Chapter 13 - The Birth of the Universe
___________ was a dramatic expansion of the universe thought to have occurred when the universe was only a fraction of a second old.
inflation
Helium originates from
mostly from the Big Bang with a small contribution from stellar nucleosynthesis.
How long after the Big Bang was the Planck time, the time at which our current theories become completely unable to describe conditions any earlier in the universe?
10^-43 seconds
What was the approximate temperature of the universe when the universe was just 1 second old?
10^10 K
The Big Bang theory is closely linked to Hubble's discovery that the universe is expanding, which seems to imply that there was a time in the past when the expansion first began. Nevertheless, the Big Bang theory did not gain widespread acceptance among scientists until the 1960s. Why wasn't expansion alone enough to convince scientists that the Big Bang really happened?
Although expansion seems to imply a Big Bang, no other specific predictions of the Big Bang theory were tested and confirmed until the 1960s.
During the history of the universe, what important event occurred about 0.001 seconds after the Big Bang?
Most matter was annihilated by antimatter.
The electromagnetic force is stronger than gravity. Why is it over-powered by gravity on large scales?
Most objects are electrically neutral.
During the history of the universe, what important event occurred about 380,000 years after the Big Bang?
Light began to travel freely through the universe.
What happens when a particle of matter meets its corresponding antiparticle of antimatter?
The combined mass of the two particles is completely transformed into energy (photons).
Given that the electromagnetic force is far stronger than gravity on a per-particle basis, why doesn't the electromagnetic force dominate the interactions of large objects like planets, stars, and galaxies?
because there are both positive and negative electric charges
Essentially all the hydrogen nuclei that will ever exist in our universe were created __________.
by the time the universe was about 3 minutes old
Which of the following does not provide strong evidence for the Big Bang theory?
observations of the amount of hydrogen in the universe
Which of these pieces of evidence supports the idea that inflation really happened?
observations of the cosmic microwave background that indicate a flat geometry for the universe
holds protons and neutrons together in atomic nuclei
strong force
holds quarks together in protons and neutrons
strong force
The Big Bang theory is supported by two major lines of evidence that alternative models have not successfully explained. What are they?
(1) The theory predicts the existence of and the specific characteristics of the observed cosmic microwave background; (2) the theory correctly predicts the observed overall chemical composition of the primeval universe.
"Seeds" Inferred from CMB
• We can compare the structures we see in detailed observations of the microwave background with predictions for the "seeds" that should have been planted by inflation • So far, our observations of the universe agree well with models in which inflation planted the "seeds" - Overall geometry is flat. • Total mass + energy has critical density. - Ordinary matter is ~ 4.4% of total. - Total matter is ~ 27% of total. • Dark matter is ~ 23% of total (Ordinary matter 4%). - Dark energy is ~ 73% of total. • Age is 13.7 billion years. In excellent agreement with observations of present-day universe and models involving inflation and weakly interacting massive particles (WIMPs)!
The vertical axis of the graph measures the temperature of the universe in kelvin. If we move up one tick mark to the next, by what factor does the temperature increase?
100
Today, most scientists accept the Big Bang theory because its predictions agree so well with observations. But a scientific theory can always be revised or discarded if future observations do not agree with its predictions. Consider the following hypothetical future observations. Which one(s) would be inconsistent with the Big Bang theory?
- Astronomers discover distant protogalactic clouds with a helium abundance below 20%. - Careful studies of quasar spectra show that, 12 billion years ago, the temperature of the cosmic microwave background was slightly lower than it is today. - Over the next 10 years, the temperature of the cosmic microwave background falls to 1 K.
Primary Evidence for the Big Bang
1) We have detected the leftover radiation from the Big Bang. 2) The Big Bang theory correctly predicts the abundance of helium and other light elements.
Mysteries Needing Explanation
1) Where does structure come from? 2) Why is the overall distribution of matter so uniform? 3) Why is the density of the universe so close to the critical density? An early episode of rapid inflation can solve all three mysteries!
The horizontal axis of the graph measures the time since the Big Bang in seconds. The extreme left of the horizontal axis, where it meets the vertical axis, is labeled 10−45 seconds (that is a decimal point followed by 44 zeroes and then a one). If we move along two tick marks to the right on the horizontal axis, it is labeled 10−35 seconds. By what factor does the time increase from one tick mark on the horizontal axis to the next tick mark toward the right?
1*10^5
According to the Big Bang theory, how many forces-and which ones-operated in the universe during the GUT era?
2 forces: gravity and a single force that later became the strong, weak, and electromagnetic forces
To get your bearings on the horizontal time axis of this plot, it is useful to calculate the age of the universe in seconds today. If you multiply the age of the universe in years by the number of seconds in a year, approximately what is the current age of the universe in seconds?
