Chapter 17 The Beginning of Time

How far/back in time can we see with our current telescopes? What is the farthest we could in principle see? Only until 380,000 years after the beginning…

17.1 The Big Bang Our Goals for Learning • What were conditions like in the early universe? • What is the history of the universe according to the Big Bang theory?

What were conditions like in the early universe?

Universe must have been much hotter and denser early in time

The early universe must have been extremely hot and dense

Photons converted into particle-antiparticle pairs and vice-versa E = mc2 Early universe was full of particles and radiation because of its high temperature

Today, In the labs we created conditions up to 10-10 seconds after the Big Bang. And we have a theory for up to 10-38 seconds after the big bang. The very instant of creations we do not know how to describe.

What is the history of the universe according to the Big Bang theory?

There are four known forces in universe: Let’s step back: There are four known forces in universe: Gravity: Electromagnetism: it acts among charged particles, in atoms and molecules, and is responsible for all chemical and biological interactions. Strong Force: important only on very small distances, binds nuclei together. Weak Force: important in nuclear fusion and fission. There are some particles which interact only through this force (and gravity). Like neutrinos and WIMPs (dark matter).

Thought Question Which of the four forces keeps you from sinking to the center of the Earth? A. Gravity B. Electromagnetism C. Strong Force D. Weak Force

Thought Question Which of the four forces keeps you from sinking to the center of the Earth? A. Gravity B. Electromagnetism C. Strong Force D. Weak Force

Do forces unify at high temperatures Do forces unify at high temperatures? (think about ice, liquid and vapor as just a various states of water)

Do forces unify at high temperatures? Yes! (Electroweak)

Do forces unify at high temperatures? Yes! (Electroweak) Maybe (GUT)

Do forces unify at high temperatures? Yes! (Electroweak) Maybe (GUT) Who knows? (String Theory)

Planck Era Before Planck time (~10-43 sec) Random fluctuation of energy/particle/space - we have no theory of quantum gravity. At the end of Plank era, gravity froze out – separated from other forces.

GUT Era Lasts from Planck time (~10-43 sec) to end of GUT force (~10-38 sec). At the end of GUT era strong force froze out. There are some theories proposed which link GUT forces. Still unconfirmed, but at least we have some idea how that could work.

Since this time, space was field with photons (radiation) and all elementary particles we know of (electrons, quarks, …and their anti-matter counterparts). Universe was extremely hot and photons had enough energy to produce even the heaviest particles (most of these particles do not exist as free particles today). Particles would then annihilate back to photons – the universe consisted of the matter-radiation sea (soup).

Electroweak Era Lasts from (10-38 sec) to end of electroweak force (10-10 sec). After this instant all forces became forever distinct in the Universe.

We have direct experimental evidence of the transition from electroweak force to two separate forces: electromagnetic and weak. We probed in the lab physics of the Universe when it was just 10-10 seconds old! To get a better idea: Temperature at the end of this era (1015K) was 100 million times hotter than in the Sun!

Particle Era In particle era it became cool enough so that quarks had combined producing protons and neutrons! The era ended when universe became too cold (1012 K) for photons to produce protons and neutrons. The total number of protons and neutrons (and antiprotons/ antineutrons) was sealed at that time. Photons were still producing electrons, and neutrinos, and got produced back by their annihilation…

Universe consisted of protons, neutrons, on one hand, and the soup of photons producing electrons, neutrinos… on the other. But, amounts of matter (protons/neutrons) and antimatter (antiprotons/antineutrons) was still nearly equal. How much anti-matter is left in today's Universe? If the Universe had exactly the same amount of protons and antiprotons how would it look like today?

Era of Nucleo-synthesis There were roughly 1 extra proton for every 109 proton-antiproton pairs! This era begins with matter annihilating remaining antimatter at ~ 0.001 sec. What was left was universe containing only matter, as we know it today!

After that, p and n began to fuse, making He and some deuterium and Li. This era ended when Universe was 3 minutes old. All elements the Universe started off with (75% H, 25% He, trace amounts of deuterium and Li) where made in the first 3 minutes! The Universe expanded so much by than, that p&n, became to far apart, and synthesis of nuclei ceased. Why elements heavier than He did not form in the early Universe?

Era of Nuclei After the era of nucleosynthesis the Universe consisted of p, He nuclei and free electrons. (no neutral atoms existed yet) Photons were bouncing off these charged particles, never managing to travel long between collisions (in what layer of the Sun does the similar condition exists?)

Era of Atoms Universe has cooled enough, at age of ~ 380,000 years, so that atoms could form (and photons had not enough energy to ionize them again). The Universe suddenly became transparent for photons, they didn’t have any free charged particles to bounce from anymore. They just flashed through the Universe, and amazingly enough, we still can see this photons today. All Universe is bathed in this, so called, cosmic microwave background radiation. It arrives to us from every point in space.

