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A Tour of the Universe Stanford SPLASH Instructor: Jack Singal Goals: give an overview of what we currently know about the cosmos and our place in it Touch on both: 1) What we know 2) How we know it I want to get some of these ideas out there. Most of the public is unaware of what those of us in astrophysics now know about the universe (could be said of any current science!) After this class, hopefully you’ll have a basic grasp of these things if you encounter them in the news or conversation Talk to your friends, parents, grandma, etc… Where are we? We live on a planet… You are here 8000 miles Earth is 92 million miles from the sun… The solar system You are here 3 Billion miles The solar system: 1 star 8 main planets many moons 5 dwarf planets many small planets 1000s of asteroid rocks Millions of comets The sun is a star. It is 25 Trillion miles to the nearest other star. Are there other solar systems out there? Yes! We now know of 100s of known planets around other stars increasing mass Earth Jupiter decreasing orbital distance We mostly detect them through their gravitational effects on their stars Although a few we can image directly Most that we can detect are huge and close to their star In the next ~20 years, we will detect planets that are more like Earth The Milky Way galaxy There are ~10 Billion stars in our galaxy You are here 5.7x1017 miles = 5.7 million trillion miles There are ~100 Billion galaxies You are here So we belong to one of 10 billion stars in our galaxy, and there are 100 billion galaxies… 4x1021 miles But that’s only a small part of the story… Everything we know about and everything we have ever seen (stars, planets, interstellar matter, intergalactic matter, and all of the light and radiation out there), is only 4% of what makes up the universe! Ordinary Matter (You are here) 3.2% free H and He gas 0.3% neutrinos 0.5% stars and big planets 0.005% heavy elements (you are here) Has it always been this way? no - The universe is ~14 billion years old - At the start, everything was in a very dense state, and it has been expanding since - The expansion is now accelerating A brief history t=0 Big Bang – beginning of universe Major milestones: 10-8 seconds: “inflation” makes universe blow up in size 10-6 seconds: Quarks combine to form protons and neutrons 300,000 years: Neutral hydrogen forms 400 million years: Matter collapses to form first stars 400 million years thru present: the elements are formed by stellar fusion and explosions 1 billion years thru present: Galaxies and super-galactic structures form ~4 billion years ago: Sun and planets coalesce from dust ejected from earlier stars ~2.5 billion years ago: Life appears on Earth ~1 billion years ago: Multicellular life appears ~750 million years ago: First animals ~100 million years ago: First mammals ~1 million years ago: First humans ~5000 years ago: First human civilizations Question 1: Which is the farthest from the Earth? a) The sun b) Neptune c) The center of our galaxy d) The Andromeda galaxy Question 2: Approximately how many times more dark matter is there than ordinary matter in the Universe? a) 2 b) 5 c) 20 d) 50 Ordinary Matter Question 3: The Earth has been around for approximately what fraction of the time history of the Universe? a) 1/10 b) 1/3 c) 1/2 d) 2/3 Universe: 14 billion years old Earth: 4.5 billion years old How do we know all this? - We study the light that comes to us from distant objects - We investigate the laws of physics here on Earth and believe they apply everywhere - We get some particles (electrons, protons, neutrons, neutrinos, etc…) from celestial objects Step 1: Telescopes Our eyes are small and don’t collect much light, so only very bright astronomical objects are visible to the naked eye. Also astronomical objects are far away so they appear very small and can’t be resolved in detail by the eye. Telescopes solve both problems: they gather the light falling on a much larger area than the eye, focus it to a small area, then magnify it. The resulting image is then viewed or recorded. Galileo uses telescope to look at Jupiter, 1609 Hubble space telescope, present LSST is one of a new generation of huge telescopes, 2015 Telescopes First people looked with their eyes and drew what they saw Galileo’s drawing of the Moon Then they recorded images with photographic film Actual photograph from ~1900 Now we record images digitally - like with your digital camera and store them as data Hubble Space Telescope image Step 2: Spectra We can also split up the light from an object that we get with a telescope into the different colors (wavelengths) and record them individually This spectroscopy gives us extra useful information… Spectra Spectroscopy allows us to see the brightness as a function of