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Modern Cosmology

Modern Cosmology. Prof Geraint Lewis Sydney Institute for Astronomy University of Sydney. Historically, “cosmology” was the realm of philosophers. Isaac Newton. First mathematical laws of gravity and motion. Thought that an isolated group of stars would collapse in on itself.

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Modern Cosmology

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  1. Modern Cosmology • Prof Geraint Lewis • Sydney Institute for Astronomy • University of Sydney

  2. Historically, “cosmology” was the realm of philosophers.

  3. Isaac Newton • First mathematical laws of gravity and motion. • Thought that an isolated group of stars would collapse in on itself. • An infinite universe of stars should collapse into isolated islands of mass. • A finely tuned universe could be balanced and static.

  4. Albert Einstein • General Relativity: viewing gravity as curved space time (1915). • “Cosmological considerations on the general theory of relativity” (1917). • Einstein thought the universe was static and unchanging, although his equations were dynamic. • Added a cosmological constant term which acts as an repulsive force, balancing gravity.

  5. Alexander Friedmann • Friedmann wrote “On the curvature of space” in 1922. • He came to the conclusion that Einstein’s cosmological equations predicted that the universe evolved with time, either expanding or collapsing. • Einstein wrote that Friedmann had made a mathematical error and his results were invalid. • In 1923, Einstein retracted his objection and agreed relativistic universe was dynamic.

  6. Einstein’s Biggest Blunder? • After Friedmann’s work, Einstein threw away his Cosmological Constant, calling it his biggest blunder. • There is a persistent myth that Einstein fudged the equations of relativity, adding anti-gravity to make a static universe. • However, this is not correct. The addition of a cosmological constant term was a completely legitimate mathematical exercise. • Einstein’s blunder was choosing a specific value for the cosmological constant to balance gravity, not its addition. It was not discarded, just set to zero.

  7. In the 1920s, Hubble measured the speeds of nearby galaxies. He found nearly all were rushing away from us, with their velocity increasing with distance, exactly as predicted in the relativistic model of the universe. Edwin Hubble

  8. The search for Hubble’s Constant, the rate of the expansion of the universe, has dominated astronomy since Hubble’s day. Velocities are easy to measure, distances are hard. The issue was only resolved in the last decade with use of the Hubble Space Telescope. Modern Measurement

  9. A good way to understand expansion is with a “conformal diagram”. It simply has us and all other objects in the universe as a series of straight lines. Understanding Expansion

  10. Friedmann’s equations give us the “Scale Factor” and the distances as a function of time are; Understanding Expansion

  11. Notice how as we go back in time, R(t) goes to zero. This means the distance between any two objects also goes to zero. This is the location of the “Big Bang”. Understanding Expansion

  12. The shape of the scale factor depends upon the mix of energies (matter, radiation, other stuff) in the universe. Universes only containing matter slow down over time, while other universes slow and then accelerate. Which is our universe? Which Scale Factor? www.astro.ucla.edu/~wright/intro.html

  13. On conformal diagrams, light rays travel at 45 degrees and it’s simple to see that light we receive now set out from distant objects long ago. Which Scale Factor?

  14. The velocity of an object (its redshift) tells us the scale factor at the time the light set out, while the brightness of an object tells us how far the light has travelled. Which Scale Factor?

  15. Supernovae are exploding stars whose true brightness is well known. Using the Keck and Hubble Space Telescope, ten years ago we were able to do this experiment. Cosmological Supernovae

  16. Which Universe? • The supernovae appeared fainter than expected, showing that the universe does not contain only matter. • A third of the cosmos is matter, the most of which is dark (does not radiate, but we can feel its gravitational pull). • Heavy elements (that’s us!) make up 0.03% of the universe. • Some mysterious substance, dark energy, make up 70% of the universe. www.lsst.org

  17. Interlude: Dark Matter • Everywhere we look in the cosmos we see the gravitational influence of dark matter, from the rotation curves of galaxies, large scale motions, gravitational lenses, hot gas in clusters to the evolution of the entire universe. • Is it a physical substance or physical fudge?

  18. Interlude: Dark Matter • This is the Bullet Cluster (actually a colliding pair of clusters). The galaxies are obvious, while the pink is the hot gas in the clusters. The blue is the dark matter, imaged with gravitational lensing. The majority of the mass is in the dark matter component.

