Astro105 Lecture13

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Published on November 14, 2007

Author: Bernadette

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Slide1:  Astro 105: Our Place in the Universe Lecturers: J.P.Ostriker A.E.Shapley J.E. Gunn P. Steinhardt Lecture 13 Reading:  Reading Chapters 8-11 in Rees, Just Six Numbers Scientific American article, “Reading the Blueprints of Creation” by Michael Strauss Problem set #5 is due this Thursday, November 17th Overview:  Overview What stuff is the Universe made of? How much of it is there? The total amount of “stuff” (mass and energy) determines the the expansion history (future) of the Universe and the overall geometry of spacetime. Today: Predictions and discovery of the Cosmic Microwave Background (CMB) Radiation; How does the nature of the CMB constrain the contents of the Universe? What do the fluctuations in the CMB mean for the formation of structure in the Universe? Review:  Review Methods for measuring Wm Different types of Dark Matter (WIMPS, MACHOs, neutrinos) Summary of matter density (baryons and dark matter) Cosmological redshift Type Ia Supernovae as probes of the Expansion History Evidence that the Universe is accelerating The acceleration is caused by “Dark Energy,” which constitutes most (70%) of the energy density in the Universe. Express this as WL=0.7. Wm Summary:  Wm Summary Baryonic matter density corresponds to Wb=0.04-0.05 Dark Matter density corresponds to WDM=0.23 Total matter density corresponds to Wm~0.3 Our current estimate is that ordinary matter (protons and neutrons), only makes up a small fraction of the total matter density in the universe. Ordinary matter Review:  Review Methods for measuring Wm Different types of Dark Matter (WIMPS, MACHOs, neutrinos) Summary of matter density (baryons and dark matter) Cosmological redshift Type Ia Supernovae as probes of the Expansion History Evidence that the Universe is accelerating The acceleration is caused by “Dark Energy,” which constitutes most (70%) of the energy density in the Universe. Express this as WL=0.7. Cosmological Redshift:  Cosmological Redshift So then what does cosmological redshift mean? Redshift is due to the expansion of the universe, which stretches the wavelength of light passing between galaxies. Light from more distant objects travels for a greater time (because light has a finite speed), the Universe expands more while the light travels to us, and the wavelengths get more stretched. Size of the Universe when the light was emitted Size of the Universe now, when we observe the light Cosmological Redshift:  Cosmological Redshift The important formula is: observed wavelength/emitted wavelength= (size of the Universe now)/(size when the light was emitted) lobs = observed wavelength, lem=emitted wavelength rnow=size of the Universe now rem=size of Universe when light was emitted lobs/lem = rnow/rem Remember, lobs/lem =1+z, so: 1+z = rnow/rem For any given object, z is easy to measure (remember redshifts are the easy part). Now we’re saying that the redshift tells you by how much the Universe has expanded in the time it’s taken for the light to get to us. Review:  Review Methods for measuring Wm Different types of Dark Matter (WIMPS, MACHOs, neutrinos) Summary of matter density (baryons and dark matter) Cosmological redshift Type Ia Supernovae as probes of the Expansion History Evidence that the Universe is accelerating The acceleration is caused by “Dark Energy,” which constitutes most (70%) of the energy density in the Universe. Express this as WL=0.7. Expansion History:  Expansion History We had a simple picture for Hubble’s law: v=H0xd. This holds in the nearby Universe. However, it turns out that H0 is just the value of the expansion rate today (the current change in size per unit time divided by the current size of the Universe). At earlier times, the expansion rate was different (depending on the matter and energy density in the Universe). At earlier times, t, H=H(t)≠H0. Even if we interpret redshifts as the expansion factor of the Universe since the light was emitted some distance from us (rather than as recession velocities), we can still use redshifts and distances to probe how H(t) evolves. The Supernova Story:  The Supernova Story White Dwarf What are Type Ia Supernovae? Explosion when dead star (white dwarf) becomes a natural thermonuclear bomb. Progenitor is probably normal star; when star uses up its fuel, it evolves into a white dwarf (size of the earth), which cools. Type Ia SNe occur when a white dwarf has a companion star, which dumps material onto the WD, causing the WD to become denser and denser, until a major explosion results when the WD crosses a certain mass and density threshold. The Supernova Story:  The Supernova Story If the Universe is accelerating, supernovae at a given redshift will look fainter than if the Universe is decelerating (because they are further away). Big result: In 1998, both teams found that the Universe is accelerating!!! The Supernova Story:  The Supernova