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Modern Cosmology

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Page 1: Modern Astronomy

Modern Cosmology

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22.1 The Universe Has a Destiny and a Shape

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Cosmologyis the study of the large-scale universe, including its origin, evolution, and ultimate destiny.

WHAT IS the fate of the universe?

Fate 1 Fate 2 Fate 3

The simplest answer depends in part on the amount of mass distributed across the universe on very large scales. The gravitational effect of this distributed matter is only one of the factors —the first one we will discuss—that determines how the universe evolves.

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If Ωmass is greater than 1, then gravity is strong enough to turn the expansion around. The expansion will slow and eventually stop, and the universe will then fall back in on itself.

FATE 1: Ωmass >1

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FATE 2: Ωmass <1

Conversely, if Ωmass is less than 1, that universe will be slowed by gravity, but it will still expand forever.

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FATE 3: Ωmass = 1

The dividing line, where Ωmass equals 1, corresponds to a universe that expands more and more slowly—continuing forever, but never quite stopping. Such a universe is expanding at exactly the “escape velocity.”

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22.2 The ACCELERATING Universe

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During the 1990s, some groups of astronomers began using the Hubble Space Telescope, telescopes in Chile, and the giant Keck Observatory telescopes in Hawaii to test this prediction.

They measured the brightness of Type Ia supernovae in very distant galaxies and compared the brightness of those supernovae with their expected brightness based on the redshift distances of those galaxies.

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How does the possibility of a nonzero value affect the possible fate of the universe?

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What if the cosmological constant is not constant with time?

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BIG RIPThe Theory: The universe will expand so much that eventually galaxies, solar systems, planets and even individual atoms will be ripped apart.How Likely Is It? Probably not going to happen. Though the latest results from the Planck satellite do push it slightly more into the realm of possibility than it was before.

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BIG Freeze This is what will happen if the universe keeps expanding forever, but not enough to rip itself apart. The stars will go out and the black holes will evaporate. Also known as the “heat death” of the universe, it’s the sweet spot between a big crunch and a big rip. 

All evidence points to a this as the fate of the cosmos. The end of the universe is going to be a cold and lonely place. But at least the atoms won’t be ripped apart.

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BIG CrunchThe Theory: The universe will end in a “big crunch” as galaxies are pulled back together by gravity, ending in a kind of reversed Big Bang.How Likely Is It? This theory requires gravity to overpower the force pushing the universe apart. That doesn’t look likely to happen, so we should escape this scenario.

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Does the universe have shape?

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FLAT UNIVERSEA flat universe is describedoverall by the rules of the basic Euclidean geometry thatyou learned in high school. As shown in Figure 22.8a,circles in a flat universe have a circumference of 2π timestheir radius (2πr), and triangles contain angles whose sumis 180°. A flat universe stretches on forever.

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Open Universe

In an open universe thecircumference of a circle is greater than 2πr, and triangles contain less than 180°.

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The circumference of a circle on a sphere is less than(2πr), and triangles contain more than 180°. This possibilityis called a closed universe because space is finite andcloses back on itself.

Closed Universe

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22.3 Inflation

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* Uncertainty Principle* Flatness problem

says that as a system is studied at extremely smallscales, the properties of that system become less and less well determined.

* Horizon Problemstates that different parts of the universe are too much like other parts of the universe that should have been “over their horizon” and beyond the reach of any signals that might have smoothed out the early quantum fluctuations

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22.4 The earliest moments

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There are four fundamental forcesin nature

Chemistry and light are products of the electromagnetic force acting between protons and electrons in atoms and molecules.The energy produced in fusion reactions in the heartof the Sun comes from the strong nuclear force that binds together the protons and neutrons in the nuclei of atoms.Beta decay of nuclei, in which a neutron decays into a proton, an electron, and an antineutrino, is governedby the weak nuclear force.

Finally, there is gravity, whichhas played such a major role throughout astronomy.

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The description of light as an electromagnetic wave resulting from electric and magnetic fields, and the quantum mechanical description of light as a stream of particles called photons

These descriptions of electromagnetism have to coexist. The branch of physics that deals with this reconciliation is called quantum electrodynamics,or QED.

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The central idea of QED—forces mediated by the exchange of carrier particles—provides a template for understanding two of the other three fundamental forces in nature.

The electromagnetic and weak nuclear forces have been combined into a single theory called electroweak theory.

This theory predicts the existence of three particles—labeled W+, W–, and Z0—that mediate theweak nuclear force.

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Sheldon Glashow, Abdus Salam, and Steven Weinberg received the 1979 Nobel Prize in Physics for their work on the theory of the unified weak and electromagneticforces.

In the 1980s, physicists identified these particles in laboratory experiments and confirmed the essentialpredictions of electroweak theory.

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The strong nuclear force is described by a third theory, called quantum chromodynamics, or QCD.This theory states that particles such as protons and neutrons are composed of more fundamental building blocks called quarks, which are bound together by the exchange of another type of carrier particle, dubbed gluons.

Together, electroweak theory and QCD comprise the standard model of particle physics.

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A universe of Particles and antiparticles

Every type of particle in nature has an antiparticle that is its opposite.