  19. Interlude: Dark Matter • It has been proposed that dark matter can decay into normal matter, with an equal mix of matter and anti-matter. The PAMELA (http://pamela.roma2.infn.it) space-craft has been searching for the signal of this additional flux of anti-matter.

  20. Interlude: Dark Matter • There was a big fuss in 2008 where it was announced that PAMELA had found excess positrons (the anti-particle of the electron) and this may be the signature of decaying dark matter. However, the anti-proton signal does not agree.

  21. The mix of matter and energy imply that the expansion of the universe is beginning to accelerate, and in the future, the universe will dilute and dim. The Future of the Universe

  22. In the future, there will be no cosmology as there will be nothing to see out there. The Future of the Universe

  23. What is dark energy? • Well, we know what it isn’t. It isn’t dark or normal matter. • It has to possess a negative pressure (a tension) to cause the universe to accelerate. • With quantum physics, the vacuum is not empty but seethes with particles popping in and out of existence. Such a vacuum possesses precisely the tension of dark energy • Only problem is that the density is wrong by a factor of

  24. What now? • Particle physics is trying to understand what dark energy is (although they still haven’t sorted out dark matter). • Observations reveal that dark energy has the same properties as Einstein’s cosmological constant. • However, the properties could have changed with time and the goal of several proposed telescopes will be to uncover any such change. • Any change will have a big impact on our understanding. Proposed Thirty Metre Telescope

  25. Georges Lemaître • Friedmann died soon after he published his work. Georges Lemaitre examined the equations of cosmology, especially the point where the scale factor goes to zero. • Running the universe backwards, he realized that it must have been hotter in the past. • He proposed the “Hot Big Bang” model of the universe, where the universe was born in a hot, dense state and has been cooling and expanding ever since.

  26. particleadventure.org/frameless/chart_cutouts/universe_original.jpgparticleadventure.org/frameless/chart_cutouts/universe_original.jpg

  27. Large Hadron Collider CERN, on the Swiss-French Border, will recreate the conditions when the universe was 0.0000000001 seconds old.

  28. particleadventure.org/frameless/chart_cutouts/universe_original.jpgparticleadventure.org/frameless/chart_cutouts/universe_original.jpg

  29. Cosmic Microwave Background Penzias & Wilson won the 1978 Nobel Prize for detecting the cosmic microwave background radiation. Mather & Smoot won the 2006 Nobel prize for showing this radiation has a blackbody spectrum (2.7K) and for revealing that it is not smoothly distributed over the sky.

  30. Cosmic Microwave Background While the mean temperature of the sky is 2.7K, some regions are hotter and some cooler, with a temperature difference of 0.001K. Where did these temperature differences come from?

  31. At the time of reionization (when the universe became neutral) there were regions of slightly higher and slightly lower density. Where did these come from? CMB Hot & Cool Spots

  32. The quantum fluctuations we met earlier were stretched out during a period of rapid expansion know as “Inflation”, with the pattern we see matching theoretical expectations. These slight over densities became the seeds of stars and galaxies, including our own Milky Way galaxy. Inflation & the Quantum

  33. It is too difficult to follow the complex evolution of matter on a piece of paper. This is especially true for gas which can collapse and form stars, which can then explode. A large number of astronomers now build their own universes within computers. Universe in a Computer Swinburne Green Machine

  34. Mapping out the locations of galaxies show that they sit on just the kind of foamy structure we expect from the lumps in the Cosmic Microwave Background. This directly connects the universe today to its very birth. 2dF Galaxy Redshift Survey

  35. Given our mix of dark energy and matter, the universe is infinite in extent, but we can only see the “observable universe”. As time proceeds, more and more is revealed. How Big is the Universe?

  36. The truth is we just don’t know. We have a major problem in physics, namely our best and most accurate theories (quantum mechanics and general relativity) just do not work together; we don’t have a theory of “quantum gravity”. Without this, we cannot work out what happened before inflation. Where did it come from? Bohr & Einstein

  37. Possibilities? Reproducing Universe: Linde Colliding Branes: Turok & Stienhardt Perhaps one of your students will work this out. The End

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