Cosmology What is Cosmology? The study of the structure and evolution of the entire universe The idea is to form picture of the entire Universe: origin, size, and future We will make assumptions that what we learn here can be applied everywhere 1
The Universe is vast... as we have seen means 10-30 meters Proton Astronomy Student Cell State To Nearest Star Visible Universe -30-20 -10 0 10 20 Planck Scale Current Experimental Limit Atom Comfort Zone Distance To Sun Milky Way We must keep in perspective these distances as we try to understand the entire Universe Olber s Paradox Every time you go outside at night and notice that the sky is dark, you are making a profound cosmological observation Here s why... If the universe had the same structure everywhere, looked the same in every direction, was infinite in extent, and unchanging, then any line of sight from Earth should eventually run into a star and the entire night sky should be bright The laws of probability dictate that, sooner or later, any line drawn outward from Earth will run into a bright stellar surface 2
So...Why is the sky dark at night? Olber gave a wrong explanation Dust hides the distant stars Would become white hot when subjected to so much radiation The sky is dark at night because the Universe does not go on forever All light rays do not lead to stars or galaxies The Universe has a size (It is finite)!! The Universe is also evolving in time The Birth of the Universe We have seen that all the galaxies in the universe are rushing away from us as described by Hubble s law: Recession velocity = H 0 * distance We used this as a way of determining the distances to galaxies, but it is much more than that Assuming that all velocities have remained constant in time, we can ask how long it has taken for any given galaxy to reach its present distance from us The time taken is simply the distance traveled divided by the velocity: 3
For H 0 = 73 km/s/mpc, this time is about 13.7 billion years Estimates range from 11.2 20 billion years 13.7 billion years is the current accepted age of the Universe using both Hubble s Law as well as other types of age calculators In reverse, the time equation Implies that 13.7 billion years ago ALL the galaxies in the universe lay right on top of one another In fact, astronomers believe that everything in the universe was confined to a single point at that instant A Singularity! Then the point exploded, flying apart at high speeds This gargantuan explosion is known as the Big Bang The Big Bang 4
The Big Bang...continued The Big Bang provides the resolution of Olber s paradox Whether the universe is actually finite or infinite in extent is irrelevant, at least as far as the appearance of the night sky is concerned The key point is that we see only a finite part of it the region lying within roughly 14 billion lightyears of us What lies beyond is unknown its light has not had time to reach us Recall that light has a finite speed! This is not the only theory but more and more evidence suggests that this is the best theory thus far! What about the center of the Universe? Consider the expansion of coins taped to the outside of a balloon Each coin will see every other coin moving away from it with a speed that depends on its distance The coins themselves do not expand nor do the actual galaxies they represent Thus, Hubble's law implies only that the whole universe is expanding, not that we are the center of it Universe 2x larger Galaxies are 2x further apart 5
From the beginning... The Big Bang An explosion of matter and space Universe Shaped Becomes less dense and cools The most basic forces in nature become distinct (gravity, the strong force, the weak force, and the electromagnetic force) By the first second, we have fundamental particles and energy: quarks, electrons, photons, neutrinos and less familiar types These particles smash together to form protons and neutrons Basic Elements Form Protons and neutrons come together to form the nuclei of simple elements: hydrogen, helium and lithium It will take another 300,000 years for electrons to be captured into orbits around these nuclei Nucleosynthesis of helium & lithium was possible only during the first 3 minutes, when the density was high enough Most helium dates from the big bang, not from the stars The Radiation Era Most of the energy is in the form of radiation -- different wavelengths of light, X rays, radio waves and UV rays 6
Matter Domination Era Energy in matter and the energy in radiation are equal Neutral atoms are formed as electrons link up with hydrogen and helium nuclei Radiation from this period can still be detected Primordial background radiation or microwave background radiation Stars and Galaxies Form Even as the universe continues to expand rapidly, pockets of gas become more and more dense Stars ignite within these pockets, and groups of stars become the earliest galaxies Background Radiation The COsmic microwave Background Explorer [COBE] satellite COBE All-sky image looks completely uniform 2.728 Kelvin EVERYWHERE!!! The Universe has expanded 1000 times since this light began its journey This is us looking at the Big Bang! Earth elliptical projection All-sky elliptical projection 7
Subtracting off the 2.728 K shows a dipole, indicating motion Indicates the Earth is moving Moving towards blue, away from red (Doppler shift) at a speed of 368 km/s Subtracting the dipole map makes the Milky Way Galaxy stand out The Galaxy light can also be removed Red means cooler, blue means warmer, but only by tens of millionths of a Kelvin We see structure at this scale, representing density variations when atoms first formed These density variations reveal the seeds of galaxy formation This is as far back as we see the wall to our vision 8
The Edge of the Universe Possible Geometries of The Universe Einstein showed that mass caused space to curve, and objects traveling in that curved space have their paths deflected, exactly as if a force had acted on them These relate to the ultimate fate of the Universe 9
The Fate of the Universe Depends on the average density being 4 x 10-30 g/cm 3 The critical density Four possible fates are theorized: The Closed Universe The Critical Universe The Open Universe The Accelerating Universe Average density > the critical value = Closed Universe Positive space curvature (ball) Finite in extent Will stop the present expansion Will eventually collapse Average density = critical value = Critical Universe Zero space curvature (flat) Infinite in extent Present expansion slows to zero in infinite time Will expand forever Average density < the critical value = Open Universe Negative space curvature (saddle) Infinite in extent Will not stop present expansion Will expand forever 10
The Accelerating Universe Current observations suggest that the Universe is actually accelerating! What could cause an overall acceleration of the universe? Cosmologists do not know They do have theories The mysterious field causing the universe to accelerate is neither matter nor radiation It is sometimes referred to as dark energy The amount of dark energy in the universe may exceed the total mass-energy of matter (luminous and dark matter) by a wide margin By opposing the attractive force of gravity, the repulsive effect of the dark energy suggests that the universe will expand forever Or... 11
Stellar Era Ends So what will happen? Astronomers assume that the universe will gradually wither away, provided it keeps on expanding and does not recollapse under the pull of its own gravity At this point no new stars are forming Degenerate Era Ends Most of the mass is locked up in stars that have blown up and collapsed into black holes and neutron stars, or have withered into white dwarfs Black Hole Era Begins The only stellar-like objects remaining are black holes of widely disparate masses, which are actively evaporating during this era Dark Era Begins Protons have decayed and black holes have evaporated Only the waste products from these processes remain: mostly photons of colossal wavelength, neutrinos, electrons, and positrons For all intents and purposes, the universe as we know it has dissipated 12
Final Thoughts I want you to understand that it is now possible to use science to address questions that were once only addressed by philosophy and religion The main difference between the scientific approach and the philosophical / religious one is the approach taken to finding the answer One depends on observing, experimenting, and modifying one's views in the light of experience The other depends on handed-down information and divine guidance Reconciling the scientific approach with the philosophical / religious one is a personal matter End Topic Cosmology 13