to see due to their proximity to the Milky Way’s dusty core.

15. The Large and Small Magellanic Clouds The next closest galaxies (also dwarf galaxies) to ours are the Large and Small Magellanic Clouds, distorted by the gravity of the Milky Way). They were known to mid-east astronomers from the middle ages, but were brought to Europeans by Ferdinand Magellan. Large Magellanic Cloud (180,000 light-years away) Small Magellanic Cloud (200,000 light-years away)

16. The Andromeda Galaxy Andromeda is the closest galaxy to ours and is about a million light-years away. There are about 100 billion galaxies in the visible universe.

17. Galaxy shapes

18. Galaxies and their shapes

19. Galaxies and their shapes Elliptical galaxies

20. Quasi-stellar objects: Quasars Quasars are sub-arc-sec objects with tremendously strong radio signals and strange optical spectra. They can outshine galaxies. They are among the most distant and oldest objects in the universe. They appear to be massive black holes in the centers of galaxies in early times, eating stars and other matter and emitting massive amounts of energy.

21. Active Galactic Nuclei (AGN) • Active galactic nuclei are a category of exotic objects that includes: luminous quasars, Seyfert galaxies, and blazars. • It’s likely that the core of an AGN contains a supermassive black hole surrounded by an accretion disk. As matter spirals in the black hole, electro-magnetic radiation and plasma jets spew outward from the poles. Blazars are AGN with jets spewing relativistic energies toward the Earth.

22. Gamma-ray bursts emit massive amounts of gamma rays. Gamma rays are absorbed in the atmosphere, so GRBs must be observed by satellites. A new one appears almost every day, and it persists for <1 second to ~1 minute. The gamma-ray sky In 10 seconds, they can emit more energy than our sun will in its entire lifetime. When shining, some are brighter than all the other stars in the visible universe combined.

23. Gamma Ray Bursters • They were recently discovered to come from hypernovae (an explosion of a >20-solar-mass star present only in the early universe) in distant galaxies. Jets emerge from the poles, unable to escape from the rotation disk. We only see one when one of the jets is pointed at us.

24. GRB Hypernova Gamma Ray Bursters and Hypernova An interesting property of GRBs is the afterglow of lower-energy photons including x rays, light, and radio waves that last for weeks. The spectra of GRBs are nearly identical to the jet of a hypernova pointing in our direction.

25. But there’s another type of gamma ray burster insideour galaxy! • Recently, it was realized that abinary neutron star collapsing in on itselfis also a gamma-ray burster that could fry anything within 1000 light-years. These yield much shorter (few-second) bursts. Binary neutron star in the M15 globular cluster in our galaxy

26. Gamma Ray Bursts and Life on Earth It’s thought that a nearby gamma ray burst caused the second biggest mass extinction on earth 450 million years ago, and which killed off more than half earth’s species at the time. The trilobites disappeared at this time.

27. What about the large-scale structure of the universe?

28. The cosmological constant • Attempting to achieve a static universe, Einstein realized that he could modify his Field Equations by introducing a term proportional to the metric: • He called the constant Λ the cosmological constant. • Einstein’s effort was unsuccessful: the static universe described by this theory was unstable, and observations of distant galaxies by Hubble a decade later confirmed that our universe is, in fact, not static, but expanding. • So Λ was abandoned, with Einstein calling it the "biggest blunder [he] ever made."

29. Standard Candle #3: Galaxies Once established as independent entities, entire galaxies have output powers that could be considered about the same. Galaxies are great standard candles because they shine continuously and are numerous. Spiral galaxy NGC 4414

30. Hubble’s measure-ments Hubble also measured spectra of standard candles, observing that most were red-shifted. He realized that this was a Doppler shift. The universe is expanding!

31. Hubble also found a linear relation between distance and recession velocity! Hubble’s Law Hubble’s law: v = HR where H is called Hubble’s constant. Hubble’s constant is related to a scale factor a that’s proportional to the distance between galaxies:

32. The Expansion of the Universe • Distances between galaxies are increasing uniformly. • There is no need for a center of the universe. The expansion of the universe resolves Olbers’ Paradox: The red-shift reduces the power from distant sources. And very distant sources are receding faster than c, so no light reaches us form them! This is called a horizon.

33. How can the universe be expanding, yet continue to look the same? Proposed in 1948 by Hermann Bondi, Thomas Gold, and Sir Fred Hoyle, theSteady-State Theory held that the universe is infinite and expanding, and matter is continuously created with net constant density. Only a few atoms per cubic meter per century would be required, so no one would ever notice. Unfortunately, no mechanism has ever been found for matter creation, as required by the Steady-State theory. Also, the universe should look the same at all distances, but it doesn’t. We don’t see galaxies forming nearby. Quasars exist only a billion light-years away or more. So the universe doesn’t actually look the same as it expands.

34. Extrapolating backward in time, there had to be one hell of a Big Bang! And because pv = nRT, it must’ve been very dense and hot at t = 0! The Big Bang occurred about 13 billion years ago.

