Lecture 17: Dark Energy & The Big Bang

Transcription

1 Lecture 17: Dark Energy & The Big Bang As with all course material (including homework, exams), these lecture notes are not be reproduced, redistributed, or sold in any form.

2 Solution? ~1998 astronomers discovered something very strange The Universe is not decelerating. It is in fact accelerating!

3 So how do we measure the expansion history of the Universe?

4 So how do we measure the expansion history of the Universe? How do we observe the expansion of the Universe?

5 From Lecture 14: Hubble showed that more distant galaxies appear to be moving away from us more quickly. Straight line given by this equation: Today s value: 70

6 If the Universe is expanding at a constant rate (over time), then we would expect.

7 If the Universe is expanding at a constant rate (over time), then we would expect.

8 If the Universe is expanding at a constant rate (over time), then we would expect. velocity (or redshift) distance

9 If the Universe is expanding at a constant rate (over time), then we would expect. velocity (or redshift) distance

10 what if the expansion was slowing down (as you d expect due to gravity) velocity (or redshift) distance

11 what if the expansion was slowing down (as you d expect due to gravity) velocity (or redshift) Decelerating (faster in past) distance

12 Now if the expansion is accelerating? velocity (or redshift) distance

13 Now if the expansion is accelerating? velocity (or redshift) Accelerating (slower in past) This is exaggerated the effect is actually quite small. distance

14 So, to measure expansion history, we need to be able to measure redshifts (velocities) of distant galaxies and measure their distances.

15 So, to measure expansion history, we need to be able to measure redshifts (velocities) of distant galaxies and measure their distances. Redshifts are relatively easy with a big telescope like Keck

16 So, to measure expansion history, we need to be able to measure redshifts (velocities) of distant galaxies and measure their distances. Redshifts are relatively easy with a big telescope like Keck measuring distances is difficult

17 Measuring the acceleration SUPERNOVAE can be standard light bulbs Certain kinds of supernovae have a relationship between how fast they get bright and how intrinsically bright they are -- similar to cepheid variable stars. By identifying distant Supernovae, we can determine how far away they are and measure the expansion rate of the Universe back very far into the past.

18 Remember Einstein s Greatest Blunder? Friedmann Einstein, your equations give rise to an expanding Universe. Einstein I will change my equations to keep the Universe static. Λ Einstein proposes a cosmological constant that acts to keep the Universe static.

19 Current Ideas to explain Dark Energy Λ? I told you so! Einstein s Cosmological Constant Quintessence Varying Vacuum Energy

20 Current Ideas to explain Dark Energy Λ? I told you so! Einstein s Cosmological Constant Quintessence Varying Vacuum Energy Is Einstein s theory of gravity (General Relativity) valid on the largest scales probed by cosmology? New physics? a 5th fundamental force?

21 Composition of the Cosmos Normal Matter 5% Dark Matter 25% We ll discuss this soon Dark Energy 70%

22 Expanding Universe leads to the idea of a Big Bang? If the Universe is expanding, we expect that at some time in the past the size was nearly zero. This implies that the Universe has a finite age (not infinitely old).

23 The Big Bang Theory Hot Cold Bang! ~14 billion years Image: NASA/WMAP

24 The Big Bang Theory Universe was smaller and hotter in the past. At ~1 sec after the big bang, the Universe was very hot, T~10 10 K. So hot, that atoms were ionized. Electrons were roaming free without being bound to nuclei. 300,000 years after the Big Bang, the Universe had cooled down to T=3000 K. Now, electrons combined to make neutral atoms & the Universe became transparent to photons. It is now about 14 billion years since the big bang, and the universe is very cold, T=3K.

25 History of the Universe Observable Universe, Galaxies Light travels freely light nuclei & free electrons quarks & primordial particles e p n n p Big Bang e e n e p - e - + Cold! 300,000 years 3 min s s Hot!

26 Evidence for the big bang? Just because the Universe is expanding, it doesn t mean that there really was a big bang. What are the observational predictions of a model where the Universe was very much smaller in the past?

27 The Big Bang Theory If the Universe was much smaller in the past, it would have been much hotter in the past. Theorists like Robert Dicke in the 1960 s predicted that this glow from the Big Bang could still be observed. The glow from the big bang should be observable as a T=3K Blackbody curve, seen in all directions from empty space.

28 History of the Universe For the first 300,000 yrs the Universe was so hot that light could not propagate freely. Electrons were not bound to atoms (atoms were ionized) and since light tends to scatter off of free electrons, the Universe was like a thick fog. microwave background light nuclei & free electrons Quarks & primordial particles e e p n n p Big bang e n e p - e - + Observable Universe, Galaxies 300,000 years 3 min s s

29 History of the Universe For the first 300,000 yrs the Universe was so hot that light could not propagate freely. Electrons were not bound to atoms (atoms were ionized) and since light tends to scatter off of free electrons, the Universe was like a thick fog. microwave background n e p n n p eno light from this time can reach us e e p - e - + Big bang Observable Universe, Galaxies 300,000 years 3 min s s