1. The Birth Of Our Universe The Big Bang And Inflation

2. What were conditions like in the early universe?

3. The universe must have been much hotter and denser early in time.
Estimating the Age of the Universe

4. The early universe must have been extremely hot and dense.

5. Photons converted into particle–antiparticle pairs and vice versa.
E = mc2
The early universe was full of particles and radiation because of its high temperature.

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

7. Primary Evidence for the Big Bang
We have detected the leftover radiation from the Big Bang.
The Big Bang theory correctly predicts the abundance of helium and other light elements in the universe.
The Hubble Expansion Law (we discussed this previously).

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

9. The cosmic microwave background— the radiation left over from the Big Bang— was detected by Penzias and Wilson in 1965.

10. Background radiation from the Big Bang has been freely streaming across the universe since atoms H formed at temperature ~3000 K: visible/IR.

11. Background has perfect thermal radiation spectrum at temperature 2
Background has perfect thermal radiation spectrum at temperature 2.73 K.
Big Bang photons had a blackbody temperature of 3000K at decoupling. The expansion of the universe has redshifted thermal radiation from that time to ~1000 times longer wavelength: microwaves.

12. Full Sky in All Wavelengths

13. The WMAP satellite gives us detailed baby pictures of structure in the universe.

14. How do the abundances of elements support the Big Bang theory?

15. Protons and neutrons combined to make long-lasting helium nuclei when the universe was ~5 minutes old.

16. Big Bang theory prediction: 75% H, 25% He (by mass)
Matches observations of nearly primordial gases

17. 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% normal matter—more evidence for WIMPS!

18. 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

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

20. Mysteries Needing Explanation
Where does structure come from?
Why is the overall distribution of matter so uniform?
Why is the density of the universe so close to the critical density?

21. Mysteries Needing Explanation
Where does structure come from?
Why is the overall distribution of matter so uniform?
Why is the density of the universe so close to the critical density?
An early episode of rapid inflation can solve all three mysteries!

22. How does inflation explain these features of the universe?

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

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

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

26. The overall geometry of the universe is closely related to total density of matter and energy.
Density = Critical
Expand forever
Density > Critical
Will eventually stop expanding and collapse ‘big crunch’
Density < Critical
Will expand forever

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

28. Patterns of structure observed by WMAP show us the “seeds” of the universe.

29. Observed patterns of structure in the universe agree (so far) with the “seeds” that inflation would produce.

30. “Seeds” Inferred from CMB
Overall geometry is flat
Total mass + energy has critical density
Ordinary matter ~4.6% of total
Total matter is ~28% of total
Dark matter is ~23% of total
Dark energy is ~72% of total
Age of 13.7 billion years
In excellent agreement with observations of present-day universe and models involving inflation and WIMPs! Recall when we discussed dark matter in galaxy clusters and in individual galaxies … we found a similar result for dark matter.

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

32. Olbers’ Paradox
infinite
unchanging
everywhere the same
If the universe were
then stars would cover the night sky.

33. As we look out in space, we can look back to a time when there were no stars.
Solution to Olbers’ paradox is the universe has a finite AGE.