Was Einstein Wrong? - Speaker Deck

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Was Einstein Wrong?! Testing Gravity with the Dark Energy Survey! 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.!

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COSMOLOGY!

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Cosmology in the early 20th Century! The Universe is:! •  “Small”! •  Static! •  Homogeneous!

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Einstein’s Universe! •  Einstein makes initial attempts to apply his 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

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Einstein’s Universe! •  Einstein makes initial attempts to apply his 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

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Hubble and the Big Bang! •  Soon thereafter, 1931, Edwin 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.!

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Einstein’s “Biggest Blunder”! •  Einstein abandoned his cosmological constant and embraced a dynamic, expanding Universe.! •  He called it his “biggest blunder”, feeling it harmed the elegance of his equations.!

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Attempts to measure deceleration! •  Once the Universe was known 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.!

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Expansion of the Universe! •  Supernovae as Standard Candles.!

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TYPE IA SUPERNOVAE!

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Expansion of the Universe! •  Supernovae as Standard Candles.! •  Measure deceleration of the ! Universe…! !

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Expansion of the Universe! •  Supernovae as Standard Candles.! •  Measure deceleration of the ! Universe…! ! … but ﬁnd accelerating expansion.!

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Accelerated Expansion! •  What can drive the acceleration?! •  Ordinary matter + general relativity?! ! A new type of exotic matter/energy with negative pressure.!

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Accelerated Expansion! •  What can drive the acceleration?! •  Ordinary 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.!

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Dark Energy! •  Today we have lots of energy for a “standard model” of the Universe…! •  … but the resulting picture is quite strange.!

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Problem Solved?! •  QFT calculations predict space contains a vacuum energy due to billions of tiny interactions.! •  This has all the properties that dark energy needs to make the Universe accelerate.!

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Cosmological Constant Problem! •  Not quite- this is our own 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.!

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Observation:! Accelerating Expansion!

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Observation:! Accelerating Expansion! Theory!

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+ Observation:! Accelerating Expansion! Theory! Contents of the Universe!

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+ Observation:! Accelerating Expansion! Theory! Contents of the Universe!

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+ + dark energy ! Observation:! Accelerating Expansion! Theory! Contents of the Universe!

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Theory! Contents of the Universe! + change gravity! Observation:! Accelerating Expansion! + dark energy !

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The Duhem-Quine Thesis! If you have an observation you really 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.!

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Uranus!

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Neptune! Uranus!

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MG for Cosmic Acceleration! •  There are historical precedents for 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.!

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Uranus or Mercury?! Dark Energy or Modiﬁed Gravity?!

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Can we change gravity instead?! We understand ! gravity here…! … and ! probably here…! … but out here?!

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Can we change gravity instead?! We#understand## gravity#here…# …#and## probably#here…# … but out here?! Modify Gravity on large scales!

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Summary: DE v MG! •  Einstein’s GR tells us how 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.!

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Part 2: " Gravitational Lensing!

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Bending Light and Testing Gravity!

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Light bending round the Sun!

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Einstein’s Revolution! •  At the beginning of the 20th Century 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.!

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The Principe Expedition! •  How to observe stars near the 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 represented 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.!

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Principe Expedition! after the 1960s, with Irwin I. Shapiro’s time-delay 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

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Principe Expedition! •  Nice photos, story.

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Principe Expedition: Success! •  Eddington announces success to the Royal Society.! •  Einstein becomes an overnight celebrity.! •  Since then spectacular gravitational lensing has been observed throughout the Universe.!

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Donnacha#Kirk# Diploma#Club,#21st#March#2013# Strong Lensing •  Abell plot.

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Weak Gravitational Lensing! •  We saw strong lensing due to gravity.! •  Every galaxy we observe has been distorted a little.!

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Weak Gravitational Lensing! •  We saw strong lensing due to gravity.! •  Every galaxy we observe has been distorted a little.!

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Weak Gravitational Lensing! •  We saw strong lensing due to gravity.! •  Every galaxy we observe has been distorted a little.! •  Like looking at the world through a frosted window.!

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Typical star used for finding telescope response Typical galaxy used for lensing

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Formidable observational challenge!

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Part 2: " Dark Energy Survey!

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Dark Energy Survey (DES)! •  First light Sept 2012! •  Optics made in UCL.! •  570 megapixel camera for WGL over 5,000deg2.!

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56 Dark Energy Survey Collaboration Fermilab, UIUC/NCSA, University of Chicago, 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

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Donnacha#Kirk# Diploma#Club,#21st#March#2013#

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DECam at CTIO 4

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DES SV image of a deep SN field # ## # #

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1st light: 12 Sept. 2012 5 Fornax cluster

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1st light: 12 Sept. 2012 5 Fornax cluster

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Donnacha#Kirk# Diploma#Club,#21st#March#2013# 1st light: 12 Sept. 2012 6 1 chip Fornax NGC1365 galaxy cluster, image from a single CCD chip!

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Looking at images: invaluable!

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Expansion vs. Growth of Structures! Dark Energy & Modiﬁed Gravity ! can predict the same accelerated expansion…! … but they predict different growth rates as matter collapses due to gravity.!

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Universe was 0.2 Gyr old!

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Universe was 1 Gyr old!

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Universe was 4.7 Gyrs old!

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Today (13.6 Gyr)!

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Results… a work in progress! CFHTLenS: Testing the Laws of 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!

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Results… a work in progress! CFHTLenS: Testing the Laws of 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!