Quasars Quasars are small, extremely luminous, extremely distant galactic nuclei Bright radio sources Name comes from Quasi-Stellar Radio Source, as they appeared to be stars! Can have clouds of gas near them, or jets racing from their cores Spectra are heavily redshifted, meaning they are very far away Energy output is equivalent to one supernova going off every hour! The HST was able to image a quasar, showing it to be the active core of a distant galaxy Energy Source for Active Galactic Nuclei Active galactic nuclei emit a tremendous amount of radiation over a broad range of wavelengths A black hole can be both very small, and have an accretion disk that can emit enough radiation Likely that at the centers of these galactic nuclei, there are supermassive black holes Intense magnetic fields in the accretion disk pump superheated gas out into jets that leave the nucleus There are still many questions to be answered 1
Energy Source for Active Galactic Nuclei Active galactic nuclei emit a tremendous amount of radiation over a broad range of wavelengths A black hole can be both very small, and have an accretion disk that can emit enough radiation Likely that at the centers of these galactic nuclei, there are supermassive black holes Intense magnetic fields in the accretion disk pump superheated gas out into jets that leave the nucleus There are still many questions to be answered 2
Seyferts and quasars are both types of active galaxies, harboring powerful luminous nuclei. Quasar nuclei appear to be more luminous, and therefore their black holes a) are accreting matter at a higher rate. b) are more massive. c) are less obscured along our sightline. d) any of the above. Unification of AGN See into the beast Partially obscured obsecured 3
Many galaxies have flat rotation curves! Dark matter is not unique to the Milky Way! Figure 78.03 99 percent of the stars in a galaxy are within 20 kpc of the center Gas extends far out into the disk, but is not very massive! Galaxies are now thought to be embedded in a dark matter halo that surrounds the entire galaxy Unfortunately, dark matter cannot be detected directly. 4
Dark Matter in Clusters of Galaxies Missing mass is also a problem in clusters of galaxies! Not enough visible mass to hold the clusters together by gravitation, and to keep hot gas in their vicinity Cluster mass must be 100 times greater than the visible mass! Once again, dark matter seems to be the solution Gravitational Lenses Dark matter warps space just like ordinary matter does The path of light rays bends in the presence of mass A galaxy or other massive object can bend and distort the light from objects located behind it, producing multiple images This is called gravitational lensing 5
Figure 78.06 Which of the following observations about the nature of the universe can be made without using any special equipment? a) The universe is expanding. b) Most of the matter in the universe does not emit light. c) Luminous matter in the universe occurs in clumps rather than being evenly distributed. d) There is background radiation from the Big Bang. 6
The expanding Universe V = H d The expansion of the universe and the increasing distance between galaxies is similar to the increase in distance between raisins in a rising loaf of raisin bread. Problem with these analogies loaves and rubber bands have edges! We have seen no edge to the Universe; there are an equal number of galaxies in every direction! Also, galaxies can move relative to space, as sometimes gravity can accelerate one galaxy toward another faster than space expands! The expanding Universe V = H d Looks like the Milky Way is at the center of the Universe, and all galaxies are moving away from us Hubble s Law can be applied to any observer in any galaxy No matter where you are, an expanding Universe will give this appearance! This is the cosmological principle The expansion of the Universe is not like the explosion of a bomb sending fragments in all directions Space itself is expanding! No Center, No Edge! 7
Suppose the Universe were not expanding, but was in some kind of steady state. How should galaxy recession velocities correlate with distance? They should a) be directly proportional to distance. b) reverse the trend we see today and correlate inversely with distance. c) show a scatter plot with most recession velocities positive. d) show a scatter plot with equal numbers of positive and negative recession velocities. The Meaning of Redshift As light waves travel through space, they are stretched by expansion This increases the wave s wavelength, making it appear more red! An objects redshift, z, is "! z =! Here, Δλ is the change in wavelength, and λ is the original wavelength of the photon This is equivalent to: Change in average distance between galaxies z = Average distance between galaxies 8