color (wavelength). Increasing wavelength This often yields lots of information. For example, we can see the presence (or absence) of different atoms and compounds that emit light at specific wavelengths Spectra Not only that, but we can see the spectrum ‘shift’ for some objects. This shift in wavelength is caused by the motion of the source relative to us. It is analogous to the change in pitch of a car horn when the car moves. Spectra When the car is coming toward us, the waves in front are compressed, decreasing the wavelength: When the car is moving away us, the waves in back are elongated, increasing the wavelength: The same thing happens for light. Spectra we can see the spectrum ‘shift’ for some objects. Analyzing the shift tells us how the object is moving! Telescopes Ok, we can record the light from objects, and even split it into different colors. Some are bright and some are dim. They have different color patterns Some are moving How far are they? Step 3: Distances Something may appear bright or faint because it actually is bright or faint or because it is close to us or far away We need to measure distances The distances to very nearby stars can be determined by measuring angles in a technique known as “parallax” Distances Farther distances rely on “standard candles” From nearby objects whose distance is known via parallax we can determine that certain types have an intrinsic brightness (“luminosity”). For farther objects of the same type, since we know how bright they actually are, and we see how bright they appear, we can determine the distance Question 4: Relative to a source that is stationary with respect to us, a source moving away from us has its sound (and light) wavelengths: a) The same b) longer c) shorter Put it all together! Now that we have all of these stars and galaxies, and their distances, and their sizes, and how they are moving, we can start to build a clearer picture of the universe We can put all of the stars, galaxies, and everything else that we see where it all belongs. That’s what people have been doing for 100s of years, and continue to do There have been and continue to be surprises though… Discovery: Expansion of the universe and big bang Armed with a way to measure the distance to other galaxies, Hubble did so, and also took the galaxies’ spectra (1929). He saw a shift in the spectral lines The spectral lines from the more distant galaxies were shifted toward the red (longer wavelength), indicating that those galaxies were moving away from us Expansion of the universe We define the redshift (z) to quantify how much the light is shifted in wavelength toward the red obs 1 z emitted Hubble showed that there was a relation between distance and redshift. The farther a galaxy is from us, the more it is redshifted and therefore the faster it is moving away. Three major conclusions: 1) Since the farther something is the longer it takes for light to reach us, higher redshift is equivalent to saying farther back in time 2) If across the board, the farther away something is the faster it is receding, then the only conclusion is that the entire universe as whole is expanding. 3) Run the movie backwards and the Universe must have started with everything in a very small volume and has expanded since – the “big bang” Step 4: There’s more light out there than meets the eye… Visible light is only a small piece of the available light out there Increasing frequency microwave Different processes in space produce emission all over the spectrum! All the different kinds of light Different processes in space produce emission all over the spectrum! Over the past 100 years, we have developed detectors – the equivalent of optical telescopes, for all of these different kinds of light. FERMI LAT Question 5: Which of the following is not the name of a range of wavelengths of light? a) visible b) X-rays c) ultra-luminous d) microwave e) radio microwave The different light tells us so much For example, looking at the center of our own galaxy: Fast electrons and magnetic field Cold gas Dust Stars unobscured Stars Hot gas Cosmic rays (charged particles) Contents of galaxies Galaxies consist of: • Stars (and planets) • Cold gas • Hot gas •“Dust” (heavy elements and molecules) • Free cosmic rays (electrons, protons, neutrons, neutrinos) • Dark matter (!) • The space between Galaxies in clusters consists of very hot gas, and in empty space consists of free electrons and protons And it all glows (in different parts of the spectrum) so we see it But it turns out there is MUCH MORE out there… Discovery: Exotic objects There is a slew of crazy stuff out there, characterized by systematically studying the sky on many different scales at many different wavelengths. Supernovae are huge explosions when a massive star dies. Most of the elements that make up