Era of Galaxies Galaxies form at age ~ 1 billion years. We have already discussed the rest of the story in the previous chapters…

Primary Evidence for Big Bang theory We have detected the leftover radiation from the Big Bang. The Big Bang theory correctly predicts the abundance of helium and other light elements.

What have we learned? • What were conditions like in the early universe? The early universe was filled with radiation and elementary particles. It was so hot and dense that the energy of radiation could turn into particles of matter and antimatter, which then collided and turned back into radiation.

What have we learned? • What is the history of the universe according to the Big Bang theory?

17.2 Evidence for the Big Bang Our Goals for Learning • How do we observe the radiation left over from the Big Bang? • How do the abundances of elements support the Big Bang?

How do we observe the radiation left over from the Big Bang?

The cosmic microwave background – the radiation left over from the Big Bang – was detected by Penzias & Wilson in 1965. They noticed that wherever they point their antenna (designed for satellite communications) to, they get some unexpected noise, which they tried hard to get rid off. At the end, they got Nobel Prize, for providing first evidence for Big Bang theory.

Background radiation from Big Bang has been freely streaming across universe since atoms formed at temperature ~ 3,000 K: visible/IR

Background has perfect thermal radiation spectrum at temperature 2 Background has perfect thermal radiation spectrum at temperature 2.73 K Corresponds to a temperature of 2.73 K – the temperature of the night sky. Expansion of universe has redshifted thermal radiation from that time to ~1000 times longer wavelength: microwaves (part of radio waves)

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COBE detected the seeds of future structure formation: the temperature of universe varies slightly, by only about 0.01%. These variations indicate that the density of the early universe did differ from place to place – the seeds of structure formation were present during the era of nuclei.

COBE (1993) WMAP (2003)

WMAP gives us detailed baby pictures of structure in the universe

How do the abundances of elements support the Big Bang?

Before the Big Bang theory, the fact that Universe contains so much He was a puzzle. It meant that the Universe was once hot enough for nuclear fusion of H to He to happen, but people did not know how. The fact that the temperature of microwave background is 2.73 K, tells us precisely how hot was the Universe in the distant past and exactly how much He should have been made. The result, 25% He, is another success of this theory.

Abundances of other light elements agree with Big Bang model having 4 Abundances of other light elements agree with Big Bang model having 4.4% of critical density of normal matter – more evidence for WIMPS!

Thought Question Which of these abundance patterns is an unrealistic chemical composition for a star? A. 70% H, 28% He, 2% other B. 95% H, 5% He, less than 0.02% other C. 75% H, 25% He, less than 0.02% other D. 72% H, 27% He, 1% other

Thought Question Which of these abundance patterns is an unrealistic chemical composition for a star? A. 70% H, 28% He, 2% other B. 95% H, 5% He, less than 0.02% other C. 75% H, 25% He, less than 0.02% other D. 72% H, 27% He, 1% other

What have we learned? • How do we observe the radiation left over from the Big Bang? Telescopes that can detect microwaves allow us to observe the cosmic microwave background—radiation left over from the Big Bang. Its spectrum matches the characteristics expected of the radiation released at the end of the era of nuclei, spectacularly confirming a key prediction of the Big Bang theory.

What have we learned? • How do the abundances of elements support the Big Bang? The Big Bang theory predicts the ratio of protons to neutrons during the era of nucleosynthesis, and from this predicts that the chemical composition of the universe should be about 75% hydrogen and 25% helium (by mass). This matches observations of the cosmic abundances, another spectacular confirmation of the Big Bang theory.

17.3 The Big Bang and Inflation Our Goals for Learning • What aspects of the universe were originally unexplained by the Big Bang model? • How does inflation explain these features of the universe? • How can we test the idea of inflation?

What is cosmological inflation? A brief period of exponentially fast expansion of the Universe. The universe expanded from to in just seconds! Do you know for some other period in which universe was expanding exponentially fast? It presumably happened at the end of GUT era. When the strong force separated, the huge amount of energy was released and it caused this rapid expansion. The idea (like many others) sounds bizzare, but it is useful addition to the standard Big Bang theory: it helps solve some features of Universe unexplained by standard Big Bang model.

What aspects of the universe were originally unexplained by the Big Bang model?

Mysteries Needing Explanation Where does structure come from? (how did density enhancements come about) Why is the overall distribution of matter so uniform? Why is the density of the universe so close to the critical density? (it could have been whatever number and it has exactly this, critical density value)

Inflation can make all the structure by stretching tiny quantum ripples to enormous size. These ripples in density then become the seeds for all structures

How can microwave temperature be nearly identical on opposite sides of the sky?

Regions now on opposite side of the sky were close together before inflation pushed them far apart

Overall geometry of the universe is closely related to total density of matter & energy Critical Density > Critical Density < Critical

Inflation of universe flattens overall geometry like the inflation of a balloon, causing overall density of matter plus energy to be very close to critical density

How can we test the idea of inflation?

