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Dark Matter, Dark Energy and the Cosmology Revolution!

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Dark Matter, Dark Energy and the Cosmology Revolution! ! Dark Matter and the Universe Topic 1 Dark Matter, Dark Energy and the Cosmology Revolution! What is the real structure of our Universe and how do we know it is so? ! Contents of Topic 1 This first Topic provides an introduction to the Dark Matter Problem, putting it in the context of current understanding of the composition of the Universe and covering some of the essential Cosmology and tools needed to study it. ‣ What do we mean by Dark Matter and why bother? ‣ Composition of the Universe & the Cosmology Revolution ‣ What could the Dark Matter be? ‣ Cosmology and tools for Dark Matter? ‣ The Cosmological Principle, Isotropy and Homogeneity ‣ The Big Bang ‣ Hubble’s Law and Redshift ‣ Critical Density ρc and the Density Parameter Ω0 ‣ The geometry of space and Inflation What! is Dark Matter and Why Bother?! ‣ The name Dark Matter attests to the fact it doesn’t give off light. Yet it provides the gravitational pull to hold together the galaxies and clusters we see in the Universe today.! ‣ We see the effects of Dark Matter everywhere – it bends light, it makes gas move at great velocity, without it galaxies would fly apart.! ‣ It must be something other than bright stars, but what?! !‣ So we should bother very much about Dark Matter! . It is a dominant material in our Universe and yet we do not know what it is made of. It most likely will lead us to completely new physics. ‣ The term Dark Matter simply means material that is Non-Luminous. The! Composition of the Universe ‣ In more detail current results give us the Mass-Energy Composition of the Universe approximately as follows: ! ! ‣ Note again! the small fraction of visible atoms! ‣ Note how Dark Matter and Dark Energy dominate! The! Composition of the Universe ‣ The little bit of the matter in the Universe we really know about can be broken down as follows: ! ! ! ‣ Interesting fact: Neutrinos contribute about the same mass- energy content to the Universe as do all the stars. The! Composition of the Universe ‣ In more detail if we look at individual galaxies we see how Dark Matter dominates galaxies but Dark Energy has the biggest influence in the Universe as a whole. ! ! Observable! Universe Galaxies Ordinary Matter Ordinary Matter Dark Matter Dark Matter Dark Energy Dark Energy The Composition of the Universe ! ‣ These results are relatively recent - we are living through a period of cosmology and particle physics revolution.! ‣ For the first time in history we have a good picture of the Universe - called the Inflationary Big Bang Model or the Standard Model of Cosmology.! ‣ We also have a good picture of the fundamental particles and forces - the Standard Model of Particle Physics.! ‣ The merging of these areas of science, Astrophysics, Cosmology and Particle Physics, are giving us new and powerful insights, forged around the new disciplines of Particle Astrophysics and Particle Cosmology. ! ‣ However, big questions remain…:! ! The Cosmology Revolution! ‣ So we have two important models but there are gaps. Big Questions in Remain! ‣ A few of these questions are as follows: 1. How did the structure of the Universe we observe form?! 2. What is the Dark Matter?! 3. Could there be more than one kind of Dark Matter?! 4. Why is the Dark Matter distributed the way it is?! 5. Could the Dark Matter be new fundamental particles?! 6. What is the Dark Energy?! ! ! What! does the Term Dark Matter Mean? ! ‣ “ Dark Matter ” means any Non-Luminous material that shows its presence by its gravitational influence.! ‣ Here’s an old example:! ! ! Calculate the Earth’s Mass two ways! ! typical ‣ Earth’s circumference - 40,075 km! rock ‣ Typical rock density - 2.8 g/cm3! ‣ Use this to estimate the mass - ANS (1)……… ‣ Newton says F = GmM/r2 = ma! gravity ‣ g = 9.8 m/sec2, G = 6.67 x 10-11m3/(kg sec2)! ‣ Use this to estimate the mass - ANS (2)……….. ! Dark Matter in the Universe ! In the previous calculation we get a factor ~x2 discrepancy ‣ due to the gravitational effect of the hidden high density “dark matter” core of the Earth. But how do we know there is hidden matter throughout the Universe? ! ‣ Later Topics will cover many of the techniques used by Cosmologists, here is a summary: 1. Observation of Rotational Curves of Galaxies! 2. Observation of Gravitational Lensing! 3. Observation of Hot Gas in Galaxy Clusters ! 4. Computer simulations of Cluster Evolution! 5. Analysis of Large Scale Structure ! 