Dark Matter, Dark Energy, and Dark Gravity Alan Heavens Imperial College London ! Friends of Imperial College 28 January 2014
Space… “Space is big. You just won't believe how vastly, hugely, mind- bogglingly big it is. I mean, you may think it's a long way down the road to the chemist's, but that's just peanuts to space.” - Douglas Adams NASA/STScI Photo: Jill Furmanovsky
Birth of modern cosmology 100,000,000,000 stars Credit: ESO/S.Brunier
Doppler shift Credit: San Jose State University Credit: NASA/Goddard Credit: R. O’Connell
Credit: Lowell Observatory Vesto Slipher
Hubble’s law Credit: CalTech Speed ∝ distance v = Hr From: Studyblue
Big Bang
Will it always expand? Gravity should slow it down If it is dense enough it will stop Measured by Ω Ω =1 is equivalent to Ω >1 Ω <1 a few atoms per cubic metre
Ordinary matter Credit: Pearson Education £5.60. Credit: N. Wright Credit: Amazon Ω normal matter = 0 . 04 NOT ENOUGH!
Dark Matter Coma cluster Fritz Zwicky (1937) Credit: NASA/JPL
Vera Rubin Emilio Segre Visual Archives/AIP/SPL drawception.com Ω matter ' 0 . 27 Credit: Mark Whittle, University of Virginia
Bullet cluster Dark Matter (Lensing) Galaxies Markevitch et al 2002 Clowe et al 2004 Hot Gas (X-ray) What is the dark matter?
Light from the Big Bang Arno Penzias and Robert Wilson Credit: Pearson Education
“Cosmic Microwave Background (CMB)” Credit: CalTech
Spectrum of the CMB Credit: NASA Universe must have been very dense
Geometry of the Universe Planck satellite Credit: ESA and Planck team, & J. Cramer for the sounds
Universe is ‘flat’ Ω total = 1 Einstein: geometry and mass content are related What is the rest? This is referred to as DARK ENERGY
Supernovae
Type 1a Supernova Don Dixon
New one! Discovered by UCL undergraduates and supervisor These are ‘standard candles’ astronomygcse.com
Measuring the expansion history Non-decelerating Universe Fainter Decelerating Universe Further back in time Distance Should appear brighter Brighter Speed
They appear fainter… ! Universe is ACCELERATING! Credit: High-z SN team
Nobel Prize for Physics 2011 For the discovery of the acceleration of the Universe Saul Perlmutter Brian Schmidt Adam Riess
How to get acceleration Modify gravity - need something to ‘push’ Einstein’s cosmological constant “The biggest blunder of my life”
General Relativity Credit: wikipedia Matter tells space-time how to curve Space-time tells matter how to move
Possibilities 1 and 2 G µ ν = T µ ν curvature : matter G µ ν − Λ g µ ν = T µ ν Possibility 1: Einstein was right: the cosmological constant exists G µ ν = T µ ν + Λ g µ ν G µ ν = T µ ν + T new µ ν Possibility 2: the vacuum has energy and other strange properties (negative pressure, repulsive gravity)
Vacuum energy Particle-antiparticle pairs 10 120 times too much energy If there is a vacuum energy, we have no idea why it has the Tim Jones energy it has
Dark Energy Possibility 3: some new field or form of matter might be driving the acceleration ‘Quintessence’ or ‘Dark Energy’ Expansion history may be a little different
Exotic possibilities Einstein Gravity works fine on small scales
Possibility 4: Extra dimensions Universe may have more than 3+1 (space+time) dimensions Gravity ‘leaks out’ into the bulk Gravity in our 3+1D universe is modified Credit: DAMTP Braneworlds
How can we tell? If the gravity law is changed: ! - it changes the way the Universe expands - it changes the rate at which structures grow - it changes how light gets bent by matter
Weak gravitational lensing Credit: S. Colombi
Credit NASA/STScI
Testing gravity with lensing and galaxy surveys Growth rate agrees with Einstein Light bending agrees with Einstein Simpson et al 2013
So far, it seems Einstein may have been right Λ
A thorough test: EPFL Launch 2020
Credit: George Dvorsky
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