More to the universe than meets the eye Everywhere we look in the Universe, we see Galaxies, each filled with hundreds of thousands of stars. Amazingly, the matter that we can see accounts for only ~4% of the mass of the Universe! Where is the other ~96%? Scientists think that part of the missing mass is made up of Dark Matter – matter that we have not yet been able to detect. Image from: http://hubblesite.org/newscenter/newsdesk/archive/releases/2004/07/image/a Credit: NASA, ESA, S. Beckwith (STScI) and the HUDF Team

What is the universe made up of? NASA’s WMAP (Wilkinson Microwave Anisotropy Probe) mission studies the fundamental structure of the Universe by measuring the Microwave Background radiation of the Early Universe. Based on those observations, WMAP reveals the make-up of the Universe. Atoms, which make up just 4% of the Universe, are the building blocks of all visible matter. The other 96% is thought to be a combination of Dark Matter and Dark Energy Pie Chart from WMAP website: http://map.gsfc.nasa.gov/m_mm.html Credit: NASA/ WMAP Science Team

The Evidence Although we cannot detect Dark Matter or Dark Energy, we know that it exists because of its’ gravitational effect on stars and galaxies. The velocity of stars in a spiral galaxy tells us how much gravity there is, and therefore how much mass the galaxy has. This Rotation Curve for a typical spiral galaxy shows the prediction (A) that stars move very fast near the center of the galaxy, and slower on the outer edges. The observed Rotation Curve (B) shows that stars are moving much faster than expected on the outer edges of the galaxy. This must be caused by mass in the galaxy that we cannot detect – Dark Matter. Rotation Curve copied from Wikipedia: http://en.wikipedia.org/wiki/Galaxy_rotation_problem

Evidence in the Milky Way Galaxy Our own Sun should be traveling at 60 km/hr in order for it to stay in orbit around the center of the Milky Way Galaxy. Observations show that the Sun is moving at a speed of 225 km/hr! Using this velocity to measure the mass of the Milky Way shows that the mass is about 10 times of what is expected from visible stars, gas, and dust within the galaxy. Image from: http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question18.html Velocity of Sun data taken from WMAP: http://map.gsfc.nasa.gov/m_uni/uni_101matter.html Drawing of Milky Way Galaxy seen “Face-On” Courtesy: NASA

More Evidence Another way to measure the amount of mass in a galaxy is to look at Gravitational Lensing. A massive galaxy acts as a lens – magnifying the light from galaxies behind it. Lensed galaxies are seen as thin wisps in the image at left. Those galaxies are actually behind the foreground galaxies, and appear distorted. Results from this method confirm that the amount of visible matter accounts for only a small percentage of mass that a galaxy actually has. Image from: http://hubblesite.org/gallery/album/entire_collection/pr2001032b/ Credit: NASA, ESA, Richard Ellis (Caltech) and Jean-Paul Kneib (Observatoire Midi-Pyrenees, France)

Dark Matter Map Image from: http://www.yale.edu/opa/newsr/05-01-06-01.all.html Citation: Astrophysical Journal Letters 617: L13-L16 (December 10, 2004) Yale Astronomers, analyzing gravitational lensing images from the Hubble Space Telescope, have made this map showing where clumps of dark matter (in blue) surround the visible galaxies (yellow to red).

WIMPS AND MACHOS Astronomers do not know what Dark Matter is made up of. The leading candidates are whimsically named MACHOs and WIMPs. MACHOs are Massive Compact Halo Objects, which can include Brown Dwarfs, White Dwarfs, and Black Holes Brown Dwarf image: http://coolcosmos.ipac.caltech.edu/cosmic_classroom/ir_tutorial/missing.html White Dwarf Image: http://chandra.harvard.edu/xray_sources/white_dwarfs2.html Supermassive Black Hole: http://www.nasa.gov/multimedia/imagegallery/image_feature_132.html Credit NASA/CXC/M.Weiss White Dwarfs are the remnants of stars like our Sun, which have collapsed after burning all their energy. They are very compact and dim. Brown Dwarfs are failed stars – stars that never gained enough mass to start burning. They are very cool and dim, making them difficult to detect. Supermassive Black Holes, like the one shown in this artist depiction, may contain much of the “missing mass” in the Universe.

WIMPS AND MACHOS WIMPs are Weakly Interacting Massive Particles, with exotic names such as axions, massive neutrinos, and photinos. Theoretically, they could have been created during the Big Bang, but none have ever been detected. Experiments are underway to try to detect WIMPs: Fermilab photo: http://www.fnal.gov/pub/presspass/vismedia/gallery/buildings.html Permission required from siri@fnal.gov CDMS Photo: http://cdms.berkeley.edu/public_pics/DSCN0145.JPG CDMS Homepage (with links to pictures): http://cdms.berkeley.edu/ The Cryogenic Dark Matter Search Experiment hopes to detect WIMPs. The experiment is located in the Minnesota’s Soudan Mine, which is now used as a deep underground Science Laboratory. Particle Accelerators, such as this one at Fermilab near Chicago, are being used to discover the existence of WIMPs.

The Dark Energy Mystery Dark Energy makes up about 70% of the Universe. This energy is causing the Universe to expand at an increasing rate of acceleration, pulling galaxies apart from one another. The expansion rate increased significantly about 7.5 Billion Years Ago. The nature of Dark Energy remains one of the biggest scientific mysteries! Image courtesy of NASA/STScI/Ann Feild http://www.nasa.gov/missions/deepspace/f_dark-energy.html Credit: NASA/STScI/Ann Feild

The Dark Energy Mystery This Diagram shows the possible fates of the Universe, depending on whether Dark Energy remains stable, or becomes unstable. In the Big Crunch, the Universe would compress, and possibly implode. In the Big Rip, stars, planets and even atoms would expand violently, and become “unglued”. Luckily, we don’t have to worry about this for about 30 Billion Years! Image courtesy of NASA/STScI/Ann Feild http://www.nasa.gov/missions/deepspace/f_dark-energy.html Credit: NASA/STScI/Ann Feild

The Dark Energy Mystery While the Search for Dark Matter is taking place in the physics laboratory, the mystery of Dark Energy will most likely be solved using direct observation with current and future observatories. These are some of the observatories currently searching for Dark Energy: CHANDRA X-Ray Observatory Credit: NASA/CXC/SAO Hubble Image: http://hubblesite.org/gallery/spacecraft/06/ Chandra Image: http://chandra.harvard.edu/resources/illustrations/craftIllustrations.html WMAP image: http://map.gsfc.nasa.gov/m_ig.html Hubble Space Telescope Credit: NASA WMAP Microwave Anisotropy Probe Credit: NASA/WMAP Science Team