If the universe is expanding, won't the solar system eventually expand apart? a) The solar system may actually be shrinking now, which makes the Universe LOOK like it's expanding. b) No, its gravity holds it together. c) No, because there is no planetary redshift. d) Eventually, but only after a very long time. The Age of the Universe Thanks to the Hubble Law, we can estimate the age of the universe At some point in the distant past, matter in the universe must have been densely packed. From this point, the universe would have expanded at some high speed to become today s universe Assuming a constant expansion over time, we find that the age of the universe is around 14 billion years. 9
Olber s Paradox Over very large distances, galaxies in the universe are more or less uniformly distributed (homogeneous) If there are galaxies in every direction, however, why do we not have a fully-lit sky? We should see a star in any direction we look! This is called Olber s Paradox If there is an edge to the universe, we should be able to see our way out of the woods Olber s Paradox 10
A Solution? In a sense, there is an edge to the universe, an edge in time Light travels at a finite (though fast) speed The size of the visible universe is defined as the distance light can travel in the age of the universe Galaxies exist at greater distances, but light from them has not reached us yet. The edge is called the cosmic horizon If we wait long enough, the night sky might become bright! Suppose you've accepted that the universe is expanding, and will always expand. You must then accept that a) there is no center to the universe b) the observable universe is infinite c) the distance between stars in the galaxies grows. d) either the density of the universe always decreases or new matter must be continuously created. 11
The Last Scattering Epoch Minutes after the Big Bang, the Universe was opaque High temperatures kept all matter ionized Photons could only travel a short distance before being absorbed After 400,000 years, the Universe cooled enough for electrons and ions to recombine, allowing light to pass Now the Universe was transparent! Light from the Early Universe So what should light from 400,000 years after the Big Bang look like? It should have a spectrum that corresponds to the temperature of the Universe at that time, 3000 K. Expansion of space will stretch this light, however The Universe has expanded by a factor of 1000 since this time, so the wavelength will have stretched by the same amount Spectrum will correspond to a temperature of 3K. This light from the early Universe has been found, and is called the Cosmic Microwave Background 12
Clumpiness in the CMB Clumpiness in the CMB 13
Clumpiness in the CMB The Curvature of the Universe Remember that mass and energy can curve the space around it. As the Universe expands, the distances between the galaxies increases, like galaxies painted on the surface of an inflating balloon If the universe was like an expanding balloon (but with the galaxies distributed in three dimensions), travel in any direction would eventually bring you back to your starting place (a closed universe) No Center, No Edge! 14
Other Possible Curvatures of Space In addition to a closed, or positive curvature of space, there are two other options Space could be flat, or have zero curvature Space could be curved away from itself, or have negative curvature Geometry behaves differently with each curvature! Expansion Forever? Or Collapse? The fate of the universe is ultimately controlled by its total amount of energy Energy of expansion (positive) Gravitational energy that can slow the expansion (negative) Binding energy If the total energy is positive or zero, the expansion continues forever If the total energy is negative, the expansion will halt, and the universe will contract and eventually collapse. 15
Density of the Universe If we can measure the density of the universe, we can predict how much gravitational energy the universe has, and therefore whether it will collapse or keep expanding The critical density of the universe, ρ C, is the density at which the total energy of the universe is zero " C 3H 2 = 8! G Ω M = ρ / ρ C, where ρ is the measured density of the universe If Ω M > 1, the universe will recollapse If Ω M < 1, the universe will expand forever If Ω M = 1, the universe is exactly at the critical density If the Big Bang theory is correct, and there is not enough mass to close the universe, then a) more Big Bangs will occur. b) there is no "dark matter". c) the universe will eventually be entirely cold. d) the expansion will slow to a halt. 16
Supernova Type Ia Findings We also need to know how the universe is expanding this can help us determine the value of Ω M We can measure the recession velocity of distant galaxies using Type Ia supernovae as standard candles It appears that the expansion rate at a time when the universe was half its current size (z=1) was slower than it is today! This shows that the expansion rate is increasing with time! Very puzzling! Dark Energy! Dark energy may provide the solution to the mystery Dark energy remains constant everywhere, regardless of the universe s expansion Provides an outward push to accelerate expansion: antigravity! In order for dark energy to balance the equation, it must make up around 70% of all of the energy in the universe Much work remains to be done on this frontier 17