17. 3 The Big Bang and Inflation 17 17.3 The Big Bang and Inflation 17.4 Observing the Big Bang for yourself 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? How is the darkness of the night sky evidence for the Big Bang?

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

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

How can the microwave background temperature be identical to 1 part in 10,000 on opposite sides of the sky? It can’t – something must be wrong with the observations Very small motions, producing very small Doppler shifts Opposite sides of the sky must have been in contact with each other :00

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

Mysteries Unexplained by the original Big Bang model Where does structure come from?

Mysteries Unexplained by the original Big Bang model Where does structure come from? 2) Why is the overall distribution of matter so uniform?

Mysteries Unexplained by the original Big Bang model 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?

Mysteries Unexplained by the original Big Bang model 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 of the universe can solve all three mysteries!

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

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

Our Universe’s Properties, as Inferred from the Cosmic Microwave Background 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

Our Universe’s Properties, as Inferred from the Cosmic Microwave Background 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 as dark matter!

But what caused inflation? Good question. Currently (always?), science runs out of answers to “why?” questions at this point. But cosmologists have lots of ideas! String Theory The Multiverse …

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? • 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.

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.