Dark Matter and Dark Energy


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Dark Matter and Dark Energy

  1. 1. Dark Matter& Dark Energy Dr. Bryan J. Higgs 28 November, 2012
  2. 2. Dark Matter & Dark Energy Over the past 35 years or so, cosmologists’ and physicists understanding of the universe has been turned on its head. It is now generally accepted in the scientific community that ‘normal matter’ — the matter that we experience in our everyday lives, and that scientists have been studying since the time of the ancient Greeks — comprises only about 4% of the matter in the universe. So, what is the other 96%?28 Nov 2012 2
  3. 3. Goals of the Talk » To describe some of the evidence (and history) for why scientists believe that Dark Matter and Dark Energy exist. » To describe what scientists have proposed to explain these observations. » To describe the implications for the beginning and the end of the universe.28 Nov 2012 3
  4. 4. History & BackgroundFirst, we need to look at some backgroundon the history and observations that led usto this point. Cast your minds back about 100 years... (Yup, buggy whip time!)28 Nov 2012 4
  5. 5. Einsteins Theory of General Relativity In 1916, Albert Einstein published his Theory of General Relativity. It provided a unified description of gravity as a geometric property of space and time.28 Nov 2012 5
  6. 6. Assumptions After the introduction of General Relativity a number of scientists, including Einstein, tried to apply the new theory to the universe as a whole. This required an assumption about how the matter in the universe was distributed. The simplest assumption to make is that if you viewed the contents of the universe with “sufficiently poor vision”, it would appear roughly the same everywhere and in every direction.28 Nov 2012 6
  7. 7. The Cosmological Principle That is, they assumed that the matter in the universe is: – Homogeneous and – Isotropic when averaged over very large scales. This is called the Cosmological Principle.28 Nov 2012 7
  8. 8. The Static Universe Model One hundred years ago, astronomers thought: The universe was unchanging through time. The stars of our galaxy (the Milky Way) made up the whole universe The galaxy was nearly motionless Physicists trying to create a model for the universe had to match these "facts".28 Nov 2012 8
  9. 9. Einsteins Theory of Gravity Einstein created his model of the universe, based on his General Theory of Relativity, using these assumptions. He came up with his famous Field Equations.28 Nov 2012 9
  10. 10. Einsteins Field Equations The Einstein Field Equations are a set of 10 equations in Albert Einsteins general theory of relativity which describe the fundamental interaction of gravitation as a result of space- time being curved by matter and energy. The expression on the left of the = sign representsNote, in particular, the the curvature of space-time.second term on the left, The expression on the right of the = sign representsthe one that includes the the matter/energy content of space-time.famous Λ (Greek capitalletter lambda)...28 Nov 2012 10
  11. 11. The Cosmological Constant Λ is the famous Cosmological Constant. It is equivalent to an energy density in otherwise empty space (the vacuum). It was originally proposed by Einstein as a modification of his original theory to achieve a stationary universe, to match what he thought was the known situation.28 Nov 2012 11
  12. 12. The Fate of the Universe There are many possible solutions to Einsteins Field Equations, and each solution implies a possible ultimate fate of the universe. Alexander Friedman proposed a number of such solutions in 1922, as did the Belgian Jesuit priest Georges Lemaître in 1927.28 Nov 2012 12
  13. 13. Fate depends on DensityEssentially, the various models of the evolution of theuniverse depend on whether or not there is enoughmass in the universe to cause it, through gravitationalattraction, to contract unto itself (the “Big Crunch”). So how much mass is there in the universe? How do we weigh the universe?28 Nov 2012 13
  14. 14. The Density Parameter The density parameter, Ω, is defined as the ratio of the actual (i.e. observed) mass density, ρ , of the universe to the critical density, ρcrit , of the universe. To date, the critical density is estimated to be approximately five atoms (of hydrogen) per cubic meter. Not so much! So whats the significance of the critical density?28 Nov 2012 14
  15. 15. The Shape of the Universe The Friedmann–Lemaître–Robertson –Walker (FLRW) model has become the most accepted theoretical model of the universe. It is sometimes called the Standard Model of modern cosmology. This model describes a curvature (often referred to as geometry) of the space- time of the universe. The curvature of space depends on the value of Ω, the density parameter.28 Nov 2012 15
  16. 16. Closed Universe If Ω > 1 (i.e., the density is above the critical density), the geometry of space is closed like the surface of a sphere. In a closed universe, gravity eventually stops the expansion of the universe, after which it starts to contract until all matter in the universe collapses to a point, a final singularity termed the "Big Crunch" – maybe!28 Nov 2012 16