Story The red curve is our current best estimate of what the Universe’s expansion history looks like. It is not decelerating, but actually starting to ACCELERATE!! We attribute this acceleration to DARK ENERGY, or the COSMOLOGICAL CONSTANT flat closed open flat, L Cosmological Constant:  Cosmological Constant Acceleration of Universe attributed to non-zero cosmological constant, L L is associated with vacuum energy density (energy density of the vacuum), which has negative pressure Positive energy density in the vacuum causes repulsion (not contraction) Vacuum energy density doesn’t change with time, so as matter density decreases in expanding Universe, importance of L increases. History of the Hot Big Bang Model:  History of the Hot Big Bang Model The Big Bang Model:  The Big Bang Model George LeMaitre, (1894-1966), Belgian, ordained Roman Catholic priest, who also studied at Cambridge and MIT (science and religion do not have to be at odds) 1927 -- proposed expanding model of the Universe (before Hubble’s 1929 discovery), as did Alexander Friedmann (Russian astronomer) Natural consequence is an initial highly compressed state (i.e. galaxies must have been closer together at earlier times….), density approaches infinity as t->0 Also championed idea of “primeval atom” which exploded (predicted remnant radiation would be cosmic rays…not quite right) “A Day without yesterday.” Side note on the Big Bang:  Side note on the Big Bang Last time, the balloon analogy was used to show the expanding Universe. Note: this is a 2D analogy for 3D space. Where is the center of the explosion on the surface of the balloon? (not the center of the balloon!!!!!) There is no center of the Big Bang explosion -- every galaxy sees the same thing. At t=0, space is infinitely dense EVERYWHERE!! Not just at one singularity. Don’t think of it like a bomb exploding. The Hot Big Bang Model:  The Hot Big Bang Model George Gamow (1904-1968), born in the Ukraine, moved to the US (after studying in Russia, Cambridge, and Copenhagen) Sought to explain all elements heavier than Hydrogen as being produced in the Hot Big Bang (1940s) Elements had to be produced in a hot environment (T>109 K) in order for nuclei to stick together Great insight, as you evolve Universe back in time, it not only gets denser, but also hotter Envisioned hot, dense mix of protons, neutrons, and electrons. Photons scatter off of electrons and are trapped by the plasma. Cosmic Elemental Abundances:  Cosmic Elemental Abundances Cosmic abundances of all elements relative to Hydrogen After Hydrogen, Helium is most abundant (~25% by mass, 10% by number) Then Carbon and Oxygen The Hot Big Bang Model:  The Hot Big Bang Model With graduate student Ralph Alpher, Gamow predicted abundances of elements produced in hot, dense Big Bang Alpher and Robert Herman then predicted (1948) that there should be leftover radiation from this Hot Big Bang The idea is that the Universe is a blackbody that cools as it expands. T(z) = T0 (1+z) (T0 is the temperature today) At early times (z->∞), T is really high! Blackbody Radiation:  Blackbody Radiation Radiation emitted by opaque surface that absorbs all the light that falls on it, and is characterized by a certain temperature, T. Temperature measures the amount of microscopic motion within a system. Object at temperature, T, emits a very specific spectrum, that is a function of the temperature, T, and the wavelength, l. Spectrum has a peak at a characteristic wavelength, which is bluer for higher temperatures and redder for lower temperatures Blackbody Radiation:  Blackbody Radiation Examples: Live Human: 310°K (38° C, 98.6° F), peak is in the infrared 10,000 nm The Sun: 5800° K, peak is in the visible The Universe when it was 3 minutes old: 109° K, peak is in gamma ray regime (wavelength is 10-13 m) The Universe today: 3° K, peak is in the millimeter range, can detect radiation with radio telescopes Man holding up a match The Hot Big Bang Model:  The Hot Big Bang Model Alpher, and Herman predicted that the universe should have cooled and expanded to the point where it would be characterized by a temperature of ~5°K The universe should then be filled with relic radiation from the Hot Big Bang phase. This radiation should have the spectrum of a blackbody with a temperature of a few degrees above absolute zero. Everywhere you look, you should see this relic radiation (i.e., it should be isotropic). The Hot Big Bang Model:  The Hot Big Bang Model The key is that something important happens when the Universe cools to ~3000°K, which is about 300,000 years after the Big Bang. Protons and electrons combine to form atoms, and photons no longer scatter off of electrons. The primeval fireball cools to the point that it is no longer opaque. Photons can free stream to us, from z~1100. Along the way they redshift, and reflect the overall expansion and cooling of the universe. These photons make up the microwave background radiation. We can’t see farther back than z~1100, because the Universe was opaque. The Hot Big Bang