For the proton there is the antiproton; for the neutron, the antineutron; and so on down thelist. Collectively these antiparticles are called antimatter.

One property of these particle-antiparticle pairs isthat if you bring such a pair together, the two particles annihilate each other.

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When a particle-antiparticle pair annihilates, the mass of the two particles is converted into energy in accord with Einstein’s special theory of relativity (E = mc2).

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The frontier of Physics

In the process of pair production there is a symmetry between matter and antimatter: for every particle created, its antiparticle is created as well.As the universe cooled, there was no longer enough energy to support the production of particle pairs, so the particles and antiparticles forming the swarm that filled the early universe annihilated each other and were not replaced.

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The frontier of Physics

When this cooling happened, first every proton should have been annihilated by an antiproton. Then at still cooler temperatures, every electron should have been annihilated by a positron. This was almost the case, but not quite.For every electron in the universe today, there were10 billion and one electrons in the early universe, but only 10 billion positrons. This one-part-in-10-billion excess of electrons over positrons meant that when electron positron pairs finished annihilating each other, some electrons were left over—enough to account for all the electrons in all the atoms in the universe today

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There are theories that combine three of the four fundamental forces into a single grand, unified force are called grand unified theories, or GUTs.Grand unified theories break the particle/antiparticle symmetry and explain why the universe is composed of matter rather than antimatter. There are several competing GUTs, and only the very simplest of GUTs have been ruled out.GUTs unify three forces but not the fourth, gravity.

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Toward the theory of Everything

To understand the earliest moments of the universe, physicists need a theory that combines general relativity and quantum mechanics into a single theoretical framework unifying all four of thefundamental forces. Such a theory is called a theory ofeverything (TOE).

In physical cosmology, the Planck epoch (or Planck era), named after Max Planck, is the earliest period of time in the history of the universe, from zero to approximately 10^(−43) seconds (Planck time), during which quantum effects of gravity were significant.

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Superstring theory

According to superstring theory, differenttypes of elementary particles are like different “notes” played by vibrating loops of string.

A theory, in which elementary particles are viewed not as points but as tiny loops called “strings.”

In principle, superstring theory provides a way to reconcile general relativity and quantum mechanics.

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22.5 Multiple Multiverses

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Is our universe the only one? Since we’ve defined the universe as “everything,” what does it mean to say “multiple universes”? Are there parallel universes either separated in space or even occupying exactly the same space as “our” universe? These ideas are quite speculative, but many cosmologists think seriously about the idea of multiple universes— or multiverses—collections of parallel universes.

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Figure 22.16observable universe is a sphere with a radiusequal to the distance light has traveled since the Big Bang (13.7billion light-years). Since the universe is infinite, there must be aninfinite number of similar spheres. The rules of probability dictatethat some of these are exactly like our own.

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What are these other parallel universes like?

The collection of parallel universes we just describedrepresents the first of four types of multiverses theorized by cosmologists

First, if the cosmological principle holds, then on large scales each of these observable universes should lookpretty much like our own, although the details couldbe very different.

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The inflationary universemodel forms the basis of the second type of multiverse.Imagine a universe that undergoes eternal inflation, withno beginning or end to the inflation. This idea was hypothesized by physicist Andrei Linde (1948– ), who realized that if such a universe exists, then quantum fluctuations may cause some regions to expand more slowly than the rest of the universe. As a result, such a region may form a bubble whose inflating phase will soon end

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In this scenario, Earth is inside such a region, and “our” Big Bang would just be the condensation of our bubble within the eternally inflating universe. Since the universe has been inflating and will continue to inflate forever, there is no beginning or end. “Our” own bubble or parallel universe separated from the rest of the universe at a time called the Big Bang, but other bubbles are constantly separating and becoming their own parallel-universe big bangs.

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The third type of multiverse has its origins purely inquantum mechanics

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The fourth type of multiverse is characterized by paralleluniverses that have different mathematical structureto describe the different physics within these universes

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22.6 Origins: Our Own Universe Must support Life

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anthropic principle

states that this universe (or this bubble in the universe) must have physical properties that allow for the development of intelligent life. Since humans exist, are (presumably) intelligent, and can observethe surrounding universe, this universe must have theproperties that would allow intelligent life to evolve. That is, this universe must have had the right physical properties and existed long enough for atoms, stars, galaxies, planets, and life to have formed.

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SUMMARY22.1 The shape and the age of the universe depend on the amount of mass and therefore gravity in the universe.

22.2 Observations suggest that the expansion of the universe is accelerating. Both gravity and the cosmological constant (or dark energy) determine the fate of the universe.

22.3 The very early universe may have gone through a brief but dramatic period of exceptionally rapid expansion, called inflation. If true, inflation would explain both the flatness and the homogeneity of the universe observed today.

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SUMMARY22.4 The earliest moments in the universe were determined by the four fundamental forces of nature, which were all unified into one basic phenomenon at the moment the universe began.

22.5 The observable universe may be only one of an infinite number of simultaneously existing universes. Scientists disagree about whether the theories of multiverses are testable and falsifiable.

22.6 Our observable universe must be one in which physics can support the formation of life.

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HAY Salamat Human najod ko.

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