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© Sierra College Astronomy Department Cosmology Dark Matter, Dark Energy, and The Fate of the Universe
© Sierra College Astronomy Department2 Cosmology: Dark Matter, Dark Energy, Fate of the Universe Introduction It is difficult beyond description to conceive that space can have no end; but it is more difficult to conceive an end. It is difficult beyond the power of man to conceive an eternal duration of what we call time; but it is more impossible to conceive a time when there shall be no time. Thomas Paine, The Age of Reason (1796)
© Sierra College Astronomy Department3 Cosmology: Dark Matter, Dark Energy, Fate of the Universe Introduction Cosmology – The study of the overall structure and evolution of the Universe. – Basic Questions How did the Universe begin? What is the structure of the Universe? What is the fate of the Universe? What observational evidence is there to support our cosmological ideas and models?
© Sierra College Astronomy Department4 Cosmology: Dark Matter, Dark Energy, Fate of the Universe Two Important Ingredients The Structure and Fate of the Universe Depend on Two Important Ingredients – Dark Matter : Discovered in galactic halos, it is the dominant form of mass in the Universe, but other than the shape of galactic rotation curves, it has not been detected, and hence identified, by any other means. (Possible causes: MACHOs, black holes, WIMPs, neutrinos) – Dark Energy : An unidentified form of energy that is causing the Universe to increase its rate of expansion with time, contrary to all expectations before its discovery. (Possible causes: vacuum energy, cosmological constant, quintessence)
© Sierra College Astronomy Department5 Cosmology: Dark Matter, Dark Energy, Fate of the Universe Evidence for Dark Matter Evidence for Dark Matter in Galaxies – The Milky Way Galaxy The rotation curve of the Galaxy as determined by 21-cm radiation from clouds and optical lines from stars. Sun’s orbit encompasses about 100 billion solar masses; whereas a distance twice as far encompasses twice the mass. Detailed analysis shows that most of the Galaxy’s mass is in a spherical halo, tens of thousands of light-years in radius, and about 10 times the total mass of the stars in the disk. Very little radiation has been detected from this halo.
© Sierra College Astronomy Department6 Cosmology: Dark Matter, Dark Energy, Fate of the Universe Evidence for Dark Matter Evidence for Dark Matter in Galaxies (continued) – Other Spiral Galaxies Rotation curves determined by measuring 21-cm radiation from atomic hydrogen clouds since this radiation can be detected at large distances from the galactic centers. Rotation curve used to determine a galaxy’s total mass. Total luminosity of galaxy used to determine galaxy’s mass due to stars and this is subtracted from the total mass to obtain the dark matter mass. Typical spiral galaxies are 90% dark matter and 10% stellar matter.
© Sierra College Astronomy Department7 Cosmology: Dark Matter, Dark Energy, Fate of the Universe Evidence for Dark Matter Evidence for Dark Matter in Galaxies (continued) – Elliptical Galaxies Rotation curves are not possible due to random motion of stars and 21-cm radiation analysis not possible due to the lack of gas. The random motion of the stars creates a broadened spectral line and the broader the line the faster the stellar motion. Spectral lines maintain a fairly constant width, and hence the star velocities remain fairly constant as we look at greater distances from the galaxy center. Consequently, ellipticals show evidence of dark matter. Globular clusters orbiting around ellipticals also lend evidence to the dark matter being present.
© Sierra College Astronomy Department8 Cosmology: Dark Matter, Dark Energy, Fate of the Universe Evidence for Dark Matter Evidence for Dark Matter in Galaxy Clusters – Orbits of Galaxies in Clusters The recessional velocities of a cluster of galaxies are found and these velocities are used to determine the cluster center. The radial velocity of each cluster relative to the cluster center is then determined, and through an averaging process, the average orbital velocity of the cluster galaxies is determined. The average orbital velocity of the galaxies then gives the cluster mass and this mass is compared to the cluster luminosity. Cluster luminosities are found to be far too low for the amount of mass present. The amount of dark matter is found to be up to 50 times more than the mass in the stars, a multiplication factor significantly greater than what is found for individual galaxies.