Model:  The Hot Big Bang Model You can think of it like photons coming to us from the inside surface of a sphere The photons are coming to us from a time when the Universe became transparent, when the Universe was only a few hundred thousand years old (rather than 13.7 billion years old). Each point in the Universe is surrounded by a last scattering surface ( where the distance to the surface corresponds to the speed of light times the time since the Universe became transparent). In the time it’s taken the photons to get to us from our last scattering surface, the Universe has expanded by a factor of 1100, and cooled by that factor too. The Hot Big Bang Model:  The Hot Big Bang Model These predictions were made in 1948. The Relic Radiation from the “primeval fireball” was not found for another 17 years (1965)? Why the delay? It turned out that parts of the theory for making elements heavier than hydrogen in the Big Bang were wrong. The theory is correct for making deuterium and helium (most importantly), but not for making heavier things like carbon and iron. The Universe does not stay hot and dense long enough to build up these heavier elements. Heavier elements are made in the interiors of stars and in supernova explosions (other hot, dense environments) The Hot Big Bang Model:  The Hot Big Bang Model 1957: Burbidge, Burbidge, Fowler, and Hoyle showed that heavy elements could be built up in stars and supernovae. Main problem, however, was that this model couldn’t explain 25% Helium abundance by mass (energy released in this reaction would be much greater than observed in stars emission). Primordial Big Bang origin is correct for light elements: Hydrogen, Helium, Deuterium, Lithium Detection of the CMB Radiation: New Jersey and the Pigeons:  Detection of the CMB Radiation: New Jersey and the Pigeons Detection of CMB: Princeton:  Detection of CMB: Princeton Robert Dicke (1916-1997), professor at Princeton, key figure in the development of radar technology during the war (also developed alternative gravity theories) Expert radio astronomer, also favored theory of oscillating Universe (universe periodically expanding and collapsing in a Big Crunch) Hot, dense state appeared to be a way of destroying heavy elements By 1964, Princeton group started to look for radiation left over from hot Big Bang Apparently independent of Gamow, Alpher, Herman predictions…. Detection of CMB: Princeton:  Detection of CMB: Princeton David Wilkinson and Peter Roll (young Princeton physicists) start building radio telescope to look for leftover radiation from Big Bang look at 3 cm wavelength build radio receiver on top of pigeon coop Detection of CMB: Princeton:  Detection of CMB: Princeton Meanwhile, Princeton theorist Jim Peebles figures out that Hot Big Bang leads to 25% of ordinary matter in the Universe being Helium (agrees with what is found in many stars). Solves problem of why there’s so much helium in the Universe Also, that there should be relic radiation of ~10K (unaware of Gamow, Alpher and Herman) that is isotropic (the same in every direction) Elements made both in Big Bang and in stars. Gamow had been discredited because he said they were all made in the Big Bang. (Peebles circa 2000) Big Bang Nucleosynthesis:  Big Bang Nucleosynthesis One of the predictions of the Big Bang model is that, for a given baryon density, Wb, the primordial abundances of Helium, Deuterium, and Lithium are all fixed (reaction rates depend on Wb). The predictions of the Big Bang model for the abundances of these light elements are consistent with the observed values, for a single Wb (value of Wb is consistent with other estimates) Major success of Big Bang Model Detection of CMB: Bell Labs:  Detection of CMB: Bell Labs 30 miles away, in Holmdel, NJ, 2 radio astronomers at Bell Labs were trying to use a very sensitive radio antenna to map the Milky Way at radio wavelengths Very similar set-up to the one being designed at Princeton In order to make their desired measurements, they needed to understand how big all the possible unwanted signals were that might contaminate their measurements First tried experiment at 7.35 cm (MW should be invisible, so good way to calibrate telescope) Giant ice cream cone laid on its side. Detection of CMB: Bell Labs:  Detection of CMB: Bell Labs Thought they accounted for all the noise, and expected to detect no signal from the sky at this wavelength (Milky Way invisible) But radio telescope generated more static than they expected, as if they were pointed at a body that had a temperature of 3.5°K What could be the source of this signal? Man-made signal from New York City? No. Noise in the electronics? No. Giant ice cream cone laid on its side. Detection of CMB: Bell Labs:  Detection of CMB: Bell Labs Didn’t matter which direction they pointed the radio telescope -- signal was always the same. Then there was the question of the “white dielectric material”… 2 pigeons roosted deep inside the antenna, and had “left their mark” They tried ejecting the pigeons and sending them