the Earth and us were fused in supernovae. Many stars are in closely orbiting binary or many-star systems Pulsars are rapidly rotating neutron stars (very dense supernova remnants) that send out beams of radio energy What happens to stars? At the end of a star’s life, there are several possibilities, depending on the size: Stars like the sun or smaller throw off most of their material and what is left is a small very dense core held up by the pressure of all the electrons. This is a white dwarf More massive stars usually have a violent supernova explosion where they spray out a huge amount of matter and light Stars above ~1.4 times the mass of the sun have so much gravity that it overcomes the electron pressure and the star collapses to where only the neutrons are holding it up. This is a neutron star. Stars above ~5 times the mass of the sun have so much gravity that nothing can stop their collapse. They become black holes. Black Holes Black holes are objects so dense that light cannot escape. Outside of a certain radius away from it (the event horizon), they gravitate like any massive object. Things can (and do) orbit them, and can orbit closely. We see the light from this stuff and it makes for some of the most spectacular phenomena in the universe. Current physics cannot explain what happens inside the event horizon, but nothing (not even light) can escape. Central black holes The big black holes at the centers of Galaxies (including ours) are a million or more times as massive as the sun Active Galactic Nuclei are matter orbiting a massive black hole at the center of a galaxy. They sometimes send out huge jets of very fast particles. The jets of particles slam into the neighboring gas and magnetic fields and radiate x-rays, UV, optical, and radio Because they are so bright, we can see some of them at a great distance. Those are called Quasars Active Galactic Nuclei If the jet is pointed right at us, it looks like a really bright, flaring point, and we call it a “Blazar” If the jet is pointed perpendicular to us, we can sometimes see the jets shooting way out. The far lobes make a “Radio Galaxy” An AGN – the “Death Star” Galaxy “Death Star” galaxy (x-ray, radio, and optical) Another galaxy jet Galaxy with AGN X-rays – purple Optical – red Radio - blue GRBs Gamma ray bursts may happen when a neutron star falls into another neutron star or black hole. The resulting explosion sends out particles and radiation all over the spectrum They are the most luminous things in the universe In May a GRB was seen at redshift 8. It is the farthest thing ever seen and occurred only 400 million years after the big bang Cosmic Microwave Background Observing the sky at microwave wavelengths, we can even see light almost back to the big bang: A glow covers the entire sky, light from a time when the early universe was hot and dense. It lends additional proof to the big bang explaination. Those hotter and colder patches in the hot and dense sea tell us a lot. They are the seeds of what will become structure. Question 6: What is the farthest thing we have ever seen? a) a black hole in a distant galaxy b) a gamma-ray burst at redshift 8 c) the CMB Discovery: Dark Matter Galaxies and groups of galaxies behave like they have a lot more matter than just adding up the stars, gas, and dust 1) Galaxy rotation: We can measure (via spectral line shift) the velocities of stars in Galaxies. The bulk rotation rate of the galaxies is related to how much matter is in them, and there seems to be way more matter than we can see Dark Matter Galaxies and clusters behave like they have a lot more matter than just adding up the things we can see: stars, gas, and dust 1) Galaxy rotation 2) Galaxy cluster dynamics: The motion of galaxies within groups of galaxies also indicates there is much more matter there (first claimed by Fritz Zwicky in 1933!) Dark Matter Galaxies and clusters behave like they have a lot more matter than just adding up the things we can see: stars, gas, and dust 1) Galaxy rotation 2) Galaxy cluster dynamics 3) Gravitational Lensing: Matter can bend light. The amount of light bending we see for objects behind galaxies and galaxy clusters indicates a lot of unseen matter in the clusters. Dark Matter Galaxies and clusters behave like they have a lot more matter than just adding up the things we can see: stars, gas, and dust 1) Galaxy rotation 2) Galaxy cluster dynamics 3) Gravitational Lensing 4) Colliding galaxy clusters: check out the picture Lensing says the mass is here 2 clusters passed through each other: So the majority of the matter passed right through each other without interacting, and it doesn’t give off X-ray light.observations say the gas (ordinary matter) is here. The gas from the two clusters collided and stayed in the middle Dark Matter Galaxies and