Modifications to Cosmology Topics Big Bang successes and shortcomings Inflation The Universe’s ultimate fate Motivation Now, let us refine our Big Bang concept by incorporating the concept of inflation. 1

In case you were asleep or sick…. The Big Bang Theory follows the Universe from its origins, when the distance between all objects was zero. As a function of time, the theory tracks… Temperature; Matter density; Energy density; Composition (what the matter is doing); The relevant physics. The Big Bang is not an explosion in space! Explosions happen in a place in space—you can run from them. In explosions, recessional speeds (Hubble’s Law) result from the Doppler Effect. The Big Bang is an expansion OF SPACE! The expansion is of space itself—you can’t run from the Universe. Recessional speeds result from the photons being continually stretched. The Big Bang: a review 2

1.Hubble’s Law The space between galaxies is expanding. (Not: “Galaxies are flying away from each other”—galaxies are ~ still.) 2.Cosmic Background Radiation The Universe is filled with a uniform glow at 2.73K. This is the cosmologically redshifted radiation from decoupling. 3.Nucleosynthesis Predicts the hydrogen ( 1 H) and helium ( 4 He) abundances. Predicts the deuterium ( 2 H), helium ( 3 He), and lithium abundances. 4.Galaxies The large scale distribution and occurrence of galaxies emerge from the theory. Furthermore, the Big Bang is a straightforward approach that—for the most part— follows standard physics, and gives us an internally consistent framework for the Universe, from just after creation to the development of protogalactic gas clouds. Big Bang predictions/explanations 3

1.Physics at t≈0 The theory is very poor at times near the Planck time (t= sec). 2.Matter-antimatter recombination Why is it that, when matter and antimatter recombined, 1 particle of matter was left over for about 10 9 recombinations? 3.Physics at t=0 Why, oh why did the Universe start its expansion? 4.Physics before t=0 What happened before the Big Bang? One can argue that the last two criticisms are not fair, as the Big Bang theory is a treatment of the Universe once it started its expansion. This dodge seems philosophically unsatisfying. Weaknesses with the Big Bang 4

Inflation is not a single theory—it is a group of theories that modify the Big Bang theory. It is better considered a paradigm. Inflation is a phase of the Big Bang Theory that was most prominently proposed in 1981 by Alan Guth, some 40 years after the Big Bang Theory was proposed. Guth was in his 4 th post-doc, nine-years post-PhD! …He was, very nearly, an astronomy washout! Inflation proposed 5

Inflation occurred at the beginning of the electroweak era, when the strong and electroweak forces separated (t ≈ sec – sec). BICEP2 results place the time of inflation at 0.5× sec. From a particle physics standpoint… Some as-yet-undiscovered particle (called an inflaton), has been proposed. The inflaton produced an inflaton field, which drove the inflation. From an energy standpoint… Some aspect of this “freezing out” of the strong and electroweak forces released an incredible amount of energy that resulted in an exponential growth in the Universe’s size; the expansion was a factor of approximately – ×! Inflation – the mechanism 6

Although the Universe was already engaged in a massive expansion, the release of the energy from the freezing out of the two forces (or from the inflaton field) resulted in a huge, short-term boost in its pre- existing expansion rate—this is inflation. As a result…in a mere instant… Regions once microscopically close to each other became separated by comparatively vast distances. A violation of special relativity? Inflation was so abrupt that objects receded from each other at speeds exceeding the speed of light! Is this a problem? No! It was the SPACE that was expanding—the matter was stationary within the mesh of the expanding Universe! Inflation – the results on space 7

1. The Smoothness Problem At the beginning of the electroweak era, the matter-energy densities were extremely high. Matter and energy densities were coupled and engaged in quantum fluctuations. Such fluctuations were poised to grow into huge, massive clumps. But such giant cosmic clumps are not seen. Before such quantum clumps grew much, the inflationary phase expanded them into huge volumes, smoothing them out. The Universe consequently became highly uniform on vast scales. As a result, when we look at the cosmic microwave background—formed when matter decoupled from energy at the end of the era of nuclei—we see an astonishingly uniform distribution. Inflation – four big benefits 8

2. The Horizon Problem When we study the cosmic microwave background, we are studying radiation emitted in the far distant past. The radiation originated from regions far from each other. As the Universe expanded, regions moved away from each other with speeds greater than the speed of light. (Remember, this is not in violation of special relativity, because it is space that was expanding.) In fact, without inflation, areas only a few degrees apart in the sky correspond to regions so far from each other, that they would never had had time to share photons with each other. They have never been in contact with each other in any way. So why, without a shared history, would they have the same characteristics? Why would they be in equilibrium? Why is the cosmic microwave background so uniform? Inflation – four big benefits 9

2. The Horizon Problem—Resolution Inflation greatly increased the distances between the parts of the Universe, but only after the Big Bang began. So it is perfectly understandable that the distant reaches of the Universe are nearly identical—they used to be in intimate contact with each other! Inflation – four big benefits 10