  17. 17. Open Universe If Ω < 1 (i.e., the density is below the critical density), the geometry of space is open – negatively curved like the surface of a saddle. An open universe expands forever, with gravity barely slowing the rate of expansion. The ultimate fate of an open universe is universal heat death, the "Big Freeze".28 Nov 2012 17
  18. 18. Flat Universe If Ω = 1 (i.e., the density is equal to the critical density), the geometry of space is flat – like a plane surface. A flat universe expands forever but at a continually decelerating rate. The ultimate fate of the universe is the same as an open universe – a “Big Freeze”. (Note that we are talking about space-time, so the shapes at left are merely analogies in lower dimensions.)28 Nov 2012 18
  19. 19. A Primeval "Cosmic Egg"? In 1927, Georges Lemaître published a model of the universe suggesting that the universe might have originated when a primeval "cosmic egg" exploded in spectacular fireworks, creating an expanding universe. Published in an obscure journal, it wasnt taken seriously at the time. But now, his contribution is highly valued.28 Nov 2012 19
  20. 20. Discovery of Galactic Redshifts In 1912, Vesto Slipher was the first to observe the shift of spectral lines of galaxies, making him the discoverer of galactic redshifts. Redshifts are analogous to the Doppler effect – think racing cars or trains passing you at speed. An observed redshift due to the Doppler effect occurs whenever a light source moves away from an observer. Conversely, light sources moving towards an observer are blueshifted.28 Nov 2012 20
  21. 21. More on Redshifts You will often see a “z value” quoted as a measure of a redshift. λobsv is the observed wavelength of a spectral line λemit is the emission wavelength of that line If z > 0, there is a redshift If z < 0, there is a blueshift28 Nov 2012 21
  22. 22. Hubbles Discovery In 1928, Edwin Hubble found that the further the distance to a nebula, the greater the receding velocity of that nebula. He used Cepheid variable stars as “standard candles” to estimate their distance, and measured their redshifts to estimate their velocity.28 Nov 2012 22
  23. 23. Galactic Redshifts Here are some examples of how spectral lines are shifted in stars and galaxies.28 Nov 2012 23
  24. 24. Einsteins “Biggest Blunder”? Evidence mounted that the universe was not static, but expanding. This was consistent with the original Einstein model; Einstein could have predicted it, but had assumed the static universe was a given. Einstein later remarked that the introduction of the cosmological constant was the biggest blunder of his life. But was it? Wait a little while...28 Nov 2012 24
  25. 25. “Big Bang” or Steady State? There were two primary explanations put forth for the expansion of the universe: » Lemaîtres “Big Bang” theory, advocated and developed by George Gamow. » A Steady State model, proposed in 1948 by Hermann Bondi, Thomas Gold, and Fred Hoyle, in which new matter would be created as the galaxies moved away from each other. In this model, the universe is roughly the same at any point in time.28 Nov 2012 25
  26. 26. Zwickys Discovery In 1933, Bulgarian-born Swiss physicist Fritz Zwicky, while investigating the Coma cluster of galaxies, stumbled upon a major discrepancy between theory and observation.28 Nov 2012 26
  27. 27. “Missing Mass?” By studying the rotation of a galaxies within the Coma Cluster, Zwicky estimated that the visible mass of those galaxies was 400 times less than the mass needed to explain their rotational motion. But Zwicky, while ahead of his time, was a pugnacious character, disliked by many of his colleagues, so his ideas were often not taken seriously.28 Nov 2012 27
  28. 28. Vera Rubins Discovery In the late 1960s and early 1970s, Vera Rubin measured the velocities at which galaxies rotate, using a telescope at the Kitt Peak Observatory in Arizona, She used a sensitive spectrometer to determine the spectrum of light coming from the stars in different parts of spiral galaxies. She discovered something unexpected: The stars far from the centers of galaxies, in the sparsely populated outer regions, were moving just as fast as those closer to the galaxys center.28 Nov 2012 28
  29. 29. Galactic Rotation View28 Nov 2012 29
  30. 30. Zwicky was Right!This was odd, because the visible mass of a galaxy does nothave enough gravity to hold such rapidly moving stars in orbit.It followed that there had to be a tremendous amount of unseenmatter in the outer regions of galaxies where the visible stars arerelatively few.Rubin and her colleague Kent Ford went on to study some sixtyspiral galaxies and always found the same thing.28 Nov 2012 30
  31. 31. Explanation: “Dark Matter”Rubins observations and calculations showed that mostgalaxies must contain about ten times as much “dark”mass as can be accounted for by the visible stars.Eventually other astronomers began to corroborate herwork and it soon became well-established that mostgalaxies were in fact dominated by "dark matter":28 Nov 2012 31
