1. The Age of the Universe

2. Remember the Grenade The ‘Hubble law’ allows us to work out the time since the expansion began

3. The Calculation Simplified Assume things began in a very dense state, all compressed together. A remote galaxy is now a distance D away from us, moving with recession velocity V. How long did it take to cover that distance?

4. The Answer Distance = Speed x Time, so Time = Distance / Speed = D / V [or: I’m driving down Highway 401, just passing Ajax, 200 km from Kingston. I have been driving steadily at 100 km/hr. When did I leave Kingston? Answer: 2 hours ago]

5. In Principle, Any Single Piece Will Do But remember: individual galaxies have small random motions in addition to the general expansion. In practice, therefore, we consider the overall behavior defined by many galaxies

6. Hubble’s Answer He calculated an ‘expansion age’ of about 1.8 billion years (He believed he was within ~10% of the correct value) This seemed plausible at the time.

7. But Consider The Earth Itself

8. A Conundrum! As the science of radioactive age-dating developed, geologists subsequently found even older rocks on Earth (up to 3.5 billion years or more) A puzzle: How can the Earth be older than the universe within which it is contained?

9. What Was the Problem? Hubble’s distance estimates were systematically wrong. The galaxies are farther away than he thought, by a factor of about seven. So he underestimated the age (using Time = Distance / Speed)

10. Why Was He in Error? (It Was Not Carelessness!) Three Reasons 1. Confusion between two kinds of Cepheids of differentluminosities.

11. Why the Error? 2. The difficulty of correcting for interstellarobscuration

12. Why the Error? 3. He was using photographic plates - hard to calibrate and measure. (Now much easier with electronic detectors!)

13. A Heroic Rescue Attempt -- Now Seen as an Irrelevant Detour The Steady-State Universe The Steady-State Universe (unchanging; lasts for infinite time) (unchanging; lasts for infinite time)

14. Fred Hoyle and Hermann Bondi

15. The Philosophical Underpinnings Remember the Cosmological Principle? The universe looks the same (on large scales) to all observers. The universe looks the same (on large scales) to all observers. i.e. we are in no privileged location

16. But Why a Special Time? The Perfect Cosmological Principle (PCP): The Universe looks the same (on large scales) to all observers and at all times. i.e. we are in no special place and live at no special time

17. The Implication If the PCP is true, then a trillion years from now the universe will look pretty much as it does today.

18. But Galaxies are Drifting Apart; the Universe is “Thinning Out” Doesn’t this immediately rule out Steady-State?

19. Solution: Continuous Creation To make up the deficit, new material could be appearing from nothing – at an unobservably small rate, particle- by-particle (not whole galaxies, as suggested here!)

20. Do You Object To This? You may well ask: How can material appear out of nothing? But don’t forget: the Big Bang model does this as well! It just assumed that it happened in one initial instant. The question of origin is a tough one!

21. Still, We Can Test This! Look far out into space, and thus back in time, to see if ‘things were different in the old days.’If the universe is evolving in its overall properties, we can rule out the Steady-State model! We see this in at least two obvious ways (and more): remote galaxies are not the same as the present- day ones (Hubble Ultra-Deep Field) remote galaxies are not the same as the present- day ones (Hubble Ultra-Deep Field) we are surrounded by the Cosmic Microwave Background, the remnant of a Hot early phase we are surrounded by the Cosmic Microwave Background, the remnant of a Hot early phase

22. Looking Back in Time