Donnacha Kirk, UCL! Thu 29th October 2015! UCL ICH Postdoctoral Society! • The Expanding Universe! • Bending of light by gravity.! • Dark Energy Survey and tests of gravity.!
new theory of gravity to model the whole Universe.! • He finds it very difficult to produce a stable static solution ! tends to expand or collapse.! ϵobs i = γI i + ϵint i ϵi = γi + ϵs i Gµν = 8πGTµν Gµν = 8πGTµν + Λ Gµν = 8πGTµν + Λ
new theory of gravity to model the whole Universe.! • He finds it very difficult to produce a stable static solution ! tends to expand or collapse.! • Has to add a new term to balance his equations, the cosmological constant.! ϵobs i = γI i + ϵint i ϵi = γi + ϵs i Gµν = 8πGTµν Gµν = 8πGTµν + Λ Gµν = 8πGTµν + Λ
Hubble discovers the Universe is far from static.! • Galaxies recede from each other, meaning the Universe had a beginning, the primeval atom or Big Bang.!
to be expanding, an obvious question was “How fast?”! • Mass attracts through gravity.! • After the big bang, this attractive force will slow the expansion.! • The future course of the Universe depends on the exact amount of mass.!
matter + general relativity?! No!! • Need some other source of energy to power the acceleration ! !! Dark Energy! • A new type of exotic matter/energy with negative pressure.!
vaguely plausible physical theory and it’s a bit embarrassing.! • QFT calculations of the vacuum energy are 120 orders of magnitude out- probably the worst prediction in all the history of physics ! solve this or back to the drawing board.!
can’t explain, there are two possible solutions:! • Your underlying theory is wrong: the rules of the game.! • You’ve missed some important ingredient: the pieces on the board.!
can’t explain, there are two possible solutions:! • Your underlying theory is wrong: the rules of the game.! • You’ve missed some important ingredient: the pieces on the board.!
can’t explain, there are two possible solutions:! • Your underlying theory is wrong: the rules of the game.! • You’ve missed some important ingredient: the pieces on the board.!
this DE v. MG dilemma.! Errors in calculation of ! Uranus’ orbit! ! Le Verrier’s prediction & discovery of Neptune, ! a new energy- momentum component.! Errors in calculation of precession of Mercury’s orbit ! ! predictions of Vulcan, ! a new planet.! Searches were fruitless and it required Einstein’s GR, a modified gravity theory, to explain the observations.!
matter/energy distort spacetime.! • Ordinary matter + Dark Matter + GR acceleration.! • But we see acceleration!! • Two possible explanations:! 1. There’s some other type of stuff ! Dark Energy! 2. The rules governing distortion are wrong on cosmic scales ! Modified Gravity! Impossible to tell which explanation to favour without more information.!
Einstein revolutionises our concept of gravity.! ! • Space/time no longer static but linked in an evolving spacetime.! ! • Gravity is the curvature of spacetime.! ! • Predicts double the bend- angle of Newton.!
sun? Use an eclipse.! 18 P. Coles Figure 6. Changes in star positions recorded during the eclipse of 1922 and published in Campbell & Trumper (1923). The eclipsed Sun is represented by the circle in the centre of the diagram, surrounded by a representation of the coronal light. Images too close to the corona cannot be used. The recorded displacements of other stars are repre- sented by lines (not to scale) of gravity. The events of 1919 also established Einstein, rightly, as one of t century’s greatest intellects. But it was to do much more than that, propelli him from the rarefied world of theoretical physics into the domain of popu culture. How did this happen? Einstein, and his theory of relativity, had appeared in newspapers befo 1919, mainly in the German-speaking world. He had himself written an arti for Die Vossische Zeitung in 1914. But he had never experienced anything li the press reaction to the announcements at the Royal Society meeting in 191 • Eddington sets out to measure the 1919 eclipse.!
techniques and the recourse t radio sources (which circumvented the dependence on solar eclipses), was it possibl Figure 2. Solar prominence – Principe, 29 May 1919, 2 hours 13 minutes 28 seconds GMT exposure 10 seconds (through cloud) (courtesy of the OAL). 16 Elsa Mota, Paulo Crawford and Ana Simo ˜es
LBNL, NOAO, University of Michigan, University of Pennsylvania, Argonne National Lab, Ohio State University, Santa-Cruz/SLAC/Stanford, Texas A&M Brazil#ConsorCum# UK#ConsorCum:# UCL,#Cambridge,#Edinburgh,# NoIngham,#Portsmouth,#Sussex# Spain#ConsorCum:# CIEMAT,#IEEC,#IFAE# CTIO# LudwigSMaximilians#Universität# LMU# ETH#Zurich# ~300 scientists US support from DOE+NSF
Gravity 9 • No detected deviation from GR but still early days…! • WGL tells us about expansion and growth.! • This allows us to tell the difference between dark energy and ! ! ! ! ! ! ! ! !modified gravity.! Simpson et al. 2013!
Gravity 9 • No detected deviation from GR but still early days…! x15 more accurate…! • WGL tells us about expansion and growth.! • This allows us to tell the difference between dark energy and ! ! ! ! ! ! ! ! !modified gravity.! • DES will give us the first percent-level measurements of gravity on cosmic scales.! Simpson et al. 2013!
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