Dark Energy in Our Expanding Universe Joe Mohr Department of Astronomy Department of Physics NCSA

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1 Dark Energy in Our Expanding Universe Joe Mohr Department of Astronomy Department of Physics NCSA 2/19/05 Saturday Morning Physics - Joe Mohr 1

2 Galaxies and Stars 2/19/05 Saturday Morning Physics - Joe Mohr 2

3 Map of Galaxies in the Local Universe de Lapparent, Geller & Huchra, ApJ Million Light Years A map of the location of 1,061 bright galaxies in the local universe. 2/19/05 Saturday Morning Physics - Joe Mohr 3

4 Modern Cosmology Cosmology is largely an observationally driven endeavor The Big Bang model of expanding, cooling universe required by the data! Recipe for the Universe What are some questions addressed by cosmology? How does the universe transform from its homogeneous, early state to its richly structured current state? When did the first objects form? What are the ingredients of the universe? What is the nature of dark matter, the nature of dark energy, gravity? 2/19/05 Saturday Morning Physics - Joe Mohr 4

5 Hubble Discovers the Universal Expansion In the 1920 s, Edwin Hubble carried out a study of nearby spiral nebulae (galaxies) to measure their distances and their line of sight velocities. He determined that the nebulae are at very large distances, well outside our own galaxy. Hubble guiding the Hooker 100 inch telescope in Photos courtesy M t. Wilson: He discovered that all but a few of our neighbor galaxies are traveling away from the Milky Way! He noted that the more distant neighbor galaxies are traveling away from us faster than our near neighbors. 2/19/05 Saturday Morning Physics - Joe Mohr 5

6 Light and Spectra Many astronomical objects emit electromagnetic radiation A spectrum provides information about the composition and temperature of emitting gas, dynamics of the object and evolution of universe since light was emitted The Hooker 100 inch telescope atop Mt. Wilson near Pasadena, CA. It was the largest telescope in the world from Sodium emission spectrum Wavelength (λ) 2/19/05 Saturday Morning Physics - Joe Mohr 6

7 Spectral Signature of Motion λ emit lab spectrum λ obs redshifted spectrum Wavelength (λ) z " # obs $ # emit # emit % v c Non-relativistic Doppler shift formula 2/19/05 Saturday Morning Physics - Joe Mohr 7

8 Redshifts Are the Easy Part Expansion of the universe from the time light is emitted to the time it is observed stretches the wavelength of light, so the pattern of absorption lines in a typical stellar spectrum is shifted to longer wavelength. Star in our galaxy v~0 km/s When we examine the light from a distant galaxy we often think of this stretching as a measure of the galaxy recession velocity. ( ) " obs = " emit 1+ z Distant galaxy v=12,000 km/s z=0.04 2/19/05 Saturday Morning Physics - Joe Mohr 8

9 What about distances? Apparent size of a standard ruler Standard ruler is an object whose intrinsic size is known Apparent (angular) size θ provides distance d given intrinsic size r r d = r tan! " r! θ Apparent brightness of a standard candle Standard candle is source whose intrinsic brightness is known Apparent brightness b provides distance d given intrinsic brightness B b = B 4!d 2 " d = B 4!b d 2/19/05 Saturday Morning Physics - Joe Mohr 9

10 Standard Rulers in Everyday Life The STOP sign is an everyday standard ruler. Because we know STOP signs are all the same size, the apparent size of a STOP sign provides us with distance information. 2/19/05 Saturday Morning Physics - Joe Mohr 10

11 Measuring the Hubble Law speed v 1 v 2 v = H 0 Hubble parameter d d 1 distance d 2 2/19/05 Saturday Morning Physics - Joe Mohr 11

12 Interpreting the Expansion Observational Fact: Galaxies are receding from us, and their recession velocities are proportional to their distances from us Two interpretations: Bad neighbor hypothesis We are at the center of the universe, and the rest of the universe is trying its best to get away from us. Homogeneous expansion hypothesis The whole universe is expanding, and observers on any other planet in any other galaxy would note the same proportionality between recession velocity and distance- the Hubble Law- for galaxies surrounding them. 2/19/05 Saturday Morning Physics - Joe Mohr 12

13 Bad Neighbor Hypothesis Like an explosion centered around us in space; galaxies fly out like shrapnel 2/19/05 Saturday Morning Physics - Joe Mohr 13

14 Homogeneous Expansion If the Universe were simply expanding uniformly (homogeneously), then observers in any galaxy would look out to observe the same Hubble Law (galaxies traveling away from them, recession velocity proportional to distance) 2/19/05 Saturday Morning Physics - Joe Mohr 14

15 A Modern Measure of the Expansion Measurements to 19 Galaxies Riess, Press & Kirshner ApJ 1996 All studies provide consistent results in the local universe: other galaxies are receding from us, and their recession velocities are proportional to their distances. The farther away the galaxy, the faster it travels away from us (or the more the universe has expanded during the time it took the light to reach us) Blue points: 19 SNe Red line: Hubble Law with H o =19.6 km/s/mly v r = H o d so H o = v r d The Hubble parameter has units of velocity over distance. 2/19/05 Saturday Morning Physics - Joe Mohr 15