© Sierra College Astronomy Department9 Cosmology: Dark Matter, Dark Energy, Fate of the Universe Evidence for Dark Matter Evidence for Dark Matter in Galaxy Clusters (continued) – Hot Gas in Clusters Gas within a cluster (intercluster medium) has been found from X-ray studies to be very hot (10s to 100s of millions of degrees) and have 7 times as much mass as the stars in the cluster. Since most clusters are is a state of gravitational equilibrium, the temperature of a cluster’s intercluster medium is dependent on the cluster’s total mass. Specifically, the speed v H (in m/s) at which hydrogen nuclei move around the center of a cluster given a gas temperature of T (in Kelvin) is: The amount of dark matter is found to be up to 50 times more than the mass in the stars, the same amount as found with the orbiting galaxies technique, and is the “glue” that holds clusters together.
© Sierra College Astronomy Department10 Cosmology: Dark Matter, Dark Energy, Fate of the Universe Evidence for Dark Matter Evidence for Dark Matter in Galaxy Clusters (continued) – Gravitational Lensing All methods to this point are based on Newton’s laws to measure galaxy and cluster masses. General Relativity predicts that mass can “bend” spacetime and this prediction has been confirmed by numerous observations. A consequence of this bending of spacetime is that a concentration of mass can act like a lens and bend light beams by a process known as gravitational lensing. The amount of bending is directly related to the amount of mass causing the bending. From the analysis of photos showing galaxy clusters (acting as the mass lenses) and the multiple images of very distant galaxies, the amount, it is once again found that dark matter exceeds the mass of the stars by almost 50 times.
© Sierra College Astronomy Department11 Cosmology: Dark Matter, Dark Energy, Fate of the Universe Is Dark Matter for Real? Does Dark Matter Really Exist? – All evidence for dark matter rests on our understanding of gravity, either from the Newtonian view or the Einsteinian view. – It is possible that our theory of gravity is wrong on cosmological scales? – Perhaps, but all attempts to explain dark matter with alternate theories of gravity, do not succeed in explaining all the other systems involving mass which are explained by Newton/Einstein. – Thus, until some other solid idea comes along to the contrary, dark matter needs to be taken seriously.
© Sierra College Astronomy Department12 Cosmology: Dark Matter, Dark Energy, Fate of the Universe The Stuff of Dark Matter Dark Matter Composition – “Ordinary” Dark Matter Most ordinary matter is made of protons and neutrons, which in turn belong to a group of particles called baryons. Objects like you, the planets, brown dwarfs, and faint red main- sequence stars, all baryonic matter, will appear as dark matter since our technology cannot detect the radiation from them. –They do not emit enough light even if they were a few light years away. –These objects are called MACHOs = MAssive Compact Halo Objects. Lone black holes would also go undetected from observations of radiation. Back-of-the-envelope calculations and probing the Milky Way’s halo with gravitational lensing techniques (more properly micro- lensing), shows that MACHOs and black holes cannot account for all of the dark matter mass.
© Sierra College Astronomy Department13 Cosmology: Dark Matter, Dark Energy, Fate of the Universe The Stuff of Dark Matter Dark Matter Composition (continued) – “Extraordinary” (Non-Baryonic) Dark Matter Neutrinos –Produced in large quantities by stellar fusion. –No electrical charge and give off no radiation. –A neutrino has very low mass and is only subject to weak and gravitation forces. »Known as a weakly interacting particle. –Very high speeds do not allow a galaxy to trap them gravitationally. –Calculations show they can only make up a small part of the dark matter outside galaxies. Other Particles –What about more massive and weakly interacting particles? –These types of particles, called Weakly Interacting Massive Particles (WIMPs) are theoretical. –Believed to constitute most of the dark matter in the Universe. –Explains why dark matter in halo and not disk (WIMPs lack a cooling and collisional mechanisms to bring them into disk).
© Sierra College Astronomy Department14 Cosmology: Dark Matter, Dark Energy, Fate of the Universe Large-Scale Structure of the Universe A Hierarchy of Large-Scale Structure in the Universe – Galaxies Gravitationally bound systems of stars, gas/dust, and dark matter. About 100,000 light-years across. – Groups and clusters Gravitationally bound systems of galaxies and dark matter Up to 10 million light-years or more across. Generally located at the intersection of large-scale galactic chains. – Superclusters Associations of groups/clusters that may be gravitationally bound and in the early stages of formation. Sizes range up to a few 100 million light-years across. – Chains, sheets, and voids May extend up to one billion light-years across. On larger scales, Universe finally takes on a more uniform look.