6. Analysis of the Cosmic Microwave Background ! ‣ All these show us that Dark Matter pervades the Universe Dark Matter and Structure Formation ! ! ‣ A particularly interesting result comes from computer simulations which model Structure Formation in the Universe. It is impossible to get a match to the observed structure, with voids and clumps, without accounting for Dark Matter.! ‣ This example simulation is from the EAGLE group. So What Could the Dark Matter be? ! When considering Dark Matter it turns out we can think two ‣ ! broad classifications: Baryonic Dark Matter (normal matter but non-luminous) ! ! Some examples to be covered in later Topics are:! (1) Dim Stars (<0.1M⊙)! (2) Hydrogen Ice! (3) Black Holes! ! Non-Baryonic Dark Matter (new fundamental particles) ! ! Some examples to be covered in later Topics are:! (1) Weakly Interacting Massive Particles (WIMPs)! (2) Axion Particles! (3) Other exotic particles! What is Dark Matter Most Likely to be? ! ‣ It turns out that it’s hard to avoid the conclusion that Dark Matter is weakly interacting Non-Baryonic “cold” particles, fundamentally new physics in action in the Universe. ‣ An amazing bit of new evidence for this is from observation of the Bullet Cluster - two galaxy clusters merging: Blue - Dark Matter inferred Bullet by gravitational lensing Cluster Red - normal baryonic matter traced by the x-ray emission from hot gas ‣ We will return to the Bullet Cluster in later Topics ‣ The blue is DM reconstructed from Gravitational Lensing. The red is normal matter. The DM has carried on moving through the merger, the baryonic matter has slowed down. The Universe in Summary so Far ! ‣ So we now know the Cosmic Recipe. Mostly the Universe is invisible “Dark Matter” (~25%) and “Dark Energy” (~70%). The remainder is normal atoms (baryons) that are also hidden (~4%), plus the small amount of visible material such as the stars (~0.5%), H and He. The Earth and inhabitants are made of the rarest stuff of all: heavy elements (~0.01%). ‣ The best idea for the Dark Matter is that it is so-called Cold Dark Matter (CDM) made of non-baryonic particles. Cold means slow moving, non-relativistically. Theories based on this can account for all the large scale features we see including the heat radiation of the Big Bang and the large scale distribution of galaxies. ‣ But we still don’t really know what the Dark Matter and Dark Energy are. There is lots of work to do. Cosmology and Tools for Dark Matter ! ‣ To continue our study we will need some basic knowledge of cosmology and to be familiar with relevant tools and parameters. Only then can start to make progress in understanding Dark Matter. We need to know about: The Cosmological Principle! The Big Bang Concept and Observational Evidence for it! Hubble’s Constant and Red Shift ! Angles and Distances in Astronomy ! The Critical Density Parameter! The Geometry of Space and Cosmic Inflation The Cosmological Principle ‣ The average density of galaxies is fixed throughout the Universe and does not change with distance or direction. ! ! ‣ So an observer anywhere !in the Universe sees approximately the same thing. No place is special, and there is no edge or centre. ‣ We know the Universe is expanding and since the expansion occurs evenly at every point in space, galaxies are separating from each other at about the same pace, giving the Universe a nearly uniform density and structure. Thus the Universe appears smooth at large distance scales. A Homogeneous and Isotropic Universe ‣ Thus the Universe is said to be homogeneous & isotropic: - Uniform with respect to position (homogeneous), ! - Uniform with respect to viewing angle (isotropic). ‣ In the left image we have an Isotropic Universe - i.e. if you stand at the centre and look in every direction, the Universe will look the same. On the right is a Homogeneous Universe. This means that if you stand in any one place and look around, the Universe looks the same. The Big Bang ‣ The Universe is observed to be expanding with all galaxies receding from each other due to the expansion of space itself. The recession is not due to their own “peculiar” motion. Imagine what happens to the dots on a balloon being blown up - a 2 dimensional analog of the 3D Universe. ‣ If the Universe is expanding now, it's logical to assume it was smaller before - i.e. that all galaxies and stars started from a hot, high density plasma, the Hot Big Bang comprising a soup of fundamental particles and energy. ! ‣ The term Big Bang is a very bad name as there is no explosion into space. It is space itself that is expanding! !Big Bang Observational Evidence ‣ The! two main bits of observational evidence for the Big Bang: (1) Galaxies are receding - the Universe is expanding! (2) The cosmic microwave background radiation (CMBR) ‣ The CMBR is a pervasive microwave radiation, the same in all directions, that can only have arisen when the Universe became transparent 300,000 years after the Big Bang. ‣ The CMBR has an almost perfect blackbody form with T = 2.7 K. ‣ Very small variations in the intensity of the CMBR from different directions can tell us a lot about dark matter. We will return to this in later Topics.
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