4*10^7
Shown following are several times in the history of the universe. Imagine that you were able to watch a single photon that has been part of the cosmic microwave background since it first became present in the universe. Rank these times from left to right based on the wavelength this photon would have at each time, from shortest to longest.
500,000 years after the Big Bang 1 million years after the Big Bang 100 million years after the Big Bang 1.5 billion years after the Big Bang today
Shown following are several times in the history of the universe. Rank these times from left to right based on the peak wavelength in the spectrum of the cosmic microwave background, from shortest to longest.
500,000 years after the Big Bang 1 million years after the Big Bang 100 million years after the Big Bang 1.5 billion years after the Big Bang today
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
B. 95% H, 5% He, less than 0.02% other
Which of the four forces keeps you from sinking to the center of Earth? A. gravity B. electromagnetism C. strong force D. weak force
B. electromagnetism
In principle, if we could see all the way to the cosmological horizon we could see the Big Bang taking place. However, our view is blocked for times prior to about 380,000 years after the Big Bang. Why?
Before that time, the gas in the universe was dense and ionized and therefore did not allow light to travel freely.
Why did the Big Bang not produce heavier elements?
By the time helium could survive, the temperature had become too low for heavier elements to form.
Shown following are the same eras shown in Part A. Rank the eras from left to right based on the temperature of the universe during each one, from coolest to hottest.
Coolest to hottest: era of galaxies, era of atoms, era of nuclei, era of nucleosynthesis, particle era, electroweak era, GUT era, Planck era
According to the Big Bang theory, why do we live in a universe that is made of almost entirely of matter rather than antimatter?
During the first 0.001 second after the Big Bang, particles and antiparticles were made in almost but not perfectly equal numbers. Everything annihilated except the very slight excess of matter particles.
The Big Bang produced no elements heavier than helium because it was never hot enough for helium to undergo fusion.
False
Shown following are the names of several eras that are used to describe the history of the universe. Rank the eras from left to right in the order in which they occurred, from first to last.
From first to last: Planck era, GUT era, electroweak era, particle era, era of nucleosynthesis, era of nuclei, era of atoms, era of galaxies
What happens to gas as it freely expands?
It gets less dense and cools.
Classify each statement below as an observation or as an inference based on the current Big Bang model. (Note that the helium abundance is defined as the mass of helium relative to the mass of hydrogen.)
Observations - Large-scale structure looks about the same in all directions. - The cosmic microwave background temperature is 2.73 K. - The temperature of the cosmic microwave background varies slightly with direction. - The helium abundance is at least 25% in every galaxy studied so far. Inferences from a model - Photons of the cosmic microwave background have traveled through space for almost 14 billion years. - Large-scale structure grew around density variations present in the early universe. - The cosmic microwave background is radiation left over from the Big Bang. - Fusion during the universe's first five minutes produced 75% hydrogen and 25% helium (by mass).
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.
__________ forces us to think about why the sky is dark at night.
Olbers' paradox
Geometry of Universe
Overall geometry of the universe is closely related to total density of matter and energy.
Which of the following statements can NOT be tested by science today?
Prior to the Planck time, our universe emerged from another universe.
Shown following are the same eras shown in Parts A and B. Rank the eras from left to right based on the size of the observable universe during each one, from smallest to largest.
Smallest to largest: Planck era, GUT era, electroweak era, particle era, era of nucleosynthesis, era of nuclei, era of atoms, era of galaxies.
Which of the following important events occurred earliest in the history of the universe?
Space-time rapidly expanded during a brief period of inflation.
According to current understanding of physics, which four of the following are the four fundamental forces in nature?
Strong force, gravity, electromagnetism, and weak force
What was the significance of the end of the era of nucleosynthesis, when the universe was about 5 minutes old?
The basic chemical composition of the universe had been determined.
Why do we expect the cosmic background radiation to be almost, but not quite, the same in all directions?
The overall structure of the universe is very uniform, but the universe must have contained some regions of higher density in order for galaxies to form.
Why did the era of nuclei end when the universe was about 380,000 years old?
The universe had expanded and cooled enough for stable, neutral atoms to form.
Which of the following is not consistent with observations of the cosmic microwave background?
The universe is at least 20 billion years old.
In basic terms, what does the graph show?
The universe is getting cooler with time.
Which of these options is the best explanation for why the night sky is dark?
The universe is not infinite in space.
What happens when a proton collides with an antiproton?
They convert into two photons
Galaxies formed from tiny density perturbations that formed prior to the inflationary period.
