SCALES: TIME

1. SCALES: TIME

2. COSMIC HISTORY

3. t = 0:The Big Bang The moment in time when the universe started expanding from its initial extremely dense state. How do we know that this happened? Universe was denser in the past; extrapolating back to infinite density, that was a finite timeago.

4. t = 0:The Big Bang Why do we care that this happened? If the universe had remained dense, it wouldn’t have cooled enough for nuclei, atoms, galaxies, and usto form. (Speaking to an audience of humans, I make no apologies for my human chauvinism.)

5. t = 10-34 sec: Inflation A brief period when the universe rapidly inflated. The cause is speculative, but is associated with the unification of forces in the early universe.

6. t = 10-34 sec: Inflation How do we know that this happened? The universe is nearly flat. To be this close to flat now it must have been extremelyclose to flat very early in the history.

7. t = 10-34 sec: Inflation Why do we care? If the universe hadn’t been flattened, it would have long since collapsed in a Big Crunchor fizzled out in aBig Chill. No inflation, no galaxies.

8. t = 3 minutes Big Bang Nucleosynthesis A period when protons and neutrons fused to form helium, ¼ of the universe by mass.

9. t=3 min: Big Bang Nucleosynthesis How do we know that this happened? The earliest stars contain 75% hydrogen, 25% helium, exactly as predicted from the theory of Big Bang Nucleosynthesis. (Later stars contain morehelium, made in previousgenerations of stars.)

10. t=3 min: Big Bang Nucleosynthesis Why do we care? It shows we understand what the universe was like when it was less than 10 minutes old. Big Bang theory has predictive power. No nucleosynthesis, no periodic table (until the 1st stars).

11. t = 400,000 years Transparency A period when the universe had cooled so that protons & electrons joined to form neutral atoms. after before

12. t = 400,000 years: Transparency How do we know that this happened? Cosmic Microwave Backgroundis the “leftover light” from when the universe was hot/opaque, stretched to long wavelengths by the expansion.

13. t = 400,000 years: Transparency Why do we care? If the universe were still opaque, we wouldn’t be able to see distant galaxies. No transparency, no astronomers.

14. t = 750 million years The First Galaxies A period when gas cools, falls to center of dark halos, and fragments into stars.

15. t=750 million years: First Galaxies How do we know that this happened? We see galaxies with large redshift (implying large distance, implying distant past). They appear young.

16. t=750 million years: First Galaxies Why do we care? We live in a galaxy, orbiting a star. We are made of ejecta from stars. No stars, no photosynthesis.

17. t = 13.7 billion years Now A period when (more-or-less) intelligent life on Earth wonders about how the universe works.

18. Geshe Activity Work in groups to order events in the history of the universein the roughly correct order. If events are contemporaneous put them side by side. When they’re done, each group will have their order critiqued by the other groups.

19. COSMIC CALENDAR

20. How do our lifetimes compare to the age of the universe? • The Cosmic Calendar: a scale on which we compress the 13.7 billion year history of the universe into 1 year. • This is a time scale model that used a scale factor of 14,000,000,000:1. • Our lives would scale similarly, so 80 years goes down by a factor of 14 billion too. • In the scale model, a human life lasts about two tenths of a second!

22. Geshe Activity Let’s get a sense of how human time scales compare to the enormous cosmic time scales. Make a timeline in the classroom, the wall is 14 billion years, from big bang to now. Place the sheets of paper at appropriate locations.