away, and removing the “white dielectric material” but the pigeons came back. Finally, they had to take more drastic measures. Giant ice cream cone laid on its side. Detection of CMB: Bell Labs:  Detection of CMB: Bell Labs Still, after a year (and with no more pigeons) the excess signal would not go away. At this point, Penzias ended up on the phone with Bernie Burke, prominent radio astronomer. Penzias started complaining about the excess signal. Burke had heard about the search for the CMB radiation at Princeton and put Penzias in touch with the Princeton people Dicke et al. realized that the excess signal was exactly what they’d been looking for. Giant ice cream cone laid on its side. Detection of CMB: Bell Labs:  Detection of CMB: Bell Labs Indeed, the relic radiation from the Big Bang (I.e. the primeval fireball) had been found, with T~3° K. Companion papers reporting detection (Penzias and Wilson) and interpretation (Dicke and Princeton group) Penzias and Wilson got the Nobel Prize in 1978. Experiment required very sensitive radio telescope, and very careful calibration of all the possible sources of noise Giant ice cream cone laid on its side. Detection of CMB: Bell Labs:  Detection of CMB: Bell Labs Princeton group detected background radiation soon afterwards at a different wavelength (3.2 cm), obtained the same temperature for the background radiation. Supports the picture of blackbody radiation, I.e. the fact that measurements at two different wavelengths yield the same temperature must mean that the spectrum follows a blackbody (more on this later) Giant ice cream cone laid on its side. COBE: The Next Big Advance:  COBE: The Next Big Advance What is COBE?:  What is COBE? Cosmic Background Explorer, launched in 1989 (developed by NASA’s Goddard Space Flight Center) 3 instruments: Diffuse Infrared Background Experiment (DIRBE); Differential Microwave Radiometer (DMR); Far Infrared Absolute Spectrophotometer (FIRAS) 2 Big Results From COBE:  2 Big Results From COBE FIRAS showed that the spectrum of the CMB radiation was that of a blackbody with T=2.725°K (consistent with prediction that Universe cooled from initial hot big bang). DMR showed fluctuations of 1 part in 100,000 (once the motion of the sun, and the emission from the galaxy are subtracted off) The fluctuations are regions slightly hotter or colder than the average temperature of 2.725°. What are we looking at?:  What are we looking at? This is an all-sky map in Galactic coordinates from COBE. The plane of our Milky Way runs horizontally across the middle of the map. Top map shows the solar system motion in the Universe (we are not at rest wrt the CMB in every direction); middle map shows our Galaxy; bottom map shows density fluctuations in the early Universe (denser--> intrinsically hotter, less dense--> intrinsically colder), but gravitational redshift reverses it This is a map of the surface of last scattering. These photons have traveled to us since the Universe was a few 100,000 yrs old. Remember: Working Timeline since BB:  Working Timeline since BB Current CMB Experiments: What are the ripples and what do they tell us? :  Current CMB Experiments: What are the ripples and what do they tell us? Post-COBE: Lots of Experiments:  Post-COBE: Lots of Experiments Lots of ground-based and balloon experiments to try to detect CMB radiation and characterize the anisotropies (i.e. fluctuations) on different scales. Big breakthrough in 2000, with the BOOMerang experiment. This was a balloon flight at the South pole, (37 km off the ground, 1998-1999), mapped a few percent of the sky at higher resolution than COBE, best constraint on the global geometry of the universe, based on the characteristic angular size of the temperature (i.e. density) ripples. Current State of the Art: WMAP:  Current State of the Art: WMAP Wilkinson Microwave Anisotropy Probe (NASA/Goddard/Princeton), launched in 2001, still collecting data, named for Princeton physicist David Wilkinson All-sky map, but at much higher angular resolution than COBE (sharper image) Allows us to answer questions about the geometry of the Universe (as well as a bunch of other parameters). Is it closed, flat, or open? COBE’s angular resolution was about 7 degrees (14 times the diameter of the full moon), while WMAP has resolution of 0.3 degrees (a bit less than the diameter of the full moon) Current State of the Art: WMAP:  Current State of the Art: WMAP Resolution more than 20 times better with WMAP A Characteristic Scale:  A Characteristic Scale It turns out that there is a specific angular scale (hot/cold patch size), on which we expect to see the most contrast. This scale corresponds to the angular size that the observable Universe at z~1100 subtends on the sky today (about 1 degree) Figuring out the exact angular size where we see the most variation tells us about the global geometry of the Universe (closed/flat/open) A Characteristic Scale:  A Characteristic Scale To be continued…:  To be continued…

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