The Accelerating Universe

1. The Accelerating Universe

2. The Hubble Law According to the Hubble Law, the space between the galaxies is constantly increasing, with Velocity = H0 Distance By observing how the expansion rate has changed over time, we can measure how much effect gravity has had on the universe, i.e., its deceleration. When we do this with supernovae, we find

3. The Accelerating Universe!!! The universe is not slowing down at all. In fact, it’s speeding up!!! We live in an accelerating universe! It’s as if there’s another force pushing the universe apart – a Cosmological Constant!!!

4. The Accelerating Universe!!! Whatever this force is, we think that it is growing stronger as the universe evolves. The more empty space in the universe, the greater the acceleration – as if the vacuum of space has pressure!

5. The Accelerating Universe!!! Whatever this force is, we think that it is growing stronger as the universe evolves. The more empty space in the universe, the greater the acceleration – as if the vacuum of space has pressure!

6. The Accelerating Universe!!! We appear to live in a universe with a flat shape, but which will go on accelerating forever. The universe is 13.7 billion years old, and is now dominated by Dark Energy. And it will only get worse – the more empty space, the more Dark Energy. This explains the age mismatch between globular clusters and the universe. The universal expansion is getting faster!

7. The Accelerating Universe!!! We appear to live in a universe with a flat shape, but which will go on accelerating forever. The universe is 13.7 billion years old, and is now dominated by Dark Energy. And it will only get worse – the more empty space, the more Dark Energy. The Dark Energy even dwarfs dark matter! Regular matter is really insignificant. We really don’t know anything about what’s going on!!

8. What is the Dark Energy? We’re clueless. There is one “traditional” theory– that particles and anti-particles are constantly being created and annihilated in the empty space (due to the uncertainty principle). For the instant these particles exist, they would act as a repulsive force. But our estimate of this force is off by a factor of 10122.

9. History of the Universe • The Big Bang occurred 13.7 billion years ago. Since then • +9,000,000,000 years: Birth of the Sun • +2,000,000,000 years: era of galaxy formation/interaction • +400,000,000 years: Milky Way begins to form • +100,000 years: release of the microwave background

10. Helium in the Universe If the universe began as a high density soup of protons and neutrons, some of those particles must have undergone fusion. In the Big Bang, about 1 of every 10 hydrogen atoms should have been changed to helium. That’s almost exactly the helium abundance we observe for the universe!

11. History of the Universe • The Big Bang occurred 13.7 billion years ago. Since then • +9,000,000,000 years: Birth of the Sun • +2,000,000,000 years: era of galaxy formation/interaction • +400,000,000 years: Milky Way begins to form • +100,000 years: release of the microwave background • +3 minutes: fusion of hydrogen to helium ends • +0.00001 seconds: protons, neutrons form • +10-12 seconds: particles form and annihilate • +10-35 seconds: quarks form; gravity begins to exist • +10-43 seconds: ???? Grand unification

12. In the First 10-35 Seconds • Why is the universe flat? • Why does one side of the sky look like the other side of the sky? (They were never in contact with each other.) • Why are there no monopoles? (Magnets always have a north pole and a south pole.)

13. Inflation A theory which explains these puzzles (and others) is that, very early on (10-35 sec after the beginning), the universe expanded much faster than now (1030 instead of 64). This is called inflation. The universe we see now is just a small region of a “bubble”. It therefore just looks flat. (the observable universe is in red)

14. Multiverses Inflation allows that our bubble may not be the only bubble. Bubbles may be forming all the time in a multi-universe. (But these other universes can never be observed.)

15. Multiverses Inflation allows that our bubble may not be the only bubble. Bubbles may be forming all the time in a multi-universe. (But these other universes can never be observed.)

16. Active Galactic Nuclei or The Monster Within

17. The Discovery In 1962, Cambridge University just completed a radio survey of the sky. Maarten Schmidt took their radio positions and looked for optical counterparts. He found a few peculiar “radio” stars. 3C 273 looked like an ordinary, fairly-bright star (with possibly a little fuzz). But ordinary stars do not emit much in the radio part of the spectrum.

18. The Spectrum The spectrum of the star was odd. It had • Emission lines instead of absorption lines • Broad (~10,000 km/s) emission lines, instead of narrow lines • Emission lines at “strange” wavelengths The solution: the emission lines were those of hydrogen, but at enormous redshift. The object was moving away at 10% of the speed of light!

