If we total up all the normal baryonic matter in the Universe we get a density

# If we total up all the normal baryonic matter in the

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There are 5 constituents of the universe, atomic matter, dark matter, light (photons), neutrinos, and dark energy. - If we total up all the normal (baryonic) matter in the Universe, we get a density about 5% of the critical density. - Dark matter seems to make up about 26% of the critical density. - If matter is only 31% of the critical density, and there is no other contribution, then the universe is open. The Flatness Problem - Astronomers and physicists have noted that the universe is suspiciously close to the critical density. - Universe could have had any density; one thousands, millions, or billions of times smaller or larger than critical density General Relativity
- The presence of matter and energy in the Universe warp the fabric of space and time. - A closed universe results in spherical geometry - An open universe results in hyperbolic (saddle shaped) geometry - A critical universe results in flat geometry Since the expansion rate of the universe changes with time depending on the geometry, we can actually hope to measure the geometry of the universe by plotting how the expansion velocities of galaxies change with distance. Einstein’s calculations showed that an infinite static universe was unstable. - The gravity of all the bodies would cause it to collapse. - Therefore, there must be some repulsive force, called the cosmological constant, that balances gravity After Lemaitre and Hubble showed that the Universe was expanding, Einstein realized that his equations showed that an expanding Universe was stable. - Lemaitre predicted the expansion of the Universe well before it was discovered observationally. - He later referred to the introduction of the cosmological constant as “the biggest blunder of my life.” Until about 20 years ago, most cosmologists did not think that the cosmological constant was necessary. Recent observations indicate that the Universe is not only expanding, but that it is accelerating. The acceleration could be explained with a cosmological constant. The current best fit model of our universe looks like: - The universe is flat. - The density of normal matter is 5% of the critical density. - The density of dark matter is 26% of the critical density. - The energy density in the cosmological constant (now being called dark energy) is about 69% of the critical density. - When this energy is combined with the normal and dark matter, we get a flat universe. The Ultimate Fate of the Universe - As time goes on, galaxies will recede from one another with ever increasing speed. - Eventually, even the nearby galaxies will be receding from us at a speed greater than the speed of light. - The Universe will grow cold and dark. Eschatology - The current age of the Universe is 13.8 billion (1.3x10 10 ) years - If Grand Unified Theories are correct, protons are unstable. Their lifetime is at least 10 33 years. - By the time the Universe is 10 40 years old, all atomic matter will have disintegrated Week 8, NO CLASS: SPRING BREAK Week 9, Day 1 (March 13th, 2018) Quiz 2 on Thursday evening 6:30-7:45 in this room - ~50 multiple choice and everything from Thursday of last quiz until today (Thursday’s material not on quiz) - Sign Up for makeup quiz AND send e-mail -