16 Evolution of the Expansion Rate Gravity is an attractive force that acts on all mass Galaxies are massive, and an expanding universe of galaxies should slow over time By measuring distances and redshifts of objects located ever farther away, one can determine exactly how the expansion rate has changed over cosmic time In a matter filled universe, one expects higher expansion rates in the past. 2/19/05 Saturday Morning Physics - Joe Mohr 16

17 SNe Ia Distances and Cosmic Acceleration Distance Results confirmed with several independent experiments that did not employ direct distance measurements. Velocity 2/19/05 Saturday Morning Physics - Joe Mohr 17

18 Implications of Cosmic Acceleration A problem with Einstein s s theory of gravity? But it has passed so many tests Is it possible to change gravity to explain acceleration without messing up all the other successes? Universe dominated by dark energy? Not a subtle effect- requires twice as much DE as DM per unit volume A constant and non-zero energy density associated with space itself? 2/19/05 Saturday Morning Physics - Joe Mohr 18

19 Dark Energy is Strange Stuff Cosmological constant: Form of DE that has constant energy density. As the universe expands it is as though energy is created! This requires that DE has negative pressure! Negative pressure gives DE an effective gravitational repulsion! DE is just another form of energy within the context of General Relativity only bizarre because it has non-intuitive behavior! Albert Einstein 2/19/05 Saturday Morning Physics - Joe Mohr 19

20 Negative Pressure Consider a piston chamber containing normal gas Expansion leads to reduction in energy density/cooling of gas (gas pressure is doing work on piston) For DE the energy density remains unchanged, so energy must be created requires negative pressure Gas or DE Gas or DE Movement of piston expands the volume in the piston chamber. This does work dw= P*dV. 2/19/05 Saturday Morning Physics - Joe Mohr 20

21 Gravitational Repulsion In GR, it s s not only the mass that couples to gravity it s s the kinetic energy of a system (i.e. pressure). Example: mass of object depends on its velocity! (confirmed in laboratory) For DE, the negative pressure then offsets the positive energy density leading to a gravitational repulsion rather than attraction Stress-energy: T = " + 3p = " + 3 #" ( ) = #2" 2/19/05 Saturday Morning Physics - Joe Mohr 21

22 Next Steps Make precise measurement of the cosmic expansion history over a large range of cosmic time This will limit available options for theorists and ultimately lead to breakthrough Such a measurement is fundamental and will stand for all time If the cosmic acceleration is caused by DE, then we must use this measurement to determine precisely how the energy density is changing with time Requires new and more powerful techniques cannot rely on simple distance estimates 2/19/05 Saturday Morning Physics - Joe Mohr 22

23 Structure Formation Based Techniques The story of the Big Bang is the story of a transition from an early, hot and homogeneous universe to a late, cold and structure-filled universe The galaxies and larger scale structures form through gravitational amplification from tiny dark matter density perturbations in the early universe The rate of structure formation is extremely sensitive to the cosmic expansion history (Hubble parameter as function of time). 2/19/05 Saturday Morning Physics - Joe Mohr 23

24 2/19/05 Saturday Morning Physics - Joe Mohr 24 Movie by Andrey Kravtsov

25 Daffodils appear as time moves forward. and disappear as time moves backward. How Does One Measure the Rate of Structure Formation? 2/19/05 Saturday Morning Physics - Joe Mohr 25

26 We use the most massive tracers: Galaxy Clusters Galaxy clusters are the most massive, collapsed structures in the universe ( M o ). Chandra Image of Zw3158 They contain galaxies (stars), hot ionized gas or intracluster medium- ICM- ( K- X-ray emitting!) and are dominated by dark matter. Clusters are good tracers of structure formation, because they are massive and easy to detect. 2/19/05 Saturday Morning Physics - Joe Mohr 26

27 10m South Pole Telescope (SPT) and 1000 Element Bolometer Array Will detect tens of thousands of galaxy clusters extending to a time when the universe was only 10% its present age. People Carlstrom (UC) Holzapfel (UCB) Lee (UCB,LBNL) Leitch (UC) Meyer (UC) Mohr (U Illinois) Padin (UC) Pryke (UC) Ruhl (CWRU) Spieler (LBNL) Stark (CfA) This new telescope will be deployed in Nov 2006 at the South Pole! 2/19/05 Saturday Morning Physics - Joe Mohr 27

28 The Dark Energy Survey SPT will detect the clusters (daffodils), but we need to measure the cluster redshifts to understand how quickly the clusters formed over time Blanco 4m on Cerro Tololo The Dark Energy Survey is a project to do just this by carrying out the most extensive optical imaging survey every undertaken The Survey will use an existing 4m telescope in Chile together with the largest CCD camera ever built. Fermilab is leading the camera building, and U Illinois is building the software system to process and store the data at NCSA. Image credit: Roger Smith/NOAO/AURA/NSF DES collaboration is currently Fermilab, U Illinois, U Chicago, LBNL and CTIO 2/19/05 Saturday Morning Physics - Joe Mohr 28

29 Summary The Big Bang model has been wildly successful as a framework for understanding a vast array of cosmological data/experiments. The cosmic acceleration, discovered in 1998, poses a new challenge. This challenge is the most exciting thing happening in physics or astronomy today, and a major revolution in our understanding of the universe may be unfolding. New tools have been developed and are being applied to precisely measure the expansion history of the universe (Hubble parameter as function of time), and we expect to have dramatically improved observational constraints within a few years. 2/19/05 Saturday Morning Physics - Joe Mohr 29