clusters behave like they have a lot more matter than just adding up the things we can see: stars, gas, and dust 1) Galaxy rotation 2) Galaxy cluster dynamics 3) Gravitational Lensing 4) Colliding galaxy clusters 5) Patterns in the CMB Dark Matter Galaxies and clusters behave like they have a lot more matter than just adding up the things we can see: stars, gas, and dust 1) Galaxy rotation 2) Galaxy cluster dynamics 3) Gravitational Lensing 4) Colliding galaxy clusters 5) Patterns in the CMB Dark matter interacts gravitationally, but we know that it doesn’t give off light (at any wavelength) and that it mostly passes right through itself. Hence “Dark” Dark matter is ~80% of the matter in the universe. The ordinary matter is only 20% We don’t know what it is (although there are theories). Dark Matter Dark matter is ~80% of the matter in the universe. The ordinary matter is only 20% Galaxies and clusters sit in ‘halos’ of dark matter. The large scale structure of the Universe consists of vast ‘filaments’ of dark matter in which the galaxies and clusters sit at the vertices of the web The dark matter can’t be as clumped like ordinary matter because it can’t radiate energy or angular momentum, so it stays ‘swimming’ in large structures Discovery: Dark Energy Question 1: how is the expansion of the universe evolving with time? Is it steady, or has it slowed down or sped up? Question 2: What is the total energy content? According to Einstein’s General Relativity, Energy can be in the form of: Matter Light “Vacuum energy” – pushes things apart The answers to these two questions are related, because the energy content controls the expansion By quantifying how distances change with time (or equivalently, redshift) we can determine the expansion history and shape, and answer both questions Dark Energy Until recently, we didn’t have a method for determining very large distances, only those to relatively nearby galaxies In the 90s, people determined that a certain type of supernova (Ia) could be used as a standard candle in the late 90s they were able to determine the actual distance to high redshift (z>1) galaxies and found a surprising result… The relationship between distance and redshift (time) shows that: The expansion of the universe is now accelerating with time! Dark Energy Meanwhile, observations of the tiny differences (anisotropy) in the CMB show the primordial seeds of structure in the early universe. We have theories that relate the size of those seeds to what we see today, and they depend on the energy content… Adding up all the ordinary matter and dark matter leaves us with 75% of the energy missing! Dark Energy Accelerating expansion + missing energy some sort of vacuum energy is present “Dark energy” We have no idea what it is or where it comes from. But it’s large, and it’s (apparently) in charge Supernova Ia, CMB, and cluster evolution all agree: ~70% Dark Energy, ~30% Matter Dark Energy Also, knowing the expansion history, we can relate redshift to distance accurately. Knowing the speed of light, once we have the distance we can calculate the time it took for light from something to get here. We know the redshift of the CMB. We can get the distance, and then the time it took for the light from the CMB to get here. That is almost the age of the universe age of the universe 13.7 billion years Question 7: Which of the following is not a source of evidence for Dark Matter? a) motion of the planets b) rotations of galaxies c) gravitational lensing of galaxies d) pictures of galaxy clusters Question 8: Which of the following is not a source of evidence for Dark Energy? a) supernova as standard candles to determine far distances b) patterns of small variation in the CMB c) galaxy rotation measurements “ΛCDM” Cosmology All of this leads to the standard so called concordance cosmology, or “ΛCDM” (Lambda stands for vacuum energy and ‘CDM’ is “cold dark matter”). • The universe started from a very dense state 13.7 Billion years ago and has expanded ever since • In the very beginning the expansion was enormous (inflation). The expansion then resumed a lower value • The gravitational formation of large scale structure is dominated by dark matter • Ordinary matter forms the stars, galaxies, and intergalactic gas. It is what we see. • As the universe expands and the matter density drops, dark energy is increasingly taking over and causing the expansion to accelerate • At present, the universe is 70% Dark Energy, ~25% Dark Matter, and ~5% Baryons Outstanding Questions in cosmology • What is dark matter? • What is dark energy? Does it evolve with time? • What is the fate of the universe? • Why did the universe start in the first place? What caused the big bang? • Is there life out there? • Are there other universes? • What happens in a black hole? • Many others !