  32. 32. Why is it called “Dark” Matter? Dark matter cannot be seen directly with telescopes; evidently it neither emits nor absorbs light or other electromagnetic radiation at any significant level. Hence “dark” (as opposed to luminous) matter.28 Nov 2012 32
  33. 33. Evidence for Dark Matter A gravitational lens is formed when the light from a very distant, bright source (such as a quasar) is "bent" around a massive object (such as a cluster of galaxies) between the source object and the observer. Studies of many cases of lensing by galaxy clusters show evidence for large amounts of dark matter.28 Nov 2012 33
  34. 34. Gravitational Lensing28 Nov 2012 34
  35. 35. The Bullet Cluster The most direct observational evidence to date for dark matter is in a system known as the Bullet Cluster, a collision between two galaxy clusters. » X-ray observations show that much of the baryonic matter (in the form of gas, or plasma) in the system is concentrated inThe Bullet Cluster: Hubble Space the center of the system.Telescope image with overlays.The total projected mass distribution » However, weak gravitational lensingreconstructed from strong and weak observations of the same system show thatgravitational lensing is shown in much of the mass resides outside of theblue, while the X-ray emitting hotgas observed with the Chandra X- central region of baryonic gas.ray Observatory is shown in red.28 Nov 2012 35
  36. 36. Summary of Evidence Observations of the rotational speed of spiral galaxies The confinement of hot gas in galaxies and clusters of galaxies The random motions of galaxies in clusters The gravitational lensing of background objects, and The observed fluctuations in the cosmic microwave background radiation All require the presence of additional gravity, which can be explained by the existence of dark matter.28 Nov 2012 36
  37. 37. Dark Matter Candidates Dark matter candidates are usually categorized as: Baryonic Composed of baryons, i.e. protons and neutrons and combinations thereof. Non-Baryonic Hot Dark Matter (HDM) Particles that have zero or near-zero mass, and so move relativistically.Cosmological simulations withCold Dark Matter and Warm Cold Dark Matter (CDM)Dark Matter. Halos selected atenvironments which could Particles sufficiently massive that theyrepresent the Milky Way, the move at sub-relativistic velocitiesAndromeda nebula M31 andM33. 28 Nov 2012 37
  38. 38. MACHOs? One potential baryonic form of dark matter is MACHOs (MAssive Compact Halo Objects): A MACHO is a small chunk of normal baryonic matter, far smaller than a star, which drifts through interstellar space unassociated with any solar system. Recent work has suggested that MACHOs are not likely to account for the large amounts of dark matter now known to be present in the universeRAMBOs (Robust Associationof Massive Baryonic Objects)have also been postulated.These are dark clusters of browndwarfs or white dwarfs.28 Nov 2012 38
  39. 39. Brown Dwarfs? Stars with below 8% of the Suns mass are called brown dwarfs. They are not hot enough to ignite the nuclear burning that keeps ordinary stars shining. Other candidates for dark matter include:  Cold "planets" moving through interstellar space, unattached to any star, could exist in vast numbers without being detected  So could comet-like lumps of frozen hydrogen  So could black holes.28 Nov 2012 39
  40. 40. WIMPs? One potential non-baryonic form of dark matter is WIMPs (Weakly Interacting Massive Particles) The main theoretical characteristics of a WIMP are: Interaction only through the weak nuclear force and gravity Large mass compared to standard particles28 Nov 2012 40
  41. 41. Axions? There are strong reasons for suspecting that dark matter isnt made of ordinary atoms at all. This argument is based on an isotope of hydrogen, deuterium (1 proton + 1 neutron). It turns out that if dark matter were made from ordinary atoms, then theory predicts that there should be much less deuterium in the Universe than we actually observe. So, dark matter could consist of some form of exotic particle. One possibility is the Axion, a hypothetical particle whose existence would explain what is otherwise a puzzling feature of quantum chromodynamics (QCD), the leading theory of strong interactions.28 Nov 2012 41
  42. 42. Neutrinos? Another particle has been regarded as a candidate for dark matter: the elusive neutrino. Neutrinos have no electric charge, and hardly interact at all with ordinary atoms: almost all neutrinos that hit the Earth go straight through it. Because neutrinos so greatly outnumber atoms, they could make up the dominant dark matter, even if each weighed only a hundred millionth as much as an atom.The best evidence for neutrinomasses comes from the Super- Experiments imply a non-zero mass for theKamiokande experiment in Japan, neutrino, but one that is too small to account forwhich used a huge tank in a former much of the dark matter.zinc mine. 28 Nov 2012 42