© Sierra College Astronomy Department15 Cosmology: Dark Matter, Dark Energy, Fate of the Universe Large-Scale Structure Formation The Creation of Large-Scale Structure – Creation of the large-scale structures requires “centers of gravity” – areas of higher mass density than surrounding regions to draw matter into “proto- structures”. – Consequently, early Universe could not have been perfectly uniform in mass density. – The existence of large amounts of dark matter suggests that high density regions of dark matter in the early Universe acted as the catalysts for the creation and evolution of the large-scale structures.
© Sierra College Astronomy Department16 Cosmology: Dark Matter, Dark Energy, Fate of the Universe Large-Scale Structure Evolution Hierarchical Structure Formation and Evolution – The large scale structures are believed to form from a bottom-up approach – galaxies form first, then clusters, etc. – Gravitationally bound systems (galaxies, clusters/groups, and perhaps superclusters) do not engage in the expansion of the Universe. Space is not expanding within bound systems. Expansion occurs primarily in the voids and unbound structures (portions of the chains and sheets and superclusters)
© Sierra College Astronomy Department17 Cosmology: Dark Matter, Dark Energy, Fate of the Universe The Fate of the Universe Gravity and the Ultimate Fate of the Universe – Observations show that the Universe is expanding – The gravitational attraction of the mass in the Universe seems to suggest two possible futures Gravity is strong enough to slow the Universe to a halt, initiate its collapse, and then bring the Universe to a “Big Crunch”. Gravity is not strong enough and only slows the expansion, never ending it. The Universe simply expands “out of existence” as it becomes a cold dark place. – So what will it be: Death by fire or Death by ice?
© Sierra College Astronomy Department18 Cosmology: Dark Matter, Dark Energy, Fate of the Universe The Fate of the Classical Universe A “Classical” Picture of the Universe’s Fate (Dark Energy Not considered) – The fate of the Universe rests on how “strong” gravity is and this strength is dependent on the mass density of the Universe. – For the current observed expansion rate of the Universe, the theoretical critical density is found to be g/cc. For actual densities greater than the critical density, the Universe will eventually collapse. For actual densities less than or equal to the critical density, the Universe will expand forever. – The observed density of the Universe, determined from just the luminous matter is found to be 0.5% of the critical density, this is 200 times short of what is needed to halt the expansion. – Assuming that dark matter may contribute up to 50 times more mass to the Universe, the actual density of the Universe may be about 25% of critical. – The Universe appears destined to expand forever.
© Sierra College Astronomy Department19 Cosmology: Dark Matter, Dark Energy, Fate of the Universe The Fate of the “Real” Universe Discovery of Dark Energy in 1990s – Using white dwarf supernovae to determine the distances to very distant objects, Universe was found (surprisingly) to be accelerating with time. – A repulsive force, referred to as dark energy, was hypothesized as the agent of the acceleration. With dark energy possibility, four fates of the Universe are envisioned: – Recollapsing Universe No dark energy, matter density exceeds critical Universe is closed (finite) and ends in Big Crunch – Critical Universe No dark energy, matter density equals critical Universe is flat (infinite) and stops expanding at infinite time – Coasting Universe No dark energy, matter density less than critical Universe is open (infinite) and never stops although it continuously slows – Accelerating Universe Finite dark energy (that does not evolve in time) Geometry of spacetime dependent on mass density Universe expansion accelerates with time
© Sierra College Astronomy Department20 Cosmology: Dark Matter, Dark Energy, Fate of the Universe Details on Fate More on the Dark Energy Observations – To determine which of the four Universes we live in (recollapsing, critical, coasting, and accelerating), theoretical curves of all four are first plotted on a graph of “average distance between galaxies” vs “time before and after now”. – Supernovae data points are then placed on the graph Distance measurements give lookback time. Redshift gives how much Universe has expanded since time of supernova explosion. – Data appears to match accelerating Universe the best. – It appears the fate of the Universe is to expand forever. The accelerating force of dark energy may eventually rip all matter apart. Mystery of what dark energy is keeps astronomers from being absolutely sure of the Universe’s fate (will dark energy evolve away with time and allow gravity to regain control?)
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