3. The Flatness Problem The future behavior of the Universe will be one of two possibilities. Recollapse (works in a high-density universe) –The matter in the Universe produces gravity. –This binds the Universe like an infinite number of little rubber bands. –These cumulative gravitational forces will halt the expansion and reverse it. – OR – Eternal expansion (works in a low-density universe) –The matter in the Universe produces gravity. –The gravity slows the expansion, but not as dramatically as in a high density Universe. –The gravity fails to halt the expansion. Inflation – four big benefits 11

3. The Flatness Problem The critical density—the boundary between these two alternatives, is ρ crit ≈ g/cm 3 What was the density of the Universe measured to be? There was nothing in the Big Bang theory to force the density of the Universe to any special value! × ρ crit ?10 -6 × ρ crit ?10 10 × ρ crit ?10 60 × ρ crit ? By 1997, measurements had found that the Universe had, in very approximate terms, about the right density to put it at the tipping point: ρ = 0.04 × ρ crit That seems to be a very lucky coincidence. And scientists do not like coincidences! Inflation – four big benefits 12

3. The Flatness Problem Jumping ahead to General Relativity for a moment, we note that space is curved by the gravity of matter. The further from critical density the Universe is, the greater the Universe’s curvature. For example, if the Universe was extremely dense, the curvature of space would be strong. Inflation – four big benefits 13 Inflation would drive the curvature of the Universe to a flat structure. Inflation would force space to exactly the correct density to put the Universe to put it at the tipping point between never-collapsing and recollapse.

4. Structure in the cosmic microwave background In the GUT era, tiny but potent quantum variations in the matter- energy density existed throughout space. Inflationary theories say that inflation expanded these variations to large sizes. At the moment of decoupling (the era of nuclei) the inflated/smeared-out energy density variations were fossilized as variations in the radiation field. We should still be able to see these variations in the cosmic microwave background today! Inflation – four big benefits 14

4. Structure in the cosmic microwave background Recapping… According to inflationary theory, the cosmic microwave background should have tiny variations on the order of the total intensity---we see them. Furthermore, inflationary theory predicts how the sizes of the blob- variations in the cosmic microwave background could vary. (The largest temperature fluctuations are predicted to be about 1° apart.)---we see the variations in the blobs. The experimental results match the predictions so close that it is quite frightening! Finally, BICEP2 (in Antarctica) has detected a “B-Mode” (tensor) polarization component 20% as large as the B+E Mode polarization in the CMB, all in fulfillment of scripture! Inflation – four big benefits 15

Remember “the flatness problem?” Inflation not only predicts that the Universe should be near the critical density to make space flat, it says the density of the Universe was PRECISELY at the critical density. By 1997, the density of the Universe (with something called Dark Matter added) was only 0.3 × ρ crit. While close to ρ crit, the density of the Universe is not precisely ρ crit. Inflation was in trouble, and many scientists were abandoning it. In 1998, studies of Type Ia supernovae revealed “Dark Energy,” a new form of energy—and therefore density—for the Universe. This brought the density of the Universe to the critical value, and inflation was vindicated! Inflation theory nearly dies in the 1990s 16

Perhaps the strangest consequence of inflation theory is the explanation of where galaxies came from… –Quantum fluctuations existed in mass and energy (GUT era). –Inflation expanded the size of the fluctuations (end of GUT era). –The radiation decoupled (end of nuclei era); hence the cosmic microwave background. –The quantum fluctuations in the matter density continued to enlarge and eventually became protogalactic clouds (era of galaxies). Galaxies are echoes of quantum variations from the GUT era! In this era, the extreme gravity imprinted polarization patterns onto the cosmic microwave background. The imprints from these ancient gravitational waves are visible today by the BICEP2 instrument. Inflation and galaxy formation 17

Because of inflation, we realize that everything we see in the Universe looks the same because it all used to be in contact—it was a tiny piece of what is part of a much larger space. Is it possible that, in these other spaces that we have never yet seen, the Universe is a very different place, with very different densities, expansion or collapse rates, and very different physics altogether? Is our part of the Universe strange, or normal? Ruminations—multiverses? 18

Conventional unification theories (GUT) predict that objects called magnetic monopoles should exist. These have not been found. This has been a long-standing sore point in GUT research. Where are the monopoles? Inflation would spread the monopoles over such a large area that they would be exceedingly rare, and hard to find. So…we just haven’t found them yet. Ruminations—magnetic monopoles? 19

The acceptance of inflation—a weird modification to the Big Bang Theory, was painful and slow—but in a way paved the way for the acceptance of Dark Energy (we’ll get to that). However, they are probably different. Inflation became important long ago, when physical conditions are very different from now. Inflation ended with a phase called “reheating.” Meanwhile, Dark Energy is still with us. Ruminations—inflation and dark energy 20