35. Age of the Universe Extrapolating backward, the universe must be 13.7 ± 0.2 billion years old. Radioactive decay of elements in the oldest meteorites hitting earth suggest that the universe is between 8 to 17.5 billion years old. • Radioactive dating of stars showed that stars were formed as early as 200,000 years after the Big Bang. • Examining the relative intensities of elemental spectral lines of old stars yields ratios of thorium/europium and uranium/thorium isotopes, indicating an average age of 14 billion years.

36. Evidence for the Big Bang: Cosmic Microwave Background Radiation • In 1964, Penzias and Wilson observed microwave background radiation that permeates the universe. • The blackbody radiation of several billion years ago has Doppler-shifted to 3 K today. • Satellite measurements show a nearly isotropic 3 K radiation background. Arno Penzias and Robert Wilson and their microwave antenna in Crawford Hill, NJ

37. Early Universe Nucleo-synthesis • In the early universe, it was hot and dense enough to create the lightest few elements. • And by considering current ratios of isotopes, we can learn more about the early universe.

38. Supernovae and nucleo-synthesis • Supernovae provide the ultrahigh temperature and pressure to produce all the heavier elements. Steady-state theorists did much of this theory in an attempt to save their theory!

39. The Cosmological Principle The cosmological principle says that the universe looks roughly the same everywhere and in every direction. Specifically, the universe is both isotropic and homogeneous.

40. Looking into the distance is looking back in time. While light travels fast, it’s not that fast. We can look about 13 billion light-years away, so we’re looking back 13 billion years in time. Early stars Proto-galaxies Quasars Modern-day galaxies Quasars Proto-galaxies Early stars This is consistent with the Cosmological Principle.

41. The Big Bang • The Big Bang model rests on two theoretical foundations: • The General Theory of Relativity • The Cosmological Principle • What does General Relativity have to say? • Robertson–Walker metric is the simplest space-time geometry consistent with an isotropic, homogeneous universe. Ds2 = a(t)2 [Dx2 + Dy2 + Dz2] – c2Dt2

42. Closed Open Flat Possible geometries of the universe The density, r, of matter in the universe determines which shape it has. W0≡ r / rcrit where rcrit = 3H2/8pG is the critical density for which the universe is flat.

43. The Future of the Universe • Looking into the past, there is little question that the age of the universe is about 13.7 billion years. • What about this future? • There are three possible futures…

44. Microwave background fluctuations in the different universe shapes microwave background intensity vs. q and f. Recent measurements of the angular variation in the microwave background by the Wilkinson Microwave Anisotropy Probe (WMAP) indicate a flat universe.

45. The Missing Mass and Dark Matter Luminous matter only accounts for 3% to 4% of the critical density of the universe. What about dark matter that we can’t see? Using this simple relation, we can estimate the total mass of the galaxy, M. The dark matter halo covers the space between 100,000 to 300,000 light-years from the galactic center. About 70% of the Galaxy is composed of dark matter. 100,000 light-years

46. The Missing Mass About 70% of the universe is composed of unknown dark matter. Some of the Universe's missing mass may be hiding in clusters of galaxies. Astronomers have discovered previously unseen clouds of hot gas being pulled into the clusters. The gas has far greater mass than the observable stars in the galaxies and so may make up a fraction of the mass of the universe, but little is known about it. And most don’t think it will suffice.

47. The Missing Mass and Dark Matter One possibility for dark matter is: MAssive Compact Halo Objects (MACHOs). Example: brown dwarves (objects whose mass is between twice that of Jupiter and the lower mass limit for nuclear reactions (8% of the mass of our sun). Brown dwarfs are failed stars with insufficient density to start nuclear fusion. Brown dwarves (artist’s rendition)

48. The Missing Mass and Dark Matter Another possibility for dark matter is much smaller particles: Weakly Interacting Massive Particles (WIMPs). WIMPs are elementary particles with very tiny interactions with ordinary matter (likely only the weak force). They don't emit or absorb photons. With hardly any interactions, they would be very hard to detect. But they have gravity. Neutrinos almost fill the bill: But their mass is too low. Maybe the WIMPS needed are a kind of particle that hasn't been discovered yet.

49. The various key constants are related by the Friedmann Equation: Dividing both sides by H2 yields: Each of the terms in this equation has special significance: The Friedmann Equation where k is the curvature parameter

50. The universe expansion seems to be accelerating! The universe appears to be expanding at an accelerated rate. In 1998, observations of Type Ia supernovae suggested that the expansion of the universe is speeding up. In the past few years, these observations have been corroborated by several independent sources: the cosmic microwave background, gravitational lensing, the age of the universe, and large scale structure, as well as improved measurements of the supernovae. Supernova/Acceleration Probe (SNAP) satellite observatory proposed to further study Type Ia supernovae in distant galaxies to better measure the universe acceleration.