True
In the particle era, particles of matter outnumbered particles of antimatter by about 1 particle in one billion.
True
Why can't current theories describe what happened during the Planck era?
We do not yet have a theory that links quantum mechanics and general relativity
How do we determine the conditions that existed in the very early universe?
We work backward from current conditions to calculate what temperatures and densities must have been when the observable universe was much smaller in size.
Although we can divide the history of the universe into many distinct "eras," sometimes it's useful just to have a broad perspective on what events occurred in the very early universe and what events occurred later. Listed following are events that occurred either before or after the first five minutes in the history of the universe. Match these events to the appropriate time period.
Within the first five minutes - antielectrons (positrons) as common as electrons - strong force and electroweak force first became distinct - temperature fell to 10^15 K - inflation occurred Sometime after the first five minutes - galaxies formed - stars formed - photons of cosmic microwave background released - carbon nuclei formed by fusion
What do we mean by inflation?
a sudden and extremely rapid expansion of the universe that occurred in a tiny fraction of a second during the universe's first second of existence
This figure shows the temperature of the universe over time, according to models of the Big Bang. At what age did the universe cool to 1 million degrees?
about 10^7 seconds after the Big Bang
Suppose you want to know what the temperature of the universe was 1 billion years ago. Where along the horizontal axis should you look?
almost all the way to the far right
When a particle of ordinary mater meets its precise opposite particle of antimatter, the result is __________ with complete conversion of mass into energy.
annihilation
What kinds of particles were present in the universe during the particle era?
approximately equal amounts of matter and antimatter
Observations of the ________ provide a way to test our theory of the Big Bang.
cosmic microwave background
explains the attraction between a proton and an electron
electromagnetism
explains why two protons will repel each other
electromagnetism
governs virtually all chemistry and biology
electromagnetism
The _________ is a single force that unifies the electromagnetic and weak forces.
electroweak force
Which of the following unifications as been directly tested and confirmed through experiments in particle accelerators?
electroweak unification
Compared to when the cosmic microwave background was first released, the radiation of the cosmic microwave background today is __________.
fainter and has most of its photons at longer wavelengths
From Part C, you know that in basic terms, the graph shows that the temperature of the universe decreases with time. In more specific terms, this graph shows that the temperature of the universe __________.
fell very rapidly when the universe was young, but is changing very gradually today
A _________ predicts that the strong, weak, and electromagnetic forces should become indistinguishable at high temperatures
grand unified theory
dominates the universe at the largest scales
gravity
the weakest of the four forces on a per-particle basis
gravity
In the past, the temperature of the universe was _____
hotter than it is today
If observations had shown that the cosmic microwave background was perfectly smooth (rather than having very slight variations in temperature), then we would have no way to account for ________.
how galaxies came to exist
The Big Bang theory seems to explain how elements were formed during the first few minutes after the Big Bang. Which hypothetical observation below (these are not real observations) would call our current theory into question?
the discovery of a galaxy with a helium abundance of only 10% by mass
Which of the following observations cannot be explained by the Big Bang theory unless we assume that an episode of inflation occurred?
the fact that the temperature of the cosmic microwave background is almost the same everywhere
What is the Big Bang theory?
the idea that all matter and energy in the universe began in an unimaginably dense state, and then space itself began expanding
Which of the following does inflation help to explain?
the uniformity of the cosmic microwave background
Consider this statement from Part A: "Photons of the cosmic microwave background have traveled through space for almost 14 billion years." This statement follows from our model of the Big Bang, because the Big Bang model is based on the idea that __________.
the universe began very hot and dense and has been cooling as it expands
One of the inferred statements from Part A is that "Large-scale structure grew around density variations present in the early universe." Observational evidence that such density variations really existed comes from the fact that the cosmic microwave background exhibits __________.
tiny temperature variations in different directions
Shown following are several times in the history of the universe. Rank these times from left to right based on the average temperature of the universe at each time, from coolest to hottest.
today 1.5 billion years after the Big Bang 100 million years after the Big Bang 1 million years after the Big Bang 500,000 years after the Big Bang
A "GUT" (grand unified theory) refers to theories that _________.
unify the strong force with the electromagnetic and weak nuclear forces
the only force besides gravity that affects neutrinos
weak force
Which of the four forces that exist today would be merged together at high energies in what scientists call the "GUT force"?