  43. 43. SuperPartner Particles? Supersymmetry arises naturally from the combination of the two cornerstones of 20th century physics: quantum mechanics and relativity. It is the unique symmetry that relates the two fundamental kinds of particles: Bosons, which act as the carriers of forcesIf Supersymmetry is realized in nature, every fermion inthe SM must have a bosonic partner particle and viceversa. Fermions, which act as the constituents of matterNo such “superpartner particle” has been observed sofar, and recent LHC experiments have cast doubt on thetheory.28 Nov 2012 43
  44. 44. MOND? In 1983, Mordehai Milgrom, a physicist (another Bulgarian-born!) at the Weizmann Institute in Israel, proposed Modified Newtonian dynamics (MOND), a modification of Newtons law of gravity, to explain the galaxy rotation problem. While MOND provides an explanation for the observed galactic rotations, and has been extensively examined by many others, it does not appear to be consistent with other observations.28 Nov 2012 44
  45. 45. So What is Dark Matter? So, dark matter could be composed of any number of particles, both known and exotic: MACHOs WIMPs Massless neutrinos Axions Neutralinos Photinos Or who knows what else?28 Nov 2012 45
  46. 46. How Fast is the Universe Decelerating? We have known since Hubble that the universe is expanding. We know that gravity should cause this expansion to slow down, depending on how much matter is present in the universe. If we measure this deceleration, we could determine the fate of the universe.28 Nov 2012 46
  47. 47. Type Ia Supernovas To do this, we need to find a set of standard candles which can be used to determine the distance to extremely remote objects. It turns out that one class of supernovae, Type Ia supernovae, can be used as standard candles. A supernova results from the violentMulti-wavelength X-ray /infrared image of SN 1572 or explosion of a white dwarf star.Tychos Nova, the remnant of aType Ia supernova28 Nov 2012 47
  48. 48. Type Ia Supernova Creation28 Nov 2012 48
  49. 49. Two Supernova Teams In 1998/9, published observations of Type Ia supernovae by The High-z Supernova Search Team The Supernova Cosmology Project suggested that the expansion of the universe is actually accelerating – aBrian Schmidt,Saul Perlmutter, total surprise to everyone.& Adam Riess. The 2011 Nobel Prize in Physics was awarded for this work. 28 Nov 2012 49
  50. 50. A New Paradigm of the Universe So, it seems from all the evidence that the universes evolution doesnt fit the original models!28 Nov 2012 50
  51. 51. Corroboration of Results Since then, these observations have been corroborated by several independent sources: Cosmic microwave background radiation Gravitational lensing Large scale structure of the cosmos Improved measurements of supernovae28 Nov 2012 51
  52. 52. Dark Energy Evidence for Dark Matter and Dark Energy has accumulated, and it is now estimated that only about 4% of the matter/energy in the universe is ordinary matter. In other words, we have no real clue what the other 96% consists of! This is most embarrassing!28 Nov 2012 52
  53. 53. So, what is Dark Energy? Candidates for Dark Energy include: Einsteins cosmological constant – dark energy is a property of space itself. An unidentified energy field, called “quintessence” – fills space like a fog and is similar to what drove inflation None of the above – perhaps its an illusion created by incorrect theories.28 Nov 2012 53
  54. 54. Cosmological Constant? Remember that Einstein thought it was his biggest blunder? The Cosmological Constant has returned, and is the leading candidate for a Dark Energy explanation. Maybe Einstein didnt blunder?28 Nov 2012 54
  55. 55. A Slight Problem... The Cosmological Constant being nonzero means that the vacuum can contain energy! However, when physicists calculate the vacuum energy using our best theory, the Standard Model, they come up with an estimate that is 120 orders of magnitude (10120) too large! This is even more embarrassing!28 Nov 2012 55
  56. 56. Quintessence The name Quintessence (“fifth essence”) dates back to the Ancient Greeks (Earth, Water, Fire, Air and...) It has been proposed by some to be a fifth fundamental force. The main difference between quintessence and the cosmological constant is that quintessence can vary with space and time.28 Nov 2012 56
  57. 57. Implications Cosmologists estimate that the acceleration began roughly 5 billion years ago. Before that, it is thought that the expansion was decelerating, due to the attractive influence of dark matter and baryons. The density of dark matter in an expanding universe decreases more quickly than dark energy, and eventually the dark energy dominates.28 Nov 2012 57
  58. 58. The “Big Rip” The Big Rip is a cosmological hypothesis first published in 2003, about the ultimate fate of the universe, based on phantom energy, an extreme form of quintessence. It predicts that the matter of the universe will progressively be torn apart by the expansion of the universe at a certain time in the future.28 Nov 2012 58
  59. 59. “The End of the Universe is Nigh!” Dont worry! It wont happen for billions of years. We all have more immediate worries!28 Nov 2012 59
  60. 60. SummaryWe have significant evidence for large quantitiesof something in the universe we call:Dark MatterandDark EnergyAnd we dont really know what either of them are!28 Nov 2012 60