Transcript Slide 1
Dark matter and dark energy Gavin Lawes Wayne State University DCMST May 22nd, 2007 Outline •Physics on cosmic length scales: special and general relativity •Astrophysical measurements Measuring distance Measuring velocity •History of the universe (on a single viewgraph) •Open problems in cosmology Dark matter Dark energy Large scale structure of the universe DCMST May 22nd, 2007 Special relativity c=300,000 km/sec is a hard speed limit for the universe. The time interval or space interval between events depends on the speed of the observer. Causality Time B Space A Slope=c •The interval between A and B is timelike; everyone agrees that A occurs before B (so A could cause B). C •The interval between A and C is spacelike; some people think A happened first and some people think C happened first (so A cannot have caused C). DCMST May 22nd, 2007 General Relativity Rab Rgab gab 8Tab 1 2 •This equation describes how space is distorted by mass and energy. • is called the cosmological constant. Einstein put this in to force a static universe. DCMST May 22nd, 2007 Cosmological Distance Units Astronomical unit (AU): Earth-Sun distance (way too small to be useful). Lightyear (ly): Distance light travels in a year (still too small). Parsec (ps): 3.26 lightyears (ditto). Megaparsec (Mpc): One million parsecs (this is about right for talking about distant galaxies). Our galaxy (Milky Way) is about 100,000 ly in diameter, the Andromeda galaxy is about 1 Mpc away, and the “edge” of the universe is ~10,000 Mpc away. DCMST May 22nd, 2007 How do we measure distance? DCMST May 22nd, 2007 •Parallax can be used to find the distance to close objects (~300 ly). •Astronomers often measure cosmic distance using apparent luminosity measurements on Cepheid variables and Type 1a supernovae. Cepheid variable “Pulsing” star works to ~20 MPC Type 1a supernova “Exploding” star works to >1000 MPC DCMST May 22nd, 2007 Cepheid Variables •Period of variable luminosity (brightness) is related to actual brightness of star (Henrietta Leavitt, 1908). •Problems: This estimate is affected by space dust, and these stars are not bright enough to see huge distances. DCMST May 22nd, 2007 Type 1a supernova These supernovae are believed to follow a standard intensity vs time curve (after taking into account well-known “fudge factors”). Problems: There are not too many Type 1a supernovae known. DCMST May 22nd, 2007 Supernovae seen by SDSS DCMST May 22nd, 2007 How do we measure velocity? DCMST May 22nd, 2007 •Elements in distant stars emit light at very specific wavelengths (spectrum). •The wavelength of light we measure on Earth (lmeasured) will be different than the wavelength emitted by the star (lstar) is the star is moving relative to the Earth (just like a Doppler shift). •This shift in wavelength is parameterized by “z” defined by: l 1 z measured lstar If z<<1 then z=v/c DCMST May 22nd, 2007 Hubble’s Law v H 0d H0=70 km s-1/Mpc Age of the universe is 1/H0, or about 13 billion years. DCMST May 22nd, 2007 NB: Using Hubble’s law, astronomers will often use “z” when talking about velocities, distances, or time after the start of the universe. Hubble’s law tells us that the universe is expanding. DCMST May 22nd, 2007 NB: Using Hubble’s law, astronomers will often use “z” when talking about velocities, distances, or time after the start of the universe. Hubble’s law tells us that the universe is expanding. ... ... DCMST May 22nd, 2007 NB: Using Hubble’s law, astronomers will often use “z” when talking about velocities, distances, or time after the start of the universe. Hubble’s law tells us that the universe is expanding. ... ... ...but what is it expanding from? What is it expanding into? DCMST May 22nd, 2007 How did the universe start? DCMST May 22nd, 2007 Big Bang •15 billion years ago, the universe was very small, very hot ball of matter and energy. •In the intervening time, the universe has expanded, but this original energy can be seen in the cosmic microwave background. In 1963 isotropic background radio waves were measured. These could be fit to 2.7 degree background radiation, and proved strong evidence for the Big Bang theory. DCMST May 22nd, 2007 History of the Universe (Inflation) Matter/antimatter annihilate (0.001 sec) Protons/neutrons form (3 minutes) ? 2.7 K Today (15 billion years) 3000 K First galaxies form (1 billion years) DCMST May 22nd, 2007 Neutral atoms form (380,000 years) Just one more thing.... DCMST May 22nd, 2007 Why was there more matter than antimatter? •The early universe should have had equal amounts of matter and antimatter, which would then annihilate completely. •This didn’t happen. •About 1 out of every 1 billion matter particles survived to the universe today. DCMST May 22nd, 2007 Why do galaxies show flat rotation curves? GM ( r ) v 2 r 2 •Most of the light from galaxies comes from the center. •The flat rotation curves suggest that there is a uniform mass distribution. Galactic rotation curve (measures the rotation velocity as a function of distance from the center of the galaxy) •This led to the suggestion of missing mass, later called “dark matter”, because it provides gravitational attraction, but doesn’t emit light. DCMST May 22nd, 2007 This dark matter is believed to surround most galaxies, and the massto-light ratio for certain galaxies can exceed 300 times that of the sun. (What is dark matter?) •Doesn’t really exist―the theory of gravity doesn’t work on very, very large length scales. •Black holes, neutron stars, brown dwarfs (collectively MACHOS). •Neutrinos. •Weakly interacting massive particles (WIMPS). DCMST May 22nd, 2007 Will the universe keep expanding forever? Einstein put an extra term (called the cosmological constant) in his equations for General Relativity to produce a static universe. Universe expands forever “Flat” universe Universe collapses DCMST May 22nd, 2007 Accelerating Universe The intensity of the most distant objects (largest z) is smaller than we expect using Hubble’s Law, so they must be moving away faster. DCMST May 22nd, 2007 Why is the universe accelerating? •Einstein’s theory of general relativity is wrong (or incomplete) and gravity works differently at long length scales. •The speed of light has changed since the Big Bang. •A cosmological constant (first proposed by Einstein) that provides a negative pressure causing the universe to accelerate. •Quintessence: like the cosmological constant, but not uniform in space and time. The last two are forms of dark energy, which would need to have an energy density of 10-29 g/cm3 (NB: E=mc2) DCMST May 22nd, 2007 Why did galaxies and clusters form? DCMST May 22nd, 2007 Small density variations right after the Big Bang could coalesce into galaxies, clusters, and superclusters. Pillars of creation ~4 ly long dustclouds Great Wall ~500 Mly long cluster of galaxies DCMST May 22nd, 2007 Cosmic microwave background The cosmic microwave background shows temperature differences of about 1 part in 100,000 over the universe. DCMST May 22nd, 2007 This plot allows astronomers to analyze the microwave background, and gives information about the geometry (flat, open, closed) and matter and energy density of the universe. DCMST May 22nd, 2007 What is the universe made of? Atoms: Normal matter. We can see this. Dark Matter: Invisible mass inferred by measuring speed of galaxy rotations. Dark Energy: Provides negative pressure for accelerating universe. DCMST May 22nd, 2007 Further research directions in cosmology DCMST May 22nd, 2007 May be possible to observe dark matter using gravitational lensing. DCMST May 22nd, 2007 Intersection of particle physics and cosmology •There was a lot of energy present during the Big Bang. •Looking at the very distant parts of the universe allows astronomers to take a snapshot very early universe to understand how matter and energy interact at these high energies. •Some specific theories also predict particles that can provide the dark matter and dark energy seen in the universe. •The Large Hadron Collider (LHC) set to start this year will allow physicists to investigate matter and antimatter at high energies, similar to . DCMST May 22nd, 2007 end DCMST May 22nd, 2007