Patterns of structure observed by WMAP tell us “genetic code” of universe

Observed patterns of structure in universe agree (so far) with what inflation should produce

“Genetic Code” Inferred from CMB Overall geometry is flat Total mass+energy has critical density Ordinary matter ~ 4.4% of total Total matter is ~ 27% of total Dark matter is ~ 23% of total Dark energy is ~ 73% of total Age of 13.7 billion years

“Genetic Code” Inferred from CMB Overall geometry is flat Total mass+energy has critical density Ordinary matter ~ 4.4% of total Total matter is ~ 27% of total Dark matter is ~ 23% of total Dark energy is ~ 73% of total Age of 13.7 billion years In excellent agreement with observations of present-day universe and models involving inflation and WIMPs!

What have we learned? • What aspects of the universe were originally unexplained by the Big Bang model? (1)     The origin of the density enhancements that turned into galaxies and larger structures. (2)     The overall smoothness of the universe on large scales. (3) The fact that the actual density of matter is close to the critical density.

What have we learned? How does inflation explain these features of the universe? (1)     The episode of inflation stretched tiny, random quantum fluctuations to sizes large enough for them to become the density enhancements around which structure later formed. (2)     The universe is smooth on large scales because, prior to inflation, everything we can observe today was close enough together for temperatures and densities to equalize. (3) Inflation caused the universe to expand so much that the observable universe appears geometrically flat, implying that its overall density of mass plus energy equals the critical density.

What have we learned? • How can we test the idea of inflation? Models of inflation make specific predictions about the temperature patterns we should observe in the cosmic microwave background. The observed patterns seen in recent observations by microwave telescopes match those predicted by inflation.

17.4 Observing the Big Bang for Yourself Our Goal for Learning • Why is the darkness of the night sky evidence for the Big Bang?

Why is the darkness of the night sky evidence for the Big Bang?

Olbers’ Paradox If universe were 1) infinite 2) unchanging 3) everywhere the same Then, stars would cover the night sky

Olbers’ Paradox If universe were 1) infinite 2) unchanging 3) everywhere the same Then, stars would cover the night sky

Night sky is dark because the universe changes with time

Night sky is dark because the universe changes with time

What have we learned? • Why is the darkness of the night sky evidence for the Big Bang? Olbers’ paradox tells us that if the universe were infinite, unchanging, and filled with stars, the sky would be everywhere as bright as the surface of the Sun, and it would not be dark at night. The Big Bang theory solves this paradox by telling us that the night sky is dark because the universe has a finite age, which means we can see only a finite number of stars in the sky.

How do we determine the conditions that existed in the very early universe? [Hint]   A. From the current expansion rate we can work backward to estimate temperature and densities at various times in the early universe. B. We can only guess at the conditions, since we have no way to calculate or observe what they were. C. The conditions in the very early universe must have been much like those found in stars today, so we learn about them by studying stars. D. By looking all the way to the cosmological horizon, we can see the actual conditions that prevailed all the way back to the first instant of the Big Bang.

  Why can't current theories describe what happened during the Planck era? [Hint] A. We do not understand the properties of antimatter. B. We do not yet have a theory that links quantum mechanics and general relativity. C. We do not know how hot or dense the universe was during that time. D. The Planck era was the time before the Big Bang, and we cannot describe what happened before that instant.

When we say that the electroweak and strong forces "freeze out" at 10--35 seconds after the Big Bang, we mean that _________. [Hint]   A. these forces are important only at temperatures below the freezing point of water --- a temperature that the universe reached at an age of about 10--35 seconds B. prior to this time, the electroweak and strong forces were indistinguishable from each other, but after this time they behaved differently from each other C. following this time, neither the strong nor electroweak forces are ever important in the universe again D. freezing out was a term coined by particle physicists who think that the Big Bang theory is really cool

What was the significance of the end of the era of nucleosynthesis, when the universe was about 3 minutes old? [Hint]   A. It marks the time at which the first stars formed. B. The proportions of dark matter and luminous matter had been determined. C. It marks the time at which the expansion of the universe had settled down to its current rate. D. The basic chemical composition of the universe had been determined.

If astronomers had discovered that the cosmic microwave background was precisely the same everywhere, instead of having very slight variations in temperature, then we would have no way to account for _________. [Hint]   A. how galaxies came to exist B. the relationship between the strong and the weak force C. the existence of helium in the universe D. the fact that our universe is expanding

In stars, helium can sometimes be fused into carbon and heavier elements (in their final stages of life). Why didn't the same fusion processes produce carbon and heavier elements in the early universe? [Hint]   A. Helium fusion occurred, but the carbon nuclei that were made were later destroyed by the intense radiation in the early universe. B. By the time stable helium nuclei had formed, the temperature and density had already dropped too low for helium fusion to occur. C. Temperatures in the early universe were never above the roughly 100 million Kelvin required for helium fusion. D. No one knows --- this is one of the major mysteries in astronomy.