weak force, electromagnetism, strong force
Era of atoms
• Atoms form at approximately 380,000 years • Background radiation released • Mixture of neutral atoms and radiations • Density of matter is slightly different in different regions - Gravity assembles protogalactic clouds
Planck era
• Before Planck time (~10^-43 second) • Temperature: above 10^32 K • Current theories of physics cannot make predictions about this time - No theory of quantum gravity • Single force? -"freezing out" of gravitational force at the end of Planck era
Era of nucleosynthesis
• Begins when matter annihilates remaining antimatter at ~ 0.001 s • Ends at about 5 minutes • Temperature: about 10^9 K • Nuclear fusion - When fusion ceases at the end of the nucleosynthesis era, the Universe is about 75% H, 25% He • (roughly the present day composition of the Universe)
Fundamental Forces
• Four known forces in universe: Strong force Electromagnetism Weak force Gravity Do forces unify at high temperatures? YES (Electroweak) Do forces unify at high temperatures? MAYBE (GUT) Do forces unify at high temperatures? Who knows? (String Theory)
Era of galaxies
• Galaxies form at approximately 1 billion years • First generation star formation • Era of galaxies continues to this day
Era of nuclei
• Helium nuclei form at age ~ 3 minutes • Universe became too cool to blast helium apart - Temperature: about 3000 K by the end • Timeframe: until 380,000 years
Inflation and Cosmic Microwave Background
• How can microwave temperature be nearly identical on opposite sides of the sky? • Regions now on opposite sides of the sky were close together before inflation pushed them far apart
Inflation
• 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 in the universe. • Observable universe became smooth before inflation, when it was very tiny. • Inflation flattened the curvature of space, bringing expansion rate into balance with the overall density of mass-energy. • Inflation of the universe flattens its overall geometry like the inflation of a balloon, causing the overall density of matter plus energy to be very close to the critical density • Observable universe became smooth before inflation, when it was very tiny • Inflation flattened the curvature of space, bringing expansion rate into balance with the overall density of mass-energy
GUT era
• Lasts from Planck time (~10^-43 second) to end of GUT force (~10^-38 second) • Temperature: above 10^29 K • Splitting of the GUT force into strong and electroweak - May have released an enormous amount of energy causing inflation (dramatic expansion) • In 10^-36 second, the universe went from the size of atomic nucleus to the size of the solar system Lasts from Planck time (~10^-43 second) to end of GUT force (~10^-38 second) • Temperature: above 10^29 K • Splitting of the GUT force into strong and electroweak - May have released an enormous amount of energy causing inflation (dramatic expansion) • In 10^-36 second, the universe went from the size of atomic nucleus to the size of the solar system
Electroweak era
• Lasts from end of GUT force (~10^-38 second) to end of electroweak force (~10^-10 second) • Temperature: 10^15 K by 10^-10 s • In the beginning, electromagnetic and weak forces were still joined together • At 10^-10 s, the four forces are forever separate • Intense radiation still fills all the space • Expansion continues
Did inflation really occur?
• Patterns observed by WMAP show us the "seeds" of structure in the universe. • Observed patterns of structure in universe agree (so far) with the "seeds" that inflation would produce.
Particle creation and annihilation
• Photons converted into particle-antiparticle pairsand vice versa. E= mc^2 • The early universe was full of particles and radiation because of its high temperature.
Abundances of elements
• Protons and neutrons combined to make long-lasting helium nuclei when universe was ~ 3 minutes old • Big Bang theory prediction: 75% H, 25% He (by mass). This prediction matches observations of primordial gases
Cosmic Microwave Background
• The cosmic microwave background—the radiation left over from the Big Bang—was detected by Penzias and Wilson in 1965 • Background radiation from Big Bang has been freely streaming across universe since atoms formed at temperature ~ 3000 K: visible/IR • Background has perfect thermal radiation spectrum at temperature 2.73 K • Expansion of universe has redshifted thermal radiation from that time to ~1000 times longer wavelength: microwaves • The Wilkinson Microwave Anisotropy Probe(WMAP), originally known as the Microwave Anisotropy Probe (MAP), was a spacecraft operating from 2001 to 2010 which measured temperature differences across the sky in the cosmic microwave background (CMB) -the radiant heat remaining from the Big Bang. • WMAP gives us detailed baby pictures of structure in the universe.
The Early Universe
• The early universe must have been extremely hot and dense
Observing the evidence of Big Bang
• The night sky is dark because the universe changes with time. As we look out in space, we can look back to a time when there were no stars
Particle era
• Time: before 0.001 s • Temperature: 10^12 K (too cool for spontaneous exchange of matter and energy) • Amounts of matter and antimatter nearly equal (roughly 1 extra proton for every 10^9 proton-antiproton pairs!) - This excess proton accounts for all present matter in the Universe • Photons are turning into quarks - By the end of particle era, quarks are combining into protons and neutrons