19. Quasars Properties of quasi-stellar radio sources (quasars, or QSOs): • Star-like appearance (with possibly some “jets”) • Emission-line spectra with internal motions of ~10,000 km/s • Does not emit as a blackbody (at least, not at a single temperature). The objects emit light in x-rays, ultraviolet, optical, infrared, and sometimes microwave and radio • Irregularly variable on timescales of days/months • Enormous redshifts (can be more than 90% of the speed of light) Stars in the Milky Way cannot move that fast. The only way to achieve such a redshift is through the Hubble Law. So, through v = H D, the objects must be incredibly far away. They are therefore incredibly bright – as bright as 1000 supernovae.

20. Size and Variability • Imagine that there is some mechanism near the center of the QSO that controls the object’s brightness. It says “get bright”. • That command goes forth no faster than the speed of light. • Within a few months, the object gets bright. • Since no signal can go faster than the speed of light, the object must be no bigger than a few light-months across! Since many quasars vary in brightness we have a crude way to estimate their size.

21. The Energy Source What can outshine ~1000 supernovae for millions of years, and be just slightly larger than our Solar System? Theoretically, not much – only a very, very big black hole. • Start with a 10,000,000,000 M black hole • Have a star come close enough to be tidally disrupted • Have the material form into an accretion disk. Energy is released via the friction in the disk. If you accrete ~ 1 M per year, the friction you get will produce the luminosity of a quasar.

22. Feeding the Monster If a star comes too close, the enormous gravity of the black hole will cause tides on the star and rip it apart. Some of that material will be trapped in orbit about the hole.

23. Feeding the Monster If a star comes too close, the enormous gravity of the black hole will cause tides on the star and rip it apart. Some of that material will be trapped in orbit about the hole.

24. Explaining a Quasar’s Properties • Near the event horizon, the gas is moving close to the speed of light. Any emission lines which are produced will be broad. • Because of the high speed of the gas, there is a lot of friction in the disk. A lot of light is produced. • The temperature of the disk depends on the speed of the gas. Near the event horizon, the friction produces x-rays. At larger radii, where the gas revolves more slowly, optical and infrared light is made.

25. Black Holes and Jets As matter accretes onto the black hole, particles can get ejected out the poles of the system at 99.999% of the speed of light. How this occurs is almost a complete mystery. But it’s often observed.

26. Where are the Quasars Today? The nearest quasar is 25% of the way across the universe; most belong to an era when the universe was only 15% of its present age. If supermassive black holes existed then, where they now? In the centers of galaxies!

27. The Quasar-Galaxy Connection When a supermassive black hole is accreting, it can be thousands of times brighter than its surrounding galaxy. On the other hand, if the black hole is not accreting, it will be invisible.

28. Active Galactic Nuclei Many nearby galaxies have some activity in their nucleus: they may have an extremely bright nucleus, or show a jet of emission, or have broad emission lines, or emit at radio wavelengths. These objects (which are probably just accreting a little mass) are said to have an Active Galactic Nucleus. The energy produced by an AGN is still often many times that of the stars.

29. Galaxies with Active Galactic Nuclei

30. Sleeping Monsters When a black hole is not accreting matter, then it’s invisible. But its gravitational influence on its surroundings can still be detected – the stars surrounding the hole must move fast (due to Kepler’s and Newton’s laws).

31. Sleeping Monsters There’s even a 2,000,000 M black hole at the center of the Milky Way. We can measure its mass by the motions of stars which pass close to it.

32. AGN and Starbursts In the present day universe, AGN are rare. However, they are more common in interacting galaxies. This suggests that the orbits of some stars have been perturbed enough to pass close to the black hole. It also suggests that all galaxies possess supermassive black holes.

33. AGN and the Universe Since quasars can be seen 90% of the way across the universe, they allow us to detect gas throughout the universe. We can therefore examine galaxies (and proto-galaxies) that we can’t even see! Any time the light from a quasar goes through a galaxy that has hydrogen gas, there will be absorption at the wavelength appropriate to hydrogen. But remember – this hydrogen is moving, due to the Hubble Law. So …

34. AGN and the Universe Each absorption is due to hydrogen gas at a different redshift (i.e., distance). Quasars allow us to probe structure throughout the universe!

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